Carbon‐13 NMR line width and relaxation studies of tert‐butyl bromide and tert‐butyl cyanide

Abstract

AbstractProton and 13C line widths and 13C spin‐lattice relaxation times (T1) were measured over a wide temperature region in liquid and solid tert‐butyl bromide (1) and tert‐butyl cyanide (2). A striking feature of the proton and 13C NMR spectra of 1 is the abrupt line narrowing at the phase I → phase II transition point. In 2, however, the line width does not vary with temperature in phase I, indicating that the translational motion is too slow to affect the line width.An analysis of spin‐lattice relaxation times in both the liquid and solid phases is reported. It is shown that 13C T1 measurements can distinguish between internal methyl group reorientation (C3 motion) and uniaxial molecular reorientation of tert‐butyl groups (C3′ motion) if these motions are independent and responsible for the T1 relaxation over a large part of the temperature region. The 13C spin‐lattice relaxation mechanism in the liquid and disordered phases of 1, and the liquid phase of 2, is governed by overall molecular tumbling with an activation energy of 8–11 kJ mol−1. The 13C T1 data show that the faster motion in the ordered phases of 1 and 2 is the C3′ reorientation with an activation energy of 15–18 kJ mol−1 while the slower motion is the C3 reorientation with an activation energy of ca 19 kJ mol−1.