Analytical Chemistry and Spectroscopy
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Ion-molecule reactions in a pentaquadrupole mass spectrometer are used to generate cluster ions in which neutral pyridine molecules are bound to a linear polyatomic cation, [OCNCO]+. The dimeric adduct, viz. Py1[OCNCO]+Py2 where Py1 and Py2 represent substituted pyridines, formed upon reaction of mass-selected [OCNCO]+ with a mixture of pyridines, has a loosely bound structure, as suggested by triple stage mass spectrometric (MS3) experiments. Dimeric adducts comprised of meta- and/or para-substituted pyridines (unhindered pyridines) display an excellent linear correlation between the fragment ion abundance ratio ln [Py1[OCNCO]+/Py2[OCNCO]+] and the proton affinity difference of the pyridines. On the assumption that the effective temperature of the [OCNCO]+ -bound dimers is similar to that of the corresponding Cl+-bound dimers, [OCNCO]+ affinities of the substituted pyridines relative to pyridine are estimated to be 3-MePy 2.3, 4-MePy 3.2, 3-EtPy 3.5, 4-EtPy 4.1, 3,5-diMePy 4.9 and 3,4-diMePy 5.6 kcal mol-1 (1 kcal = 4.184 kJ). A linear relationship between the relative [OCNCO]+ cation affinity and the relative proton affinity (PA) is derived as relative [OCNCO]+ affinity (kcal mol-1) = 0.96 ΔPA, using the assumed effective temperature of 555 K. Dimers consisting of ortho-substituted pyridines display substantial steric effects between the ortho-substituted alkyl group and the central [OCNCO]+ cation. A set of gas-phase steric parameters (Sk) is determined and steric effects are ordered 2-MePy (-1.39) 〈2,5-diMePy (-3.02) 〈 2,4-diMePy (-3.15) 〈 2,3-diMePy ( -3.29) 〈 2,6-diMePy (-5.09) 〈 2,4,6-triMe (-6.13). A greater steric effect is experienced in the [OCNCO]+ system than in the corresponding Cl+ system, owing to the larger central ion in Py[OCNCO]+ Py. Structural and electronic information regarding the bond angles, bond lengths and charge density distributions in [OCNCO]+, Py[OCNCO]+ and Py[OCNCO]Py+ was obtained from ab initio calculations. The calculations show that [OCNCO]+ is linear with high positive charge densities on the carbon atoms. The calculations also reveal that the Py[OCNCO]+ monomer is a planar ion with the nitrogen atom of pyridine bound to a carbon atom of the [OCNCO]+ cation, and that the Py[OCNCO]Py+ dimer is a symmetrical ion in which the nitrogen atoms on the pyridine molecules bind to the carbon atoms of the central cluster ion, [OCNCO]+.
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