Optical enrichment of a racemic chiral crystal by X-ray irradiation
GENERATION of optical activity in a racemic mixture of enantiomers, a process relevant to the origin of life and of naturally chiral compounds1, requires chirality in the interaction responsible for the transformation or the environment in which it occurs. Without violating this strict requirement, we have found a way in which optical enrichment of a racemic mixture can occur without any chiral specificity in the mechanism. A racemic mixture of (1-cyanoethyl)(piperidine)cobaloxime, which contains the chiral 1-cyanoethyl group, crystallizes in a chiral space group (P212121) in which R and S enantiomers occupy crystallographically distinct (diastereoisomeric) positions in the asymmetric unit cell. We have prepared single crystals of the D configuration. Irradiating the crystal with X-rays at 343 K causes racemization of the S en-antiomer while the R enantiomer remains unaltered. This racemization is thought to be due solely to the different volume constraints on the two enantiomers in their different crystal environments. The result is that the number of R molecules is increased at the expense of S, and so the overall composition of the crystal changes from racemic to enriched in the R enantiomer.
Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. III. Photoracemization of 1-cyanoethyl cobaloxime complexes.
The H atoms bonded to the chiral C atoms (stereogenic center) of the 1-cyanoethyl groups in two cobalt complexes, [(R)-1-cyanoethyl]bis(dimethylglyoximato)(pyridine)cobalt(III) (2) and [(R,S)-1-cyanoethyl]bis(dimethylglyoximato)(piperidine)cobalt(III) (3), were replaced with D atoms, such as Co--C*D(CH(3))CN. The crystals of the two cobalt complexes were irradiated with a xenon lamp for 72 h and 27 d, respectively. The unit-cell dimensions were gradually changed with retention of the single-crystal form. The crystal structures after irradiation were determined by neutron diffraction. In each crystal the chiral 1-cyanoethyl group of one of the two crystallographically independent molecules was partly inverted to the opposite configuration, whereas that of the other molecule kept the original configuration. The C*--D bond in the inverted group was completely conserved in the process of the inversion of the chiral alkyl group. This suggests that the inversion of the chiral 1-cyanoethyl group proceeds with the rotation of the cyanoethyl radical after the Co--C bond cleavage by photo-irradiation so that the opposite side of the radical faces the Co atom. This is followed by recombination of the Co--C bond to form the inverted 1-cyanoethyl group.
Heat Recovery Systems
Geothermal energy is recovered from an underground zone of relatively hot strata via a borehole into which is inserted a heat pipe system containing a working fluid, the heat pipe system comprising an assembly of two tubes in concentric relation, the inner tube being constituted by a relatively small diameter capillary for the return of working fluid to the vicinity of the zone of the relatively hot strata. The working fluid is preferably assisted in its return under gravity by means of a pump. The system includes heat exchange means operable to recover thermal energy from the working fluid. A third and outermost tube may be used to reduce heat losses, this third tube enclosing the two tubes over all but that part of their length which is in the zone of relatively hot strata.