Study of hydracids trapped in monatomic matrices. III. Pressure effects on the vibrational frequencies of the aggregates and evolution of the charge transfer

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Abstract
The modifications induced by pressure on the vibration–orientation spectra of the various hydracid polymers in rare gas matrices are discussed from an experimental and theoretical point of view. The main experimental results obtained are the blue shifts of the monomer and one of the dimer bands and the red shifts of all the other poylmeric bands under increasing pressure. The interpretation in terms of the intermolecular potential is based on a hydrostatic pressure model which contracts the lattice parameter and shortens the intermolecular distance between hydracids. When pressure increases, the angular geometries of the polymers determined at P = 0 are nearly unchanged although the overlap of the electronic clouds of two neighboring molecules becomes more and more important. On the contrary, the potential derivatives with respect to the internal vibrational coordinates are considerably amplified by a charge transfer process due to this overlap. The quasicontinuous study of this evolution with the pressure enables a quantitative discussion of this little known phenomenon in weakly hydrogen bonded species. A model of the molecular parameter derivatives modification due to this charge transfer is proposed and found in quantitative agreement with experiment.