Proton spin-lattice relaxation study of molecular motion in tertiary-butylammonium chloride, bromide and iodide

Abstract

The spin-lattice relaxation times (T1) of the Cl–, Br– and I– salts of (CH3)3CNH+3 and (CH3)3CND+3 have been measured over a range of temperatures by pulsed proton magnetic resonance. All six salts show two T1 minima. For the (CH3)3CND+3 salts these have been interpreted in terms of C3 reorientations of methyl groups (lower temperature minimum) and of the tertiary-butyl group about its C3 axis (higher temperature minimum). The —NH+3 group relaxes most efficiently in the same temperature region as the t-butyl group which consequently modifies the higher temperature minimum in the (CH3)3CNH+3 salts, and by considering the two sets of results the —NH+3 contribution can be separated. The T1 minimum values for the methyl group reorientations are all above the calculated value and the curves show unusual broadening, both effects decreasing in the order of salts Cl–, Br– to I–. This has been interpreted in terms of non-identical environments for the three methyl groups and confirmed by spin-lattice relaxation in the rotating frame (T1ρ measurements) and far infrared spectra. Activation energies have been worked out for all three types of motion. The iodide undergoes a phase transition at higher temperatures. This was checked by differential scanning calorimetry.