An ab initio molecular dynamics study of supercritical aqueous ionic solutions: Hydrogen bonding, rotational dynamics and vibrational spectral diffusion Bhabani S. Mallik 1 , Amalendu Chandra ⇑ Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India article info Article history: Received 27 March 2011 In final form 22 June 2011 Available online 7 July 2011 Keywords: Ions in supercritical water Ab initio molecular dynamics Time series analysis Hydrogen bond fluctuations Spectral diffusion abstract The structure and dynamics of supercritical aqueous ionic solutions are investigated through ab initio molecular dynamics simulations. Explicit results are obtained for two different densities and, for the higher density, two different concentrations are considered. The structure of the solutions is investigated through ion–water correlations, hydrogen bond and coordination numbers of the ions and water mole- cules and also through frequency–structure correlations of water molecules in the hydration shells and bulk phases. It is found that the stretch frequencies of water in the anion hydration shell are lower than those in the bulk water contrary to the behavior of the corresponding ambient solutions. On the dynam- ical side, calculations are made for diffusion, orientational relaxation, dynamics of hydrogen bonds and dangling OD groups. The dynamics of vibrational spectral diffusion is also investigated and its connec- tions to the underlying molecular dynamics of water in hydration shells and bulk phases are established. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Supercritical fluids have unique solvent characteristics which primarily result from the fact that the high compressibility and lack of a liquid–gas phase transition as a function of pressure allow the solvent properties to be tuned continuously over a wide range simply via the adjustment of density. A supercritical fluid pos- sesses the diffusion characteristics of a gas and many solvation properties of a liquid and there have been many industrial and environmental applications using supercritical fluids [1–4]. In par- ticular, supercritical water has attracted increasing attention as an environmentally benign solvent medium with unique solvation properties different from those of ambient water. For the specific case of aqueous solutions, chemical and physical processes involving charged species are of natural interest and, therefore, a number of existing studies of supercritical aqueous solutions deal with the behavior of ions in supercritical water. The experimental studies primarily deal with ion hydration and transport properties. The existing experimental studies of solute hydration are based on UV–visible and NMR spectroscopic meth- ods and also X-ray and neutron diffraction techniques [5–8]. There have also been a number of theoretical studies of supercritical aqueous ionic solutions using the methods of molecular dynamics simulations and integral equation theories [8–16]. We note, in particular, the work of Cummings and coworkers [11–13] and of Rasaiah and coworkers [14,15]. These authors investigated the hydration structure and diffusion coefficients of alkali and halide ions over a wide range of density and temperature under supercrit- ical conditions and reported many interesting results. However, many of their results could not be directly compared with experi- ments, such as the almost equal hydration number of ions and drastic change of the hydration structure of neutral solutes in supercritical water (temperature 683 K and solvent density 0.35 g cm 3 ) as compared to that in ambient water at room tem- perature because of the lack of neutron diffraction data of these hydration structures. The hydration structure and diffusion of hydrophobic solutes, such as oxygen and methane, have also been investigated in recent studies [17]. Very recently, the dynamics of water–water and ion–water hydrogen bonds and also effects of solute size on ion diffusion in supercritical water have been inves- tigated through classical molecular dynamics simulations [18,19]. We note that all these simulation studies were based on models which are parametrized to reproduce structural, thermodynamic and dynamical properties of aqueous solutions under ambient con- ditions and it may not be accurate to use such models at supercrit- ical conditions, especially when the density is tuned from high to low values which can result in a rather significant changes in the molecular dipole moments and hence water–water and ion–water interactions. Polarization effects are generally believed to be important for aqueous solutions and these effects also vary with thermodynamic state of the solutions. The effects of state-dependent polarization and many body interactions enter naturally in ab initio molecular 0301-0104/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2011.06.031 ⇑ Corresponding author. Tel.: +91 512 2597241; fax: +91 512 2597436. E-mail address: amalen@iitk.ac.in (A. Chandra). 1 Present address: Department of Chemical and Biomolecular Engineering, Univer- sity of Notre Dame, Notre Dame, IN 46556, USA. Chemical Physics 387 (2011) 48–55 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys