Evidence for two kinds of hydrogen-bonding networks in ionic solutions by 2D infrared correlation spectroscopy

IR spectra from 4000 to 100 cm^-1 were measured for four kinds of alkali chloride solutions (LiCl, NaCl, KCl, CsCl solutions) with different concentrations. Their second derivative spectra revealed two bands around 3400 and 3160 cm^-1 attributed to the OH stretching mode of weakly hydrogen-bonded (WHB) and strongly hydrogen-bonded (SHB) water species, respectively. Two-dimensional correlation spectroscopy (2D-COS) was applied to analyze the IR spectra. Two autopeaks in a 2D synchronous spectrum correspond to the WHB and SHB bands, respectively. Moreover, 2D-COS confirmed the existence of two bands around 700 and 350 cm^-1 in the 1000 -100 cm^-1 region. The bands at 700 and 350 cm^-1 are assignable to liberational L2 modes and correlated with SHB (~ 3160 cm^-1) and WHB band (~ 3400 cm^-1), respectively, revealed by the hetero 2D-COS spectrum. These results suggest the two kinds of hydrogen-bonding networks are constructed by SHB or WHB water species in the alkali chloride solutions.