Raman and DRIFT spectra, vibrational assignments and quantum mechanical calculations of centrosymmetric meso-2,3-Dimercaptosuccinic acid.

The Raman spectrum (3700-100cm-1 ) of meso-2,3-Dimercaptosuccinic acid (meso-DMSA; C4 H6 O4 S2 ) was recorded in the solid phase using 514.5 and 785nm excitation lines. Whereas, the DRIFT spectrum (4000-400cm-1 ) of the sample powdered in KBr was obtained. Moreover, DFT-B3LYP/6-31G(d) geometry optimization and frequency calculations were carried out for centrosymmetric trans (Ci ), gauche (C1 ; G+ /G- ) and eclipsed (Cs ; Ef and C1 ; E+ /E- ) rotational isomers in favor of a trans conformation, the least energy with real frequencies. However, other conformers were found at either local minima or local maxima as a result of the rotation of carboxyl, hydroxyl and thiol groups according to a potential energy surface scan. Moreover, an imaginary wavenumber was predicted; therefore, they are considered transition states. On the other hand, the mass spectrum of the sample dissolved in an acetonitrile/methanol mixture reveal 4-6% dimer through intermolecular hydrogen bonding interactions via the dicarboxylic groups. Therefore, we have modeled the complex structure obeying Ci restricted symmetry for an isolated dimer unit using DFT-B3LYP/6-31G(d) and for two molecules per unit cell in the solid phase implementing DFT-PBE functional. Thus, the meso-DMSA forms long strands in which individual molecules are bonded together at each termini through hydrogen bonding. Aided by normal coordinate analysis, complete vibrational assignments were provided herein which support Ci configuration of meso-DMSA in the solid state which found consistent with the observed broadening, composite, split bands, and the mutual exclusion rule.