Proton transfer reactions and dynamics in CH₃OH–H₃O⁺–H₂O complexes

Abstract

Proton transfer reactions and dynamics in hydrated complexes formed from CH(3)OH, H(3)O(+) and H(2)O were studied using theoretical methods. The investigations began with searching for equilibrium structures at low hydration levels using the DFT method, from which active H-bonds in the gas phase and continuum aqueous solution were characterized and analyzed. Based on the asymmetric stretching coordinates (Deltad(DA)), four H-bond complexes were identified as potential transition states, in which the most active unit is represented by an excess proton nearly equally shared between CH(3)OH and H(2)O. These cannot be definitive due to the lack of asymmetric O-H stretching frequencies (nu(OH)) which are spectral signatures of transferring protons. Born-Oppenheimer molecular dynamics (BOMD) simulations revealed that, when the thermal energy fluctuations and dynamics were included in the model calculations, the spectral signatures at nu(OH) approximately 1000 cm(-1) appeared. In continuum aqueous solution, the H-bond complex with incomplete water coordination at charged species turned out to be the only active transition state. Based on the assumption that the thermal energy fluctuations and dynamics could temporarily break the H-bonds linking the transition state complex and water molecules in the second hydration shell, elementary reactions of proton transfer were proposed. The present study showed that, due to the coupling among various vibrational modes, the discussions on proton transfer reactions cannot be made based solely on static proton transfer potentials. Inclusion of thermal energy fluctuations and dynamics in the model calculations, as in the case of BOMD simulations, together with systematic IR spectral analyses, have been proved to be the most appropriate theoretical approaches.

Dynamics and mechanism of structural diffusion in linear hydrogen bond

“…Similar results were presented for the CH 3 OH þ 2 (H 2 O) n complexes, n ¼ 1-4, [7] in which BOMD simulations at 350 K suggested the CH 3 OH [12,18] to be approximated from the vibrational energy for the interconversion between the oscillatory shuttling and structural diffusion motions (Dm…”

“…It was iterated that static proton transfer potentials cannot provide complete description of proton transfer reactions and the thermal energy fluctuation and dynamics must be incorporated in the model calculations. [7][8][9] To continue the series of theoretical studies on small H-bond systems, [7][8][9]19] as well as to obtain additional explanations for the proposed mechanisms, [13,15] structures, energetic and dynamics of proton transfer processes in protonated H-bond chains were investigated in the present work, using the CH 3 OH þ 2 (CH 3 OH) n complexes, n ¼ 1-4, as model systems, and B3LYP/TZVP calculations and BOMD simulations as model calculations. Since the effects of electron correlations could be important in the present model systems and it has been our intention to apply the DFT method in larger H-bond complexes, some static and dynamic results, obtained based on B3LYP/TZVP calculations, were examined using ab initio calculations at the RIMP2/TZVP level of accuracy.…”

“…[7][8][9][10] Born-Oppenheimer MD (BOMD) simulations showed that a quasi-dynamic equilibrium is established between the Zundel and Eigen complexes and considered to be the rate-determining step in the structural diffusion process. [12][13][14] It was demonstrated that the structural diffusion in aqueous solution is not concerted due to the thermal energy fluctuation and dynamics in the H-bond network.…”

“…[11] CH 3 OH has therefore been frequently selected as model molecule for the investigation of proton transfer in H-bond chain. [6,7,[12][13][14][15][16] The high mobility of excess proton in liquid CH 3 OH has been extensively studied using theoretical and experimental methods, [13][14][15][16] from which the structural diffusion of cationic defect was proposed. [13,15] The results obtained from density functional theory (DFT) calculations and vibrational predissociation spectroscopy suggested three fundamental structures in the structural diffusion pathway namely, the methyloxonium ion (CH 3 OH þ 2 ), the protonated methanol dimer (C 2 H 9 O þ 2 ), in which the H-bond proton is equally shared between two CH 3 )).…”

“…[1][2][3] This is due to the fact that the transport of excess proton, especially in hydrogen bond (H-bond), plays significant roles in important chemical and biochemical processes, ranging for examples from proton conductions in proton exchange membrane fuel cell to enzymatic reactions of proteins. [4][5][6][7] Although theoretical and experimental results have been accumulated, dynamics and mechanisms of proton transfers in liquids and aqueous solution are not completely known. Since basic chemistry of proton transfer reactions has been discussed in many review articles, only the theoretical and experimental information pertinent to this study will be briefly summarized.…”