Magnetic field-cycling NMR and 14N, 17O quadrupole resonance in the explosive pentaerythritol tetranitrate (PETN)

The explosive pentaerythritol tetranitrate (PETN) C(CH2-O-NO2)4 has been studied by 1H NMR and 14N NQR. The 14N NQR frequency and spin-lattice relaxation time T1Q for the ν+ line have been measured at temperatures from 255 to 325 K. The 1H NMR spin-lattice relaxation time T1 has been measured at frequencies from 1.8 kHz to 40 MHz and at temperatures from 250 to 390 K. The observed variations are interpreted as due to hindered rotation of the NO2 group about the bond to the oxygen atom of the CH2-O group, which produces a transient change in the dipolar coupling of the CH2 protons, generating a step in the 1H T1 at frequencies between 2 and 100 kHz. The same mechanism could also explain the two minima observed in the temperature variation of the 14N NQR T1Q near 284 and 316 K, due in this case to the transient change in the 14N…1H dipolar interaction, the first attributed to hindered rotation of the NO2 group and the second to an increase in torsional amplitude of the NO2 group due to molecular distortion of the flexible CH2-O-NO2 chain which produces a 15% increase in the oscillational amplitude of the CH2 group. The correlation times governing the 1H T1 values are approximately 25 times longer than those governing the 14N NQR T1Q, explained by the slow spin-lattice cross-coupling between the two spin systems. At higher frequencies, the 1H T1 dispersion results show well-resolved dips between 200 and 904 kHz assigned to level crossing with 14N and weaker features between 3 and 5 MHz tentatively assigned to level crossing with 17O.