ORDER-DISORDER EFFECTS OF SMALL GUESTS IN CLATHRATES STUDIED BY NUCLEAR-QUADRUPOLE RESONANCE AND CRYSTALLOGRAPHY .1. CARBON-TETRACHLORIDE AND RELATED MOLECULES IN NI(SCN)2(3-METHYLPYRIDINE)4

NQR spectroscopy and X-ray crystallography have been jointly applied to the study of the small guest molecules CH2Cl2, CHCl3, CCl4, CBrCl2, C(CH3)2Cl2 etc. in the Werner host complex Ni(SCN)2(3-methylpyridine)4. Specific host-guest dipole-dipole interactions play an important role in order disorder effects for guest molecules in the cavities of orthorhombic system, Fddd. Thus a weakly polar molecule such as CBr2Cl2 exhibits a disordered alternation, both the bromine and the chlorine atoms occupying the two possible sites in the C2 cavity, whereas for the polar molecule C(CH3)2Cl2, an ordered structure is observed. In addition. two new types of crystal structures, C2/c and P1BAR, are reported here for dichloromethane and chloroform clathrates.