Properties of the hydracids trapped in a nitrogen matrix: Experimental and static field approximation study

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Abstract
The properties of the hydracid monomers and dimers trapped in N2 matrices are studied through an experimental reinvestigation of the near and far infrared spectra. In the nir spectra, two kinds of dimers are identified: a nearest‐neighbor (nn) dimer in which the two molecules are nonequivalent and a next‐nearest‐neighbor (nnn) dimer giving rise to a specific absorption very close to the monomer signal. New data in the fir confirm the previous assignment of the librational modes of the monomer but do not allow the identification of the corresponding dimer bands. These results are interpreted according to intermolecular potential calculations, taking into account the angular distortion of the matrix molecules around the HX impurity. An analytical description of the orientation quantum states for the impurities is developed and conveniently explains the fir monomer data by the lifting of the symmetry properties of the perfect crystal. The theoretical frequencies are in much better agreement with experimental when the N2 crystal dynamics is included. In the same way, calculations performed on the dimers show they lose their entity properties in the N2 crystal. Moreover, the librational response of the (HX)2 species, found in the same frequency range as the monomer, may account for a surprising discrepancy between experimental and calculated fir monomer band intensity ratio. The results obtained in the nir lead to a crucial modification of the electric multipole derivatives with respect to the internuclear coordinate in the hydracid dimers, which is explained in terms of a charge transfer process between the two molecules.