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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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