Infrared Spectra of Highly Compressed Gas Mixtures of the Type HCl+X. A Theoretical Study

Abstract

A theory is proposed to explain the ir spectra of highly compressed gas mixtures of the type HCl+X (or HBr, HF with X=He, Ar, Xe, N2, CO, etc.). This theory is based on the following three hypotheses: (1) The compressed mixture is formed of three molecular species, free HCl (or HBr, HF, etc.), free X, and bound van der Waals molecules (HCl···X). (2) In thermodynamic equilibrium, the system is subject to the law of mass action: HCl+X⇌(HCl···X). (3) The equilibrium constant is given by the standard formula K(T) = exp(—ΔF/RT). Using these hypotheses, the spectral perturbations are explained in terms of the calculated properties of the bound van der Waals molecules. The theory predicts that three limiting types of van der Waals molecules exist according to whether the energy of coupling between the rotation of HCl and the over-all rotation of the complex is (1) smaller than the energy of the over-all rotation of the complex [Coupling Case (1)], (2) larger than the energy of the over-all rotation of the complex but smaller than the energy of the rotation of HCl [Coupling Case (b)], (3) much larger than the energy of rotation of HCl [Coupling Case (c)]. The energy expressions are given for the three coupling cases, the equilibrium constants K (T) are calculated and the spectral perturbations are analyzed. In particular, the fine structure of the induced band is discussed.

A new He–CO interaction energy surface with vibrational coordinate dependence. I. Ab initio potential and infrared spectrum

“…Subsequently, a poten tial with a different functional form was determined from a fit to the same set of spectroscopic data.8 The starting point of the new potential surface was the exchange-Coulomb (XC) model potential, which is partly based on ab initio information on the exchange-repulsion energy and longrange induction and dispersion coefficients. This " X C" po tential reproduced the observed spectrum as well as the V o 33) potential. Moreover, because of its sounder theoreti cal basis the XC potential is expected to be more realistic in the regions not sampled by the spectroscopic data.…”

Calculated rovibrational energy levels and infrared spectrum of He-C2H2

“…For most eigenstates of the DMKD surface, the dominant channel contributes over 90%, with the ground state having a 96% contribution from thej = 0, l = 0 channel. We therefore conclude thatj and l are nearly-good quantum numbers in the DMKD model of the helium-acetylene complex, and hence that the best zeroth order description for its states is provided by the free-internal-rotor picture of Bratoz and Martin's ease-1 [14]. In contrast, for analogous states of the Ar-HC1 system, the wavefunction contributions from coupled channels associated with the same value of j are often approximately equal [15].…”

“…For example, will the weak binding or the strong anisotropy determine the qualitative behaviour of the complex exhibited through spectroscopic selection rules and ordering of energy levels? Bratoz and Martin have identified three schemes for characterizing the angular momentum coupling or zeroth order descriptions of the states of Van der Waals complexes [14], but it is not immediately clear which is most appropriate here. The proper zero order description of the states of the complex will facilitate assignment of exact and approximate quantum numbers, which in turn will allow meaningful selection rules to be formulated and the spectra to be assigned.…”

Hindered rotation of water molecule in van der Waals complex

“…At thermodinamic equilibrium the amount n ~ of the water molecules in clusters of #-modification may be calculated by the following formula [24]:…”