Infrared Absorption of Diatomic Polar Molecules in Liquid Solutions

Abstract

A theory of the infrared absorption spectrum of light dipolar diatomic molecules dissolved in simple liquid solvents is presented and discussed. This is done by keeping the quantum character of the orientational degrees of freedom of the absorbing molecules. Moreover, the low frequency part of the density fluctuations around this molecule is taken into account by introducing an almost static polarizing action which strongly disturbs the free rotation wavefunctions and therefore eliminates from the zeroth order of the calculation the usual selection rules of the diatomic rotator. The presence of this polarization is suggested by a study of the factorization of the density operator; the fluctuations of the true coupling of the active molecule with the liquid in respect to the quasistatic coupling considered above implies a broadening of all the possible transitions of the active molecule already polarized. Models are examined to represent the correlation of this fluctuation. An example of HCl molecule dissolved in liquid CCl4 is then studied, and this leads to a satisfactory comparison of the theoretical results with the experimental band profiles.

Experimental analysis and modified rotor description of the infrared fundamental band of HCl in Ar, Kr, and Xe solutions

“…One class of systems that attracted a considerable interest were solutions of hydrogen halide compounds in chemically inert media, widely studied during last decades. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] The vibration-rotation absorption bands of these molecules typically show a triplet structure composed of a pair of side shoulders, resembling the P and R branches observed in the gas phase spectra, and a central component ͑the so-called Q branch͒, which is dipole symmetry forbidden for isolated diatomics.…”

“…Most important for revealing the nature of the processes contributing to formation of such complicated spectral band profiles were the following: ͑i͒ observation of a resolved fine rotational structure in the P and R branches indicating persistence of a quantized rotational motion of light solutes in dense fluids; 3 ͑ii͒ significant changes in the integrated intensities detected for the vibrational fundamental and first overtone Hhal bands upon dissolution; [4][5][6] ͑iii͒ disappearance of the Q branch in the spectrum of HCldoped solidified Xe solvent; 11 ͑iv͒ the effects of temperature and solute concentrations on the fundamental band of HCl in liquefied rare gas ͑Rg͒ solutions; 12 ͑v͒ demonstration of a continuous enhancement of the Q branch on going from gaseous to liquid HF-Xe mixtures; 13 ͑vi͒ recent data on the evolution of the Q branch with temperature at constant densities in the HCl-SF 6 and HCl-Xe systems. 14,15,19 Theoretical approaches to interpretation of the observed triplet spectral bands included explanation for the appearance of the Q branch in terms of lifting the ⌬j = ± 1 selection rule for the rotational quantum number j, caused by an effective "orientational" field generated by the solvent, 7 by introducing a fluctuating liquid cell model that accounts for stochastic fluctuations of the solute-solvent anisotropic interactions, 9 by building theoretical spectra as sums of weighted contributions due to the free rotation and rotational diffusion motions of the solute species, 8,10 and by molecular dynamics ͑MD͒ simulations. [16][17][18] As was recently demonstrated, 15 the intensity distribution in the range of discrete rotational features observed in the spectra of HCl diluted in dense inert fluids can be realistically reproduced by a quantum-statistical model of the quasi-free rotation of light diatomic molecular probes.…”

Far-infrared permanent and induced dipole absorption of diatomic molecules in rare-gas fluids. I. Spectral theory

“…5,6 The application of the rotational relaxation of a quantized rotor embedded in a classical fluid as a model for calculating the permanent contribution to the FIR spectrum of diatomic polar molecules in nonpolar solvents was laid 20 years ago mainly by Neilsen and Gordon,7 the Amsterdam group, 1,8 -10 and the Besançon group. [11][12][13] In most of these works a central role is played by the time correlation functions ͑TCF's͒ associated to the solute-solvent intermolecular potential, which is formally expressed as an expansion in Legendre polynomials P J ͑cos k ͒, where k is the angle between the intermolecular line and the axis of the diatomic molecule. Within an impact approximation, Neilsen and Gordon 7 concluded that at sufficiently low densities the FIR spectrum is a superposition of Lorentzian rotational lines with shift and width proportional to the density of the fluid.…”

The libron–phonon coupling and the IR absorption spectra of diatomic molecules in a simple liquid

“…Dynamical approaches of diatomics in liquid solutions started with Robert and Galatry. 8 They used a cage model to mimic the liquid structure perturbing the molecular orientation within a quantum formulation. The atoms in the liquid surrounding the impurity are viewed as a thermal bath represented by a fluctuating potential.…”