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 (T_1ρ 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.