Mixed classical-quantum simulation of vibro-rotational absorption spectra of HCl diluted in dense Ar: Anisotropic interaction and the Q-branch

Two-Dimensional Infrared Spectroscopy From the Gas to Liquid Phase: Density Dependent J-Scrambling, Vibrational Relaxation, and the Onset of Liquid Character

“…This growing intensity in the nominally forbidden gas phase Q-branch region is phenomenologically analogous to the triplet structure previously observed in the numerous studies of the absorption spectra of hydrogen halides in nonpolar liquids. 2,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] 2DIR studies of these diatomics in these dense nonpolar fluids can also establish the free rotor character in these diatomic probe systems and their state point dependence. MD simulations and calculations have been carried out to understand the origins of these absorption spectra and what they reveal about solvation in the liquid and high-pressure environments.…”

“…analysis. 2,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] A characteristic phenomenological feature of these IR spectra is the observation of a broad and overlapping "triplet" structure for vibrational bands of diatomics in these liquids and high pressure fluids. The two feature or shoulders on the red and blue of the vibrational absorption band are attributed to unresolved P (DJ = -1) and R (DJ = +1) gas-phase-like rovibrational branches.…”

“…6 There is an extensive literature on the observation and molecular-level interpretation of the vibrational spectra of small molecules, especially IR absorption spectra of hydrogen halides in high pressure and simple non-polar liquids, that closely relates to this 2DIR analysis. 2,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] A characteristic phenomenological feature of these IR spectra is the observation of a broad and overlapping "triplet" structure for vibrational bands of diatomics in these liquids and high pressure fluids. The two feature or shoulders on the red and blue of the vibrational absorption band are attributed to unresolved P (DJ = -1) and R (DJ = +1) gas-phase-like rovibrational branches.…”

“…Despite the simplicity of these fluid solutions, various theoretical and computational approaches have been applied to understand the appearance of these observed complex IR absorption lineshapes, characterized by the density dependent triplet structure and Q-branch like absorption intensity in high density environments. Explanations offered for these diatomics in liquid and dense fluid vibrational bandshapes span the range from dipole-induced-dipole effects, 36 J-mixing due to anisotropic interaction potential, 37 van der Waals complex formation or long lived anisotropic spatial correlations in the fluid, 2,38,39,48 "modified rotor" or hindered rotor descriptions, 42,43,50 to P and R spectral interference effects. 49 Some analyses have speculated that these complex lineshapes are due to both mixed free rotor character and modified rotor character resulting from slowly varying anisotropic solute-solvent potentials and the role of barriers on the lower lying rotational levels.…”