Dangling OH Vibrations of Water Molecules in Aqueous Solutions of Aprotic Polar Compounds Observed in the Near-Infrared Regime

Abstract

Near-infrared (NIR) absorption spectrum measurements over a frequency range from 4000 to 12000 cm(-1) were employed to investigate the effects of the presence of solute compounds to vibrational modes of water molecules in aqueous solutions of some aprotic hydroneutral polar compounds with large dipole moments, such as nitro compounds and nitriles. The obtained NIR spectra for the aqueous solutions were decomposed into three components: free water, solute, and water molecules affected by the presence of solutes. Newly determined NIR spectra of affected water molecules were well-described with at least four absorption modes observed at 7040, 6850, 6450, and 5640 cm(-1) for both the nitro compounds and nitriles. The highest frequency mode at 7040 cm(-1) possessing the strongest intensity was assigned to the first stretching overtone of affected water hydroxy (O-H) groups, which are nonhydrogen bonded to other water molecules and dangling. The second highest frequency mode at 6850 cm(-1) was assigned to the first stretching overtone of affected water O-H groups hydrated to other (free) water molecules. The third mode at 6400 cm(-1) was attributed to a combination mode of the fundamental stretching of O-H and the first overtone of the O-H bending mode of the affected water molecules. The lowest frequency mode at 5640 cm(-1) was assigned to the combination mode of the fundamental O-H stretching mode, the fundamental O-H bending mode, and the hindered rotational (libration) mode of the affected water molecules. Because absorption intensities of the third and lowest frequency modes for water molecules affected by the solutes depended on the sizes of alkyl groups of polar solutes, these two modes possibly result from the contribution of hydrophobic hydration effects.

Diclofenac Ion Hydration: Experimental and Theoretical Search for Anion Pairs

“…IR intense bands caused by combination mode of the fundamental O–H stretching vibrations are located in the range of ~6000–~6500 cm −1 ( Figure 7 and Figure S4 ). The obtained values of the wavenumbers are in reasonable agreement with the literature data [ 33 ]. According to Figure 7 and Figure S4 , wavenumbers of IR intense vibrations of monomer and dimer practically do not differ from each other in the NIR region.…”

“…Valuable information about intermolecular interactions is provided by near-infrared (NIR) spectroscopy [ 31 ]. H-bonding effects on the wavenumbers and absorption intensities of the OH overtones of alcohol in aprotic solvents [ 32 ], dangling OH vibrations of water molecules in aqueous solutions of aprotic polar compounds [ 32 ], etc., [ 33 ] are usually investigated. The study of the first hydration shell of aqueous solutions of organic molecules with unshared electron pairs by NIR was also carried out [ 34 , 35 ].…”

Aquaphotomics Investigation of Electrolyzed Hydrogen Water: Understanding the Water Molecular Structure and Its Role in Scavenging Reactive Oxygen Species

“…The high absorbance at band 1447 nm can be related to the physi-adsorbed water, another type of water involved with hydration of the present structures (but not necessarily charged) [46]. Alternately, it can also be assigned to secondary, more bulk-like hydration layer [95], OH-metal vibration [96,97], dangling water molecules [98] or more or less ordered water at some interface [99]. The band at 1452 nm can similarly be assigned to physi-adsorbed water [46,100], or bulk water [101].…”