X-ray diffraction and inelastic neutron scattering study of 1:1 tetramethylpyrazine chloranilic acid complex: temperature, isotope, and pressure effects

The x-ray diffraction studies of the title complex were carried out at room temperature and 14 K for H ∕ D (in hydrogen bridge) isotopomers. At 82 K a phase transition takes place leading to a doubling of unit cells and alternation of the hydrogen bond lengths linking tetramethylpyrazine (TMP) and chloranilic acid molecules. A marked H ∕ D isotope effect on these lengths was found at room temperature. The elongation is much smaller at 14 K . The infrared isotopic ratio for O-H(D)⋯N bands equals to 1.33. The four tunnel splittings of methyl librational ground states of the protonated complex required by the structure are determined at a temperature T = 4.2 K up to pressures P = 4.7 kbars by high resolution neutron spectroscopy. The tunnel mode at 20.6 μ eV at ambient pressure shifts smoothly to 12.2 μ eV at P = 3.4 kbars . This is attributed to an increase of the strength of the rotational potential proportional to r − 5.6 . The three other tunnel peaks show no or weak shifts only. The increasing interaction with diminishing intermolecular distances is assumed to be compensated by a charge transfer between the constituents of δ e ∕ e ∼ 0.02 kbar − 1 . The phase transition observed between 3.4 and 4.7 kbars leads to increased symmetry with only two more intense tunneling bands. In the isotopomer with deuterated hydrogen bonds and P = 1 bar all tunnel intensities become equal in consistency with the low temperature crystal structure. The effect of charge transfer is confirmed by a weakening of rotational potentials for those methyl groups whose tunnel splittings were independent of pressure. Density functional theory calculations for the model TMP ∙ ( H F ) 2 complex and fully ionized molecule TMP + point out that the intramolecular rotational potential of methyl groups is weaker in the charged species. They do not allow for the unequivocal conclusions about the role of the intermolecular charge transfer effect on the torsional frequencies.