Abstract
Molecular orbital calculations have been carried out of the basicities of substituted pyridines in the gas phase at the HF/6-31G*, MP2/6-31G*, and B3LYP/6-31G* levels and in solution based on the isodensity surface-polarized continuum model (IPCM). The correlated gas-phase MO basicities, especially at the MP2 level, agree well with the experimental gas-phase results. The IPCM model with MP2/6-31G* and B3LYP/6-31G* geometries also reproduces well the solvation free energy difference, ΔG°s, between the pyridinium ion and neutral pyridine in water, although it leads to a somewhat greater negative slope for the pKa(aq) vs σ plot (ρaq = −8.6 and −9.0 at the MP2/6-31G* and B3LYP/6-31G* levels, respectively, in contrast to the experimental slope of ρaq = −5.8). The model predicts a linear correlation between the theoretical (IPCM) ρ values in solution and the Onsager dielectric function (e − 1)/(2e + 1). The estimate of ion solvation free energy for H+ in acetonitrile (G°s = −250.4 kcal mol-1) indicates that the...
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