Proton transfer in phenol-amine complexes: Phenol electronic effects on free energy profile in solution

• Published in: Journal of computational chemistry Vol. 31; no. 16; pp. 2924 - 2931
• Main Authors: Aono, Shinji, Kato, Shigeki
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01.12.2010; Wiley Subscription Services, Inc
• Subjects: electronic structure; Electrons; free energy profile; Hydrogen; Hydrogen Bonding; Hydrogen bonds; Methyl Chloride - chemistry; Methylamines - chemistry; Molecular Structure; Organic chemicals; phenol-mine complex; Phenols - chemistry; proton transfer; Protons; Quantum Theory; Solutions; solvation effect; substitution effect; Surface Properties; Temperature; Theory; Thermodynamics
• ISSN: 0192-8651; 1096-987X
• DOI: 10.1002/jcc.21588

Free energy profiles for the proton transfer reactions in hydrogen-bonded complex of phenol with trimethylamine in methyl chloride solvent are studied with the reference interaction site model self-consistent field method. The reactions in both the electronic ground and excited states are considered. The second-order Møller-Plesset perturbation (MP) theory or the second-order multireference MP theory is used to evaluate the effect of the dynamical electron correlation on the free energy profiles. The free energy surface in the ground state shows a discrepancy with the experimental results for the related hydrogen-bonded complexes. To resolve this discrepancy, the effects of chloro-substitutions in phenol are examined, and its importance in stabilizing the ionic form is discussed. The temperature effect is also studied. In contrast to the ground state, the ππ* excited state of phenol-trimethylamine complex exhibits the proton transfer reaction with a low barrier. The reaction is almost thermoneutral. This is attributed to the reduction of proton affinity of phenol by the ππ* electronic excitation. We further examine the possibility of the electron-proton-coupled transfer in the ππ* state through the surface crossing with the charge transfer type πσ* state.