19F spectroscopy and relaxation behavior of trifluorovinyldichloroborane

The 19F NMR spectra and spin-lattice relaxation rate, R1, of trifluorovinyldichloroborane as shown in were studied as a function of temperature, T, and magnetic field, B. All logR1 vs 1/T plots show a minimum at 299K indicating the presence if dipolar relaxation at lower T and spin-rotation relaxation at higher T. The R1 values increase with increasing B due to chemical shift anisotropy relaxation. Estimates of the fluorine chemical shift values for F3 (cf. Fig. 1) suggest that there is pi character in the F-C bond. The other two C-F bonds are largely single bonded. No evidence was found for intermolecular exchange of the trifluorovinyl group. Two of the three fluorine atoms show large increases in their NMR linewidth with increasing temperature while the third shows only a small increase but the activation energy for the process is the same for all. The increase in linewidths is due to scalar coupling to the boron atoms. The boron linewidths were measured between 253 and 363K and decreased with increasing temperature. A plot of logR2, where R2 is the linewidth of the boron as a function of 1/T shows some curvature indicating a second relaxation mechanism. This is ascribed to spin-rotation but not enough data are available to be conclusive. In all cases there is a second small set of fluorine peaks that are due to 10B interactions separated from the 11B peaks by amounts varying from 1 to 4 ppm depending on the field and fluorine atom.