Structure and vibrational spectra of the azabenzenes. A density functional study including exact exchange contributions

The geometry and vibrational spectra of pyridine, pyridazine, pyrimidine, pyrazine, s-triazine, 1,2,3-triazine, 1,2,4-triazine, and s-tetrazine have been computed using the B3LYP density functional method and correlation consistent basis sets of spd and spdf quality. Empirical corrections have been applied to the computed bond distances. Computed harmonic frequencies are in very good to excellent agreement with the available experimental data; the only real deficiency seen is for the Kekule-type vibration, which appears to have some multireference character. Significant basis set effects are seen for the Kekule modes and for ''pinwheel''-type hydrogen motions. Empirically corrected B3LYP/cc-pVDZ and B3LYP/cc-pVTZ bond distances are in essentially complete agreement for CH distances, excellent agreement (0.001 Angstrom) for CC distances, and good agreement (0.002 Angstrom) for CN distances, but differ markedly (0.003-4 Angstrom) for NN distances. Computed B3LYP/cc-VTZ dipole moments are in excellent agreement with the available experimental values (about 0.05-0.1 D too low). The calculations are found to be valuable in verifying a number of less certain experimental vibrational assignments.