Microsolvation of the Sodium and Iodide Ions and Their Ion Pair in Acetonitrile Clusters: A Theoretical Study
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- 作者：Tao-Nhâ ; n V. Nguyen ; Sean R. Hughes ; Gilles H. Peslherbe
- 刊名：Journal of Physical Chemistry B
- 出版年：2008
- 出版时间：January 17, 2008
- 年：2008
- 卷：112
- 期：2
- 页码：621 - 635
- 全文大小：405K
- 年卷期：v.112,no.2(January 17, 2008)
- ISSN：1520-5207
文摘
The structural and thermodynamic properties of Na+(CH3CN)n, I-(CH3CN)n, and NaI(CH3CN)n clusters havebeen investigated by means of room-temperature Monte Carlo simulations with model potentials developedto reproduce the properties of small clusters predicted by quantum chemistry. Ions are found to adopt aninterior solvation shell structure, with a first solvation shell containing ~6 and ~8 acetonitrile molecules forlarge Na+(CH3CN)n and I-(CH3CN)n clusters, respectively. Structural features of Na+(CH3CN)n are found tobe similar to those of Na+(H2O)n clusters, but those of I-(CH3CN)n contrast with those of I-(H2O)n, forwhich "surface" solvation structures were observed. The potential of mean force calculations demonstratesthat the NaI ion pair is thermodynamically stable with respect to ground-state ionic dissociation in acetonitrileclusters. The properties of NaI(CH3CN)n clusters exhibit some similarities with NaI(H2O)n clusters, with theexistence of contact ion pair and solvent-separated ion pair structures, but, in contrast to water clusters, bothtypes of ion pairs adopt a well-defined interior ionic solvation shell structure in acetonitrile clusters. Whereascontact ion pair species are thermodynamically favored in small clusters, solvent-separated ion pairs tend tobecome thermodynamically more stable above a cluster size of ~26. Hence, ground-state charge separationappears to occur at larger cluster sizes for acetonitrile clusters than for water clusters. We propose that thelack of a large Na+(CH3CN)n product signal in NaI(CH3CN)n multiphoton ionization experiments could arisefrom extensive stabilization of the ground ionic state by the solvent and possible inhibition of the photoexcitationmechanism, which may be less pronounced for NaI(H2O)n clusters because of surface solvation structures.Alternatively, increased solvent evaporation resulting from larger excess energies upon photoexcitation ormajor solvent reorganization on the ionized state could account for the observed solvent-selectivity in NaIcluster multiphoton ionization.