X-ray diffraction and inelastic neutron scattering study of 2,6-dimethylpyrazine chloranilic acid complex

The DMP (.) CLA and DMP (.) CLA-d(2) crystals below 100 K are monoclinic, space group P2(1)/c with four molecules in the unit cells. Infinite chains of hydrogen bonded counterparts are formed with 0(...)N distances equal to 2.767(2) and 2.639(2) for non-deuterated DMP (.) CLA. Deuteration leads to the Ubbelohlde effect particularly well manifested in the second short O-(HN)-N-... bridge (elongation by ca. 0.02 angstrom). In the IR spectra a Hadzi's trio of the broad absorption is observed characteristic of strong hydrogen bonds. In the INS spectra the vibrational density of states and methyl rotational tunnel splittings were determined. The temperature dependence of tunneling bands enabled to make mode assignments and to determine the methyl rotational potentials. Comparison of the results to the pure electron donor DMP was made and the difference found can be almost completely assigned to the steric changes of the environment. A weak isotope effect with deuteration of the (OHN)-N-... bridges of the DMP (.) CLA complex is assigned to a charge transfer of bel e = 0.006. (c) 2006 Elsevier B.V. All rights reserved