Order-disorder effects of small guests in clathrates studied by nuclear quadrupole resonance and crystallography. Part I. 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, CBr₂Cl₂, 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 andP1¯, are reported here for dichloromethane and chloroform clathrates