Molecular Motions in Plastic Phase of 2-Methyl-2-nitropropane, (CH3)3CNO2, by Pressure and Temperature Dependences of 1H-NMR Spin-Lattice Relaxation

Abstract

Abstract Proton NMR spin-lattice relaxation time, T1, in the plastic phase of 2-methyl-2-nitropropane (CH3)3CNO2 was measured at 10 MHz up to 150 MPa between 262 and 320 K. The overall molecular reorientation with an activation enthalpy of ΔHr*=6.8±1.0 kJ mol−1 and an activation volume of ΔVr*=(0.07±0.02)Vm did not show significant pressure dependence up to 140 MPa, where Vm is the molar volume of (CH3)3CNO2 at 273.2 K. The self-diffusion, which was treated by the Torrey’s isotropic diffusion model, showed large pressure dependence; the activation enthalpy, ΔHd* changed from 46±4 kJ mol−1 under saturated vapor pressure of the sample to 95±8 kJ mol−1 under 100 MPa and the activation volume, ΔVd* changed from Vm at 278.0 K to 0.8Vm at 320.0 K. These results strongly suggest monovacancy mechanism for self-diffusion. The phase diagram of liquid-plastic and plastic-brittle phase transitions is also given.