Knowledge Bank

The vibrarional optical themselves of small alkyl groups is of particular stereochemical interest. Achiral themselves, these groups are frequently present in chiral molecules and therefore can serve as powerful probes for the chirality of their environment. We have synthesized a fair number of model molecules, many of them deuterium substituted, with such groups, with the goal of a detailed understanding of the mechanisms which govern their optical activity. The terms entering the formulae for the chirality numbers q are of the form [ (^{\partial\alpha}\mu \nu/\partial Q_{P}){O}, (^{\partial G}\mu \nu/\partial Q{P}){O}, and\ (^{\partial A}\mu \nu\lambda/\partial Q{P})_{O}, ] where $Qp$ are normal coordinates and $\alpha u\nu ,Gu\nu$, and $Au\nu \lambda$ stand for the compronents of the polarizability, the potical activity, and the electric dipole electric quadrupoletensors. In order to assess the mechanical contribution to the chirality numbers, normal coordinate analyses have been performed for some of the small molecules like 3-methylcyclopentene, while the electronic part has been estimated from direct calculations based on approximate wavefunctions. The theoretical results are compared to the experimental spectra recorded in our laboratory on an improved multichannel Ranman optical activity spectrometer.