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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: The magnitude ρx(ρnuc) is shown to decrease with increase in the extent of bond making estimated by the kinetic isotope effect for the reactions of Y-benzoyl chlorides with anilines, XC6H4NH2, in acetonitrile at 25·0°C.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The intramolecular cyclizations of ω-haloenolate anions, -CαH2—C(=O)—(CH2)n-3—CωH2—X with X = F, Cl and Br and n = 3-7, were investigated by the AM1 method. In most cases, cycloketone formation proceeds more favourably than cycloether formation, as predicted by the HSAB principle. The reactivity increases in the order X = F 〈 Br 〈 Cl for both processes, and for cycloether formation it is in the order n = 3 〈 4 〈 7 〈 6 〈 5 for all X but the relative order for n = 5 and 6 reverses in favour of n = 6 for cycloketone formation with X = Cl and Br. The softness of the acid centre, Cω, decreases for a harder X( = F) whereas it increases for a softer X( = Br). Thus the reactivity order with respect to X suggests that the softness of the base centres O and Cα belongs to the borderline class. The transition state (TS) structures indicate that the BEP principle is obeyed in all cases, and a less reactive process has a more product-like TS.
    Additional Material: 3 Ill.
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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 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.
    Additional Material: 3 Ill.
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  • 4
    ISSN: 0192-8651
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The relative stabilities of the keto and enol forms [Δ E0 (enol-keto)] and the energy barriers to enolization of the keto forms [Δ E≠ (transition state-keto)] for CH3COR (R = CH3, H, F, and CN) and CH3CHY (Y = CH2, NH, and S) are investigated theoretically by Hartree-Fock and Möoller-Plesset second-order calculations with 6-31G** basis sets. Specific and bulk solvent effects are considered by incorporating one water molecule and applying the self-consistent reaction field (SCRF) method to the reaction system, respectively. The Δ E0MP2 values are all positive, in agreement with the lower stability of the enol form in the gas phase as well as in solution. In contrast to a relatively small effect of specific as well as bulk solvation on Δ E0, there is a large lowering of Δ E≠ (by ca. 30 kcal/mol) when solvent effects are accounted for. In general, both Δ E0 and Δ E≠ are depressed in solution and hence enolization is favored thermodynamically as well as kinetically. The keto form is strongly stabilized by a π donor, whereas the enol isomer is stabilized by a π as well as a σ-acceptor substituent, R. As a result, substituent R = F is the most unfavorable whereas R = CN is the most favorable for the enolization. The water catalyzed enolization in the neutral water proceeds concertedly, but carbon deprotonation is more important than carbonyl-oxygen protonation by water in the rate determining step. © 1997 by John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 5
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The mechanism of the A2 acid hydrolysis of methyl carbamate was investigated using MNDO method. The reaction was found to proceed in two steps: (1) the rate-determining nucleophilic attack of water on the carbonyl carbon of the N-protonated tautomer involving the tetrahedral TS; and (2) the fast subsequent proton abstraction by the leaving group, NH3, to form products. The mechanism is similar to that involved in the A2 hydrolysis of acetamide. Effects of substituents, R1, R2, and R3 in R1OCONR2R3, on rates can be predicted by the changes in electron densities on alkoxy oxygen and N, in complete agreement with the experimental results. We concluded that there is no need for invoking two different mechanisms for amides and carbamates since a common mechanism can easily accommodate all the experimental results.
    Additional Material: 7 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 2 (1989), S. 281-299 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Mechanisms of the hydrolysis of urea have been investigated using the MNDO and AMI methods. All geometries were fully optimized and the transition states were characterized by calculating force constants. The results showed that: (i) The unimolecular decomposition process via the direct intramolecular proton transfer is preferred to both the A1 and the bimolecular nucleophilic attack by water, in agreement with the experimental results of Shaw et al. in the low acidity medium. (ii) The diprotonated form of urea exists as an equilibrium species, which undergoes the A2 type hydrolysis more favorably than the monoprotonated form, as Moodie et al. found in the intermediate acidity medium. (iii) The A2 hydrolysis of the monoprotonated form is very similar to those of acetamide and methyl carbamate. (iv) As the number of the solvate water molecules increases, the activation barrier for the A2 process of the monoprotonated form increases while that for the unimolecular decomposition of the free base form decreases, indicating a possibility of the barrier height reversal in the bulk solvent in favor of the latter process, thus accommodating all the experimentally found trends in the urea hydrolysis. The A1 mechanisms involving six-membered ring type intermediates can be ruled out as untenable since no such equilibrium species was obtained by both the MNDO and AM1 calculations.
    Additional Material: 17 Ill.
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