Study of hydracids trapped in monatomic matrices. I. Near infrared spectra and aggregate structures

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Abstract
The infrared spectra of H35Cl, D35Cl, HBr, DBr and their mixtures in solid argon and krypton in the range 10–30 K are presented. Studies have been made over a wide concentration range, for matrix/hydracid ratio from 1000 to 20 which, together with annealing experiments and H/D isotopic dilutions, allow a complete discussion of polymeric species spectra. In the monomer region, Q features are clearly identified at low M/R ratio and assigned to lattice distortion effects on isolated molecules. A new assignment for dimer bands is proposed, in which the two molecules are not equivalent and characterized by different values of force constants and moment derivatives. The trimer is found cyclic, a configuration already mentioned, the three molecules being equivalent and oriented along the sides of an equilateral triangle. After annealing, a new polymeric species associated with a strong band at lower frequency appears and is identified, in the light of H/D mixture spectra, as a cyclic planar tetramer. From accurate frequency measurements one concludes that the H/D isotopic substitution slightly changes the hydracid force constant, a DX molecule acting as a stronger perturber than a HX one on any partner belonging to the same aggregate. At last the analysis of mixed H35Cl–HBr and HCl–HI spectra enables a comparison between the perturbing power of these three molecules, the decreasing electron donor power being in the order HI, HBr, HCl.