14N NQR STUDIES OF STABILIZERS DPA, METHYL- AND ETHYL-CENTRALITE

1
14N NQR STUDIES OF STABILIZERS DPA, METHYL- AND
ETHYL-CENTRALITE J. Luznik, J. Pirnat T. Apih and
Z. Trontelj Institute of Mathematics, Physics and
Mechanics and Institute J. Stefan, University of
Ljubljana, Slovenia

Introduction Nuclear Quadrupole Resonance (NQR)
with its ability of identification specific
molecules in measured sample is potentially
powerful method in solid state physics, chemistry
and pharmacy. Namely, molecules and atoms with
quadrupole nuclei are in each chemical compound
in different electric environment, hence they are
characterized with different NQR transition
frequencies. Nitrogen nuclei are present in many
organic chemical compounds and thus enable their
nitrogen NQR spectroscopy studies. 14N NQR
transition frequencies for different substances
are covering a broad frequency range from around
100 kHz to about 5 MHz. Unfortunately the
nitrogen resonance frequencies of some most
interesting chemical compounds are found at the
very low frequencies. Hence, their detection is
difficult because of the very poor
signal-to-noise (S/N) ratio. The required signal
averaging and measuring times are therefore often
of the order of several hours and thus too long
for practical applications. With the recently
improved techniques (proton-nitrogen level
crossing polarization transfer combined with
proper pulsed spin-locking sequencies) the NQR of
very low frequency 14N lines in some energetic
materials and explosives became interesting for
different applications. We will demonstrate some
results of 14N NQR signal temperature dependence
and correlation between 14N NQR signal intensity
and concentration studies of stabilizers
Diphenylamine, Methyl- and Ethyl-centralite. This
kind of 14N NQR application is promissing for
studies of quality, stability and degradation of
explosives and propellants.
Figure2 Temperature dependence of 14N NQR in
Diphenylamine
Experimental 14N NQR investigations of three
widely used stabilizers Diphenylamine,
Methylcentralite and Ethylcentralite were
performed. Their chemical furmulae and molecular
structures are shown on Fig. 1. 14N NQR signals
were measured from room temperature to 77 K in
all three samples. Temperature dependences of NQR
transition frequencies in all three samples are
similar and do not show any irregularity. Fig. 2
shows the example of NQR temperature dependence
in Diphenylamine. Approximately 10 g of samples
were used in a solenoidal coil with 16 mm
diameter. Very strong and easily detectable 14N
NQR signals were obtained within the whole
measured temperature interval. Some problems were
with Diphenylamine at higher temperature (around
room temperature) where the signal intensity in
this sample is reduced. In our experiments the
multipulse pulse spin - locking (PSL) sequence
was applied a0 - ( t - a90 - t - )n , where n
refers to the number of pulse-train repetitions
and the pulse width a is chosen to optimize the
signal. Fig.3 shows a typical signal of the n
transition in Methylcentralite obtained in a
single shot experiment with averaging 100 echos
within one PSL sequence.
Figure3 14N NQR signal at 3768 kHz in
Methylcentralite at room temperature
Discussion In all three samples we were able to
get very good 14N NQR spectra. The NQR transition
frequencies for them are collected in Table 1.
The numbers of different frequencies
(corresponding to different positions of nitrogen
atoms in crystal unit cell) for all the samples
are in agreement with the known crystal
structures 1,2.
Methylcentralite C15H16N2O
Table 1 NQR frequencies in DPA, MC and EC
Ethylcentralite C17H20N2O
Stabilizers are added to explosives and
propellants in small amounts mostly less than 5
. To follow the aging and degradation of
explosives and propellants accurate determination
of these low concentrations is important. We
tried to determine the accuracy of such
measurement with diluted samples of
Methylcentralite down to 0,5 (Fig. 4).

Diphenylamine C12H11N

Figure 1 Structural and chemical formulae of
tested stabilizers
References 1 Mark A. Rodriguez and Scott D.
Bunge, Acta Cryst. E59, o1123-o1125 (2003) 2
P. Ganis, G. Avitabile, E. Benedetti, C. Pedone,
and M. Goodman, Proceedings of the National
Academy of Sciences, Vol. 67, No. 1, pp. 426-433,
September 1970
Figure 4 Intensity of the NQR signal in
Methylcentralite versus concentration