Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds

Abstract: Publisher Summary This chapter discusses chiral metal atoms in optically active organo-transition-metal compounds. An important approach to stereochemical problems is to make use of the concept of chirality. The chiral center most frequently encountered is the asymmetric carbon atom––a tetrahedral C atom––bonded to four different substituents. Chiral molecules may be studied by a great many techniques. Occasionally, optically active ligands have been employed in organo-transition-metal chemistry to demonstrate the stereospecificity of reactions at metal–ligand bonds with respect to the α-carbon atom of the ligand. The chapter focuses on organo-transition-metal compounds having chiral metal atoms whose optical activity have been demonstrated. The principle of introducing a diastereoisomer relationship into a pair of mirror-image isomers is the basis for each optical resolution. After the preparation of diastereoisomers by the introduction of an optically active resolving agent, the next problem in an optical resolution is to separate the diastereoisomers. The optical purity of enantiomers has been determined by nuclear magnetic resonance (NMR) spectroscopy with the help of optically active shift-reagents. Thus, for racemization, the steric effect seems to be most important. Metal–alkyl bonds can be carbonylated, and metal–acyl bonds can be decarbonylated. Optically active organo-transition-metal compounds exhibit extremely large specific rotations, usually exceeding the specific rotations encountered in organic chemistry by a factor of one or two powers of ten.

Observation of steric effects in gas–surface scattering

Abstract: Molecules adsorbed on a surface are known to show preferential orientations, and in particular, the interaction potential between a linear molecule and a surface depends on the orientation of the molecular axis. But the fact that the molecules eventually adsorbed are orientated with respect to the surface is not evidence that the dynamics of gas–surface interactions is governed by the initial molecular orientation in the gas phase. For example at very low velocities the molecule might achieve its optimum orientation adiabatically during its approach to the surface. Dependence of scattering dynamics on molecular orientation can also be degraded as a result of surface structure and surface vibrations. There have been, experimental studies, however, which suggest that orientational or steric effects could influence gas–surface scattering1–4. Thus, the much larger rotational excitation for NO- than for N2-scattering from Ag(lll) indicates that large anisotropies can occur in a potential for gas–surface dynamics (G. O. Sitz et al. manuscript in preparation). The double rotational rainbow observed in the rotational state distribution for NO/Ag(111) (refs 3 and 4) can also be interpreted as a manifestation of this anisotropy5–9. These studies all indicate that steric effects could be important and here we report on scattering of orientated NO from Ag(111) to provide direct experimental evidence for the importance of such effects in gas–surface interactions.