LOW-TEMPERATURE NMR-STUDIES OF PHENYL DITHIOACETATE AND ETHYL DITHIOACETATE

The 300.52 MHz proton NMR spectrum of phenyl dithioacetate in CD2Cl2 a t - 91 degrees C shows separate methyl signals for the E and Z conform ations, with populations of 0.09 and 0.91, respectively. The free-ener gy barriers at coalescence (-65 degrees C) are 9.8 (1) and 10.6(4) kca l/mol for the E --> Z and Z --> E conversions, respectively. Slow-exch ange C-13 NMR spectra were also obtained for solutions in several solv ents. 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 carbo n signals, and free-energy barriers at coalescence (-68 degrees C) wer e 9.4(8) and 10.2(0) kcal/mol. Decoalescence was not observed at low t emperatures for ethyl dithioacetate, probably due to a low population of the E isomer. The finding of an observable population of the E isom er for phenyl dithioacetate, but not for ethyl dithioacetate, is sugge sted to be related to the difference in hybridization and electronegat ivity of the two groups, with higher electronegativity favoring the E conformation. (C) 1998 Elsevier Science B.V. All rights reserved.