CONFORMATIONAL ANALYSIS OF ETHANE-1,2-DITHIOL AND MERCAPTOETHANOL AN AB INITIO SCF HF/3-21G∗ AND 6-31G∗∗ STUDY

Abstract

A conformational analysis study has been carried out on ethane-1,2-dithiol and mercaptoethanol, using ab initio 3-21G∗ and 6-31G∗∗ bases set. The optimized geometries are very similar in both cases and suggest that very good results can be obtained by adopting the less time consuming 3-21G∗ basis. In contrast with literature findings based on electron diffraction and microwave spectra, the most stable conformer of ethane-1,2-dithiol, is the gauche-trans-gauche structure, but the gauche-gauche-gauche rotamers have less than 2 kJ mol⁻¹ energy difference. On the contrary, mercaptoethanol is predicted to exist mainly as a mixture of gauche-gauche-gauche conformers, the gtg form being unfavoured. This is attributable to different hydrogen bond strengths due to different hydrogen bridges (S—H…S in the former and O—H…S in the latter molecules, respectively). Approximate hydrogen bond energies were also calculated. They are in line with experimental estimation and with expectation on the ground of literature semiempirical relations. Rotation barriers and interconversion pathways are also discussed.

Key Words:

• Ethane-1,2-dithiol
• mercaptoethanol
• conformational analysis