Internal Rotation in Liquid 1-Fluoro-1,1,2,2-Tetrachloroethane

Abstract

Fluorine and proton high‐resolution NMR lineshapes have been observed at 56.4 and 60 MHz, respectively, for a 50 mole % solution of 1‐fluoro‐1,1,2,2‐tetrachloroethane in carbon disulfide, between −120° and −14°C. Fluorine spin echoes were obtained at 25.27 MHz and proton spin echoes were obtained at 26.8 and 17.7 MHz. An iterative complete‐lineshape‐fitting computer program was used with the experimental lineshapes to obtain best‐fit exchange rates for interconversion of gauche and trans rotamers. A value of 18.9 ppm was found for the fluorine chemical shift from the low‐temperature high‐resolution spectrum and was used to fit the lineshapes at higher temperatures, as well as to fit the observed dependence of the fluorine relaxation rate on rf pulse spacing in the Carr—Purcell experiments. Other parameters required in the analysis, i.e., the energy difference between trans and gauche rotamers and the proton‐fluorine coupling constants in each rotamer, were obtained from the temperature dependence of the proton spectrum under conditions of complete averaging (−90°−25°C). The values obtained for the coupling constants are in good agreement with an earlier determination by the same method but over a different temperature range. At −120°C the fluorine resonance consisted of a singlet and a doublet, with splittings in satisfactory agreement with the high‐temperature results. The proton spin echoes at −112°C are sensitive to the proton‐fluorine coupling constants, and can be calculated satisfactorily with coupling constants obtained from the high‐temperature proton lineshape. Because of the large fluorine chemical shift, four orders of magnitude of the exchange rate were measurable. Exchange rates obtained from all four sets of data fall on the same, linear Arrhenius plot, with frequency factor (7.0±0.5)×10¹¹ sec⁻¹ and activation energy 7.85±0.05 kcal mole⁻¹. The frequency factor is low compared with a value computed by assuming immediate deactivation from the eclipsed transition state, but agrees with a value calculated by assuming that the molecules undergo ``free'' internal rotation before deexcitation to any one of the stable rotamers.