HIGH-FIELD NMR-STUDIES OF TERT-BUTYL CYANIDE IN LIQUID AND SOLID-PHASES

H-1, H-2 and N-14 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 H-1 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 H-2 and N-14 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 x 10(12) 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.