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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
ABSTRACT - Internet Electron. J. Mol. Des. January 2003, Volume 2, Number 1, 1-13

Proton Transfer Reactions of R-OH•(H2O)n (R = H5C2 and C6H5, n = 2, 3, 4, 6 and 12). A Computational Study
Shinichi Yamabe and Noriko Tsuchida
Internet Electron. J. Mol. Des. 2003, 2, 1-13

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Abstract:
Hydroxyl groups in aliphatic and aromatic compounds are readily subject to proton exchanges. Aliphatic alcohols are neutral, while aromatic ones are weakly acidic. It is a question whether those well-known phenomena in aqueous media may be described uniformly in a framework of molecular interactions, i.e., hydrogen bond functions. A systematic analysis of proton transfer reactions in EtOH(H2O)n and PhOH(H2O)n has been carried out Density-functional theory calculations, B3LYP/6-31G* and B3LYP/6-311+G(2d,p), with the Onsager's SCRF solvent effect were performed for the title reactions, and proton-relay reaction paths were determined. First, the minimal and optimal model of a proton exchange reaction, Et-OH•(H2O)n→ Et-OH•(H2O)n, was sought. The n = 3 model was found to give a strain-free hydrogen bond network with the smallest activation energy for the concerted proton transfer. In larger models, such as Et-OH•(H2O)6 and Et-OH•(H2O)12, the Et-OH•(H2O)3 unit was confirmed to involve the concerted proton-relay movement. The transition states of proton transfers in Et-OH•(H2O)n (n = 6 and 12) are of the ion-pair character. Similar proton-relay reactions of Ph-OH•(H2O)n→ Ph-OH•(H2O)n were traced. A crucial difference between Et-OH•(H2O)n and Ph-OH•(H2O)n is the absence or presence of an ion-pair intermediate. That is, the ion-pair intermediates with Cs-symmetric structures were obtained in Ph-OH•(H2O)n (n = 6 and 12). Proton-exchange reactions and absence or presence of the electrolytic dissociation were suggested to be described uniformly by the R-OH•(H2O)3 unit.

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