An ab-initio study of the magneto-optical rotation of diastereoisomers

Experimental studies on the natural optical activity and Faraday rotation of the three different stereoisomers of tartaric acid were reported recently by Ruchon et al. [Chem. Phys., Lett. 2005, 412, 411]. The authors noted that the Faraday rotation of the meso (R,S) system differed from those of the (S,S) and (R,R) enantiomers, and derived a simple dipole-dipole interaction model to describe what they claim to be a "new property". We present the results of both density functional theory (DFT) and coupled cluster calculations for a structurally elementary model system composed of two chiral carbon atoms presenting three diastereoisomers (C2H2Cl2F2), as well as a detailed DFT study of the natural and magnetic-field-induced optical rotation of tartaric acid. The effects of electron correlation, basis set, and conformational flexibility are analyzed. It is found that the specific Faraday rotations of the chiral (R,R) and mesa forms of tartaric acid (for lambda = 632.8 nm) differ by about 3%, a value which is quite close In magnitude-but of opposite sign-to that obtained with the simplified model proposed by Ruchon and co-workers.