Populations and free-energy differences for the
E and
Z conformations of
S-methyl,cyclopropyl,isopropyl, and cyclopentyl thioformate were determined bylow-temperature
1H NMR spectroscopy, and free-energy barriers of 10.63 and 11.84 kcal/mol were obtained forinterconversion of
E and
Z conformationsof
S-methyl thioformate at -52.4
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C. Populations andfree-energy differences were also determined at roomtemperature by using
13C NMR for a series of N-substitutedformamides and
N-cyclopropylacetamide in 1%solutions inCD
2Cl
2/CH
2Cl
2.In both sets of compounds, electron-withdrawing groups attached tosulfur ornitrogen appear to favor the
E conformations. Theelectronegativities of the groups are ta
ken to increase inthe order methyl < vinyl ~ phenyl ~ cyclopropyl < hydrogen <ethynyl. Data from the literature are discussedin these terms, including the
E-
Z energydifferences for formic acid and its ethynyl, vinyl, and methylesters.