Vibrational spectroscopy of hydroxy-heterobiaryls I. Low frequency modes

Vibrational structure of four molecules known to undergo an extremely rapid excited state proton transfer: [2,2′-bipyridyl]-3,3′-diol, 5,5′-dimethyl[2,2′-bipyridyl]-3,3'-diol, [2,2′-bipyridyl]-3-ol and 2-(2-pyridyl)phenol was studied with FTIR and Raman spectroscopy and ab initio quantum chemical calculations. The assignments for all the observed vibrations lying below 600 cm^-1 were proposed, based on the comparison of experimental and computational results of transition energies, shifts upon deuterium and methyl substitution, and the analysis of the evolution of individual bands along the series. The calculations appear to be very reliable in predicting the vibrational frequencies, and in reproducing frequency shifts resulting from deuteration and methylation. The assignment of low-frequency modes may be helpful in understanding of the phototautomerization mechanism, as well as in interpretation of the complicated structure of the band corresponding to the OH stretching vibration.