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  • Atomic, Molecular and Optical Physics  (441)
  • Physical Chemistry  (227)
  • 1990-1994  (668)
  • 1970-1974
  • 1965-1969
  • 1992  (668)
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  • 1990-1994  (668)
  • 1970-1974
  • 1965-1969
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  • 1
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: With the aid of a thermochemical cycle consisting of acidity and new redox data in dimethyl sulfoxide (DMSO) solution, homolytic N—H bond dissociation energies (ΔBDE values) relative to acetamide (1) (where the N—H BDE for 1 is defined as 0 kcal mol-1) were determined for diacetamide (0), biuret (+ 1), 3,3-dimethylglutarimide (-3), diacetylhydrazine (-16), 4,4-dimethylpyrazolidine-3,5-dione (- 25), 4-dimethylurazole (-29), hydrazoic acid (-15), succinimide (-15) and 1,2-dimethylurazole (-13) (all values in kcal mol-1). These ΔBDE data provide (a) additional evidence for the minimal N—H bond weakening effects of adjacent carbonyl groups, (b) evidence for relatively large N—H bond weakening effects of adjacent  - NHC(O)R moieties in both cyclic and acyclic hydrazides and (c) evidence suggesting that urazolyl radicals are more stable than pyrazolidinedionyl radicals, relative to their hydrogenated precursors. Inserting the appropriate acidity and redox data for hydrazoic acid into a thermochemical cycle that includes a constant that permits comparison of DMSO solution BDEs with gas-phase BDEs yields estimates of 93 kcal mol-1 for the homolytic strengths of the N—H bonds present in succinimide and H—N3. The DMSO N—H BDE determined in this way for H - N3 is in remarkable agreement with a determination of its gas-phase value, whereas the DMSO N—H BDE for succinimide places it intermediate between three published estimates of its gas-phase value.
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  • 2
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    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 3
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The molecular recognition properties of 18 acyclic imides were studied to evaluate the relative contributions of conformational, hydrogen bonding and crystal packing forces to the stabilization of specific aggregate patterns in the solid state. The crystal structure of diisobutyramide and the aggregate patterns of the 18 imides are presented. The stabilization by hydrogen bonding was found to override the conformational preferences of imides, while packing forces often precluded the formation of the most stable hydrogen bonded aggregate. The aggregate patterns of imides were also found to be a function of the type of substituents present as R groups. Imides with R groups of similar shape and size prefer to pack as bifurcated hydrogen bonded chains, whereas dimers or singly hydrogen-bonded chains form when the R groups have significantly different spatial requirements. Analysis of imide aggregate patterns revealed the similar spatial requirements of isopropyl and phenyl groups. The molecular recognition properties of acyclic imides are summarized as a set of hydrogen bond rules, which can be used to design new imide aggregates.
    Additional Material: 3 Ill.
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  • 4
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rates of loss of NO2- from the radical anions of 2-(4-nitrophenyl)-2-nitropropane (α,p-dinitrocumene) and 2-phenyl-2-nitropropane (α-nitrocumene) were compared by three different methods. The rate constant for the apparently heterolytic process (DN) undergone by the radical anion of the dinitrocumene was measured by pulse radiolysis to be 2 × 103 s-1 in dimethylformide at 25°C. This value is in reasonable agreement with values estimated from an indirect chemical method and from cyclic voltammetry at low temperature. The rate for the formally homolytic dissociation process (DR) undergone by the radical anion of the mononitrocumene could not be determined by pulse radiolysis but could be shown to be between 50 and 1000 times faster than the dinitro case using the other two methods. Possible reasons for the faster rate for the DR process are discussed with reference to related systems for which the DR process had been found to be unexpectedly slow.
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  • 5
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reaction between α-tocopheroxyl radical (VE·) and ascorbic acid (VC) and its lipophilic derivatives ascorbyl-6-caprylate (VC-8), 6-laurate (VC-12) and 6-palmitate (VC-16) was studied by stopped-flow ESR spectroscopy in cetyltrimethylammonium bromide (CTAB) micelles. The second-order rate constants for the reaction were found to be 9 · 0, 3 · 0, 0 · 7 and 0 · 03 × 105 1 mol-1 s-1 for VC, VC-8, VC-12 and VC-16, respectively, indicating a remarkable influence of the aliphatic side-chain on the reactivity. The lifetime of the reaction intermediate, ascorbate radical anion, was greatly enhanced by the lipophilic side-chain, being 0 · 4, 5 and 110 s for VC-8-., VC-12-. and VC-16-. respectively. Kinetic analysis shows that the inter- and intramicellar diffusion may be the rate-limiting steps for the reaction carried out in micelles.
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  • 6
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Long-lived trapped radicals produced during the photopolymerization of butane-1, 4-diol diacrylate (BDDA) were studied by EPR and ENDOR spectroscopy and their thermal decay was followed by EPR measurements at different temperatures (40-120 °C) for kinetic study. The EPR signal showed the superimposition of two different patterns, a three-line and a single-line spectrum. Both EPR patterns decayed following first-order kinetics in the investigated temperature range. Activation parameters of the decay were obtained. The EPR patterns were attributed to the same radical species situated in fluid and in cross-linked regions of the photopolymerized BDDA. Radicals of the latter kind undergo electron spin exchange strong enough to wash out the hyperfine splitting. The single-line width is mainly determined by electron spin dipole-dipole interactions. The ENDOR response is only of the matrix kind, typical of radicals in a solid phase. The present model was also compared with recent literature reports.
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  • 7
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The application of cavity theories of solution and linear solvation energy relationships to the interpretation of enthalpy of solution values of tert-butyl halides in hydroxylic media, measured at 25 °C and infinite dilution, is deserted, in order to define the intermolecular forces acting between solutes and solvents. It is concluded that solvent dipolarity and solvent HBD acidity effects dominate the solution process. An empirical equation relating the enthalpies of solution with selected properties of both solvents and solutes is proposed. New experimental Δ Hs∞ values for tert-butyl iodide in 13 alcohols are reported.
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  • 8
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Cyclopropylmethoxycarbene undergoes ambiphilic-nucleophilic intermolecular reaction with alkenes and methanol, but its intramolecular chemistry (1,2-carbon migration) is suppressed (k 〈 3 × 103 s-1) by the α-methoxy substituent.
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  • 9
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 10, 10-Dimethyl-9-anthrylidene (DMA) was studied by laser flash photolysis (LFP) techniques. LFP of the requisite diazo precursor in pentane or carbon tetrachioride produced transient spectra attributed to the hydrocarbon and monochlorinated diarylmethyl radicals, respectively. In the presence of pyridine and oxygen, ylides were formed upon LFP of 10, 10-dimethyl-9-diazoanthrone. However, ylides are not formed in acetonifrile or acetone. The carbene itself was not detected by transient absorption spectroscopy. The absolute rate constants for reaction of DMA with pyridine, methanol and triethylsilane are 1·96 × 109, 1·8 × 1010, and 1·0 × 108 I mol-1 s-1, respectively. It is concluded that the singlet and triplet states of DMA are essentially degenerate.
    Additional Material: 6 Ill.
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  • 10
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The different possible sources of the intraannular hydrogens in the dehydroxylated calixarenes obtained by reductive cleavage of the calix [4] arene diethyl phosphate esters 2 and 6 are analysed. Two calixarene diethyl phosphate esters (4 and 5) full deuterated in the ethyl groups were synthesized. Reductive cleavage of 4 and 5 (potassium-ammonia) resulted in the formation of the OH-depleted calixarenes 3 and 7, respectively, which did not incorporate any deuterium at the intraannular positions, as judged by integration of the NMR signals. Quenching with D2O of the reaction mixture of either 2 or 6 and potassium-ammonia did not result in any deuterium incorporation in the products. The labelling experiments rule out the possibility that the source of the intraannular hydrogens is the diethyl phosphate ester groups or the quencher. It is concluded that the most probable sources of the hydrogen atoms in the OH-depleted calixarenes is the ammonia molecule.
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  • 11
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Peroxymonosulfate ion (HSO5-) rapidly oxidizes 2-chloroethyl phnyl sulfide (1). Dilute micelle-forming cationic surfactants slightly speed the reaction but the reaction rate decreases sharply with increasing surfactant concentration. Rate-surfactant concentration profiles in solutions of cetyltrimethylammonium hydrogensulfate can be treated quantitatively by estimating concentrations of 1 and HSO5- at the micellar surface with allowance for competition between HSO5- and HSO4-. The second-Order rate constant in the micellar pseudo-phase is lower that in water by a factor of ca 400. Similar rate-surfactant concentration profiles were observed for the oxidation of di-n-propyl sulfide by periodate ion in solutions of cetyltrimethylammonium bromide, chloride and mesylate, and second-order rate constants in the micellar pseudo-phases are lower than in water by factors of ca 400. These oxidations are slowed by a decrease in the water content of aqueous acetonitrile and solvent, and micellar rate effects are consistent with transition states in which positive charge builds up on sulfur.
    Additional Material: 6 Ill.
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  • 12
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The hydrogen-bond pattern of 4-mercaptopyridine-4-thiopyridone in solution and in the solid state consists of extended chains of 4-thiopyridone associated through NH…S hydrogen bonds. The solution structure in chloroform was studied by UV-visible spectrometry and vapor pressure osmometry. A tautomerization constant of 1 · 8, favoring 4-thiopyridone, and a self-association constant of 2600 ml-1 were determined. Over 50% of the sample is aggregated near saturation with significant amounts (〉15%) of the sample present as oligomers containing four or more 4-thiopyridone monomer units. In its crystal structure, 4-mercaptopyridine-4-thiopyridone is composed of infinite chains of 4-thiopyridone associated by NH…S hydrogen bonds between glide related molecules, with N…S = 3 · 219(3) Å, H…S = 2 · 42(4) Å, N - H…S angle = 175(4)° and H…S = C angle = 96(1)°. Relationships between solution aggregation, crystal nucleation, and crystal propagation are discussed. Crystal data are as follows: space group = P21/c, a = 7 · 183(5) Å, b = 6 · 132(5) Å, c = 11 · 618(7) Å, β = 90 · 51(5)°, Z = 4, Dc = 1 · 44 g cm-3, 1623 reflections, R = 0 · 050, Rw = 0 · 072.
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  • 13
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The ESR spectra of 2-arylindan-1,3-dionyl radicals in various solvents at 343 K were obtained and assignments of the hyperfine structures were made. The hyperfine coupling constant (hfcc) of the dimethylamino nitrogen increases whereas the 2',6'-hydrogen hfcc decreases as the ET value, a solvent polarity parameter, increases. MO calculations were made according to the semi-empirical procedure of Rieger and Fraenkel; the nitrogen and oxygen Coulomb integrals were adjusted so that the calculated spin density on the nitrogen atom agrees with that estimated from its hfcc. It is found that the π-moment of the radicals increases from 9·13 D in toluene to 9·85 D in acetic acid. The solvent dependence of the equilibrium between the radicals and its dimer has been explained in terms of the solventinduced π-moment in polar solvents.
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  • 14
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The protonation of diphenyl sulfoxide and several substituted diphenyl sulfoxides was probed by the MNDO-PM3 method to gain a theoretical insight into the experimentally observed preference for stable diphenyl sulfoxonium ion formation (O-protonation) in 1:1 FSO3H - SbF5 (Magic Acid) - SO2. In agreement with solution studies, O- protonation is uniformly favored (by ca 17 kcal mol -1) over S-protonation. The differences in the heats of formation of protonated and unprotonated diphenyl sulfoxides are increased by electron-withdrawing substituents (F, CF3) and decreased by electron-donating groups (Me, OMe). Variations in the SO bond length and bond order in the onium ions are compatible with simple resonance arguments and the oxonium/sulfonium character of both the O- and S-protonated onium ions. Surprisingly, the O- and S-protonated dibenzothiophene S-oxides are predicted to have identical energetics as compared with the corresponding protonated diphenyl sulfoxides. On structure optimization dibenzothiophene S-oxide itself, if the initial geometry is somewhat twisted, rearranges by ring expansion to give a new heterocycle with lower energy. A rotational barrier study on the parent O-protonated diphenyl sulfoxide showed two minima, separated by 1.3 kcal mol-1, at HOSC dihedral angles of 60° and 240°. The two conformations correspond to syn and anti orientations of the OH proton relative to the aromatic rings, and support the lowtemperature solution observations of the presence of two distinct sulfoxonium ions in solution. The rotational barriers for diphenyl sulfoxonium cation are compared with those of O-protonated dimethyl sulfoxide.
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  • 15
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The first direct determination of a 1,2-chlorine atom shift in a chlorobenzylcarbene was achieved by nanosecond laser flash photolysis. Arrhenius activation parameters of Eact = 3.39 ± 0.14 kcal mol-1 and log [A(s-1)] = 10.98 ± 0.14 were obtained for 1,2-chlorine migration in α-methyl-α-chlorobenzyl(ch1oro)carbene. The lifetime of this carbene is considerably longer than previously estimated and the measured Eact, is in excellent agreement with that determined by product analysis.
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  • 16
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reaction between 1-fluoro-2,4-dinitrobenzene and n-butylamine in toluene shows a two-step plot of kobs, values vs the initial values of the concentration of the amine. The usual base-catalysis mechanism for HF elimination from the zwitterionic intermediate hardly explains this kinetic behaviour and the kinetic effect of addition of salts (and of 2-hydroxypyridine) to the reaction mixtures at different initial values of the concentration of n-butylamine. In contrast, the kinetic behaviours are easily explained by the presence of substrate-amine (or catalyst) interactions on the pathway of the substitution reaction.
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  • 17
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Access to spiro(bicyclo [2.2.1] heptane-2,1' -cycloprop-6-yl) derivatives was gained from the alkene spiro(bicyclo [2.2.1] hept-5-ene-2,1'-cyclopropane) via separation of positional isomers. Spiro(bicyclo [2.2.1]-2,1'-cycloprop-exo-6-yl) p-toluenesulphonate (10) and spiro(bicyclo [2.2.1] heptane-2,1'-cycloprop-exo-6-yl) trifluoroacetate were found to solvolyse faster than the analogous exo-2-norbornyl esters, as predicted by theory. Ion-pair recombination, with the formation of tricyclo [4.2.1.03,7] non-3-yl p-toluenesulphonate, accounts for previous failures to assess the true reactivity of 10. An intervening bridged carbocation (3), labelled with deuterium, was shown to achieve equivalence of C-1 and C-6 prior to ring expansion. The rate of the formal Wagner-Meerwein rearrangement is estimated to be of the order of molecular vibrations, thus supporting the symmetrical bridged structure of 3. Methyl substitution at C-6 was found to direct nucleophilic attack exclusively to the tertiary carbon, and ring expansion preferentially to the secondary carbon. An equilibrating pair of 6(1)-methylspiro(bicyclo [2.2.1] heptane-2,1′-cycloprop-6-yl) carbocations is thought to explain these observations most reasonably.
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  • 18
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of the hydrolysis of benzohydroxamic acid have been investigated in hydrochloric, sulphuric and perchloric acids in 10% (v/v) dimethyl sulphoxide - water at 55 °C. Activation parameters were also determined. Rate correlation by the Cox - Yates excess acidity method shows an A-2 mechanism involving rapid pre-equilibrium protonation of the substrate followed by rate-limiting attack of water at the carbonyl carbon atom to form a tetrahedral intermediate which collapses, in a fast step, to the products.
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  • 19
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    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Owing to the importance of the concept of aromaticity, different indices have been developed to try to quantify this property. The possible π delocalization through an X—SOn—X group (X = C, N; n = 0, 1 or 2) could explain heteroaromaticity in rings containing the moiety. For that reason, the aromaticity of five-membered sulphurcontaining rings with different oxidation numbers (S, SO and SO2) and including no (thiophene), one (isothiazole) or two (1,2,5-thiadiazole) adjacent atoms was investigated. Ab initio calculations were carried out to determine geometrical parameters (planarity of the ring, bond length and bond order), electronic structure (charge distribution and dipolar moment) and the participation of d-orbitals of sulphur. According to these calculations, only compounds with S(II) can be considered to be aromatic, whereas compounds bearing S(IV) or S(VI) are better described as ylides.
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  • 20
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A novel technique for determining the binding constants of host-guest complexes, viz. the bis-cationic electron relays methylviologen and octylviologen in the macrocyclic host of novel (bisheteroleptic) crown ether-ruthenium sensitizers, is described. This technique allows the separation of the quenching effects of bound and free electron relays. It can be used to calculate binding constants in non-covalently linked supramolecular systems evaluating the electron transfer process between guest and host. High binding constants comparable to those in many natural enzymes and the photosynthetic reaction centre were found. As an explanation for the behaviour of the sensitizers bound to the bipyridinium cations it is suggested that photoelectron transfer affects the binding between the excited sensitizer and the bound acceptor.
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  • 21
    Electronic Resource
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    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 22
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Polarographic half-wave potentials for the electrochemical reduction of a series of para-substituted iodobenzenes in acetonitrile, acetone, benzonitrile, dimethylformamide, dimethyl sulphoxide and methanol obey Hammett equations and the reaction constant found, ρ = 0·36 ± 0·02, is independent of the solvent used. The results obtained are discussed on the basis of a mechanism of the electrode process.
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  • 23
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effects of inorganic and organic salts on the rates of methanolysis of ionized phenyl salicylate, PS-, were studied at 30°C in H2O-MeOH solvents. The observed pseudo-first-order rate constants, kobs, for methanolysis of PS- represent a non-linear incrrease with increase in methanol contents in mixed H2O-MeOH solvents. The observed data are explained in terms of the proposal that methanol molecules exist in monomeric, dimeric and in general polymeric forms in H2O-MeOH solvents. The rate constants, kobs, at alkanol [ROH (R = Me, HOCH2CH2)] contents of ca ≤ 55% (v/v), fit the relationship kobs = k [ROH]T/(1 + kA [ROH]), (1 + KA [ROH]), where k and KA represent the nucleophilic second-order rate constant for the reaction of ROH with PS- and the association constant for the dimerization of ROH, respectively, and [ROH]T is the total concentration of ROH. The rate constants, k, appear to be independent of total concentrations of inorganic salts [MX]T (M = Li, Na, K and X = OH, Cl), but the values of KA increase with increase in [MX]T and this increase varies in the order Li+ 〉 Na++ 〉 K+. The values of both k and KA show a decrease with increase in the total concentrations of organic salts, [R4MX]T (R = Et, Prn, Bun and X = Br, I). The effects of [MX]T and [R4MX]T on kobs versus content of ROH (%, v/v) profiles reveal the following inferences: (i) in the water-rich region of H2O-ROH solvents [at ca 〈 55% (v/v) ROH] the solvation shells of M+ and X- contain preferentially only water molecules whereas in the methanol-rich region [at ca 〉 55% (v/v) ROH] of solvents some cosolvent methanol molecules also enter into these solvation shells; (ii) the solvation shells of tetraalkylammonium ions, R4N+, contain some methanol molecules even in the water-rich region of the solvents; (iii) Li+ cause a methanol structure-making effect whereas Na+ and K+ ions show a methanol structurebreaking effect; and (iv) organic cations such as Et4N+, Prn4N+ and Bun4N+ reveal a methanol structure-breaking effect through an interaction mechanism different from that operating for Na+ and K+. Both k and KA show significant normal deuterium isotope effects in the reactions of MeOH and PS- which indicates the probable occurrence of proton transfer in the rate-determining step.
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  • 24
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The cycloaddition reactions of 4-methylphenylthio-, methoxy-, 4-methoxyphenyl-, phenyl-, chloro- and cyanoallene with tert-butylthioacrylonitrile (BTA) in toluene-d8 solution were investigated. The relative reactivity sequence of these substituted allenes is 4-methylphenylthio ∼ methoxy- 〉 4-methoxyphenyl- 〉 phenyl- 〉 chloro- 〉 cyanoallene. Activation parameters were measured for diradical intermediate formation of BTA with 4-methylphenylthio-, methoxy- and 4-methoxyphenylallene, giving average values of Ea and ΔS± of 14.2-16.8 kcal mol-1 and ca. -33 eu, respectively. The relative reactivity sequence is consistent with a FMO allene-HOMO, BTA-LUMO dominant interaction for diradical intermediate formation. However, the regioselectivity of attack on the substituted allene appears to be thermodynamically controlled. The formation of the diradical intermediates in these cycloaddition processes appears to be irreversible. However, the ring closure of the diradical intermediates formed from 4-methylphenylthio-, methoxy- and 4-methoxyphenylallene is reversible under the conditions of the kinetic experiments. The kinetically controlled ring closure of the diradical intermediates is allyl radical SOMO controlled, while the final cycloadduct distribution is thermodynamically controlled.
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  • 25
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The relative stabilities of gaseous protonated propylene oxide and its isomers, protonated propanal, oxetane and acetone, were reinvestigated in a Fourier transform ion cyclotron resonance mass spectrometer by using multiple-stage tandem mass spectrometri experiments. The dependence of ion structure on internal energy was examined by generating the ions in proton transfer reactions with different exothermicities and then probing their structures by using energy-resolved mass spectrometry (collision-activated dissociation as a function of collision energy). In contrast to results obtained in several recent investigations, protonated propylene oxide was found to be distinct from its more stable isomers when generated with only a small amount of internal energy. When the exothermicity of the proton transfer reaction was higher than 6 kcal mol-1 (1 kcal = 4·184 kJ) or when the epoxide ion was subjected to multiple activating collisions, rapid isomerization to protonated propanal occurred. The energy required for opening of the epoxide ring estimated to be similar to that measured earlier for protonated cyclohexene oxide (5-10 kcal mol-1).
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  • 26
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The enhancement factors of the 1H polarization induced in radical pairs generated by α-cleavage of 12C=O- and C=O-labelled ketones deviate from the ratios expected on the basis of the relative abundances of the respective carbon isotopomers. For dibenzyl ketone, the ratio of absolute CIDNP intensities of the 1H(13C) doublet (90% 13C=O) to the 1H(12C) singlet (10% 12C=O) was found to be 〉9:1 at 58·3 kG and 〈9:1 at 21 kG. Similar deviations were observed for other 13C labelled ketones, viz. deoxybenzoin (13C=O) and acetophenone (13C=O or 13CH3). This novel enhancement or suppression effect on the CIDNP enhancement can be ascribed to the presence of a second hyperfine coupled nucleus in the intermediate radical pair. The individual contributions of the g factor difference (Δg), the external magnetic field strength (Ho), and the hfc constants of the observed (aA) and the interacting nucleus (ax) in the enhancement-suppression are exemplified. The experimental results are consistent with theoretical calculations of the mutual effect of Δg, H0, aA and ax on the CIDNP enhancement factor of the observed nucleus.
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  • 27
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rate, endo/exo and diastereofacial selectivities of the Diels-Alder reaction between cyclopentadiene and ( - )-menthyl acrylate were measured in a series of organic solvents and organic-aqueous mixtures. Regression analyses show that a model with the α empirical solvent parameter accounts for most of the changes in rates and diastereofacial selectivities, whereas in the case of the endo/exo selectivity, inclusion of the π* and δ parameters is needed. Theoretical calculations carried out on the model reaction between methyl acrylate and cyclopentadiene using dielectric continuum and supermolecule models do not clearly show the effect of the different solvation mechanisms on rate. However, they agree with the above-mentioned influence of α and π* on the endo/exo and diastereofacial selectivities.
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  • 28
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The pK values of some arylthiomethylbenzoic acids and the corresponding sulphones were determined spectrophotometerically. The role of substituent and the position of the carboxylic group are discussed.
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  • 29
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvent effects on the Hammett ρ value for the cathodic reduction of substituted benzophenones were determined. The electrochemistry of a series of 11 compounds was studied in acetonitrile, acetone, dimethyl sulphoxide, propylene carbonate, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylformamide and hexamethylphosphoric triamide. The ρ values for the reversible one-electron transfer are described by the Lewis acid-base model ρ = - 0.006AN + 0.003DN + 0.391, where AN = solvent acceptor number and DN = solvent donor number.
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  • 30
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The partial rate factors of the four modes of meta photocycloaddition of methyl-substituted benzenes to cyclopentene have been determined by measuring quantum yields of formation of the most important meta photocycloadducts of benzene, toluene and the three xylenes. It is demonstrated that the results can be applied to predict quantum yields of any meta photocycloadduct of methylbenzenes to cyclopentene. The predicted yields agree very well with those determined experimentally, and it is shown that the effects of two or more methyl groups are fully additive. The method is promising for the investigation of mechanisms of photochemical reactions and can be used to predict regioselectivities.
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  • 31
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rates of degradation of the triazine herbicide atrazine are reported, together with an analysis of the sequence of stages involved, using hydroxyl radicals generated by hydrogen peroxide in the presence of UV light and by the pure photochemical reaction. These conditions bring about the displacement of chlorine by hydroxyl, dealkylation of the alkylamino groups and finally deamination of the triazine ring. The significance of these results to water purification is discussed.
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  • 32
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The conformational analysis of dimethyl [2] metacyclo [2] (3,4)thiophenophane (1), which is the thiophene analogue of [2.2] orthometacyclophane, is described. The comparison between the dynamic NMR spectra of 1 and the computer-simulated spectra led to the energy barrier between syn and anti conformers of 1. This is the first conformational analysis of a [2,2] orthometa-type cyclophane.
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  • 33
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 34
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The title compounds (1 and 3, respectively) were studied by X-ray crystal structure analysis. Both are trigonal, space group R3, with lattice parameters a = 27.718(6), c = 8.161(2) Å, Z = 3 and a = 31.267(7), c = 6.560(2), Z = 18, respectively. The dodecamethyl compound 1 is isomorphous with its parent hexakis(4-benzylphenoxy)benzene (2), a known host. However, ‘collapse’ along the c-axial direction has occurred such that the appreciable clathrate cavity of 2 has been replaced by only a very small residual void for 1. Phenol 3 does not pack in discrete [OH]6 hydrogenbonded hexamers; instead, molecules are assembled in infinite chains, linked by OH…O hydrogen bonds of length 2.735(3) Å, which are propagated along threefold screw axes. A variable-temperature CPMAS NMR study of sidechain aryl group rotation in 1 gives a value ΔG≠ of 14.0 ± 0.7 kcal mol-1 (at 42 °C) for the (crystallographically unique) para -disubstituted ring, a similar 14 kcal mol-1 free energy barrier being found hindering rotation of this ring, and of the outer phenyl ring of 1, at 9°C.
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  • 35
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Structures in CD3OD—CDCl3 mixtures of pyridino-18-crown-6 and diesterpyridino-18-crown-6 complexes with primary ammonium cations containing an α-phenyl group were found to be significantly different. The π-π overlap between the α-phenyl group of the ammonium cation and the pyridine ring of the ligand occurs in the pyridino-18-crown-6 complexes but not in the diesterpyridino-18-crown-6 complexes. The solution structure of a diesterpyridino-18-crown-6 complex with an α-phenyl-containing primary ammonium cation was also found to differ markedly from the corresponding crystal structure in that π-π overlap occurs in the crystal structure, but not in the solution structure.
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  • 36
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effect of strontium and barium ions on the acyl transfer reaction to ethoxide ion from 2-acetoxy-1,3-xylyl-15-crown-4, 2-acetoxy-1,3-xylyl-18-crwon-5, 2-acetoxy-1,3-xylyl-21-crown-6 and the model compound phenyl acetate was investigated in ethanol at 25°C. Significant catalytic effects were observed in all of the studied reactions as a result of stronger metal ion associations with the transition states than with reactants. The rate enhancements observed with the crown ether substrates are much larger than those observed with phenyl acetate, the largest acceleration being displayed by 2-acetoxy-1,3-xylyl-21-crown-6, which reacts with EtOBaBr 5 × 105 times faster than with EtONMe4. These findings provide strong evidence that cooperation of electrostatic binding to the negative charge developing at the carbonyl oxygen and coordinative binding to the polyether chain is highly efficient in the metal-bound transition state. Comparison of rate data reported here with analogous data obtained in methanol solution clearly shows that both modes of interaction contributing to the stability of the metal-bound transition state are much more efficient in ethanol than in methanol solution.
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  • 37
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Three anthraquinone-containing crown ether macrocycles and one macrobicycle were reduced coulometrically to their corresponding anion radicals under vacuum. The resulting anion radical solutions in CH2CL2 were stirred with an excess of a solid 6LiClO4-7LiClO4 mixture of known isotropic composition in order to form the appropriate Li+ complexes. After equilibration, the solution was separated from the excess solid and the [6Li+]/[7Li+] ratio was determined by atomic absorption spectrometry. The values were converted into the corresponding separation factors (α) after division by the original composition ratio of the solid mixture. The values were in the range 1.04-1.18. Except for one compound, 2, all of the reduced systems studied exhibited a much larger α value than the 1.057 reported for 12. crown-4 at 0° C in a liquid-liquid extraction system.
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  • 38
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Benzophenone-mediated protolyses of five cage-shaped compounds in benzene solutions result in the formation of phenyl-substituted products with high regioselectivity. The regioselectivity is rationalized based on the relative bond strengths and structural parameters of each molecule.
    Additional Material: 1 Tab.
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  • 39
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Kinetic studies of the reactions of methallyl arenesulphonates (II) with anilines and N,N-dimethylanilines in acetonitrile at 45.0°C are reported. The sign and magnitude of the cross-interaction constants pxz (and βxz) between substituents in the nucleophile (X) and leaving group (Z) suggest that the transition state (TS) is slightly tighter than that for the corresponding reactions of allyl arenesulphonates (I). This is also supported by the observation that the magnitudes of ρx and ρz for II are uniformly greater than those for the reactions of I. These results are in line with the simple MO theory that the 2-position of the allyl system is inactive electronically. The steric effect of the 2-methyl group in II causes a rate retardation and a shift of the TS toward a later position along the reaction coordinate with a slight increase in the overall tightness of the TS structure. The large |ρxz| value obtained eliminates the possibility of an SN2′ mechanism.
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  • 40
    Electronic Resource
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    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 41
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hydrolysis of geranyl diphosphate (GPP) at pH 7 in water gives largely linalool (LOH) + geraniol (GOH) in the ratio of 3:1. Added N-3 generates mixed acylic allylic azides and increases the LOH GOH ratio to 15:1 in 2 M NaN3, but does not speed up the overall reaction. Hydrolysis of neryl diphosphate (NPP) gives largely α-terpineol (TOH) +p LOH, but their ratio is not very sensitive to NaN3 concentration although acyclic azide and small amounts of α-terpinyl azide (TN3) are formed. Hydrolysis of α-terpinyl diphosphate (TPP) gives large amounts of the cyclic alkenes, limonene and terpinolene. Added N-3 does not change the amount of elimination, but increases the ratio of limonene to terpinolene, and diverts some substitution product to TN3. Trapping of carbocationic species from GPP by N-3 is sharply increased by addition of Mn2+, which also catalyzes the overall reaction. Products of reaction of GPP are derived from acyclic intermediates and of NPP from acyclic and cyclic intermediates, and ionizations of the three substrates do not generate common carbocationic species.
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  • 42
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Dewar-anthraquinone (1a) and 1,2,3-tri-tert-butyl-5,8-Dewar-anthraquinone (1b) were synthesized. Thermodynamic parameters for their cycloreversion to the corresponding anthraquinones indicate that thermal population of the excited states of the products via non-adiabatic valence isomerization is energetically feasible. On thermolysis of 1b, the formation of a detectable amount of 1,2,3-tri-tert-butyl-1,4-Dewar-anthraquinone (3) was observed. The excited singlet state of 1,2,3-tri-tert-butylanthraquinone (2b) produced via the non-adiabatic process is responsible for the formation of 3.
    Additional Material: 1 Tab.
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  • 43
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A molecular mechanics study on some organophosphorus podands, with both rigid and flexible terminal groups, and on their complexes with Li+ and Na+ was carried out. Na+ causes smaller deformations of the ligands than Li+, but its complexes are less stable, as the interaction energy with the podands is weaker. The ligands with rigid terminal groups are, in general, more pre-organized for complex formation. The cation-ligand interaction energies, including also the changes in steric energies, are always higher in complexes of flexible molecules than in those containing rigid ligands. The lower entropy losses which accompany the formation of complexes of rigid molecules with alkali metal cations probably account for the observed higher stabilities of the complexes.
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  • 44
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Some transimination processes were studied using an intermolecular model formed by pyridoxal-5′ -phosphate (PLP) an amino acid and dodecylamine (DOD) in an aqueous medium. All the kinetic constants for the reversible reaction were determined. The results show that in these cases transimination proceeds through an addition-elimination by forming a diamine geminal intermediate. Equilibria are always shifted to dodecylamine-PLP Schiff base formation. Differences between the stability of this Schiff base and the ∊-aminocaproic Schiff base cannot be explained only on the basis of the different nucleophicities of amine groups and therefore differences in the imine double bond environment must be taken into account to explain this behaviour.
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  • 45
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 46
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A mechanistic investigation of the gas-phase reactivity of the halomethyl anions XCH2- (X = Cl, Br) towards a mixture of the corresponding halomethane and ammonia was performed using Fourier transform ion cyclotron resonance mass spectrometry. The interpretation of the experimental data is supported by high-level density functional theoretical (DFT) calculations for the chlorine-containing systems (X = Cl). When the specific isotopomer AXCH2- (AX = 35 Cl, 79Br) is allowed to react in an atmosphere of CH3X and NH3, the exclusive formation of the isotope cluster of the halide anions AX- and BX- (BX = 37Cl, 81Br) is observed. However, the intensity ratio I(AX-)/I(BX-) exceeds significantly the value expected from the natural relative isotope abundances and depends linearly on the pressure ratio p(NH3)/P(CH3X). The experimental results are interpreted in terms of three competing reaction mechanisms: (i) The by far dominating process is the more than 70 kcal mol-1 exothermic one-step SN2 substitution of AXCH2- on CH3X, generating haloethane AXCH2CH3 and X- isotopomers, the latter in the proportion of their natural abundances (direct SN2). The experimentally observed excess of AX- stems from two minor reaction pathways: (ii) in a secondary reaction, the halide X- in the primary product anion-molecule complex [AXCH2CH3 … X-] * of the SN2 substitution induces a 1,2-elimination, leading to the formation of the AX- isotopomer (two-step SN2/E2). (iii) Finally, ACH2- can react with ammonia by consecutive endothermic proton transfer (PT) from NH3 to AXCH2- and a very exothermic SN2 substitution of the resulting amide on AXCH3 leading to CH3NH2 and an excess of AX- which depends linearly on p(NH3)/p(CH3X) (PT/SN2). Theoretical calculations show that in the case of [ClCH2-…NH3] *, the PT/SN2 reaction has no stable intermediate. Therefore, it is concluded that this reaction is not a two-step but a one-step process.
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  • 47
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 48
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A bond and group equivalent scheme that allows the calculation of heats of formation for aldehydes and ketones from ab initio 6-31G* energies has been developed. For a group of 24 aldehydes and ketones, the RMS error for the calculated heat of formation was 0 · 46 kcal mol-1. Heats of formation have been predicted for an additional seven compounds for which the experimental values are believed to be either in error or unknown. There are some problems with the norbornanones.
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  • 49
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An AM1/PM3 study of several tetrahalomethane-arene molecular complexes is presented. The results indicate the existence of weak attractive non-dispersion interactions in some of these complexes, which can be rationalized in terms of multipole - multipole and multipole - induced dipole interactions. No evidence was found for the occurrence of charge transfer in the ground states of these complexes.
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  • 50
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The base-promoted nitrile-forming elimination reactions of YCH=CHCβH=NOCH=CHZ (Y=OCH3, H or Cl and Z=H or NO2) were studied by the AM1 MO theoretical method with Cl- as a base. The reaction is found to proceed by an E1cB-like E2 mechanism in which Cβ - H bond cleavage is more advanced than N - O bond breaking. The syn-elimination has a more E1cB-like transition state (TS) than the anti elimination, which is attributed to the structurally favourable nN - ó * (Cβ - H) charge-transfer interaction. An electron-withdrawing Y substituent lowers the activation barrier by stabilizing negative charge developed on Cβ in the TS. An electron-withdrawing substituent in the leaving group (Z = NO2) tends to enhance the anti relative to the syn elimination process by depressing the δ*(N - O) level, which in turn makes the nć - δ*(N - O) interaction more effective. The YCH=CH -  and  - CβH=N fragments are perpendicular in the TS, which is stabilized by delocalization of negative charge developed on the Cβ atom.
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  • 51
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rate constants of the pseudo-first-order methylation reaction of seven substituted pyridinium-N-phenoxide betaine dyes by iodomethane, were determined by UV-visible spectrophotometry in nine different solvents. The influence of substituents on the position of the long-wavelength, intramolecular charge-transfer absorption band of the unsubstituted betaine dye can be described by a modified Hammett equation. The solvent-dependent absorption constant ρA correlates satisfactorily with the acceptor properties of the nine solvents used. The substituent-dependent alkylation rate constants (In k) correlate linearly with the Hammett substituent constants, whereas the solventdependence of in k follows a more complicated pattern. Temperature-dependent measurements of the rate of alkylation exhibit an isoenthalpic behaviour for all the betaines in all the solvents investigated, i.e. the Arrhenius activation energy is almost constant for all differently substituted betaines in a given solvent. Temperature-dependent measurements of the alkylation rate of the unsubstituted betaine in different solvents led to an isosolvent relationship, i.e. the corresponding Arrhenius plots show a common point of intersection. According to the theory of isokinetic relationships, this isosolvent behaviour is interpreted in terms of a resonant energy exchange between the reacting species and the surrounding heat-bath system, which in this particular case seems to be the betaine molecule itself.
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  • 52
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The efficiency of the developed method of dissymmetry functions was studied using various model systems. The alteration of the dissymmetry function was analysed with systematic variations of the bond lengths, valence angles and masses of atoms in model tetrahedra. The behaviour of the dissymmetry function was studied for conformationally labile systems and chiral polyhedra. In general, it was found that in all cases the alteration of the dissymmetry function is in agreement with the speculative representations of the changes in degree of chirality.
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  • 53
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: For a number of chiral α,β-unsaturated carbonyl compounds, cyclohexanone and cyclohexenone derivatives, existing essentially in the form of single molecular conformation, the correlation dependences were established between the values of the calculated dissymmetry functions (DF) with respect to atomic masses and atom refractions, and the induction effectiveness of helical ordering in the nematic mesophase (twisting power). Such correlative dependencies, as the examples show, allow one to predict safely enough the twisting power of new substances with the ‘fixed’ molecular conformation. Types of dissymmetry functions and kinds of correlative dependences, the most acceptable for these reasons, were revealed. Dissymmetry functions formed on the basis of atom refractions (polarizabilities) describe the influence of the molecular structures of chiral compounds on the twisting power with higher accuracy than those that characterize the dissymmetry of the atomic masses distribution, according to the important role of the dispersion forces in the formation of helical ordering in liquid crystalline systems. The components characterizing the distribution dissymmetry of atomic refractions and masses with respect to the long and two shorter axes as the peculiar characteristics of biaxiality of chiral molecules describe the influence of the molecular structure on the twisting power with the same accuracy as the general DF do. Thus influence of chiral dopant molecules on the interactions between molecules in the uniaxial mesophases is negligible within the limits of the DF method. Based on the calculations for model systems it is established that the presence of the non-linear cinnamoyl fragment has a decisive influence on the molecular dissymmetry of the chiral α,β-unsaturated ketones. The alkyl groups influence the molecular dissymmetry only slightly, but it is important that their presence ensures the stabilization of the chiral conformer with the defined helicity of the cinnamoyl fragment.
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  • 54
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 2,2′-Furil (di-2-furyl diketone) undergoes photoreduction to give furoin on irradiation at 366 nm in triethylamine-containing benzene and ethers. On the other hand, in ethanol the photoirradiation leads to the formation of an enediol type of compound, 1,2-di(2-furan)ethene-1,2-diol. The results of quenching and sensitization both for the photoreactions and the phosphorescence demonstrate that furoin and the enediol are formed via the lowest excited singlet and the triplet states of 2,2′-furil, respectively.
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  • 55
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvolysis of 1,1-diphenyl-1-X-ethane (1-X) [X = 4-nitrobenzoate (PNB), 3,5-dinitrobenzoate (DNB), AcO, MeOH+, EtOH+ or H2O+] was studied in 20 vol% dimethyl sulphoxide in water, in which specific salt effects are very small, and in 25 vol% acetonitrile in water. Substrates with negatively charged leaving groups yield more elimination product 1,1-diphenylethene (3) than those with uncharged groups, indicating the intermedicacy of ion pairs. Thus, three times more alkene is formed in the reactions of the substrates 1-OAc and 1-PNB than in the acid-catalysed hydrolysis of 1-OMe and 1-OAc. The substitution products may be formed via the solvent-equilibrated carbocation since the measured nucleophilic selectivities are very similar with different leaving groups.
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  • 56
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The application of computational techniques to biology, chemistry and physics is growing rapidly. Quantitative structure-activity relationships (QSAR) have been used widely to relate biological activities and physicochemical properties to molecular structural features. A difficulty in this approach has been non-uniformity of parameter sets resulting in the inability to examine contributions across properties and data sets. Linear solvation energy relationships (LSER) developed by Kamlet and Taft successfully utilize a single set of parameters to correlate a wide range of biological, chemical and physical properties. The empirical LSER solvatochromic parameters have been replaced with theoretically determined parameters to permit greater ease in a priori property prediction. These TLSER descriptors have given good correlations and interpretations for some biological activities. This paper discusses the application of these descriptors to six physicochemical properties involving equilibria, kinetics and spectra. The results show good correlation and physical interpretation.
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  • 57
    Electronic Resource
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 5 (1992), S. 435-435 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 5 (1992), S. 424-428 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The O-17 NMR spectra of 13 α′-alkyl- and aryl-substituted α-diazoketones were measured. The results obtained show a clear discrimination between them. A good correlation in a series of para-substituted α-diazoacetophenones between O-17 chemical shifts and both α+p and dual substituent parameters indicated a greater contribution of resonance than inductive effects.
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  • 59
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rate constants (kB1 and kBH-1) for the reversible deprotonation of triphenyl(2-Z-9-fluorenyl)phosphonium ions (Z = H, Br, NO2) by piperidine, morpholine, n-butylamine, 2-methoxyethylamine, glycine ethyl ester, cyanomethylamine, OH- and water were determined in 50% Me2SO-50% (v/v) water at 20°C. Brønsted αCH (variation of carbon acid) and β values (variation of amine), and intrinsic rate constants [log k0 = log kB1/q when pKBHa - pKCHa + log(p/q) = 0] were obtained. αCH decreases with increasing basicity of the amine whereas βB decreases with increasing acidity of the carbon acid. These trends, which imply changes in the transition-state structure with reactivity, can be described by the interaction coefficient pxy = ∂βB/∂pKCHa = ∂αCH/ - ∂pKBHa = 0.03 (primary amines) and 0.01 (piperidine/morpholine). αCH is smaller than βB, indicating an imbalance due to a lag in the delocalization of the negative charge into the fluorenyl moiety at the transition state. The influence of the Ph3P+ group on the intrinsic rate constant is analyzed in terms of possible contributions by inductive/field (I), resonance (R), polarizability (P) and steric (S) effects. Using 9-carbomethoxyfluorene as a reference, it is shown that the stronger electron-withdrawing I effect of the Ph3P+ group relative to the COOMe group enhances log k0 substantially; the fact that the R effect of Ph3P+ is weaker than that of COOMe also contributes to an increase in k0, and so does the P effect of the phosphorus. All these increases are virtually completely offset by the rate-retarding S effect of the bulky Ph3P+ group. A similar analysis for the Me2S+ derivative studied by Murray and Jencks [J. Am. Chem. Soc. 112, 1880 (1990)] leads to similar conclusions except that the still smaller R effect is probably one of the main reasons why k0 for the Me2S+ derivative is more than ten times higher than for the Ph3P+ derivative; another potential reason is a difference in the steric effect.
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  • 60
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effect of alkali metal ions (Li+, Na+, K+, Cs+) on the reactivity of toluenethiolate anion in the demethylation of 2-methoxy-1,3-xylyl-18-crown-5, 2-methoxy-1,3-xylyl-24-crown-7 and the model compounds anisole and 2,6-dimethylanisole was investigated in dimethylformamide (+3.3 M water) at 60°C. It was found that the metal ion effects are markedly influenced by the presence of the polyether chain in the reaction system. Whereas reactions of the model compounds are slightly inhibited by the presence of alkali metal ions, the latter strongly enhance rate of demethylation of the crown ether derivatives, the observed catalytic factors ranging over nearly three orders of magnitude. These remarkable rate-enhancing effects are ascribed to a strong transition-state stabilization by metal ions through cooperation of electrostatic binding with the negative charge developing on the oxygen atom of the methoxy group undergoing nucleophilic attack and coordinative interaction with the polyether chain.
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  • 61
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Using a flourescence-detected ethidium displacement assay, the calf thymus DNA complexation properties of 27 mono-, di-, tri-, tetra- and hexacationic polyamines were determined. The DNA-binding affinity of these polyamine compounds increased with increasing cationic charge on the polyamine. Although most of the compounds exhibited no base pair binding selectivity, two of the tricationic polyamines possessing additional neutral amine groups exhibited approximately tenfold GC binding selectivities.
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  • 62
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The direct nucleophilic aromatic substitution reactions of anthraquinones have permitted the syntheses of more than 30 novel podands, crown ethers and lariat ethers. Anthraquinones having (ethyleneoxy)n sidearms were obtained by direct displacement of chloride by the anion of CH3(OCH2CH2)nOH. The ethyleneoxy-substituted anthraquinones could, in turn, undergo direct replacement by nucleophiles that failed to displace chloride. This approach has been used for the preparation of two-armed podand derivatives and several novel crown derivatives of anthraquinone. Binding comparisons are presented for several of these new anthraquinones. Direct substitution did not prove successful in the preparation of anthraquinone- [2 · 2] -cryptand which was obtained by alkylation. The crystal structure of the latter reveals an orientation of ring and anthraquinone appropriate for cation binding, a fact confirmed by cation binding constant measurements.
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  • 63
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Spectroscopic and potentiometric studies of Ag(I) binding by tris-bipyridine strands indicate that the formation of trinuclear silver trihelicates is a process displaying positive cooperativity.
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  • 64
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    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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