1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Electro, Physical & Theoretical Chemistry H-Bonding in Water of Hydration: NIR Spectral Studies of Hydration Behavior of 1-n-Alkyl-3-metylimidazolium-Based Bromide and Amino Acid Ionic Liquids at 298.15 K Dilip H. Dagade* and Seema S. Barge [a] H-bonding in ionic hydration is crucial for understanding various phenomena such as osmolyte effects, ion transport, electrochemical processes, etc. Ionic liquids (ILs) from the biomaterials have great affinity for protein solubility and stabilization due to hydrophobicity and hydrogen bonding ability. To explore potential use of ionic liquids, it is essential to understand hydration of ionic liquids with detailed information on H-bonding within the water of hydration. In this context, near-infrared spectral investigation of H-bonding in water of hydration of 1-n-alkyl-3-methylimidazolium bromides and 1- ethyl-3-methylimidazolium based amino acid ionic liquids were made through simultaneous estimation of hydration number and hydration spectra of ILs in the spectral range of 7800-5800 cm 1 . Weak H-bonding extist in the water of hydration for 1-n- alkyl-3-methylimidazolium bromides while strong cooperative H-bonding exists in case of amino acid ionic liquids leading to higher hydration numbers. The results further demonstrate that the individual resolved spectral components can be used to analyse (qualitatively and quantitatively) different types of H- bonded species at ionic surfaces. Along with all these, the effect of hydrophobicity on hydration behavior as well as on the nature and strength of H-bonding within the hydration shell of ions are also probed and discussed. 1. Introduction The structure and dynamics of H-bonded network in water is more complex and studied extensively by many scientists with the help of Raman and IR spectroscopy and simulation techniques. [1–13] These studies reveal the presence of non-H- bonded configurations (NHBs) also known as dangling hydro- gen bonds which are transient species with life-time of < 200 fs in liquid water. [1,2] Such species absorb preferentially on the high frequency side (blue) of the spectrum while strong relatively stable H-bonds absorbs at low frequency side (red) of the spectrum. Our recent work on analysis of H-bonded network in pure liquid water emphasized that water consist five different types of H-bonded species such as fully H-bonded ice-like, fully H-bonded distorted ice-like, steady state equili- brated doubly and triply H-bonded species, singly H-bonded species and NHBs absorbing at different unique frequencies. [13] Furthermore, the optimum temperature of ~ 316.5 K (43.5 8C) is observed at which equal concentration of fully H-bonded species and sum of singly H-bonded species and NHBs exists. Below this optimum temperature water is highly structured and above this temperature the H-bonded network of liquid water gets disrupted anomalously leading to abrupt increase in concentration of singly H-bonded species with increase in temperature. Survival of most of living organisms and the functioning of their metabolic activities are normal below this optimum temperature suggest that H-bonded network of liquid water is more important for existence and stabilization of life than the disrupted highly broken network of H-bonds. The existence of these two states of water (one state dominated by fully four H-bonded species below 316.5 K and other state dominated by singly H-bonded species above 316 K) in liquid water reinforce us to think over use of mixture of two different liquid waters instead of single uniform liquid water medium for studying various biomolecular processes particularly in case of computer simulation studies. The strong support for this two state behavior of liquid water is the recent work reported by Marque ´s et al. [14] in which authors have compiled various properties of liquid water at different temperatures along-with some new findings based on fluorescence study of quantum dots. These authors observed the bilinear behavior of liquid water corresponding to the existence of two states of water in nanometric and biological systems. [14] The addition of ions to liquid water results in reorientation of water molecules around the ions leading to formation of hydration shell and disruption of H-bonding network in liquid water. H-bonding plays crucial role in the dissolution and stabilization of polar species most importantly the biologically important macromolecules as well as the osmolytes. Hence, along-with hydration properties of the molecules, H-bonding in the hydration shell needs to be more understood to explain the properties of such solutions. Recently we investigated the hydrogen bonded network in pure liquid water and for water in the hydration shell of alkali and alkaline earth halide for which a method of simultaneous estimation of the spectrum of water in the hydration shell and hydration number of solutes was proposed. [13] From the [a] Dr. D. H. Dagade, Dr. S. S. Barge Department of Chemistry, Shivaji University, Kolhapur – 416 004, INDIA E-mail: dhd_chem@unishivaji.ac.in dilipdagade@yahoo.com Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201702281 Full Papers DOI: 10.1002/slct.201702281 11703 ChemistrySelect 2017, 2, 11703 –11712 # 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim