STRUCTURE AND DYNAMICS OF A SERIES OF CRYSTALS OF BUTTERFLY-LIKE MOLECULES AS STUDIED BY CL-35 NQR, AND H-1 AND F-19 NMR

4,4'-Difluorobenzophenone (DFBP), 2,2-bis(4-chrorophenyl)-1,1-dichloroethylene and dichlorodiphenylmethane have been investigated by Cl-35 nuclear quadrupole resonance, and H-1 and F-19 nuclear magnetic resonance. The spin-lattice relaxation times T-1 of H-1 and F-19 of DFBP assume minima at about 60 K, suggesting that the internal rotation of the C6H5F group is excited at low temperatures. The T-1 of H-1 changes drastically at the first-order phase transition point (approximate to 100 K), due probably to the abrupt change in the rate of internal rotation of the C6H5F group. The asymmetric temperature curve of T-1 indiates that the stochastic internal rotation cannot be described by a single correlation time of the motion. Cl-35 NQR reveals that 2,2-bis(4-chlorophenyl)-1,1-dichloroethylene and dichlorodiphenylmethane do not exhibit any phase transition between 4.2 K and their respective melting points.