Theoretical calculations on vibrational frequencies and absorption spectra of S-1 and S-2 states of pyridine
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Three harmonic potential-energy surfaces of (1)A(1), B-1(1), and B-1(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 nu(12) and nu(6a) modes in the B-1(1) state; nu(1) and nu(12) modes in the B-1(2) state, indicating that these modes have strong vibronic coupling between ground and excited states.