'Long-range' isotopic H/D effects in the infrared spectra of the centrosymmetric dimeric systems of hydrogen bonds

In this paper a mechanism is analysed of a long-distance influence of the hydrogen atoms of C-H groups (peripherial with respect to the hydrogen bonds) on the spectral properties of the centrosymmetric dimers of hydrogen bonds, along with the so called 'long-range' isotopic effects measured in the frequency range of the nu(X-H) and nu(X-D) bands. For this purpose a model of the vibronic coupling for the dimeric system was proposed, within the Herzberg-Teller approximation describing a resonance between the proton stretching vibrations nu(X-H) and the C-H group stretching vibrations nu(C-H), Which occur via the electrons of the systems. We derived the purely electronic matrix Hamiltonian of the dimer, responsible for its spectral properties. The consequence of the model is enhancement of an uncommon spectral phenomenon depending on breaking of the vibrational selection rules for the dimer in the IR range, due to promotion of the symmetry forbidden excitation of the totally symmetric protonic vibrations. This effect was found to be mass-dependent on all the hydrogen atoms in the model system. From this model it also seen that the strongest 'long-range' isotopic effects correspond with the case when all the hydrogen isotopes in the dimer are identical. (C) 1998 Elsevier Science B.V. All rights reserved.