1H-NMR, dielectric and calorimetric studies of molecular motions in m-nitroaniline crystal Original Research Article

Spin-lattice relaxation time, T1, spin-lattice relaxation time in the rotating frame, T1ρ, and the second moment of the resonance line measurements at 80 MHz and over the 190–380 K temperature range are reported for protons in the optically nonlinear material m-nitroaniline (m-NA). T1 has also been measured for samples irradiated by low energy and low intensity radiation. The real and imaginary parts of electric permittivity as well as tan δ have been recorded in the 80–380 K temperature range at frequencies ranging from 100 Hz to 1.0 MHz. DSC measurements have been performed in the 110–387 K temperature range. Two phase transitions have been found: A glassy to rotative transition at 160 K and a plastic to plastic transformation at 365 K. The main feature of the m-NA crystal is that its plasticity continues to grow as the temperature increases. The reorientations of phenyl rings, the –NH2 group proton 180° jumps, the lattice distortions caused by anisotropic thermal expansion and the cooperative reorientations of big molecular aggregates are thought to be the reasons for phase transitions and for the subsequent intermolecular charge transfer. The results are discussed with respect to optical second harmonic generation and near-IR photochemical reaction found in the m-NA crystal.