ABSTRACT
The structure and dynamic behaviour of monomeric lead(ii) dithiolate Pb(SCH(2)CH(2)NMe(2))(2) are studied by multi-nuclear NMR, IR and Raman spectroscopy in solution, as well as variable-temperature Raman spectroscopy and X-ray diffraction analysis in the solid-state, revealing an unusual dynamic "flip-flop" process in solution and reversible conformational isomerism of the chelated five-membered rings in the solid-state.
ABSTRACT
Crystallization of the hexane reaction mixture after treatment of LiGe(OCH(2)CH(2)NMe(2))(3) with Ph(3)CN(3) gives rise to a new triclinic (space group P\overline{1}) polymorph of triphenylmethylamine, C(19)H(17)N, (I), containing dimers formed by N-H...N hydrogen bonds, whereas the structure of the known orthorhombic (space group P2(1)2(1)2(1)) polymorph of this compound, (II), consists of isolated molecules. While the dimers in (I) lie across crystallographic inversion centres, the molecules are not truly related by them. The centrosymmetric structure is due to the statistical disordering of the amino H atoms participating in the N-H...N hydrogen-bonding interactions, and thus the inversion centre is superpositional. The conformations and geometric parameters of the molecules in (I) and (II) are very similar. It was found that the polarity of the solvent does not affect the capability of triphenylmethylamine to crystallize in the different polymorphic modifications. The orthorhombic polymorph, (II), is more thermodynamically stable under normal conditions than the triclinic polymorph, (I). The experimental data indicate the absence of a phase transition in the temperature interval 120-293 K. The densities of (I) (1.235 Mg m(-3)) and (II) (1.231 Mg m(-3)) at 120 K are practically equal. It would seem that either the kinetic factors or the effects of the other products of the reaction facilitating the hydrogen-bonded dimerization of triphenylmethylamine molecules are the determining factor for the isolation of the triclinic polymorph (I) of triphenylmethylamine.
