Department of Chemistry, University of California, Berkeley, California 94720-1460, United States.
J Phys Chem B. 2010 Oct 21;114(41):13189-200. doi: 10.1021/jp1068945.
An improved cluster pair correlation method that is based on the method originally introduced by Tuttle et al. ( Tuttle et al. J. Phys. Chem. A 2002 , 106 , 925 - 932 ) was developed and evaluated using a significantly larger data set than used previously. With this larger data set, values for the absolute proton hydration free energy of -259.3 and -265.0 kcal/mol were obtained using the original and improved method, respectively. The former value is ∼4.5 kcal/mol less negative than previously reported values obtained with the same method but with smaller data sets. The dependence of this value on data set size indicates that the uncertainty in the original method may be greater than previously realized. The improved method has the advantages of higher precision, and the effects of cluster size on the proton hydration free energy and enthalpy values can be more readily evaluated. Data for ions with extreme pK(a)s, many of which were included in previous estimates of the proton hydration free energy, were found to be unreliable and were eliminated from the extended data set. There is only a subtle effect of cluster size on the Gibbs free energy values, and within the limits of the approximation inherent in the cluster pair correlation method, the "best" value for the standard absolute proton hydration free energy obtained with this new method and larger data set is -263.4 kcal/mol (average for clusters with 4-6 water molecules). The absolute proton hydration enthalpy values decrease from -273.1 to -275.3 kcal/mol with increasing cluster size (one to six water molecules, respectively). This trend, along with an anomalously high value for the absolute proton hydration entropy, indicates that the enthalpy obtained with this method may not have converged for these relatively small clusters.
一种改进的簇对相关方法，该方法基于 Tuttle 等人最初提出的方法（Tuttle 等人，J. Phys. Chem. A 2002, 106, 925-932），并使用比以前更大的数据集进行了开发和评估。使用这个更大的数据集，使用原始方法和改进方法分别得到了绝对质子水合自由能的-259.3 和-265.0 kcal/mol 值。前者的值比以前用相同方法但数据集较小得到的值约小 4.5 kcal/mol。该值对数据集大小的依赖性表明原始方法的不确定性可能比以前认识到的更大。改进的方法具有更高的精度，并且更容易评估簇大小对质子水合自由能和焓值的影响。对于 pK(a)极端的离子的数据，其中许多离子被包括在以前的质子水合自由能估计中，被发现是不可靠的，并从扩展的数据集中排除。簇大小对吉布斯自由能值只有细微的影响，并且在簇对相关方法所固有的近似范围内，使用这种新方法和更大数据集获得的标准绝对质子水合自由能的“最佳”值为-263.4 kcal/mol（具有 4-6 个水分子的簇的平均值）。随着簇大小的增加（分别为一个到六个水分子），绝对质子水合焓值从-273.1 减小到-275.3 kcal/mol。这种趋势，以及绝对质子水合熵的异常高值，表明对于这些相对较小的簇，该方法获得的焓值可能尚未收敛。
