Structural investigations and modeling of cavities in clathrates

Abstract

A molecular-graphics study has been performed in order to build and visualize the shape of cavities within different clathrates from X-ray diffraction data [e.g. Dianin's compound, Werner complexes Ni(SCN)2(3-methylpyridine)4, Fe(acetylacetonate)3 and Ni(ethylxanthate)2(4,4'-dimethyl-2,2'-bipyridyl) complexes]. The algorithm of the solvent-accessible surfaces representation has been applied to a part of the whole crystal structure rather than to isolated host molecules, by using the MOLCAD molecular modeling package. This type of modelization has been found very efficient both to study the shape properties of the host cavities (cage or channel types) and to approach the structural features of the host/guest interactions.

An NQR Study of Inclusion Compounds Formed Between Small Chlorine-containing Molecules and bis(N-alkylimidazolidine-2-thione)Cu(I) Halide Complexes

“…An example of this is shown in Fig. 4 for complex 5, where three of the four lines of the CC1 4 guest fades out at around 140 K while the remaining line persists up to 175 K. This behaviour is reminiscent of the behaviour of CC1 4 included in cavities with three-fold symmetry [1][2][3] with one of the CC1 bonds -the axial chlorine atom -lying along the three-fold axis, where there are therefore only two resonances with intensityratios 3:1. The site symmetry permits easy rotation about the three-fold symmetry axis so that the more intense lines -arising from the three equatorial chlorine atoms -fade out rapidly, but since rotation about the three-fold axis does not alter the direction of the field-gradient z axis of the axial chlorine atom its resonance remains visible until a more general tumbling motion sets in.…”

“…In most cases the 35 C1 resonances of the guest molecule were visible only at low temperatures, usually fading out at temperatures in the range 100-200 K. As discussed in [1][2][3], this behaviour is due to the fact that the guest molecule can reorient within the cavity or channel of the host crystal and that, when this molecular reorientation is sufficiently rapid, the 35 C1 line broadens and disappears. In several cases one of the lines persists to higher temperature than the others.…”

“…CC1 4 formed complexes only with the chlorides or bromides. Finally 1,2-dichloroethane was included only by (N-isopropylimidazolidine-2-thione) 2 CuBr. Twenty-five complexes are listed in Table 1 and were characterised either by elemental C, H, N analysis or by H'-NMR, the analytical results being shown in Table 2.…”

“…The inclusion complexes were prepared by recrystallization from the guest solvent, or for CC1 4 by recrystallisation from a 1:2 mixture of CC1 4 2 CuX, (X = C1, Br, I), (N-isopropylimidazolidine-2-thione) 2 CuI and (Nbutylimidazolidine-2-thione) 2 CuCl did not include any of the guests. The guest CH 2 C1 2 was often lost rapidly and did not give an NQR signal.…”

“…We have previously demonstrated the utility of nuclear quadrupole resonance (NQR) for the study of the behaviour of small guest molecules in a variety of inclusion complexes [1][2][3]. The temperature-dependence of the NQR frequency depends on the temperature-variation of the mobility of the guest molecule.…”