STRUCTURAL PHASE-TRANSITION OF N,N-DIMETHYLNITRAMINE AS STUDIED BY N-14 NUCLEAR-QUADRUPOLE RESONANCE

The N-14 nuclear quadrupole relaxation time T1Q and resonance frequenc ies nu+, nu-, and nu(d) of N,N-dimethylnitramine were measured between 14 and 313 K. The frequency increase of nu+ and nu-observed below T(c ) (108 K) is explained by the increase of p pi electrons in the amino nitrogen caused by the deformation of the molecular skelton, which is planar above T(c). The sharp T1Q minima for nu+ and nu- observed at T( c) are attributed to the softening of vibration modes on both sides of T(c). The fluctuation of the nitrogen-hydrogen magnetic dipolar inter action due to CH3 reorientation was detected as a broad T1Q minimum ob served around 84 K. A logarithmic divergence of T1Q-1 observed on the high-temperature side of T(c) suggests the presence of a diffusive or overdamped-type soft mode.