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 intramolecur (IM) hydrogen atom and proton transfer in the photochemical cycles of 2-(2′-hydroxyphenyl)benzoxazole (HBO) and 5,8-dimehtyl-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 quantum-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 case of DMT. Two rotamers E_I and E_II for the enol form of DMT were located corresponding to different positions of the H atom in the hydroxyl group. In the group state the first is more stable both in the gas phase and in polar protic solvents such as diethyl ether—isopentane—ethanol (5:2:5 by volume). Therefore the reketonization reaction is treated as one-step tunneling from the rotamer E_I to the keto form, i.e. without the activated rotational equilibrium E_I↔E_II proposed by Grellmann and coworkers in an earlier study. The steep slope of the kinetic curve of this reaction is attributed to the additional activation energy resulting from the final reorganization of the low frequency 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 found for the similarity of the kinetic curves for triplet excited-state intramolecular proton transfer in HBO and DMT.