Steric Effects of Solvent Molecules on SN2 Substitution Dynamics

Xu Liu, Jing Xie, Jiaxu Zhang, Li Yang, William L. Hase

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

Abstract

Influences of solvent molecules on SN2 reaction dynamics of microsolvated F-(H2O)n with CH3I, for n = 0-3, are uncovered by direct chemical dynamics simulations. The direct substitution mechanism, which is important without microsolvation, is quenched dramatically upon increasing hydration. The water molecules tend to force reactive encounters to proceed through the prereaction collision complex leading to indirect reaction. In contrast to F-(H2O), reaction with higher hydrated ions shows a strong propensity for ion desolvation in the entrance channel, diminishing steric hindrance for nucleophilic attack. Thus, nucleophilic substitution avoids the potential energy barrier with all of the solvent molecules intact and instead occurs through the less solvated barrier, which is energetically unexpected because the former barrier has a lower energy. The work presented here reveals a trade-off between reaction energetics and steric effects, with the latter found to be crucial in understanding how hydration influences microsolvated SN2 dynamics.

Original languageEnglish (US)
Pages (from-to)1885-1892
Number of pages8
JournalJournal of Physical Chemistry Letters
Volume8
Issue number8
DOIs
StatePublished - Apr 20 2017

Bibliographical note

Funding Information:
This work is supported by the National Natural Science Foundation of China (Nos. 21573052, 21403047, 51536002) and the Fundamental Research Funds for the Central Universities, China (AUGA5710012114, 5710012014). The research of W.L.H. reported here is based upon work supported by the Robert A. Welch Foundation under Grant No. D-0005. Support is also provided by the High Performance Computing Center (HPCC) at Texas Tech University, under the direction of Philip W. Smith.

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