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 H-1 NMR and N-14 NQR. The N-14 NQR frequency and spin-lattice relaxation time T-1Q for the v(+) line have been measured at temperatures from 255 to 325 K. The H-1 NMR spin-lattice relaxation time T-1 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 H-1 T-1 at frequencies between 2 and 100 kHz. The same mechanism could also explain the two minima observed in the temperature variation of the N-14 NQR T-1Q near 284 and 316K, due in this case to the transient change in the N-14 center dot center dot center dot H-1 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 H-1 T-1 values are approximately 25 times longer than those governing the N-14 NQR TIQ, explained by the slow spin-lattice cross-coupling between the two spin systems. At higher frequencies, the H-1 T-1 dispersion results show well-resolved dips between 200 and 904 kHz assigned to level crossing with N-14 and weaker features between 3 and 5 MHz tentatively assigned to level crossing with O-17. (C) 2010 Elsevier Inc. All rights reserved.