NMR Studies of Conformational Equilibria in Substituted Ethanes
Authors: Gutowsky H. S. , Belford Geneva G. , McMahon P. E. 
Published: January 2005

Proton and fluorine magnetic resonance spectra have been observed in 12 liquid polysubstituted ethanes over temperature ranges of 250° to 450°K. In each of these compounds, reorientations about the C–C bond are fast enough to yield high resolution NMR spectra which are averages of the three rotational isomers. The change in the proportions of the isomers with temperature is large enough for CHCl2CHCl2, CHCl2CHF2, CF2ClCFCl2, CHCl2CHFCl, and CF2BrCFBrCl, and also CFCl2CHCl2 for which 〈JHF〉 has been reported, to permit a least-squares type analysis of the averaged shifts 〈v〉 and coupling constants 〈J〉 with a high-speed digital computer. The latter evaluates the physical parameters, three or five depending upon molecular symmetry, which govern the temperature dependence of 〈v〉 and 〈J〉. These parameters, in our approximation, are the values of the spectral quantity in question for each of the ``rotamers,'' and the relative energies ΔE of the latter. This procedure could not be applied to CHΦ2CH2(COΦ), CHBr2CH2Br, CH2ΦCH2Cl, and CH2ΦCH2Br for which ΔE is sufficiently small that 〈JvicHH〉 is virtually temperature independent. However, an approximation was developed for obtaining such small ΔE's (35 to 90 cal) by assuming (JtHH—JgHH) to be an average of values found in other compounds. A similar approximation was used for solutions of three CHXYCHYZ compounds in which ΔE is large, ∼1000 cal, but thermal decomposition prevented measurements of 〈JvicHH〉 over a large enough temperature range. In addition, ΔE's were determined for CHCl2CHF2 and CHCl2CHFCl by analyzing the dependence upon solvent of infrared absorption bands assigned to trans and gauche rotameters.

There is fair to excellent agreement between values for a given ΔE obtained from different NMR observables and also from vibrational spectra. The results from chemical shift data appear to be least reliable, because of molecular association effects. Also, experimental errors are compounded when second-order perturbations prevent measuring 〈v〉 or 〈J〉 directly as a splitting in the spectrum. The ΔE's in new cases appear compatible with previous results in their dependence upon steric factors and electric dipole interactions.

The vicinal coupling constants obtained for the individual rotamers are alike for H–H, H–F, and F–F in that they are smaller in magnitude when the nuclei are gauche to one another, i.e., | Jg | &lt; | Jt |, and there are instances in which Jg and Jt are of opposite as well as of like sign. JgemHF and JtHF are of the same sign. 〈JgemHF〉 and 〈JgemFF〉 have temperature independent values of 49.1±0.2 and 166.8±0.5 cps in CHCl2CHFCl and CF2BrCFBrCl, respectively. Other numerical values found are, for H–H: Jg ±2.0 to ±2.6 (4), Jt 10.2 to 16.5 (3); for H–F: Jg ±13.2 to ±2.8 (3), Jt 37.3 to 18.2 (2); and for F–F: Jg ±5.3 to ±21.2 (5), Jt 38.7 to 41.6 (3). All are in cps with the number of results given in parentheses.

The NMR methods developed appear to be very versatile in principle. With improved instrumentation, higher-quality data, and a better understanding of the factors which can affect the results, NMR may well be the best approach to ΔE for liquid or even gaseous molecules containing hydrogen or fluorine. Certainly, such studies are useful in establishing the dependence of NMR parameters upon rotational configuration.