Characterizing the local solvation environment of OH- in water clusters with AIMD

In this work, we use ab initio molecular dynamics coupled with metadynamics to explore and characterize the glassy potential energy landscape of the OH- in a 20 and 48 water cluster. The structural, energetic, and topological properties of OH- are characterized for both clusters and the molecular origins of the IR signatures are examined. We find that in both the small and large clusters, the OH- can donate or accept a varying number of hydrogen bonds confirming that the amphiphilic character does not depend on cluster size. However, we highlight some important differences found between the energetic and topological properties of both families of clusters which may have implications on understanding the changes in the solvation structure of OH- between bulk and interfacial environments. By studying the IR spectra of smaller subsets of molecules within the 20 water molecule cluster, we find that the IR spectrum of the bare OH- as well as the water molecule donating a strong hydrogen bond to it exhibits characteristic absorption along the amphiphilic band between 1500 and 3000 cm(-1) at positions very similar to those found for the entire hydroxide cluster. The results presented here will be useful in the calibration and improvement of both ab initio and semi-empirical methods to model this complex anion. (C) 2016 AIP Publishing LLC.