NUCLEAR MAGNETIC RESONANCE STUDIES OF 1,3-BUTADIENES .V. CONFORMATIONS OF 1,1-DICHLORO-3-FLUOROBUTADIENE-1,3 AND 1,1-DIBROMO-3-FLUOROBUTADIENE-1,3

The high-resolution proton magnetic resonance spectra of the title compounds in solution in mixtures of dimethyl ether and dichlorodifluoromethane at temperatures from –130 to +39° have been recorded and analyzed in terms of chemical shifts and coupling constants. The temperature dependence of the 3JHF coupling between the proton on carbon 2 and the fluorine indicates (1) that these compounds exist as mixtures of s-trans and skew conformers; (2) that 3JHF in the s-trans conformation has the value +25.7 cps (dichloro compound) and +25.2 cps (dibromo compound), while in the skew conformation 3JHF has the value – 1.8 cps (dichloro compound) and –1.9 cps (dibromo compound); (3) that for the dichloro compound ΔH = 980 ± 30 cal, while for the dibromo compound ΔH = 1020 ± 30 cal, the s-trans form being the more stable at low temperatures in each case. © 1968, American Chemical Society. All rights reserved.