Structural and conformational properties of 1,2-ethanedithiol as studied by microwave spectroscopy and ab initio calculations

The microwave spectrum of 1,2-ethanedithiol has been re-investigated in the 10.0-39.0 GHz spectral region. Ten all-staggered rotameric forms are possible for this compound. The gas phase consists of a complex equilibrium mixture of several rotameric forms of the molecule. Four of these conformers, gAg, gAg', gGg and gGg', are presumed to predominate. The assignments for one previously assigned conformer, gGg, have been extended. The assignments for two 'new' conformers, gAg' and gGg', are reported for the first time. The gAg' rotamer was found to be the most stable conformer that possesses a dipole moment different from zero. gAg' is 3.2(4) kJ mol(-1) more stable than gGg, and 1.8(4) kJ mol(-1) more stable than gGg'. In addition to these three rotamers, the gAg conformer having no dipole moment and hence not observable by microwave spectroscopy, is assumed to be a fourth stable, low-energy form of the molecule. The gGg rotamer is stabilised with one weak S-H ... S intramolecular hydrogen bond, whereas gGg' is stabilised with two such bonds. The gAg' conformer displays tunnelling in the ground vibrational state and in the first excited state of the C-C torsional vibration. The tunnelling is presumably caused by a concerted rotation by both thiol groups. The tunnelling frequency is 0.575(80) MHz in the ground vibrational state, and 2.48(5) MHz in the first excited state of the C-C torsion. Tunnelling is absent in the gGg and gGg' rotamers. The microwave work has been assisted by nb initio computations at the HF/6-311++G** and MP2/6-311++** (frozen core) levels of theory, as well as density theory calculations at the B3LYP/6-311++G** level. All ten all-staggered conformations were predicted to be 'stable' (minima on the energy hypersurface) in the HF/6-311++G** computations, nine were predicted to be stable in the B3LYP/6-311++G** calculations, while only five were predicted to be stable in the MP2/6-311++G** computations. The conformers predicted to be stable in the last-mentioned computations all have gauche arrangements for the H-S-C-C links of atoms. The relative energies of the different conformers were rather similar at all these three levels of theory in those cases where stable conformers were predicted. The agreement with experiment is found to be satisfactory. The best predictions of the rotational constants were found in the MP2/6-311++G** computations, which are therefore assumed to predict the most accurate geometries for the conformers.