Department of Chemical and Biomolecular Engineering, The Institute of NanoBioTechnology, Johns Hopkins University, Baltimore, Maryland 21218, USA.
J Chem Phys. 2011 Aug 7;135(5):054505. doi: 10.1063/1.3620077.
Thermochemistry of gas-phase ion-water clusters together with estimates of the hydration free energy of the clusters and the water ligands are used to calculate the hydration free energy of the ion. Often the hydration calculations use a continuum model of the solvent. The primitive quasichemical approximation to the quasichemical theory provides a transparent framework to anchor such efforts. Here we evaluate the approximations inherent in the primitive quasichemical approach and elucidate the different roles of the bulk medium. We find that the bulk medium can stabilize configurations of the cluster that are usually not observed in the gas phase, while also simultaneously lowering the excess chemical potential of the ion. This effect is more pronounced for soft ions. Since the coordination number that minimizes the excess chemical potential of the ion is identified as the optimal or most probable coordination number, for such soft ions the optimum cluster size and the hydration thermodynamics obtained with and without account of the bulk medium on the ion-water clustering reaction can be different. The ideas presented in this work are expected to be relevant to experimental studies that translate thermochemistry of ion-water clusters to the thermodynamics of the hydrated ion and to evolving theoretical approaches that combine high-level calculations on clusters with coarse-grained models of the medium.
气相离子-水团簇的热化学以及对团簇和水配体的水合自由能的估算被用于计算离子的水合自由能。通常，水合计算使用溶剂的连续体模型。准化学理论的原始准化学近似为这种努力提供了一个透明的框架。在这里，我们评估了原始准化学方法中固有的近似，并阐明了体相的不同作用。我们发现，体相可以稳定通常在气相中观察不到的团簇构型，同时也降低了离子的过剩化学势。对于软离子，这种效应更为明显。由于将离子的过剩化学势最小化的配位数被确定为最佳或最可能的配位数，因此对于这种软离子，与不考虑体相对离子-水簇合反应的情况下相比，最优的团簇大小和水合热力学可能不同。本工作中提出的观点有望与将离子-水团簇的热化学转化为水合离子的热力学的实验研究以及将簇上的高级计算与介质的粗粒化模型相结合的不断发展的理论方法相关。
