Nuclear relaxation and molecular motion in liquid and solid arsenic trifluoride

Abstract

The ¹⁹F spin-lattice relaxation time, T ₁, has been measured as a function of temperature in both solid and liquid arsenic trifluoride at frequencies of 9 Mhz and 18 Mhz. The transverse relaxation time, T ₂, has also been measured as a function of temperature in the liquid at 18 Mhz. Pre-melting was found to occur in the solid and both T ₁ and T ₂ measurements were made in the ‘pre-melt’. It was found that three mechanisms were responsible for spin-lattice relaxation in the liquid, namely the direct dipolar interaction, the spin-rotation interaction and the scalar interaction between the ¹⁹F and ⁷⁵As nuclei. The contributions from these three interactions have been separated and the corresponding correlation times found. In the case of the scalar interaction the relevant correlation time is the spin-lattice relaxation time of the ⁷⁵As nucleus. The scalar coupling constant is found to be 1·30 khz and the spin-rotation constant is found to be 13·2 khz. The scalar interaction is the dominant mechanism responsible for transverse relaxation. The results in the solid indicate that the molecules are not free to translate but are free to reorientate.