Abstract
Raman (3700–100 cm
) and infrared (4000–400 cm
) spectra of 2,5-Dimercapto-1,3,4-thiadiazol (DMTD) were recorded in the solid phase. Six structures (
) were initially proposed for DMTD as a result of
tautomerism and internal rotation(s) of thiol group(s) around the C–S bond. Quantum chemical calculations were carried out for an isolated molecule (
) using density functional theory (B3LYP) and
MP2(full) methods utilizing 6-31G(d) and 6-311++G(d,p) basis sets which favor
tautomerism (structure
). Relaxed potential energy surface scans of structure
revealed an additional conformer (the
group is out-of-plane, structure
) using the aforementioned methods at 6-311++G(d,p) basis set. For additional verification, plane-wave solid state calculations were carried out at PW91 and PBEsol came out in favor of conformer
. This is in agreement with the computed/observed SH in-plane bending of
(959/941 cm
) rather than the one estimated at (880 cm
) for
. Moreover, the observed split IR/Raman bands were found consistent with solid state calculated frequencies of
assuming two molecules per unit cell bonded via H-bonding intermolecular interactions. Aided by vibrational frequency calculations, normal coordinate analysis, force constants and potential energy distributions (PEDs), a complete vibrational assignment for the observed IR and Raman bands is proposed herein. Furthermore, we have estimated the frontier molecular orbitals and atomic charges to account for the corrosion inhibition efficiency of DMTD along with its binding sites to the metal surface. Our results are discussed herein and compared to similar molecules whenever appropriate.