Theoretical Calculations on Vibrational Frequencies and Absorption Spectra of S_1 and S_2 States of Pyridine

摘要

Three harmonic potential-energy surfaces of ~1A_1, ~1B_1, and ~1B_2 singlet states of pyridine have been obtained by using ab initio method. Geometric properties and force fields of these states have been determined with the complete-active-space self-consistent-field (CASSCF) theoretical method. Vibrational frequencies of two excited states have been assigned on the basis of the potential energy distribution from normal mode analysis. The resulting properties are used to simulate the experimental absorption spectrum. From the spectral simulation the totally symmetric vibrations with large Huang-Rhys factors have been identified as ν_(12) and ν_(6a) modes in the ~1B_1 state; ν_1 and ν_(12) modes in the ~1B_2 state, indicating that these modes have strong vibronic coupling between ground and excited states.