| Résumé | Multidimensional ab initio proton tunneling rate constants are reported for the tautomerization of singlet-excited 7-azaindole complexed with water, represented by discrete water molecules with and without a dielectric continuum. The results are compared with experimental observations in cold beams and in room-temperature aqueous solutions. For complexes with one and two water molecules, potential-energy surfaces are calculated at the complete active space multiconfiguration self-consistent-field [CASSCF(8,8)] level. For comparison with solution data, the structures are reoptimized inside a spherical cavity according to the Onsager model. To compare the effect of the dielectric with that of a secondary solvent shell, the structure of 1:1 and 1:2 complexes solvated by four and three additional water molecules so as to form 1:5 complexes, are optimized at the CASSCF(8,8) level with single-point Onsager corrections. Based on these potential-energy surfaces, temperature-dependent multidimensional proton transfer rate constants are calculated with a recently developed version of the instanton approach. It is found that in gas-phase 1:1 and 1:2 complexes tautomerization occurs through concerted double and triple proton transfer, respectively. The calculated low-temperature rate constants agree with the observation that in these complexes no tautomerization occurs within the fluorescence lifetime of about 8 ns. Addition of a dielectric continuum within the Onsager model cannot explain the room-temperature rate constant of about 1010 s�1 observed as the fast tautomerization component of excited 7-azaindole in protic solutions. Addition of a secondary solvent shell of four water molecules to the 1:1 complex has only a minor effect on the proton transfer rate, but addition of a secondary shell of three water molecules to the cyclic 1:2 complex yields rate constants of the observed order of magnitude. This happens because the double bridge facilitates charge separation, which stabilizes an ion-pair structure for the transition state. As a result the barrier is lowered drastically and although the proton effective mass is also increased, the effect of the lower barrier dominates, leading to much faster proton transfer. It is concluded that the fast rate component observed in room-temperature tautomerization of excited 7-azaindole in water and alcohols corresponds to proton transfer through a bridge of two hydrogen-bonded water molecules, rather than through a single-molecule water bridge as previously assumed. The predicted mechanism involves a (meta)stable intermediate state. |
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