Rotational isomerism. A gas-phase 1H nuclear magnetic resonance study of 1-bromo-2-chloroethane

The average vicinal coupling constants of 1-bromo-2-chloroethane at different temperatures have been obtained in a gas-phase ¹H NMR study of 1,2-disubstituted ethanes. Analysis of the experimental data, assuming a ”static” model with constant values for the vicinal coupling constants of the individual rotamers, yielded unacceptable results. A ”dynamic” model, which takes into account the torsional vibrations, has therefore been developed and used to analyse the gas-phase results. The values at 305 K for the vicinal coupling constants of the individual rotamers are: trans rotamer J_T = 13.6 Hz, J'_t = 4.9 Hz; gauche rotamer $\text{1}\text{2}$(J_G1 + J_G2) = 0.9 Hz, $\text{1}\text{2}$(J'_G1 + J'_G2) = 7.8 Hz. These couplings are temperature-dependent with J_T changing about four times as much as the other coupling constants. Analysis of the average vicinal coupling constants, measured in different solvents, with the “dynamic” model gave values for the vicinal coupling constants of the individual rotamers in excellent agreement with those from the gas-phase data. The value of $\text{1}\text{2}$(J_G1+ J_G2) decreases noticeably with increasing dielectric constant of the solution. This can be explained by changes in the average dihedral angle between the coupling protons, in the gauche rotamers, which leads to changes of J_G1 and J_G2 in the same direction.