Hyper‐Raman study of hydrated excess protons in water: Measurement of concentrated HCl solution

Abstract

We report hyper‐Raman (HR) spectra of concentrated hydrochloric acid solutions for the first time. HR signals from the OH stretching region to the low‐frequency region reflect the presence of chloride anions solvated in water. By contrast, it turns out that HR spectroscopy is insensitive to the presence of excess protons in water. HR spectra of HCl solutions do not show any signature of a proton continuum and shoulders on the high/low‐frequency sides of the bending mode of H2O in stark contrast to IR spectroscopy. Moreover, we found that a sharp HR signal around 500 cm−1 appear in 532 nm excited HR spectra. A laser power dependence indicates that this signal comes from a photochemical reaction. The absence of this signal under 1064 nm HR excitation and 532 nm Raman excitation suggests that the resonance (hyper‐)Raman effect enhances the signal from the photoproduct.

Is Unified Understanding of Vibrational Coupling of Water Possible? Hyper-Raman Measurement and Machine Learning Spectra

“…HR scattering is an inelastic scattering involved in three photons. A system absorbs two photons with the frequency of the incident light (ω i ) and emits one photon with 2ω i ± Ω, where Ω is the frequency of a molecular vibration. − HR spectra can provide unique spectral information because the selection rules of HR spectroscopy differ from those of IR and Raman spectroscopy. − Previous studies have shown that an HR spectrum of pure H 2 O is more similar to IR than parallel Raman spectra. , However, the interpretation of the HR spectra of water, including the impact of vibrational couplings, remains completely unexplored.…”

TMAO perturbs intermolecular vibrational motions of water revealed by low-frequency modes

“…4,29,35 In addition, two broad bands ranging from 400–900 cm −1 ascribed to librational motions of water molecules are observed as in the previous HR spectra of water and aqueous solutions. 38,50 For convenience, we name the librational bands at the lower and higher vibrational frequencies the L1 and L2 bands, respectively. There is no remarkable difference between the spectra of water and the TMAO solution at 0.1 M. In contrast, at higher concentrations of TMAO, in addition to intense signals originating from water molecules, weak intramolecular vibrational bands of TMAO molecules appear.…”

Characterization of Secondary Structures of Model Polypeptides in Solutions with Hyper-Raman Spectroscopy

“…After pulse broadening achieved by a 4-f filter composed of a pair of transmission gratings and a slit, average laser power and pulse duration of the output laser beam were tuned to 150–180 mW and several picoseconds (ps), respectively. Except for the fundamental light source mentioned above, the rest of our HR spectroscopic system in detail has been described elsewhere. ,, The laser beam was focused by a lens onto a 1 cm square quartz cuvette for PLL and PLGA or a 3 mm quartz cuvette for PLO inserted in a temperature-controlled holder (Luma 40, Quantum Northwest). Note that except for PLL α-helix and PLO α-helix, which were measured at 10 and 5 °C, respectively, measurements of other solutions were carried out at room temperature, that is, 25 °C.…”