In the present contribution a review of selected applications of spectroscopic methods, including the inelastic neutron scattering techniques, in studies of hydrogen bonded (HB) and charge transfer (CT) molecular complexes. Of particular interest is the CT interaction effect on the dynamics of methyl groups represented by the low frequency torsional modes and tunnel splittings. Our studies are focused on complexes of hexamethylbenzene (HMB) with tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE) and tetracyanobenzene (TCNB) as well as of tetramethylbenzene (durene) with TCNB and of tetramethylpyrazine (TMP) with chloranilic acid (CLA) squaric acid (H2SQ) and TCNB and finally of three isomeric dimethylpyrazines with CLA. Some of these complexes were obtained for the first time so that the standard information on the structure, phase transitions and infrared and Raman spectra was collected.

In the case of the TMP.CLA complex a remarkable drop of the barrier to internal CH₃ group rotation was found that enabled to perform detailed studies of the tunnel splitting and influence on the rotation of various parameters such like temperature, pressure and isotopic substitution. The results collected so far seem to indicate unequivocally on the substantial contribution of the charge transfer degree to the shaping of the rotational potential.

References:

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[3] W. Sawka-Dobrowolska, G. Bator, L. Sobczyk, A. Pawlukojć, H. Ptasiewicz-Bąk, H. Rudlőf, M. Prager, P. Jagielski, J. A. Janik, J. Krawczyk, M. Nowina-Konopka, E. Grech, J. Nowicka-Scheibe, O. Steinsvoll, J. Chem. Phys. 2005, 123, 124305.

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