Excited-State Tautomerization in the 7-Azaindole-(H2O)n (n = 1 and 2) Complexes in the Gas Phase and in Solution: A Theoretical Study.

A systematic study of the excited-state tautomerization of 7-azaindole-(H2O)n (n = 1 and 2) complexes in both gas and solution phases were investigated theoretically. Electronic structures and energies for the reactant, transition state (TS), and product were computed using the time-dependent density functional theory (TDDFT) and complete active space self-consistent field (CASSCF) levels with 6-31G (d,p), 6-311G(d,p), and 6-311+G(d,p) basis sets. Barrier heights and tautomerization energies were corrected by the second-order multireference perturbation theory (MRPT2) to consider the dynamic electron correlation. The solvent effect decreased the tautomerization barrier height in the 7-azaindole-H2O complex. In the 7-azaindole-(H2O)2 complex, two transition states were found for two asynchronous but concerted paths: in the first, the pyrole ring proton moved first to water; in the second, the water proton moved first to the pyridine ring. The CASSCF level with the MRPT2 correction clearly showed that the former path was much preferable to the latter. The preferable barrier height was only 1.6 kcal/mol with a zero-point energy correction, which would make the excited-state tautomerization possible. At all TDDFT levels, the TS structures and barrier heights depended on both the basis set used and the solvent effect. Most TDDFT methods failed to reproduce the CASSCF structures and MRPT2 energies. Only two methods, WB97XD/6-31G(d,p) and M062X/6-311+G(d,p), predicted two TSs for the two asynchronous paths in the 7AI-(H2O)2 complex but failed to reproduce the energetics. Further systematic study is necessary to test whether current TDDFT methods, including solvent effects, can be used to understand excited-state proton transfer reactions.