The relaxation time of the fluorine nuclei in benzotrifluoride has been measured and is found to be an only slightly varying function of temperature, ranging from 3.2 sec at minus;20deg;C to 2.4 sec at +50deg;C. The contributions to the relaxation time from the dipolemdash;dipole and spinmdash;rotation interactions have been separately calculated, and the effective spinmdash;rotation coupling constant is calculated to be 25plusmn;2 kc/sec. This coupling constant has been used to calculate the paramagnetic contribution to the chemical shift of the fluorine in C7H5F3relative to F2, and yields the value, 453 ppm, which is within 10 of the observed value or 493.3 ppm for the chemical shift.
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