O. Steinsvoll

Inelastic and quasielastic neutron scattering and IR and R spectroscopic studies of 1,2,4,5-tetracyanobenzene(TCNB)-1,2,4,5-tetramethylbenzene (durene) complex¥

The infrared and Raman spectra for durene, tetracyanobenzene and their 1 : 1 complex were studied. Simultaneously the neutron scattering experiments (INS and QENS) were performed. A detailed analysis of the modes assigned to the methyl groups vibrations were analyzed based on the simulated frequencies and intensities by using the GAUSSIAN-03 and auntie-CLIMAX programs. A good agreement between calculated frequencies and INS experimental ones was found. Moreover the calculations generate, with a quite high accuracy, the observed lattice phonons below 70 cm−1. The QENS studies have shown that the activation energy for the 120° CH3 jumps equal to 49 ± 13 meV for durene and 35 ± 12 meV for the complex confirming that the complexation leads to a decrease of the potential barrier for the methyl group rotations. ¥ Dedicated to Professor Jerzy A. Janik on the occasion of his 80th birthday.

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.

Relaxation in glass and cholesteric phase of isopentylcyanobiphenyl

Abstract The paper presents adiabatic calorimetry investigations of slow relaxation processes (enthalpy relaxation) in isopentylcyanobiphenyl (right handed (s) 4–(2-methylbutyl) 4′–cyanobiphenyl) in a vicinity of the glass transition. The preliminary neutron scattering results concerning the fast relaxation processes and low-energy excitation detected as “boson peak” are also presented. The general behaviour observed in the experiments seems to correspond to the typical picture for the molecular glasses.

Quasielastic neutron scattering (QNS) study of cation rotation in (CH3NH3)5Bi2Cl11, (CD3NH3)5Bi2Cl11 and (CH3NH3)5Bi2Br11

Abstract Rotation of (CH3NH3)+ cations in crystalline (CH3NH3)5Bi2Cl11, (CD3NH3)5Bi2Cl11 and (CH3NH3)5Bi2Br11 was studied by QNS. The model of instantaneous 120° jumps of whole cations around their C–N axes was fitted to the experimental data. The parameters describing reorientation obtained for two types of cations agree well with the results of NMR experiments.

Neutron quasielastic scattering by (CH3NH3)5Bi2Cl11

Abstract (CH3NH3)+-cation rotation in crystalline (CH3NH3)5Bi2Cl11 was studied by the QNS. The following (no doubt simplified) models were considered: 1. 1. All cations are equivalent and rotate as a whole around C-N axes 2. 2. Majority of cations behave as in point (1) and the remaining ones do not move (for the neutron observation window). 3. 3. All CH3 groups rotate, whereas all NH3 groups do not move. The QNS measurements exclude the possibilities (1) and (3). A transition at ca. 160–170 K was confirmed in the measurements, as connected with an increase of the quasielastic component intensity observed via a dip of the EISF.