High‐field NMR studies of tert‐butyl cyanide in liquid and solid phases

Abstract

Abstract1H, 2H and 14N NMR line widths and spin—lattice relaxation times (T1) of tert‐butyl cyanide (TBCN) were measured at a field strength of 9.4 T in the liquid and solid states. The NMR line width data reveal the existence of two crystalline phases between 160 K and the melting point (292 K). The ordered phase (solid II) is observed throughout most of the temperature region on warming (170–290 K), indicating that the disordered phase (solid I), observed between ca 260 K and the melting point, is monotropic. The 1H line width of solid II is large and almost constant between 171 and 285 K (19–16 kHz), indicating slow whole‐molecule motion. The line narrowing of the 2H and 14N resonances observed above ca 260 K is ascribed to the onset of the whole‐molecule motion, slow overall molecular tumbling and fast C4 reorientations of the dipole axis. Activation parameters for the whole‐molecule motions and internal methyl and tert‐butyl reorientations (C3 and C3 motions, respectively) were obtained from the T1 data. The activation energy for the overall tumbling motion increases from 9.6 to 31 kJ mol−1 on freezing. This motion is very fast in the liquid (1.6 × 1012 s−1 at the melting point), but too slow to affect T1 in the solid phase. However, in solid I the molecular symmetry axis is also undergoing rapid, low‐amplitude reorientations with an activation energy of 8.9 kJ mol−1. The faster motion in solid II is the uniaxial C3′ reorientations with an activation energy of 15 kJ mol−1, while the slower motion is the C3 reorientation with an activation energy of 19 kJ mol−1.