RAMAN OPTICAL ACTIVITY OF SELECTED MOLECULES

Vavra, Mark Roderick

Abstract

The design and construction of a Raman optical activity (ROA) spectrometer is discussed. The depolarized Raman optical activity spectra of (-)alpha-pinene, (-)alpha-cedrene, (-) beta-pinene, (+)beta-cedrene and (+) trans-pinane were measured from 200 cm^-1 to 1600 cm^-1. Vibrational circular dichroism (VCD) has been measured for these molecules from 900 to 2000 cm^-1 using a Fourier transform infrared (FTIR) based instrument.

Using frequency stability and depolarized ROA sign it was possible to show the 1082 cm^-1 band to be the same for 1S, 5S (-)alpha-pinene, and 1S, 5S (-)beta-pinene. This same band in 1R, 2R (+) trans-pinane is seen to be of the same sign. This excludes the depolarized Raman optical activity of the 1082 cm^-1 from being an indicator of the stereochemistry of the pinene series.

Both vibrational circular dichroism and depolarized Raman optical activity of the two cedrane molecules in the 1070 cm^-1 region show correlatable bands analogous to the pinene situation. These bands most likely arise from the very mixed framework modes. These spectra provide an example of vibrational optical activity being more sensitive to the absolute molecular stereochemistry than optical rotation (or near UV CD) which in this case is less sensitive.