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A brief summary of the general theory of the long-range potential energy between two molecules, previously developed by Buckingham, is first presented. Then the first detailed expressions of this potential energy are given for the three following molecular pairs : tetrahedral molecule-atom, tetrahedral molecule-linear molecule and tetrahedral molecule-tetrahedral molecule. These expressions are applied to the calculation of the spectral moments of the far infra-red pressure induced absorption band, the second virial coefficient and the intermolecular mean square torque. This allows a discussion of the magnitude of the various terms contributing to this potential energy and leads to a determination, for CH4, of the octupole and hexadecapole moments, and of the hyperpolarizability : f2 = (2"30 + 0"04) x 10 -z4 e.s.u, cm 3, ~ = --(4"55 + 0"55) X 10 -42 e.s.u, cm 4 and A = (2"35 + 0"05) x 10 -32 cm ~.

In the frame of the second order time-dependent theory of the molecular line shape, the line shifts for diatomic molecules perturbed by diatomic ones are carefully analyzed. Particular emphasis is put on two types of physical situations depending upon the nature of the perturber with respect to that of the active molecule. The available literature presents data which may be classified in two categories. The first one (here called ’’normal shifts’’) is illustrated by the vibration–rotation line shifts of HCl–HCl, HF–HF, and HCl–DCl and exhibits an aperiodic behavior of the line shifts vs the initial rotational quantum number. The second one (called ’’oscillating shifts’’) is illustrated by the vibration–rotation line shifts of DCl–HCl and DF–HF and is characterized by an oscillating behavior with a semiperiod of one rotational quantum number. These behaviors are particularly well reproduced by the present calculations in a quantitative way. A detailed analysis of the physical mechanisms responsible for these two types of behavior is presented and permits a precise determination of the nature of the collision induced transitions included in these mechanisms. Moreover, an extension of this study is proposed in order to predict changes of the phase of the oscillations. This is the case, for instance, for HCl perturbed by HF in the O–2 band.

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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.