THEORETICAL-STUDY OF INTRAMOLECULAR PROTON-TRANSFER IN SOLUTION - APPLICATION TO THE PHOTOENOLIZATION OF 5,8-DIMETHYL-1-TETRALONE

In this study, further development of the Golden Rule (GR) approach is presented for the dynamics of intramolecular (IM) hydrogen atom and p roton transfer (PT) in solution. The IM modes are treated following th e procedure reported earlier which simplifies drastically the problem of evaluating the multidimensional transfer integrals. The polar solve nt is treated as a dielectric continuum with classical Debye spectrum. In the most typical case of relation between the parameters involved, the rate constant is expressed as a product of two almost independent terms: the ''pure'' tunneling rate of the same transfer but without a ny reorganization effects taken into consideration, and a suppressing tunneling factor of Levich-Dogonadze type in a generalized form. Two m ajor effects are present: the promoting effect of the IM vibrations sy mmetrically coupled to the reaction coordinate, and the suppressing ef fect resulting from the final reorganization of-both the molecule and solvent. This approach is applied to the hydrogen atom and proton tran sfer in the photochemical cycle of 5,8-dimethyl-1-tetralone (DMT) obse rved by Grellmann and co-workers in a polar protic solvent (EPA). This compound exhibits typical non-Arrhenius temperature and isotope depen dence of the rate of triplet enolization. The kinetic curves of the gr ound-state reketonization reaction are close to Arrhenius, with signif icantly higher slopes than for typical intramolecular PT reactions. Se miempirical quantum-chemical calculations at AM1 level were carried ou t to study the relative stability, structure, and charge distribution of all states involved in the photochemical cycle, including the effec ts of solvation in a polar II-bonding solvent. Two rotamers E(I) and E (II) for the enol form were located corresponding to different positio ns of the H atom of the hydroxyl group. In ground state the first is m ore stable in both the gas phase and polar protic solvent modeled by w ater. Therefore, the reketonization reaction is treated as one-step tu nneling from the rotamer E(I) to the keto form, i.e., without activate d rotational equilibration E(I) <-> E(II) proposed by Grellmann and co -workers in an earlier study. Calculations of the rate constants were performed for both the direct and reverse reaction. Standard AM1 outpu t (structural and force field data) was used as input, and good agreem ent with the available kinetic experiments was reached for both compou nds. The high slope of the kinetic curve of this reaction is attribute d to the additional activation energy resulting from the final reorgan ization of the low-frequency oscillators, mainly those from the solvat ion layer.