Time-Weighted-Average Approximation in the NMR Spectrum of CF2BrCFBrCl

Abstract

The spectrum of CF2BrCFBrCl has been redetermined at 150°K in CFCl3 as a solvent. The coupling constants, chemical shifts, and relative energies of the three rotamers have been determined with considerable care. Assuming that high-temperature coupling constants and chemical shifts are time-weighted averages of the coupling constants and chemical shifts of the individual rotamers, these quantities have been calculated for several high temperatures, and compared with the experimental values, which have been determined in the same solvent. All of the spectra involved are typical ABX spectra. The experimental value for ½(JAX+JBX)Av is 0.6 to 1.3 cps lower than the calculated value. Excellent agreement is obtained between the calculated and observed chemical shift between the A and B fluorine atoms. At 371°K there is a large difference between the calculated and observed chemical shifts between any other pair of fluorine nuclei, including the solvent nuclei. This difference decreases linearly with temperature and extrapolates to 0 at approximately the temperature of the low-temperature data. The cause of these discrepancies is discussed.