Nitrogen-15 and carbon-13 spin-lattice relaxation in trans-azobenzene and n-butyl nitrite

Abstract

Spin-lattice relaxation of 15N in trans-azobenzene dissolved in CDCl 3 occurs by a mixture of spin-rotation, dipole-dipole, and chemical shift anisotropy mechanisms. Relaxation of 15N in n-butyl nitrite occurs almost entirely by the spin-(internal rotation) mechanism. In both molecules, 15N relaxation is remarkably fast ( T 1 < 50 sec) for nuclei not bonded to other magnetic nuclei. Whereas the spin-(internal rotation) mechanism maintains its exclusivity throughout the temperature range 5–80°C in n-butyl nitrite, the mix of relaxation mechanisms changes considerably for trans-azobenzene in the same temperature range. Relaxation of the 13C nuclei in both molecules occurs predominantly by the dipole-dipole mechanism. Differences in the relaxation times of the aromatic carbons indicate that molecular rotation in trans-azobenzene occurs preferentially about the long axis of the molecule.