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-pyr idyl)phenol was studied with FTIR and Raman spectroscopy and ab initio quantum chemical calculations. The assignments for all the observed v ibrations lying below 600 cm(-1) were proposed, based on the compariso n of experimental and computational results of transition energies, sh ifts upon deuterium and methyl substitution, and the analysis of the e volution 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 methylati on. The assignment of low-frequency modes may be helpful in understand ing of the phototautomerization mechanism, as well as in interpretatio n of the complicated structure of the band corresponding to the OH str etching vibration. (C) 1998 Elsevier Science B.V. All rights reserved.