Low-temperature NMR studies of phenyl dithioacetate and ethyl dithioacetate

The 300.52 MHz proton NMR spectrum of phenyl dithioacetate in CD2Cl2 at - 91 degrees C shows separate methyl signals for the E and Z conformations, with populations of 0.09 and 0.91, respectively. The free-energy barriers at coalescence (-65 degrees C) are 9.8 (1) and 10.6(4) kcal/mol for the E --> Z and Z --> E conversions, respectively. Slow-exchange C-13 NMR spectra were also obtained for solutions in several solvents. The populations in 3:1 acetaldehydelacetone-d(6) were 0.12 and 0.88 at -92 degrees C, as obtained from integration of the methyl carbon signals, and free-energy barriers at coalescence (-68 degrees C) were 9.4(8) and 10.2(0) kcal/mol. Decoalescence was not observed at low temperatures for ethyl dithioacetate, probably due to a low population of the E isomer. The finding of an observable population of the E isomer for phenyl dithioacetate, but not for ethyl dithioacetate, is suggested to be related to the difference in hybridization and electronegativity of the two groups, with higher electronegativity favoring the E conformation. (C) 1998 Elsevier Science B.V. All rights reserved.