Determination of enantiomeric excess using Raman optical activity

A simple method for the determination of enantiomeric excess is described. It is based on depolarized Raman optical activity (ROA) measurements in right-angle scattering employing the incident ciruclar polarization (ICP) modulation approach. The accuracy of this method was assessed utilizing α-pinene as a chiral test compound. The four α-pinene ICP ROA signals associated with the Raman bands observed at ca. 772, 787, 887 and 929 cm−1 were selected to establish a linear relationship between ROA signal strength and enantiomeric composition for a set of calibration mixtures containing both α-pinene enantiomers. Using this relationship, a (+)-α-pinene sample content of 16.3% was determined for a blind mixture containing 83.6% (1S)-( — )-α-pinene and 16.4% (1R)-(+)-α-pinene. Similary, utilizing additional spectral subtraction procedures, a (+)-α-pinene sample content of 1.5% was estimated for a test mixture consisting of 50.1% (1S)-( — )-α-pinene and 49.9% (1S)-( — )-trans-pinane