Nuclear magnetic resonance and relaxation in dl-norvaline in the solid state

Abstract

Proton magnetic resonance absorption and spin-lattice relaxation time measurements were made on solid dl-norvaline, NH 3 +CH(CH 2CH 2CH 3)COO −, in the temperature range 77 to 450 K. The absorption line measurements and second moments show that the structure is rigid on the NMR timescale at the lowest temperatures studied, while at higher temperatures the motional behavior ranges from rotation of methyl groups to composite motion involving the reorientation of the amino groups about their pseudosymmetry axes. The proton spin-lattice relaxation measurements in the Zeeman frame, T 1, reveal three distinct minima; a high-temperature minimum at 352 K due to NH 3 + motion and two closely spaced minima at 198 and 154 K, respectively, due to CH 3 motion. The appearance of the double minima is attributed to the presence of two nonequivalent methyl groups whose environments are different. Spin-lattice relaxation time measurements in the rotation frame, T 1 ϱ , taken together with the second-moment trend noted in the vicinity of room temperature, show the presence of additional molecular motion. This is also indicated by measurements of the Zeeman frame spin-lattice relaxation times for individual carbon atoms. This additional motion is thought to be due to the reorientation of the whole molecule or of the side chain. The activation energy of 36.6 kJ/mol for NH 3 + motion is characteristic of a hydrogen-bonded amino group.