Theoretical Studies for Excited-State Tautomerization in the 7-Azaindole–(CH₃OH)_n (n = 1 and 2) Complexes in the Gas Phase

Abstract

The excited-state tautomerization of 7-azaindole (7AI) complexes bonded with either one or two methanol molecule(s) was studied by systematic quantum mechanical calculations in the gas phases. Electronic structures and energies for the reactant, transition state (TS), and product were computed at the complete active space self-consistent field (CASSCF) levels with the second-order multireference perturbation theory (MRPT2) to consider the dynamic electron correlation. The time-dependent density functional theory (TDDFT) was also used for comparison. The excited-state double proton transfer (ESDPT) in 7AI-CH(3)OH 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 (ESTPT) of the 7AI-(CH(3)OH)(2) complex. In the first TS, the pyrrole ring proton first migrated to methanol, while in the second the methanol proton moved first to the pyridine ring. The CASSCF level with the MRPT2 correction showed that the former path was much preferable to the latter, and the ESDPT is much slower than the ESTPT. Additionally, the vibrational-mode enhanced tautomerization in the 7AI-(CH(3)OH)(2) complex was also studied. We found that the excitation of the low-frequency mode shortens the reaction path to increase the tautomerization rate. Overall, most TDDFT methods used in this study predicted different TS structures and barriers from the CASSCF methods with MRPT2 corrections.

Theoretical Study on the Substituent Effect on the Excited‐State Proton Transfer in the 7‐Azaindole‐Water Derivatives

“…Otherwise, the vibration-mode-specific nature of ESPT in 7AI-(H 2 O) 2 and 7AI-(CH 3 OH) 2 complexes was reported by Sakota et al (47,53). Subsequently, the vibration-mode-specific nature was reproduced theoretically (56). Our results showed that the vibration-mode selectivity shortened the reaction path of ESPT and accelerated the rate of ESPT.…”

“…. Subsequently, the vibration‐mode‐specific nature was reproduced theoretically . Our results showed that the vibration‐mode selectivity shortened the reaction path of ESPT and accelerated the rate of ESPT.…”

“…Some theoretical results indicated the concerted but asynchronous mechanism for ESPT of 7AI‐(Solvent) n complexes in the gas phase. . Sakota et al .…”

“…When the proton donor and acceptor groups in 7AI were linked by a H-bonded chain, the increased acidity and basicity of 7AI started the excited-state tautomerization of 7AI. The excited-state multiple proton transfer from N-H group in the pyrrole ring to the =Nsite in the pyridine ring in the 7AI dimer (23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37), and 7AI complexes with small protic solvent molecules (38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57) have been extensively researched. One of the most significant and interesting issues debated in the proton transfer field is whether the ESPT is concerted or stepwise.…”

“…Some theoretical results indicated the concerted but asynchronous mechanism for ESPT of 7AI-(Solvent) n complexes in the gas phase. (41,52,54,56,57). Sakota et al (53) found that the multiple protons transferred asynchronously but concertedly when they researched the gas phase 7AI-(H 2 O) 2 complex using spectroscopy and quantum chemical calculations.…”