No abstract

No abstract

Abstract:The crystal structure of the title compound was determined by X-ray analysis at 200K. Three independent molecules form a trimer joined by strong and linear N-H . . . N hydrogen bonds. There is another centrosymmetrically related trimer in the unit cell. Both tautomers (1H and 2 H ) are present in each trimer. Disorder of the NHprotons involved in the N-H . . N hydrogen bonds has been observed. Solid-state 13C CPMAS NMR was used to establish the dynamic nature of the NH-proton disorder, the title compound being the first example of proton transfer in a tautomeric mixture of pyrazoles with an equilibrium constant other than 1.

A new host compound in optically resolved and racemic forms for selective crystalline inclusion formation derived from natural mandelic acid was synthesized. Inclusion properties of the two optical species are discussed, involving comparison with a lactic acid-based host analogue. Inclusion compounds with amines, ketones and heterocyclics and specifically with small unbranched alcohols were isolated. The crystal and molecular structures of the optically resolved and racemic forms of the free host at room temperature and the methanol inclusion complex of the resolved host at 255 K were determined by x-ray analysis. Two different binding schemes characterize the packing of these structures, in which one hydroxyl group is responsible for the formation of dimers and chains while in the free host the other hydroxyl group is involved in OH-phenyl interactions. A survey of the OH-phenyl interactions based on the Cambridge Structural Data Base (October 1994 version) reveals that nelectron bonding occurs in a wide range of crystal structures. The approximation of the hydrogen appears to take place in an asymmetric way. Several calculations for the ab initio prediction of the crystal structures were performed.

The supramolecular inclusion complexes of two mandelic acid-based chiral host compounds with dimethylformamide (DMF), (1) and (2), show reversible solidsolid state phase transitions upon lowering the temperature. For both inclusion complexes, the low-[(la) and (2a) The low-temperature forms have the higher crystallographic symmetry. In (lb) half of the DMF guests are twisted by 45.0 ° with respect to those in (la). A comparative discussion of the four crystal packings is presented.