Theoretical study on excited-state proton transfer via hydrogen-bonded ethanol (EtOH) wire for 7AI in the gas phase

Abstract

Systematic studies of the excited-state tautomerization in the 7-azaindole–(EtOH) n (n = 1, 2) complexes in the gas phase were theoretically investigated. Structures and energies for reactant, transition state and product were computed at the CASSCF levels with the 6-31G(d,p) and 6-311G(d,p) basis sets. The barrier heights and reaction energies were corrected by the second-order multireference perturbation theory (MRPT2) to consider the dynamic electron correlation. The excited-state double-proton transfer in 7AI–EtOH occurs in a concerted but asynchronous mechanism. Similarly, such paths are also found in the two transition states during the excited-state triple-proton transfer of 7AI–(EtOH)2 complex. One path is that the proton from the pyrrole ring moved first to ethanol, and the other path is that the ethanol proton moved first to 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 for the 7-azaindole–(EtOH)2 complex was 4.3 kcal/mol with the zero-point energy correction. Additionally, the effect on the substitution of the ethyl group for the methyl group in the 7AI–(MeOH) n (n = 1, 2) was discussed. The replacement of the methyl group by the ethyl group obviously increased the barrier height and the asynchronousity of proton transfer in the 7AI–(EtOH)2 complex and had little effect on the 7AI–(EtOH) complex.