Potential Energy as a Function of Internal Rotation in CFClBr–CFClBr by NMR Measurements

Abstract

The NMR spectrum of CFClBr–CFClBr is reported as a function of temperature from 177° to 300°K. The high-temperature spectrum is analyzed in terms of a superposition of the spectra of the meso and dl isomers, each in rapid rotation about the C–C bond; the low-temperature spectrum on the basis of a superposition of the spectra of the three rotamers of each of the two isomers. The gauche coupling constant between fluorine atoms in this compound is 21.5±0.5 cps. Chemical shifts for the fluorine atoms were different for the three rotamers. Fluorine atoms gauche to one fluorine and one chlorine atom are at the highest field, and fluorine atoms gauche to one fluorine and one bromine atom are at the lowest field. By observing the relative areas of peaks at low temperature, relative energies of the three potential minima for each isomer are calculated. High-temperature areas determined the relative amounts of the two isomers. Using, in addition, chemical shift data from the low-temperature spectra, spectra of this mixture of isomers have been calculated as a function of temperature assigning various potential barriers to internal rotation. By comparing the calculated spectra with those observed at various temperatures, several of the potential barriers to internal rotation have been assigned. In each isomer, the three potential minima vary by at most 450 cal/mole. The potential maxima which can be measured are all approximately 10.0±0.5 kcal/mole above the potential minima.