Winter Nicole, Benjamin Ilan
Department of Chemistry, University of California, Santa Cruz, California 95064, USA.
J Chem Phys. 2005 May 8;122(18):184717. doi: 10.1063/1.1896357.
The thermodynamics and dynamics of a model S(N)1 reaction: t-BuCl --> t-Bu+ + Cl- is studied at the water liquid/vapor interface using molecular-dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. Umbrella sampling calculations are used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) in bulk water and in several locations at the interface. We find a significant increase of the dissociation barrier height and of the reaction free energy at the interface relative to the bulk. This is shown to be due to the reduced polarity of the interface. Reactive flux correlation function calculations show significant deviation of the rate constant from the transition-state theory: The transmission coefficients range from 0.49 in the bulk to 0.05 above the Gibbs surface. The low transmission coefficient at the interface despite the lower friction is shown to be due to slow vibrational relaxation.
使用分子动力学计算机模拟，在水的液/气界面研究了模型S(N)1反应：t-BuCl --> t-Bu+ + Cl-的热力学和动力学。采用经验价键方法在电子绝热极限下耦合共价和离子这两个非绝热态。采用伞形抽样计算来计算在体相水中以及界面处几个位置沿着反应坐标（定义为t-Bu到Cl的距离）的平均力势。我们发现，相对于体相，界面处的解离势垒高度和反应自由能显著增加。这表明是由于界面极性降低所致。反应通量相关函数计算表明速率常数与过渡态理论有显著偏差：传输系数范围从体相中的0.49到吉布斯表面上方的0.05。尽管界面处摩擦力较低，但传输系数却很低，这表明是由于振动弛豫缓慢所致。
