EXPERIMENTAL AND ABINITIO THEORETICAL VIBRATIONAL RAMAN OPTICAL-ACTIVITY OF TARTARIC ACID

The vibrational Raman optical activity (ROA) spectra of (2R,3R)-(+) tartaric acid-d0 in H2O and (2R,3R)-(+) tartaric acid-d4 in D2O between 300 and 1800 cm-1 measured in backscattering are reported. Ab initio Raman intensities were evaluated using basis sets at 6-31G, 6-31G* and double zeta plus polarization (DZP) levels. Ab initio ROA intensities were obtained at two levels: in one calculation both the normal coordinates and the polarizability and optical activity tensor derivatives were evaluated with the 6-31G basis set; in a second calculation normal coordinates obtained with the DZP basis set were used to evaluate the normal coordinate derivatives of polarizability and optical activity tensors from the corresponding Cartesian derivative tensors obtained with the 6-31G basis set. Sufficiently good correlation was found between many of bands in the theoretical and experimental Raman and ROA spectra for both the -d0 and -d4 species to confirm that the absolute configuration of the (+)-enantiomer is indeed (2R,3R) and to suggest that the trans COOH and trans COOD conformations are dominant. Tartaric acid-d4 shows very similar ROA to tartaric acid itself in the range 300-800 cm-1 but quite different in the range 800-1450 cm-1, which provides insight into the influence of normal mode composition on ROA spectra. It was found that the normal mode compositions are much more sensitive to the level of basis set used than the polarizability and optical activity tensor derivatives.