Solvent dependence of the chemical shifts and coupling constants of 1,2-dichlorofluoro-ethylenes

Abstract

A detailed study has been made of the solvent-induced variations in 1 J CH, 1 J CF, and 3 J HF and 1H and 19F chemical shifts in cis- and trans-1,2-dichlorofluoroethylene. A linear relationship was found between all three coupling constants and the respective δ 1 H and ø 19 F values. Based on a consideration of various factors expected to influence chemical shifts and coupling constants, it is concluded that the dominant factor affecting these parameters (in the present systems) is the reaction field of the solvent. The differences in coupling constants between geometrical isomers as well as the alteration of the coupling constant caused by solvent change are discussed and compared with empirical relationships reported in the literature. In addition, the changes in 1 J CH and 1 J CF coupling constants are rationalized in terms of the MO theory of Pople and Santry. On the basis of a correlation with coupling constants from the literature, and the solvent behavior of these coupling constants, it is proposed that the signs of the 2 J CCH coupling constants in the cis and trans isomers as well as the 3 J HF coupling in the cis isomer are positive. In an earlier study it was shown that the solvent dependence of the 1 J CH and 1 J CF coupling constants is consistent with positive and negative signs respectively. No correspondence was noted between the 2 J values and the nature of the isomer. An unusually low value of 2 J CCF = 20.0 Hz was observed for the trans isomer.