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.
Photochrome-coupled metal complexes: molecular processing of photon stimuli.
The frontiers of novel photoresponsive materials constructed with photochromes and transition metal complexes are surveyed in this review. Strategies to develop new photofunctions are categorized into four types. In the first category, intramolecular electronic interactions between photochromes and metal complexes produce entangled responses such as redox-regulated photochromic reactions or tristable photochromism. In the second, light-induced molecular structural rearrangements of photochromes induce the transformation of flexible and labile coordination structures, which can be applied to complex photomechanics or photoelectron conversion. In the third, the photochromic moiety also acts as a photonic switch, transmitting a metal-metal interaction when it is located between two metal complex moieties. The last category concerns the development of new photochromic reactions, involving metal-ligand bond rearrangements. These reactions potentially induce drastic electronic tuning of the metal center, and can be used to develop light-driven molecular machines.
Design of reinforced concrete lining in pressure tunnels, considering thermal effects and jointed rockmass
Abstract Jaeger's theory (Jaeger, Rock Mechanics and Rock Engineering , Cambridge University Press, 1972) of the design of the lining for a pressure tunnel applies to a tunnel located in an isotropic, homogeneous and elastic medium. The theory is derived from the elasticity equations for the plane stress condition. In this paper, Jaeger's theory is extended to a plane strain situation. Also included is a discussion of the effects of a uniform increase (or decrease) in the lining temperature, jointed rockmass, cracking of the tunnel lining, and cracking of the rockmass around the opening. Tables are presented for the modulus of subgrade reaction for various rockmass conditions and for rock sharing pressures for various lining conditions. The reduction factor to include the effect of joints in rockmass is derived by the analysis of a numerical model of the lined tunnel by the finite-infinite-interface element method.