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

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Abstract

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.

Introduction

Magnetic field-cycling methods can be used to measure T1 dispersion (the variation of the NMR spin–lattice relaxation time T1 with magnetic field [1], [2], [3]). Magnetic fields can be switched on and off in times of a millisecond [1], [4] enabling T1 values as short as a few ms to be measured in very low magnetic fields [1], [4]. In addition, quadrupole interactions can be studied by the phenomenon of quadrupole dips [5]. This paper presents such a study of the explosive pentaerythritol tetranitrate (PETN) C(CH2–O–NO2)4, in which quadrupole dips from 14N and possibly 17O (in natural abundance) have been detected. The temperature dependence of the zero-field 14N NQR frequencies and T1Q relaxation times has also been measured to compare with these results. The molecular structure of PETN is shown in Fig. 1.

Section snippets

Experimental methods

The T1 dispersion measurements were conducted between 1H NMR frequencies of 1.8 kHz to 15 MHz on a fast field-cycling NMR spectrometer with a maximum magnetic field of 1 T and a field stability of 1 part in 105, which could be switched in 0.7 ms, with a settling time of between 2 and 4 ms depending on the field value [4]. The high field (42 MHz) NMR coil was a solenoid consisting of eight turns of copper wire wound on a former of 10 mm diameter and 10 mm length, across which the field homogeneity was

The temperature variation of the 14N NQR ν+ line frequency

14N quadrupole resonance frequencies are already known for PETN at ambient temperatures, with ν+ = 890 and ν = 495 kHz [10]. Only one set of 14N NQR frequencies is observed as predicted by the symmetry of the molecules in the crystal, which sit on fourfold inversion axes [6], [7]. We have measured the temperature variation of ν+ line frequency, and the results are plotted in Fig. 3.

The temperature variation has two approximately linear regions indicated by the two linear fits in Fig. 3. The first

Conclusions

The results of combined 14N NQR measurements and 1H T1 NMR dispersion between 1.8 kHz and 40 MHz in the explosive pentaerythritol tetranitrate (PETN) can be explained by a hindered rotation of the NO2 group about the bond to the CO oxygen atom. The resulting transient change in the 14N…1H dipole–dipole interaction produces a step in the 1H T1 dispersion between 2 and 100 kHz characterised by a correlation time approximately 25 times longer than that of the reorientational jumps of the NO2 group,

Acknowledgment

The authors thank the Defence Science and Technology Laboratory at Fort Halstead for support of this project.

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