Rotational isomerism: XVII—solvent effects on the NMR spectra and rotamer energies of some tetrahaloethanes

Abstract

AbstractThe NMR spectra of 1,1‐dichloro‐2,2‐difluoroethane (1), 1,1‐dibromo‐2,2‐difluoroethane (2), meso and dl 1,2‐dichloro‐1,2‐difluoroethane (3) and 1,1,2,2‐tetrachloroethane (4) have been analysed in a number of solvents. The 19F spectrum of 3 in L‐bornyl acetate at 56·4 and 94·1 MHz allows an unambiguous identification of the meso and dl isomers. The spectra of the d and l isomers consist of two AA′XX′ spectra with a small chemical shift difference between the d and l forms, whilst that of the meso form is an apparent AA′XX′ spectrum at 56·4 MHz but an ABXX′ spectrum at 94·1 MHz, the 19F nuclei in this isomer being anisochronous in this solvent. The observed solvent and temperature dependence of the couplings of 1, 2, meso 3 and 4 when combined with the calculated solvation energies, allow the determination of the rotamer energies and couplings in these molecules. The rotamer energy differences (Eg – Et) in the liquid and vapour states are 0·6 and −0·2 kcal/mol (1); 0·4 and −0·5 kcal/mol (2); 0·9 and 0·2 kcal/mol meso (3) and −0·1 and −0·8 kcal/mol (4). The 3J(HH), 3J(HF) and 3J(FF) couplings for the distinct rotamers are considered together with those of similarly constituted molecules. The general agreement demonstrates that the solvation theory may be applied to multisubstituted ethanes without any basic modifications. The trans oriented HH couplings show a linear substituent electronegativity dependence, which differs appreciably from that obtained for disubstituted ethanes, however. The gauche couplings show the influence of dihedral angle variations as well as substituent electronegativity. The rotamer 3J(FF) couplings in meso 3 are −38·2 Hz (Jt) and −17·4 Hz (Jg).