Janko Lužnik

Improved N14 nuclear quadrupole resonance detection of trinitrotoluene using polarization transfer from protons to N14 nuclei

Combination of proton-nitrogen level crossing polarization transfer and pulsed spin-locking sequence makes N14 nuclear quadrupole resonance (NQR) in trinitrotoluene fast and sensitive enough to be used in routine detection of explosives. Enhancement factors for all three N14 NQR lines (the case with η≠0) were calculated and compared with experimental values. Good agreement between measured and calculated signal enhancement factors was observed. N14 NQR signals in a 15g trinitrotoluene sample of predominantly monoclinic modification were measured in 15s in different polarization magnetic fields. The conditions for optimal proton-nitrogen level crossing were determined.

14N Nuclear Quadrupole Resonance Study of Piroxicam: Confirmation of New Polymorphic Form V

A new polymorphic crystal form of piroxicam was discovered while preparing crystalline samples of piroxicam for (14) N nuclear quadrupole resonance (NQR) analysis. The new crystal form, designated as V, was prepared by evaporative recrystallization from dichloromethane. Three known polymorphic forms (I, II, and III) were also prepared. Our aim was to apply (14) N NQR to characterize the new polymorphic form of piroxicam and compare the results with those of the other known polymorphic forms. Additional analytical methods used for characterization were X-ray powder diffraction (XRPD), thermal analysis, and vibrational spectroscopy. For the first time, a complete set of nine characteristic (14) N NQR frequencies was found for each prepared polymorph of piroxicam. The consistent set of measured frequencies and calculated characteristic quadrupole parameters found for the new polymorphic form V is a convincing evidence that we are dealing with a new form. The already known piroxicam polymorphic forms were characterized similarly. The XRPD results were in accordance with the conclusions of (14) N NQR analysis. The performed study clearly demonstrates a strong potential of (14) N NQR method to be applied as a highly discriminative spectroscopic analytical tool to characterize polymorphic forms.

Magnetic properties of TDAE-C60 single crystal and powder samples: The influence of thermal annealing

Abstract Magnetization measurements in TDAE-C 60 single crystals reveal the importance of thermal annealing in these samples. Only propertly annealed single crystals and powder samples exhibit magnetism whereas freshly prepared single crystals show no magnetic signals. Extended measurements of M ( H ) and M ( T ) in TDAE-C 60 single crystals confirm the appearance of magnetic anisotropy below 16 K : the different shapes of M ( H ) signals for H∥a axis and for H∥c axis are in agreement with the previously introduced idea of canted spin orientation of unpaired electron spins on C 60 − ions in TDAE-C 60 crystals [1]. Correspondingly, the simultaneous presence of ferromagnetism along the a axis and antiferromagnetism along the c axis is possible.

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.

The radiofrequency coil design and the signal intensity for continuous-wave NQR and NMR spectrometers

The improvement in signal intensity by proper rf coil design for cw NQR and NMR spectrometers, working in the frequency interval 3 – 30 MHz, is discussed. Several sets of coils were sistematically studied in order to find the limits of the geometrical data for coils which should yield an optimal signal to noise ratio.

Capacitor-based detection of nuclear magnetization: Nuclear quadrupole resonance of surfaces

We demonstrate excitation and detection of nuclear magnetization in a nuclear quadrupole resonance (NQR) experiment with a parallel plate capacitor, where the sample is located between the two capacitor plates and not in a coil as usually. While the sensitivity of this capacitor-based detection is found lower compared to an optimal coil-based detection of the same amount of sample, it becomes comparable in the case of very thin samples and even advantageous in the proximity of conducting bodies. This capacitor-based setup may find its application in acquisition of NQR signals from the surface layers on conducting bodies or in a portable tightly integrated nuclear magnetic resonance sensor.

A study of continuous-wave two-frequency NQR

A simple continuous-wave parallel to pulse version of a multifrequency nuclear quadrupole resonance (NQR) device was constructed and tested: two orthogonal radio-frequency (RF) fields were applied simultaneously at two suitable frequencies to a multilevel quadrupole probe (I⋝1). A modified superregenerative NQR oscillator-detector (SRO) was utilized. An additional coil and RF source were used to irradiate the sample at the second frequency. After tuning the SRO to a chosen NQR line, the signal proportional to the population difference of the corresponding pair of energy levels was monitored. When another suitable transition was saturated simultaneously, sharing one level with the monitored line, the corresponding population change was also reflected in the first signal intensity change. The phenomenon was examined for Sb nuclei in Sb2S3 powder at room temperature.

Chlorine NQR of single crystal SnCl2 · 1.5 H2O

Abstract Chlorine NQR spectra revealed a new crystallohydrate of SnCl2, SnCl2 · 1.5 H2O. In a single crystal the directions of SnCl chemical bonds were determined from Zeeman perturbed NQR measurements at room temperature. To get the complete crystal structure. X-ray difraction measurements were performed. The chemical bond directions obtained from Zeeman perturbed NQR and from X-ray diffraction agree very well.

Determination of a zero-field splitting parameter D in Mn12Ac below 20 K

Abstract Molecular magnet Mn12Ac has a large molecular spin S=10. The uniaxial crystalline anisotropy splits the energy levels in zero magnetic field into 10 doublets and one singlet. The simplest Hamiltonian in magnetic field Hz for this symmetry is H =−DS z 2 −gμ B μ 0 HS z . The zero-field splitting parameter D has been determined from measured M(H) in weak magnetic field at different temperatures. The approximate value is D=0.7 K .