Alan Gregorovič

TNT detection with 14N NQR: Multipulse sequences and matched filter

Nuclear quadrupole resonance (NQR) has a distinct potential to verify the presence of nitrogen bearing substances based on the unequivocal signatures of their spectra. Therefore, this technique is especially suitable for remote detection of illicit substances and explosives. Unfortunately, the inherent signal-to-noise of the most abundant explosive trinitrotoluene (TNT) is very low. Here we present an NQR method with improved sensitivity for estimation of the probability of TNT presence in the investigated object. The method consists of a spin-lock spin-echo (SLSE) multipulse sequence for signal excitation and a time domain matched filter for signal detection. We find that the signal-to-noise increases by shortening the pulse spacings, even though this means a decrease in spectral resolution. In our case, the decrease of the pulse spacings from the typical 2 ms to 540 micros resulted in an increase of the signal-to-noise by 14 dB. A theory describing this enhancement is presented and compared to experimental results on TNT. Issues related to temperature and polymorphism variations are also discussed.

Relaxation during spin-lock spin-echo pulse sequence in N14 nuclear quadrupole resonance

In this work, we investigate off-resonance effect on the (14)N nuclear quadrupole resonance magnetization decay during the spin-lock spin-echo pulse sequence (SLSE). The compound chosen for this study is paranitrotoluene with a single (14)N site, which represents a suitable simplified model for the explosive trinitrotoluene with six nonequivalent (14)N sites. We find that the quasi-steady state magnetization exhibits dips at particular frequency offsets and more interestingly that its decay rate T(2 eff) (-1) exhibits similar dips (slower decay) at the same frequency offsets. The coexistence of dips is very important for applications where the primary use of the SLSE sequence is to increase the signal-to-noise ratio, as longer sampling times compensate for small magnetization values. A theory explaining both observations is presented which includes homonuclear dipolar interactions and spin coupling to the lattice. We show that the homonuclear dipolar interaction contributes only 20% to the total magnetization decay rate, while spin-lattice coupling is the dominant mechanism.

Improving 14N nuclear quadrupole resonance detection of trinitrotoluene using off-resonance effects

The off-resonance dependence of the amplitudes of the six dominant (14)N nuclear quadrupole resonance (NQR) lines in commercial polymorphic trinitrotoluene (TNT) sample were experimentally determined for a wide range of experimental parameters when irradiated with the spin-lock spin-echo (SLSE) pulse sequence. We find that the amplitudes off-resonance dependence follows a sinc-like function with an additional modulation due to the spacing between the RF pulses. This dependence can be very well modeled with expressions we have derived for a single site (14)N NQR in paranitrotoluene (PNT). The results can be immediately used for the reduction of the number of free parameters used in the robust signal processing models for the TNT NQR detectors.

Polymorphism and disorder in natural active ingredients. Low and high-temperature phases of anhydrous caffeine: Spectroscopic (1H–14N NMR–NQR/14N NQR) and solid-state computational modelling (DFT/QTAIM/RDS) study

The polymorphism of anhydrous caffeine (1,3,7-trimethylxanthine; 1,3,7-trimethyl-1H-purine-2,6-(3H,7H)-dione) has been studied by (1)H-(14)N NMR-NQR (Nuclear Magnetic Resonance-Nuclear Quadrupole Resonance) double resonance and pure (14)N NQR (Nuclear Quadrupole Resonance) followed by computational modelling (Density Functional Theory, supplemented Quantum Theory of Atoms in Molecules with Reduced Density Gradient) in solid state. For two stable (phase II, form β) and metastable (phase I, form α) polymorphs the complete NQR spectra consisting of 12 lines were recorded. The assignment of signals detected in experiment to particular nitrogen sites was verified with the help of DFT. The shifts of the NQR frequencies, quadrupole coupling constants and asymmetry parameters at each nitrogen site due to polymorphic transition were evaluated. The strongest shifts were observed at N(3) site, while the smallest at N(9) site. The commercial pharmaceutical sample was found to contain approximately 20-25% of phase I and 75-80% of phase II. The orientational disorder in phase II with a local molecular arrangement mimics that in phase I. Substantial differences in the intermolecular interaction phases I and II of caffeine were analysed using computational (DFT/QTAIM/RDS) approach. The analysis of local environment of each nitrogen nucleus permitted drawing some conclusions on the topology of interactions in both polymorphs. For the most stable orientations in phase I and phase II the maps of the principal component qz of EFG tensor and its asymmetry parameter at each point of the molecular system were calculated and visualized. The relevant maps calculated for both phases I and II indicates small variation in electrostatic potential upon phase change. Small differences between packings in phases slightly disturb the neighbourhood of the N(1) and N(7) nitrogens, thus are meaningless from the biological point of view. The composition of two phases in pharmaceutical material should not be any obstacle, which is relevant from the pharmaceutical industry point of view.

Relaxor Ferroelectrics: Coupled Pseudospin-Phonon Model and the Pressure Temperature Phase Diagram

A coupled spherical random bond—random field pseudospin phonon model is presented, which explains the effects of pressure on the relative stability of the ferroelectric, relaxor-, and paraelectric-like phases and thus the temperature-pressure phase diagram of relaxor systems. Within this model we derived an expression for the dynamic susceptibility of relaxors, which describes the pressure and temperature dependence of the peak in the dynamic dielectric permittivity. The NMR spectral line shapes of PMN as well agree with the proposed model.

45Sc and 93Nb NMR in relaxors

Abstract The narrow components in the 93Nb(I = 9/2) and 45Sc(I = 7/2) NMR spectra of single crystalline PMN and disordered single crystalline PST are due to the 1/2 → -1/2 NMR transition whereas the broad background components are due to satellite transitions. The macroscopic cubic symmetry is locally broken on the NMR time scale. From the NMR point of view, the structure is homogeneous and there is no need to invoke the existence of “pinned” chemical clusters. The observed NMR spectra in PMN and PST can be described in terms of reorientable dynamic polar nanoclusters-mainly due to Pb2+ site disorder-as invoked by the spherical random bond-random field model.

Polarization Enhanced Nqr Detection at Low Frequencies

In this contribution we present our current research on polarization enhanced nuclear quadrupole resonance (NQR) detection at low frequencies with the emphasis on 14 N NQR trinitrotoluene (TNT) detection at room temperature. Combination of proton-nitrogen level crossing polarization transfer and pulsed spin-locking sequence makes 14 N NQR in TNT fast and sensitive enough to be used in routine detection of explosives. Enhancement factors for 14 N NQR lines in TNT were calculated and compared with experimental values. Good agreement between measured and calculated signal enhancement factors was observed. 14 N NQR signals in a 15 g trinitrotoluene sample of predominantly monoclinic modification were measured in 15 s in different polarization magnetic fields. The conditions for optimal polarization enhancement were determined. Introduction Nuclear Quadrupole Resonance (NQR), with its ability to identify specific molecules, is potentially a powerful method in solid state physics, chemistry and pharmacy. In the last 10 to 15 years, several attempts have been made to improve the detection of military explosives, improvised explosive devices (IED) and other illicit materials – mainly narcotics by 14 N NQR [1-9]. Unfortunately, many of these substances have 14 N NQR frequencies in the low frequency domain between 100 and 1000 kHz, hence a rather low signal to noise (s/n) ratio. Therefore, the measuring times for the required signal averaging can be hours and they are thus too long for practical applications. With a combination of proton polarization transfer to nitrogen nuclei and multi-pulse spin-locking sequences the measuring time can be significantly reduced to an acceptable level of the order of 10 s, provided the proton and the nitrogen spin-lattice relaxation times (T1) are suitable. There are two ways to increase the s/n ratio by proton-nitrogen level crossing polarization transfer: a) proton-nitrogen nuclear double resonance techniques [10-13] using changes in the proton NMR signal as an indirect indication of the 14 N NQR transitions; and b) direct 14 N NQR detection where the signal is enhanced by proton polarization transfer via proton-nitrogen level crossing in a time variable magnetic field [14-19]. The first technique requires a homogeneous applied external magnetic field and is therefore not convenient for work in the

14 N NUCLEAR QUADRUPOLE RESONANCE SIGNALS IN PARANITROTOLUENE AND TRINITROTOLUENE. SPIN-LOCK SPIN-ECHO OFF-RESONANCE EFFECTS

A simple, yet effective technique to enhance the 14 N NQR tri- nitrotoluene notoriously low sensitivity is the use of multipulse sequences. Here we investigate the off-resonance effects of the Spin-Lock Spin-Echo multipulse sequence, a predecessor of many advanced pulse sequences used for the same enhancement. Two samples have been used: paranitrotoluene, with a single 14 N site as a model compound for trinitrotoluene, and tri- nitrotoluene itself, with six 14 N sites. Our main focus has been the irradiation frequency dependence of the NQR signal, which is important when 14 N NQR is used for remote detection of explosives. The two related principal issues are: the target temperature uncertainty and the existence of multiplets with several closely spaced resonance frequencies. The first applies to any explosive, since in remote detection the temperature is only approximately known, whereas the second applies mainly to trinitrotoluene, with 12 reson- ance frequencies between 837 and 871 kHz. Our frequency dependent investigation shows that the signal intensity as well as the effective spin- spin relaxation time varies substantially with irradiation frequency in both samples. We provide a theoretical explanation of this variation which describes very well the observations and can be useful for increasing the reliability of remote detection signal processing.

Giant Electrostriction in Relaxor Polymers

We present a theoretical description of organic relaxor polymers within the framework of the generalized spherical random bond−random field model of relaxor ferroelectrics. The value of the hydrostatic electrostriction coefficient is estimated using the model parameters as well as the experimental values for the pressure derivative of the effective Curie temperature and the bulk modulus, and is found to be in reasonable agreement with experiments.

Dynamics of relaxors

A dynamic spherical random bond-random field (SRBRF) model based on the coupled polar cluster picture is formulated. Assuming stochastic flips of the cluster polarization, the Langevin equations of motion are written down, from which the linear and nonlinear dynamic dielectric permittivities are derived. A coupled SRBRF pseudospin phonon Hamiltonian is presented, which models the effects of pressure on the relative stability of the ferroelectric, relaxor-, and paraelectric- like phases and thus the temperature-pressure phase diagram of relaxor systems.