PROTON-TRANSFERRING SYSTEMS STUDIED BY VIBRATIONAL SPECTROSCOPY AND THEORETICAL AB-INITIO CALCULATIONS - THE S-0 AND T-1 STATES OF [2,2'-BIPYRIDINE]-3,3'-DIOL

Tautomerization and phototautomerization of [2,2'-bipyridine]-3,3'-dio l is investigated by means of infrared, Raman, and time-resolved reson ance Raman spectroscopies and ab initio theoretical calculations. Full ab initio SCF (self-consistent field method) geometry optimization fo llowed by MBPT(2)/6-31G* (second-order many-body perturbation theory) energy calculations of both tautomeric forms in the ground state and the lowest excited triplet state is reported and discussed. Two tautom eric forms, A (dihydroxy) and B (dioxo) are found to be minima on the S-0 and T-1 potential energy surfaces. While form A is the most stable one in the ground state, the energetic ordering is reversed when goin g to the triplet state. A good agreement between the experimental and calculated vibrational spectra allows a reliable assignment of most vi brational bands corresponding to tautomer A in the ground state. To as sign the observed triplet-triplet optical transition and estimate inte nsities of resonance Raman bands in the triplet state, excitation ener gies and oscillator strengths of higher triplet states relative to T-1 are calculated. Tentative assignments for the resonance Raman bands o bserved in the lowest excited triplet state are given. All observed vi brational bands are assigned to totally symmetric modes of the most st able B form. From a comparison of the observed and calculated T-1 reso nance Raman spectra, it is concluded that two transitions, namely, T-1 --> T-5 and T-1 --> T-8, contribute to the resonance Raman scattering and to the triplet-triplet absorption band observed around 430 nm. Th ese states are both of A, symmetry and are calculated at 3.15 (T-5) an d 4.80 eV (T8) above T-1 (B-u). In view of the fact that the exciting wavelength (around 430 nm, 2.88 eV) is substantially out of resonance with the calculated T-1 --> T-8 transition, vibronic coupling between T-5 and T-8 is considered as a possibility.