27 Pure Appl. Chem., Vol. 85, No. 1, pp. 27–40, 2013. http://dx.doi.org/10.1351/PAC-CON-12-05-09 © 2012 IUPAC, Publication date (Web): 6 December 2012 A first-principles theoretical study of hydrogen-bond dynamics and vibrational spectral diffusion in aqueous ionic solution: Water in the hydration shell of a fluoride ion* Jyoti Roy Choudhuri, Vivek K. Yadav, Anwesa Karmakar, Bhabani S. Mallik † , and Amalendu Chandra ‡ Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India Abstract: We present a first-principles simulation study of vibrational spectral diffusion and hydrogen-bond dynamics in solution of a fluoride ion in deuterated water. The present cal- culations are based on ab initio molecular dynamics simulation for trajectory generation and wavelet analysis for calculations of frequency fluctuations. The O–D bonds of deuterated water in the anion hydration shell are found to have lower stretching frequency than the bulk water. The dynamical calculations of vibrational spectral diffusion for hydration shell water molecules reveal three time scales: a short-time relaxation (~100 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation (~7.5 ps) corresponding to the lifetimes of fluoride ion-water hydrogen bonds, and an even longer time scale (~26 ps) associated with the escape dynamics of water from the anion hydration shell. However, the slowest time scale is not observed when the vibrational spectral diffusion is calculated over O–D bonds of all water molecules, including those in the bulk. Keywords: fluoride ions; hydrogen bonds; ionic solution; spectral diffusion. INTRODUCTION Understanding the microscopic dynamics of water molecules in ion solvation shells has long been of major interest. In recent years, there have been a number of experimental studies on the dynamics of aqueous solutions using the technique of time-dependent vibrational spectroscopy [1–7]. These studies have looked at the dynamics of so-called vibrational spectral diffusion, which is then mapped to the dynamics of hydrogen bonds in these systems. Experimental studies have also been carried out on the changes of electronic structure of water molecules [8,9] and molecular organization of water [10,11] in ion hydration shells. On the theoretical side, there have been studies in recent years on the vibrational spectral diffusion of hydration shell water molecules in aqueous solutions using a combination of clas- sical dynamics and perturbative method [12] and also by ab initio molecular dynamics [13] simulations. It was shown that the slow escape dynamics of water molecules from ion hydration shells can show up as a long-time component of the spectral dynamics of hydration shell water in dilute solutions. In this work, we present a detailed dynamical study of water molecules inside the solvation shell of a fluoride ion. Existing microscopic studies on aqueous solutions of fluoride ions are rather limited. *Pure Appl. Chem. 85, 1–305 (2013). A collection of invited papers based on presentations at the 32 nd International Conference on Solution Chemistry (ICSC-32), La Grande Motte, France, 28 August–2 September 2011. † Present address: Department of Chemistry, Indian Institute of Technology Hyderabad, Yeddumailaram 502205 Andhra Pradesh, India ‡ Corresponding author: E-mail: amalen@iitk.ac.in