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

The explosive pentaerythritol tetranitrate (PETN) C(CH(2)-O-NO(2))(4) has been studied by (1)H NMR and (14)N NQR. The (14)N NQR frequency and spin-lattice relaxation time T(1Q) for the nu(+) line have been measured at temperatures from 255 to 325K. The (1)H NMR spin-lattice relaxation time T(1) has been measured at frequencies from 1.8kHz to 40MHz and at temperatures from 250 to 390K. The observed variations are interpreted as due to hindered rotation of the NO(2) group about the bond to the oxygen atom of the CH(2)-O group, which produces a transient change in the dipolar coupling of the CH(2) protons, generating a step in the (1)H T(1) at frequencies between 2 and 100kHz. The same mechanism could also explain the two minima observed in the temperature variation of the (14)N NQR T(1Q) near 284 and 316K, due in this case to the transient change in the (14)N...(1)H dipolar interaction, the first attributed to hindered rotation of the NO(2) group and the second to an increase in torsional amplitude of the NO(2) group due to molecular distortion of the flexible CH(2)-O-NO(2) chain which produces a 15% increase in the oscillational amplitude of the CH(2) group. The correlation times governing the (1)H T(1) values are approximately 25 times longer than those governing the (14)N NQR T(1Q), explained by the slow spin-lattice cross-coupling between the two spin systems. At higher frequencies, the (1)H T(1) dispersion results show well-resolved dips between 200 and 904kHz assigned to level crossing with (14)N and weaker features between 3 and 5MHz tentatively assigned to level crossing with (17)O.