Proton dipolar spinning-sideband intensities in the MAS NMR spectrum of a solid containing a molecular group with fast anisotropic rotation

Elastic, quasielastic, and inelastic neutron-scattering studies on the charge-transfer hexamethylbenzene-tetracyanoquinodimethane complex

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.

Errors in chemical shift tensor components and orientation in the molecular frame as obtained from MAS NMR spinning sideband analysis

It is shown how to calculate random errors in chemical shift tensor components and in the Euler angles which fix the orientation of the sigma tensor in the molecular frame, as obtained from spinning sideband analysis of MAS NMR spectra of powdered solids, when heteronuclear dipolar coupling interactions occur in a two spin system. The procedure was applied to experimental data corresponding to the chemical shift tensor of a carbon-13 bonded to a phosphorus-31 nucleus. Clues are given concerning the experimental variables to be set in order to obtain the desired accuracy in the orientation angles.