Vibronic model for H/D isotopic self-organization effects in centrosymmetric dimers of hydrogen bonds

Abstract

In this paper a theoretical model has been proposed aiming to explain a new kind of H/D isotopic effects, i.e. the isotopic ‘self-organization’ effects, recently deduced from IR spectra of hydrogen bonded molecular crystals. The existence of the new kind of co-operative effects in cyclic dimeric systems of hydrogen bonds was explained in the limits of a vibronic model in the Herzberg–Teller approximation, which assumed a strong coupling between the proton stretching vibrations in a dimer with the electronic motions. It was shown that additional ‘attracting’ forces, responsible for an additional stabilization energy of a dimer, concerning identical hydrogen isotope atoms, appear in the ground vibrational and the ground electronic state, when the proton totally symmetric vibrations in a dimer couple with the electronic movement. These self-organization effects were deduced to be absent in the case of non-symmetric ‘HD’ -type dimers, containing both X–H⋯Y and X–D⋯Y bonds in one dimer. Therefore, the symmetric dimers of the ‘HH’ and the ‘DD’ -type should be more stable when compared with the ‘HD’ -type dimer properties. This would explain characteristic spectral effects, registered in the spectra of partially deuterated molecular crystals containing cyclic dimers of hydrogen bonds in their lattices, depending on a fair invariance of the proton ‘residual’ ν X–H bands in their IR spectra.