Structure and solvation dynamics of the hydroxide ion in ice-like water clusters: a CCSD(T) and car–parrinello molecular dynamics study

Using MP2, CCSD(T) electronic structure theory and ab initio molecular dynamics simulations, we explore the structure, solvation dynamics and vibrational spectra of OH−(H2O)n clusters. Our study reports new cubic and fused cubic global minima structures of OH−(H2O)n for n = 8–26 with surface and interior solvation arrangements. In the case of OH−(H2O)26, we show that MP2 and CCSD(T) calculations predict global minima structures with the hydroxide ion occupying the interior region of a densely packed cubic cluster that is secured by ionic hydrogen bonds. More importantly, results from ab initio molecular dynamics simulations of OH−(H2O)26 demonstrate that the hydroxide ion remains in the cluster interior and hexa-coordinated, irrespective of the temperature, up to around 175 K, then incrementally transitions from a surface-exposed penta- (170–200 K), to a tetra- (225 K) to a tri-coordinated OH−(H2O)3 structure at 300 K. Building on our temperature-dependent vibrational power spectra, we are also able to disentangle structure and temperature effects on individual spectral contributions arising from water molecules located in the inner and outer shell of OH−(H2O)26. Some of these theoretical results provide valuable guidance for the interpretation of IRMPD spectra of small hydroxide-water clusters, but there are also several intriguing implications of these results, in particular, for the solvation of the OH− ion at the surface of water nanodroplets and aqueous interfaces.