RESEARCH ARTICLE Investigation of solvation of ammonium salts: A Raman spectroscopy and ab initio study Anamika Mukhopadhyay | Pankaj Dubey Department of Chemical Sciences, IISER Mohali, Sector 81, Knowledge City, SAS Nagar, Manauli, Punjab 140306, India Correspondence Anamika Mukhopadhyay, Department of Chemical Sciences, IISER Mohali, Sector 81, Knowledge City, SAS Nagar, Manauli, Punjab 140306, India. Email: anamika@iisermohali.ac.in; anamukherjeester@gmail.com Abstract The effect of dissolved salts on the hydrogen bonded network in water is extremely important to be understood, as it plays an important role in many aspects of structure and dynamics in aqueous solutions. We have undertaken a study of this phenomenon, using NH 4 Cl (AC) and (NH 4 ) 2 SO 4 (AS), as the salts for influencing the hydrogen bonded network in water. The effects of vary- ing the temperature and concentration in these aqueous solutions of both the salts, on the Raman spectra were studied, over the wavenumber range 50– 4000 cm -1 . It was found that at 25 °C, with increasing AS concentration, a monotonic increase in intensity of spectral features on the low wavenumber side (~3200 cm -1 region) of the O–H stretching band was observed, whereas AC showed the opposite effect. A parameter (χ struct ) is defined from the spectral data, which indicates that more hydrogen bonded network forms in presence of AS salt compared with AC salt, in aqueous solution. Temperature variation study also reveals that, presence of AC induces a more disordered network in aqueous solutions, than AS. To support these conclusions, we have performed ab initio calculation for the salt⋯nW species, where n =1-8, using the MP2/ 6–31+G(d,p) level of theory. Solvent separated ion pair formation has been reported for NH 4 + and Cl - ions, whereas NH 4 + and SO 4 2- ions remain as con- tact ion pair up to AS⋯8W cluster. This study helps understand the effect of salt water interaction at the molecular level and may have huge implications in atmospheric physics, geophysics, and ice crystallization. KEYWORDS ab initio calculations, Hofmeister series, ion pairs, Raman spectroscopy, salt effect 1 | INTRODUCTION Water is a ubiquitous liquid known for its important role in different biological, [1–6] chemical, and atmospheric processes. [7,8] Its role in protein folding and their stabil- ity has been well documented. [9–11] Liquid water, in which the monomers are connected through directional hydrogen bonded (H‐bond) network resulting in a high degree of structure, is the most studied liquid. [12] The anomalous behavior of water has also been one of the reasons for the interest in elicits. Over past few decades, many experimental and computer simulation studies were performed to elucidate the structure of liquid water, leading to the concepts of “continuum model” [13] and “multicomponent model.” [14–22] Although the first model describes the liquid water as O–H⋯OH‐bonded liquid with wide distribution of H‐bond angle and dis- tances, the later model describes liquid water as a combi- nation of two states, that is, free and partially H‐bonded structure. Received: 2 June 2017 Revised: 6 November 2017 Accepted: 24 November 2017 DOI: 10.1002/jrs.5322 J Raman Spectrosc. 2017;1–11. Copyright © 2017 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs 1