J. van der Elsken

Rotation-translation spectra of polar molecules in liquids

Polar liquids and polar solutes in non polar solvents show a strong absorption in the far infrared region of the spectrum below 150 cm-l [l-5]. This absorption cannot be accounted for by intra molecular vibrations of the molecules. It has been described by some authors as a pure rotational spectrum of the polar molecules [3], by others as a “liquid lattice” absorption [5]. It has also been suggested that dimer formation might be responsible for absorption in this frequency region. In order to check these assignments, a more quantitative approach seems necessary. We therefore examined the temperature dependence of these spectra and the appearance of the absorption for different polar molecules in a variety of solvents. All spectra were recorded on a Perkin-Elmer 301 grating spectrometer with an experimental limitation on the low frequency side of 16 or 12.5 cm-l depending on the quality of the detecting system. Fig. 1 shows the far infrared spectra of nitrobenzene, nitromethane, acetonitrile and benzonitrile in dilute solutions in n-heptane. The strong absorption at frequencies of 150 cm-l and higher as found in the benzene derivatives is due to internal vibrations of the phenyl group and is not characteristic for the absorption in the low frequency region. In this low region there is an almost uniform behaviour of all the solutes examined. The absorption starts around 170 cm-l and rises continuously towards longer wavelengths, in most cases till a maximum is reached between 70 end 20 cm-l, in some cases till the frequency limit posed by the instrument is reached. The most striking feature of fig. 1 is the much lower absorption intensity (per molecule) for the phenyl compounds as compared with the methyl compounds. Fig. 2 shows the influence of the solvent on the position of the band. In the more polarizable solvents the entire band shifts to shorter wavelengths. Finally in fig. 3 the influence of the ---CYCN \

The far-infrared spectra of hydrogen chloride in liquid argon and in liquid krypton

Abstract Absorption spectra of hydrogen chloride dissolved in liquid argon and liquid krypton in the region between 12 and 210 cm −1 are reported. Both spectra show a residual rotational fine structure. It is concluded that, apart from the effect of molecular collisions, some effect arising from the characteristic liquid properties must come into play.

Anisotropic density fluctuations in argon at different densities: Far infrared measurements and molecular dynamic calculations

From measurements of the pure rotational lines of a linear probe molecule dissolved in a simple fluid one can deduce the power spectra of the anisotropic density fluctuations of l=1 and l=2 symmetry of the fluid. The power spectra thus obtained for argon at various densities are compared with the results of molecular dynamics calculations and with the approximate power spectra which can be calculated starting from neutron‐scattering data. The density dependence of the high frequency components of the l=1 power spectrum and of the zero frequency component of the l=2 power spectrum give a strong characteristic of the dense fluid.

Temperature dependence of the pressure induced width and shift of the rotational lines of HCl

Abstract The pure rotational spectra of HCl mixed with argon, krypton and xenon gas of varying pressures at different temperatures have been measured. From the linewidths and shifts the cross sections can be calculated as a function of the temperature. With the insertion of a Boltzmann distribution of translational energy the temperature dependence can be converted to a dependence of the cross section σ( E ) on the relative kinetic energy of the colliding pairs. A comparison of the experimentally determined σ( E ) values with the values predicted by a semi-classical calculation by Neilsen and Gordon leads to the conclusion that the elastic contribution to the cross sections remains of importance even at energies so high that the inelastic contribution has become constant because of the attainment of the sudden perturbation limit. Furthermore it must be concluded that the coefficient of the first Legendre polynomial in the series development of the attractive anisotropic potential is considerably larger than has hitherto been assumed.

Non-linear density dependence of rotational line-broadening of HCl in dense argon

Abstract The rotational far-infrared spectra of HCl in argon at densities between 100 and 480 amagat and T = 162.5 K are presented. The observed density dependence of the width of the different rotational lines is non-linear and differs for high- and low-frequency lines. An Enskog correction to the collision frequency in the dense gas fails to account for the density dependence of the high rotational lines. It is argued that the many-body character of the relaxation mechanism should be taken into account. Comparison with the results of MD calculations on argon indicates that such an approach may explain the observed density dependence.

The far-infrared spectrum of hydrogen chloride in liquid xenon

Abstract The absorption spectrum of hydrogen chloride in liquid xenon in the region between 10 and 240 cm −1 is reported. The spectrum shows a residual rotational fine structure that is even more pronounced than the fine structure in the corresponding spectra in lighter noble gas liquids. Therefore there is no positive correlation with the atomic polarizabilities.

Far infrared absorption spectra of some solid and fused alkalimetal nitrates

Abstract The low temperature spectra of solid films of sodium and cesium nitrate show very sharp absorption bands that must be ascribed to the transversal optical modes of the crystal lattice. With increasing temperature these bands broaden considerably but no special effect at the point of fusion is noticed.

A far infrared study of the Ar–HCl van der Waals molecule

Results of far infrared measurements on the system HCl–Ar at low density and temperature are presented. Distinct spectral features are observed that must be attributed to Ar–HCl van der Waals molecules. Possible explanations of the observed spectra in terms of a simple picture of the internal motion of the complex are discussed. An estimate of the enthalpy of formation is made.

Rotational Diffusion Model with a Variable Collision Distribution

Starting with an m‐diffusion model a matrix description is given of the rotational motion of a dipole molecule undergoing frequent collisions. This treatment gives rise to an analytical expression for the dipole correlation function and for the angular momentum correlation function in which a limited number of parameters from the model appear. It is argued that the collision distribution which determines the rotational diffusion process need not necessarily be a Poisson distribution. In liquids with strong interactions the distribution is governed by the frequency distribution of the medium. This leads to the inclusion of a librational motion in the rotational diffusion model. A comparison of simulations with different collision distributions and experimental data is given.

Far infrared spectra of van der Waals molecules in HCl–noble gas mixtures

Results of far infrared measurements on HCl–noble gas mixtures in the region 14–240 cm−1 at low temperatures and densities between 0.4 and 4 amagat, are presented. Between the free HCl rotational lines distinct absorption features are observed that are assigned to X–HCl van der Waals molecules (X=HCl, Ar, Kr, Xe). Various internal motions of the complexes are considered and it is concluded that the spectra originate from internal rotatory motion of the HCl subunit. A model for an effective angular potential governing this motion is discussed. The trends in the experimental spectra suggest that the well depth increases in the series Ar–Kr–Xe, reflecting an increase in anisotropy in the above order.