Effects of isotopic substitution on spectral parameters of the ν AH and vB+H infrared bands in molecular and ionic H-complexes

Abstract

Isotopic substitution effects on frequencies, integrated intensities and absorption probabilities of the ν AH and ν B+H bands in the infrared spectra of molecular and ionic H-complexes have been investigated. It has been shown that on H-bonding strengthening (concomitant with the integrated intensity enhancement and the low frequency shift of the ν AH and ν B+H bands) a regular diminution of the centres of gravity ν H/ ν D and the mean frequency ν = H/ ν = v D isotopic ratios occurs attaining an abnormally low value of ca. 1.0 for very strong asymmetric molecular and ionic H-complexes. For symmetric ionic and nearly symmetric molecular H-complexes an increase of the frequency isotopic ratio has been observed approaching a harmonic value ∼1.32) in the case of the ν = v H/ ν = v D ratio. The integrated intensity isotopic ratio A H/ A D has been shown to be practically constant and equal to ca. 2.0 for weak, medium and strong H-bonds; however, the A H/ A D ratio noticeably decreases for very strong asymmetric and symmetric H-complexes attaining a value of ∼ 1.14. As distinct from the A H/ A D behaviour, the absorption probability isotopic ratio P H/ P D increases from 1.0 for free molecules to an anomalous value of ∼ 2.0 for very strong asymmetric H-complexes and then decreases to 0.9-1.0 for very strong symmetric H-complexes. From an analysis of the obtained results conclusions have been drawn concerning the structures of H-complexes formed, the H-bonding proton potential functions, mechanical and electrical anharmonicities of the AH and B +H groups in the H-complexes. The experimental changes of the 0 → 1 transition absorption probability isotopic ratio for the asymmetric molecular and ionic H-complexes may be explained without recourse to the electrical anharmonicity of the H-bonded groups AH and B +H.