G. B. Soifer

NQR data on the activation energy of orientational defects in crystal lattice

The nuclear quadrupole spin-lattice relaxation related to the formation of orientational defects in crystals has been analyzed. The defects arise because of thermally activated reorientational motion of molecules and their fragments between the nonequivalent potential wells. These molecular reorientations are considered for double-and triple-well potentials, both periodic and bounded by external side walls. It was shown that, for such potentials, the temperature dependence of the relaxation time provides the data on the activation energy for transitions of molecules in the crystal lattice from the equilibrium ground-state positions to those corresponding to metastable states. We systematized the known NQR data on the activation energies for orientational defects arising in the intracrystal rotations of molecules and atomic groups possessing the imperfect two-, three-, and sixfold axial symmetries.

Molecular structure and electronic effects of chlorophosphorane from mndo calculations and35Cl NQR data

The molecular geometries of eleven compounds of pentacoordinated phosphorus are optimized and charges on the central phosphorus atom and on the ligands are calculated by the semiempirical MNDO method. In cases of chlorine atoms as ligands, correlations between the chlorine-35 NQR frequencies and charges on the chlorine atoms of P-Cl bonds, on the one hand, and these bond lengths, on the other, are established. Parameters of the corresponding correlation equations are obtained and discussed. The efficiency of this calculation procedure and NQR method for analyzing the geometrical structure and electron distributions in chlorophosphorane molecules is shown.

Intramolecular Hindrances to CCl3 Reorientations in CCl3PCl4 and (CCl3)2PCl3 Crystals

The trigonal bipyramidal structure of trichloromethyl-tetrachlorophosphorane CCl3PCl4 and bis(trichloromethyl)trichlorophosphorane (CCl3)2PCl3 molecules with apical CCl3 groups is responsible for steric hindrances to reorientations of these groups around the C–P bond. The intramolecular barriers to CCl3 reorientations calculated for the two phosphoranes (Hartree–Fock method using the 6-31G(d) basis set) are 105.9 kJ/mole and 106.6 kJ/mole, respectively. These internal barriers are high enough to block the reorientational motion of the CCl3 groups, as found previously by 35Cl NQR for the stated crystals.

Intramolecular Hindrances to CCl 3 Reorientations in CCl 3 PCl 4 and (CCl 3 ) 2 PCl 3 Crystals

The trigonal bipyramidal structure of trichloromethyl-tetrachlorophosphorane CCl3PCl4 and bis(trichloromethyl)trichlorophosphorane (CCl3)2PCl3 molecules with apical CCl3 groups is responsible for steric hindrances to reorientations of these groups around the C–P bond. The intramolecular barriers to CCl3 reorientations calculated for the two phosphoranes (Hartree–Fock method using the 6-31G(d) basis set) are 105.9 kJ/mole and 106.6 kJ/mole, respectively. These internal barriers are high enough to block the reorientational motion of the CCl3 groups, as found previously by 35Cl NQR for the stated crystals.

Structure and Internal Rotation of Molecules in Chlorodifluoroacetic Acid

The potential energy profile of an isolated CF2ClCOOH molecule with a CF2Cl group rotating around the C–C bond was determined by the Hartree–Fock method using the 6-31G(d) basis set. Barriers to internal rotation were estimated for this molecule; its geometrical parameters were found for the equilibrium and transition states that are due to the torsion potential with unequal wells. Crystal effect on CF2Cl reorientations in solid chlorodifluoroacetic acid has been evaluated.

Effect of the carbon atom coordination on the thermoactivated motion of the carbon-bonded trichloromethyl group

The reorientational motion of the trichloromethyl group depending on the environment of the carbon atom is considered based on the chlorine-35 nuclear quadrupole resonance (NQR) data. The study is performed for CCl3 groups bonded to tri- and tetracoordinated carbon atoms in the crystalline compounds CCl3CCl=NR (R = CH2C6H5 and CCl3CCl3) and CCl3CXClN=CClC6H4NO2-n (X = H and Cl).35Cl NQR studies of thermoactivated motion in 70 solid trichloromethyl-containing compounds are summarized. The ranges of activation energies of CCl3 reorientations at tri- and tetracoordinated carbon atoms were determined to be 10-50 and 30-90 kJ/mole, respectively, the activation energy being markedly greater in the latter case. CCl3 reorientations may be completely frozen by the bulky substituents which may be present along with CCl3 in the tetracoordinated carbon bonds.

35Cl NQR and structural-dynamic properties of crystalline CCl3C(O)OC6H2Cl3-2,4,6

The temperature dependences of the resonance frequency and quadrupole spin-lattice relaxation time of the chlorine-35 nuclei in crystalline CCl3C(O)OC6H2Cl3-2,4,6 were studied. Owing to the presence of resonant nuclei in various fragments of the molecules, the character of dynamics of these molecular fragments and the entire molecule was established. It is shown that thermal librations of molecules are quasiharmonic over the whole temperature range from 77 K to the melting point of the crystal. The reorientational motion of the CCl3 group bonded to the three-coordinated carbon atom was revealed. This motion causes an exponential increase in the spin-lattice relaxation time of the chlorine nuclei of this group and subsequent damping of NQR signals (chlorine-35 resonance signals of the aryl radical were observed before melting of the sample). The activation energy of the reorientational motion is found; its value, which is 27.3 kJ/mole, is considered in comparison with the activation energies obtained by the Cl NQR method in solids for CCl3 reorientations in similar molecular structures.

Quantum chemical calculations and35Cl NQR study of the molecular structure of trichloroacetyl halides and trichloroacetaldehyde

An optimal geometry of CCl3C(O) X molecules (X=H, F, Cl, Br, and I) is determined by the MNDO quantum chemical method. According to the results of calculations, the molecules in the free state have an eclipsed conformation with a syn-peri-planar position of the oxygen atom and one of the chlorine atoms of the trichloromethyl group. However, as follows from the35Cl NQR spectra at different temperatures, in the crystal state of CCl3C(O)X this conformation is distorted under the influence of intermolecular interactions due to the torsional rotation of the CCl3 group around the C-C bond. Indicatory potentialities of the NQR method with respect to the structural features that are due to crystallographic effects are discussed.