Abnormal distribution of protons and deuterons between the hydrogen bonds in cyclic centrosymmetric dimers in partially deuterated samples

In this paper we discuss the problem of a stable coexistence of the X–H…Y and X–D…Y bonds in one cyclic dimer of the hydrogen bonds. Comparing the spectra of the partially deuterated dimeric systems with those of the pure hydrogen bonded systems, and considering the results of calculations of the dimeric IR spectra, based on the `strong-coupling' model, we found no convincing evidence for the existence of the nonsymmetrical dimers in mixtures of the hydrogen isotope derivative cyclic dimers, although such spectra should easily be distinguished from those of the symmetric dimers of the hydrogen bonds. This isotopic effect seems to apply to the different hydrogen bond cyclic dimeric systems and to be generally widespread in nature. We suggest that the mechanism, which restricts simultaneous existence of the two different hydrogen isotope hydrogen bonds in one dimer, is of a vibronic nature and results from the collective properties of hydrogen bonds. The nonsymmetrical, different hydrogen isotope dimers, most likely characterized by a higher energy, when compared with the symmetrical ones, are considerably rarer than predicted by a statistical approach. We roughly estimated the excess energy value for the mixed hydrogen isotope dimers as equal to 1.5–2.0 kcal mol−1 of the dimers.