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

In this paper a: theoretical model has been proposed aiming to explain a new kind of HID 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-(HY)-Y-... and X-(DY)-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' v(X-H) bands in their IR spectra. (C) 2002 Elsevier Science B.V. All rights reserved.