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

Abstract

The Raman spectrum (3700-100cm^-1) of meso-2,3-Dimercaptosuccinic acid (meso-DMSA; C₄H₆O₄S₂) 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 (C_i), gauche (C₁; G⁺/G^-) and eclipsed (C_s; E_f 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 C_i 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 C_i configuration of meso-DMSA in the solid state which found consistent with the observed broadening, composite, split bands, and the mutual exclusion rule.

Keywords: DFT theoretical calculations; Raman and infrared spectra; Vibrational assignments; meso-2,3-Dimercaptosuccinic acid.