Partial disorder and molecular motion of 4-chlorobiphenyl studied byCl35NQR and Raman spectroscopy

Abstract

Pulsed 35 Cl nuclear quadrupole resonance ͑NQR͒ and Raman spectroscopy studies were performed to get information about the structural and dynamic thermal behavior of the 4-chlorobiphenyl. NQR measurements of the line shape and the spin-lattice relaxation time ͑T 1 ͒ were obtained in the temperature range 80-320 K. An inhomogeneously broadened NQR line shape was observed, suggesting a disordered structure in all the scanned temperature range. The disorder may be of conformational type, tentatively associated with a random distribution of dihedral angle values of the molecules in the crystal. Anomalously short T 1 values, nearly temperature independent, were measured for temperatures lower than 200 K. For higher temperatures, a dynamical mechanism enhances the spin-lattice relaxation. The T 1 values follow an Arrhenius type law, with an activation energy of 26Ϯ1 K J/mol. The effects of this dynamical process are observed in the temperature behavior of the NQR frequencies between 150 and 200 K. Raman measurements between 4.5 and 300 K show an anomalous temperature behavior in the 80 cm Ϫ1 frequency zone. The thermally activated process could be assigned to hidden rotations of phenyl rings between close equilibrium dihedral angle values.

Applications of nuclear quadrupole resonance spectroscopy in drug development

“…Temperature dependencies of the relaxation times permit determination of the activation energies of molecular motions such as reorientations or rotations of molecular fragments [39,40]. For example, a sharp decrease of the spin lattice relaxation time T 1 is usually attributed to the onset of reorientation of the group over a potential barrier [41] whereas T 1 minimum is usually ascribed to the modulation effect of the EFG due to motion of a molecular fragments such as a sudden jump reorientation of an atomic group [42].…”