AB Initio calculations and vibrational spectra of CH3SC≡CH and CH3SC≡CSCH3

Methylthio- (MTE) and bis-methylthioethyne (BMTE) molecules are calculated by the SCF MO method (geometry optimization, basis set 6–31G*/MP2). The calculated internal rotation barriers of methyl groups are 7.12 kJ/lmole for MTE and 12.86 kJ/mole for BMTE (both groups are simultaneously rotated). The s-gosh-orientation of the thiomethyl fragments corresponds to a stable conformation of BMTE. The estimated values of the s-cis- and s-trans-barriers of mutual rotation of SCH3 groups about the axis of the C≡C bond are 13.61 and 12.54 kJ/mole, respectively. Conformationally sensitive MOs and vibration frequencies are established. An analysis of the experimental IR absorption and Raman spectra and the calculated vibrational spectrum makes it possible to conclude that in the liquid phase the BMTE molecules also have an s-gosh-conformation.