The Low-Lying Excited States of Pyridine

We assign the observed phosphorescence and high-resolution singlet-to-triplet absorption spectra of pyridine,
performing large model vibronic coupling calculations involving six active modes and three near-degenerate
electronic states, 3A1, (2)3A1, and 3B1. Vibronic coupling primarily through ν8a between 3A1 and (2)3A1 results
in the breakdown of the pseudo-parity selection rule:  the lower surface has a double minimum, with each
well corresponding to a diabatic single-determinant (π,π*) excitation. These states then strongly vibronically
couple to 3B1 primarily through ν16b, resulting in a very complex 3A‘ manifold. In addition, for the singlet
manifold, we detail a low-lying conical intersection between 1B1 and 1A2 and suggest that this intersection
should have observable consequences for excited-state dynamics. These conclusions are obtained through the
examination of all states below 5 eV in energy [1B1 (S1), 1B2 (S2), 1A2 (S3), 3A1, (2)3A1, 3B1, 3A2, and 3B2],
performing CIS, CASSCF, CASPT2, CCSD, CCSD(T), EOM-CCSD, CNDO/S, B3LYP, BLYP, TD-B3LYP,
and TD-BLYP calculations for the vertical excitation and emission energies; equilibrium, transition-state,
and conical-intersection structures; vibration frequencies; spin−orbit couplings; vibronic couplings; ESR atomic
spin densities; and low-resolution absorption and emission band contours. Special techniques are developed
for the application of the electronic structure methods to the evaluation of the required molecular properties,
and it is shown that the application of a wide range of methods is required both because of the diversity of
the required properties and because the intrinsic errors in the methods are of magnitudes that are chemically
significant.