Proton and fluorine nuclear magnetic resonance studies of molecular motion in dimethyl sulfoxide-boron trifluoride complex

Abstract

Proton and fluorine second moments and spin-lattice relaxation times have been studied over a wide temperature range in solid dimethyl sulfoxide-boron trifluoride (CH 3) 2SO · BF 3 and in its deuterated analog (CD 3) 2SO · BF 3. The results indicate that the two methyl groups are dynamically inequivalent within the molecule and reorient at and above 77 K about their threefold symmetry axes ( C 3) with different frequencies and with activation energies of 6.69 and 10.45 kJ/mol. The BF 3 group is rigid at 77 K, while above 135 K it reorients about its C 3 symmetry axis with an activation energy of 15.93 kJ/mol. The low-temperature fluorine relaxation is controlled by the reorientation of the methyl groups which is faster than that of BF 3. This allows one to deduce for the fluorine-proton second moment a value indicating that the BF3 group is closer to one of the two methyl groups in the proton-containing compound.