Study of hydracids trapped in monatomic matrices. II. Intermolecular potential calculations of the structure and spectral response of polymers

Intermolecular potential calculations are developed for hydracids trapped in rare gas matrices and the results concerning both geometries and spectral responses of low polymers are compared to the analysis of experimental data previously described. Geometrical structures, including matrix distortion, are calculated from a minimization of the total energy. Cyclic and planar conformations found for trimers and tetramers satisfy both criteria: the (molecular) packing tendency imposed by the isotropic Lennard-Jones potential and the highest order symmetry for this packing required by the electrostatic potential. In a second step, the corresponding librational and vibrational frequencies are calculated according to the usual techniques of separation of angular and vibrational variables. For librations, it is moreover possible to separate in-plane and out-of-plane motions, the first ones lying at higher frequency than the second; intensities of the corresponding bands are also calculated, taking into account the induction effects tied to the matrix, leading to a noticeable relative intensity change of the IR active in-plane/out-of-plane modes according to the nature of the rare gas. For vibration, the a priori calculations show that agreement with experimental data is obtained if we introduce with respect to their gas phase values an enhancement of multipolar moment derivatives with intermolecular forces. The enhancement parameter τCT is interpreted as an effect of charge transfer between interacting hydracids XH⋅⋅⋅XH and the best fit for dimers, trimers, and tetramers responses show that this parameter depends on the nature of both electron donor and acceptor molecules, the stronger effect being however observed for the acceptor one (2.5 for HCl and 6 for HBr).