The faster conversion of ethene thiol than vinyl alcohol in tautomerization reactions in aqueous solution — theoretical prediction

Abstract

Ethene thiol (ET)–thioacetaldehyde (TA) tautomerization was investigated via ab initio molecular orbital (MO) calculations. The G2, G2(MP2), and G1 methods were used to calculate the energy difference for the two tautomers and their conversion energy barrier both in the gas phase and in a cluster including an ET and a single water molecule. The results were compared with those of the similar conversion process for vinyl alcohol (VA)–acetaldehyde (AA) tautomerization. In the gas phase, ET was found to be 2.6 kcal/mol less stable in energy than TA at the G2 level. It is comparatively shown that ET is not so unstable as VA, compared with the corresponding keto-form. On the contrary, the conversion energy barrier between the two tautomers for the ET–TA system (55.2 kcal/mol) was as high as that for the VA–AA system (56.4 kcal/mol). On the other hand, for one water-mediated cluster system, the tendency in relative stability of the two systems is not so different, while the conversion energy barrier of ET to TA (35.1 kcal/mol) was slightly higher than that for the VA–AA system (31.5 kcal/mol). However, the self-consistent reaction field (SCRF) calculations with the isodensity polarizable continuum model (ICPM) method have shown that the barrier for the ET–TA system (25.0 kcal/mol) becomes lower than that for the VA–AA system (33.2 kcal/mol). This is clearly understood by the fact that the dipole moment at the TS for the ET–TA system is 4.6 D and is larger than twice that for the VA–AA system (2.2 D). In conclusion, it is suggested theoretically that the conversion of ET to TA should be faster than that of VA to AA in an aqueous environment.