COMPARATIVE THEORETICAL-STUDY OF INTRAMOLECULAR PROTON-TRANSFER IN THE PHOTOCHEMICAL CYCLES OF 2-(2'-HYDROXYPHENYL)BENZOXAZOLE AND 5,8-DIMETHYL-1-TETRALONE

We present a comparative golden rule analysis of the dynamics of the i ntramolecular (IM) hydrogen atom and proton transfer in the photochemi cal cycles of 2-(2'-hydroxyphenyl)benzoxazole (HBO) and 5,8-dimethyl-1 -tetralone (DMT). Two major effects are taken into consideration: the promoting effect of the IM vibrations which are symmetrically coupled to the reaction coordinate, and the suppressing effect resulting from the reorganization of both the molecule and solvent. Semiempirical qua ntum-chemical calculations at the AM1 level were carried out to study the energy levels of all states involved in the photochemical cycles, including the effects of solvation in a polar protic solvent in the ca se of DMT. Two rotamers E(I) and E(II) for the enol form of DMT were l ocated corresponding to different positions of the H atom in the hydro xyl group. In the ground state the first is more stable both in the ga s phase and in polar protic solvents such as diethyl ether-isopentane- ethanol (5:2:5 by volume). Therefore the reketonization reaction is tr eated as one-step tunneling from the rotamer E(I) to the keto form, i. e. without the activated rotational equilibration E(I) <--> E(II) prop osed by Grellmann and coworkers in an earlier study. The steep slope o f the kinetic curve of this reaction is attributed to the additional a ctivation energy resulting from the final reorganization of the low fr equency oscillators, both intramolecular and solvent. For the dynamic calculations, the standard AM1 output (structural and force field data ) was used as the input, and good agreement with the available kinetic experiments was reached for both compounds. No special reasons were f ound for the similarity of the kinetic curves for triplet excited-stat e intramolecular proton transfer in HBO and DMT.