Determination of the absolute stereochemistry of limonene and alpha-santalol by Raman optical activity spectroscopy.

Determining the absolute stereochemistry of organic compounds in solution remains a challenge. We investigated the use of Raman optical activity (ROA) spectroscopy to address this problem. The absolute configurations of (+)-(R)- and (-)-(S)-limonene were determined by ROA spectroscopy, which can be applied to smaller amounts of sample as compared with vibrational circular dichroism (VCD) spectroscopy. This ROA method was also applied to (+)-(E)-alpha-santalol and shown to be successful in the determination of the absolute configuration of this compound. ROA spectroscopy shows promise as a useful tool for determining the absolute stereochemistry of many natural compounds.

Fliplike motion in the thalidomide dimer: Conformational analysis of (R)-thalidomide using vibrational circular dichroism spectroscopy.

The dynamic fliplike motion in the (R)-thalidomide dimer has been reported for the first time. The vibrational circular dichroism (VCD) spectrum of (R)-thalidomide in DMSO-d6 indicates the characteristic nu(CO) bands with opposite signs and reflects the structural property of the equatorial configuration of the phthalimide ring. On the other hand, the VCD spectrum of (R)-thalidomide in CDCl3 exhibits a different pattern of nu(CO) bands and suggests the fliplike motion in dimer forms. This novel insight for the dimer forms would be helpful for the understanding of the structure-activity relationship for thalidomide.

Voronoi space division of a polymer: topological effects, free volume, and surface end segregation.

In order to investigate the topological effects of chain molecules, united-atom molecular dynamics simulations of a 500-mer polyethylene linked by 50 hexyl groups (a grafted polymer having 52 ends) are carried out and analyzed in terms of Voronoi space division. We find that the volume of a Voronoi polyhedron for a chain end is larger than that for an internal or junction atom, and that it is the most sensitive to temperature, both of which suggest higher mobility of chain ends. Moreover, chain ends dominantly localize at the surface of the globule: The striking evidence is that while the ratio of surface atoms is only 24% of all atoms, the ratio of ends at the surface is 91% out of all ends. The shape of Voronoi polyhedra for internal atoms is prolate even in the bulk, and near the surface it becomes more prolate. We propose the concept of bonding faces, which play a significant role in the Voronoi space division of covalently bonding polymers. Two bonding faces occupy 38% of the total surface area of a Voronoi polyhedron and determine the prolate shape.