title: Elastic, quasielastic, and inelastic neutron-scattering studies on the charge-transfer hexamethylbenzene-tetracyanoquinodimethane complex.
abstract: The 1:1 hexamethylbenzene (HMB)-tetracyanoquinodimethane (TCNQ) complex shows a first-order phase transition at 230/218 K (heating/cooling) with no change of the space group. The neutron-diffraction studies reveal that this transition is related to a freezing of the rotation of methyl groups. The results for 100 K enabled precise determination of configuration of HMB.TCNQ complexes. The planes of HMB and TCNQ molecules from small angle (6 degrees) so that the dicyanomethylene group approaches the HMB molecule to a distance of 3.34 angstroms. The conformation of methyl groups was exactly determined. The quasielastic neutron-scattering spectra can be interpreted in terms of 120 degrees jumps with different activation barrier in low- and high-temperature phases, equal to 3.7 and 1.8 kJ/mol, respectively. These values are lower than that for neat HMB (6 kJ/mol). The conclusion can be drawn that the methyl groups can reorient more freely in the complex. This conclusion is in agreement with the results of inelastic neutron-scattering studies of low-frequency modes assigned to torsional vibrations of methyl groups. These frequencies are lower than those for neat HMB. The analyzed increase of frequencies of these modes as compared with free molecules can be interpreted as due to formation of unconventional C-H...Y hydrogen bonds which are more pronounced in crystals of neat HMB than in those of HMB.TCNQ. The low-frequency librational modes can be treated as a sensitive measure of unconventional hydrogen bonds formed by the CH3 groups.
authors.affiliation: Faculty of Chemistry, University of Wrocław, 50-383 Wrocław, Joliot-Curie 14, Poland.
authors.forename: Wanda
authors.initials: W
authors.lastname: Sawka-Dobrowolska
authors.forename: Grazyna
authors.initials: G
authors.lastname: Bator
authors.forename: Lucjan
authors.initials: L
authors.lastname: Sobczyk
authors.forename: Andrzej
authors.initials: A
authors.lastname: Pawlukojć
authors.forename: Halina
authors.initials: H
authors.lastname: Ptasiewicz-Bak
authors.forename: Håkan
authors.initials: H
authors.lastname: Rundlöf
authors.forename: Jan
authors.initials: J
authors.lastname: Krawczyk
authors.forename: Małgorzata
authors.initials: M
authors.lastname: Nowina-Konopka
authors.forename: Piotr
authors.initials: P
authors.lastname: Jagielski
authors.forename: Jerzy A
authors.initials: JA
authors.lastname: Janik
authors.forename: Michael
authors.initials: M
authors.lastname: Prager
authors.forename: Olav
authors.initials: O
authors.lastname: Steinsvoll
authors.forename: Eugeniusz
authors.initials: E
authors.lastname: Grech
authors.forename: Joanna
authors.initials: J
authors.lastname: Nowicka-Scheibe