Conformational behaviour in S-methyl halothioacetates through NMR, FT-IR, and theoretical calculations

This work presents an investigation of the conformational behaviour of S-methyl chlorothioacetate (MCTA), S-methyl bromothioacetate (MBTA), and S-methyl iodothioacetate (MITA) through NMR and IR spectroscopies and through theoretical calculations that apply DFT/B3LYP theory. The DFT calculations showed two stable rotamers in the vapour phase, gauche and trans, for all the compounds. The coupling constants, 1 J CH , were obtained from NMR spectroscopy; these were used in the solvation theory to calculate the vapour-phase energy difference, the coupling constants for each rotamer, the solution energy difference, and the rotamer population for each solvent. These data showed that for MCTA, the most stable rotamer is trans; however, for MBTA and MITA, the most stable rotamer is gauche, both in the vapour phase and in solution. The changes in rotamer stability are because of the preference in stereo-electronic interactions; in the MCTA case the interaction is mainly electrostatic, while for MBTA and MITA, it is because of the interaction between n Br,I π * CO .Key words: S-methyl halothioacetates, conformational analysis, theoretical calculations, NMR, solvation theory.