Infrared spectral profiles in liquids and atom-diatom interactions

Abstract

Molecular dynamics simulations of the infrared spectrum of a generic simple polar diatomic in a liquid nonpolar solvent allow to reproduce the different prototypical experimental line shapes of this kind of systems. This is feasible by using different solute-solvent anisotropic potentials at fixed thermodynamic conditions. In the limit cases, the rotation of the diatomic is explained in terms of a quasifree motion or a rotational diffusion evolution and the spectra show a doublet structure formed by P and R branches or a unique collapsed branch, respectively. When the profile contains three branches, including an intense Q branch in the vicinity of the center of the band, rotational evolution presents a particular hindering that can be understood by studying the influence on rotational spectral densities of the different time scales involved in rotational relaxation. Cancellation/enhancement effects among spectral density terms arising from intermediate and long times (0.4–1 ps) are essential to understand rotational hindering.

Hindering of rotational motion of guest molecules in the Type I clathrate hydrate

“…This shows that at 48 K rotations of CO molecules are strongly hindered as a vast majority of guests preserve their orientation for 100 ps; at 68 K a transient situation between low-and high-temperature behavior occurs with a slow acf decay over 100 ps time. It should be noted that under ambient conditions the rotational autocorrelation function of a single CO molecule in the gas phase decays on the sub-ps timescale [23]. Thus the 20 ps decay of the acf at 108 K, as depicted in Fig.…”

Non-Markovian near-infrared Q branch of HCl diluted in liquid Ar

“…Finally, we can make a line-by-line study of the Q branch according to the called R(j) and P(j) effective lines (17,18). In Fig.…”

“…[11][12][13][14][15][16][17][18][19] Perhaps the most recent works about the Q branch are the classical dynamical simulations of Medina et al, [15][16][17][18] where it is shown that, unlike the previous quantum spectral theories based in the Kubo cumulant expansions, [12][13][14] the central Q branch appears in the spectral density when the site-site diatomic-solvent potential is fitted into the advanced intermolecular potentials present in the literature. 16 a) Author to whom correspondence should be addressed.…”

“…In both types of studies, [15][16][17][18][19] the calculated spectral profiles were compared with the experimental band shapes, concluding that the Q branch is a manifestation of the diatomic-solvent longlived spatial correlations which in the low density regime leads to the concept of van der Waals complex. The explicit relation between the diatomic-solvent correlation time scales and the appearance of the Q branch was stated by Padilla et al in Ref.…”

A Simulation Study of the Far-Infrared Absorption Spectra of HCl Diluted in Liquid Ar

“…An extensive classical study of the Q-branch has been accomplished by Medina et al [17][18][19][20].…”