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Four polymorphic forms of carbamazepine have been simultaneously investigated by 1 H NMR and 14 N NQR. The results show that the proton spin-lattice relaxation time and the 14 N NQR spectra can be used to differentiate between various polymorphic forms. Spontaneous transformations from Form II to Form III and from Form IV to Form III have been investigated through their influence on the 14 N NQR spectrum and the proton NMR signal and spin-lattice relaxation. The 14 N NQR spectra prove that in the observed polymorphic forms of carbamazepine the hydrogen bonded dimers of carbamazepine molecules are the basic elements of the crystal structure. The dimers are centrosymmetric in Forms III and IV and in metastable polymorphic form occurring during the transformation of Form IV to Form III. Two non-equivalent molecular positions are observed in Form II with the occupation ratio 1:1 and in Form I with the occupation ratio either 2:1 or 3:1. The 14 N NQR data are related to the published crystal structures. Possible reasons for the mismatch of the X-ray and NQR data for Forms I and II are discussed., Competing Interests: Declaration of competing interest Authors declare no conflict of interest., (Copyright © 2020 Elsevier Inc. All rights reserved.)

A modification of Slusher-Hahn's double resonance technique is described and experimentally tested. It is based on application of multiple frequency sweeps and can be used for a rapid location of nuclear quadrupole resonance (NQR) frequencies. The resolution of the present technique is relatively low but, when the NQR frequencies are located, it is easy to use either the Slusher-Hahn's technique or pulse NQR to determine the NQR frequencies with a higher precision., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Here we describe a method for the extraction of (14)N quadrupole parameters from a (1)H-(14)N cross-relaxation spectrum by fitting the lineshapes of the (14)N quadrupole transitions. The procedures used typically to fit quadrupole lineshapes are not directly applicable to fit the (1)H-(14)N cross-relaxation spectrum, because the presence of proton homonuclear dipolar interaction broadens the lineshapes considerably and prevents a reliable determination of Cq and η from a single lineshape. Instead, one must fit two or even three lineshapes originating from the same nitrogen site simultaneously. The problem is to identify which lineshapes belong together when many are observed due to the existence of several nitrogen sites. We solve this problem by fitting the spectrum for all possible combinations and find the best-fitting one. This combination then most likely correctly identifies lineshapes belonging to the same nitrogen site. There are two main advantages of our method compared to the typically used method, which relies only on lineshape singularities: (i) the method is "automatic" and does not require knowledge of nitrogen quadrupole parameters in similar environments to aid dip pairing and (ii) the accuracy of quadrupole parameters is better, as proton linewidth is included in the fits. We use sildenafil and sildenafil citrate as model compounds, each with six non-equivalent nitrogen sites., (Copyright © 2016 Elsevier Inc. All rights reserved.)

¹⁴N nuclear quadrupole resonance (NQR) is a promising method for the analysis of pharmaceuticals or for the detection of nitrogen based illicit compounds, but so far, the technique is still not widely used, mostly due to the very low sensitivity. This problem is already acute in the preliminary NQR stage, when a compound is being examined for the first time and the NQR frequencies are being searched for, by scanning a wide frequency range step-by-step. In the present work, we experimentally show how to increase the efficiency of this initial stage by using a combination of a wideband excitation achieved with frequency swept pulses (WURST) and a "spin-lock" state obtained with a quadrupolar-CPMG (QCPMG) sequence. In the first part we show that WURST pulses provide a much larger excitation bandwidth compared to common rectangular pulses. This increased bandwidth allows to increase the frequency step and reduces the total number of steps in a scanning stage. In the second part we show that the "spin-lock" decay time T2eff obtained with the WURST-QCPMG combination is practically identical with the T2eff obtained with the most common "spin-lock" sequence, the SLSE, despite a very different nature and length of excitation pulses. This allows for a substantial S/N increase through echo averaging in every individual step and really allows to exploit all the advantages of the wider excitation in the NQR frequency scanning stage. Our experimental results were obtained on a sample of trinitrotoluene, but identical behavior is expected for all compounds where a "spin-lock" state can be created., (Copyright © 2013 Elsevier Inc. All rights reserved.)

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., (Copyright © 2010 Elsevier Inc. All rights reserved.)

The use of high-Q probes to increase the sensitivity in NMR and NQR is a well-known technique, however very high Q values are associated with several limitations. This paper explores the (14)N NQR multipulse detection of trinitrotoluene (TNT) signal-to-noise ratio as a function of the pickup coil Q factor, with a particular emphasis on the "super-Q" regime, where probe bandwidth becomes narrower than the NQR linewidths. We have used a mixed experimental-theoretical approach to find the TNT Q-dependent signal-to-noise value which avoided the inconvenient construction of a probe at every Q. The process has been repeated for a range of excitation/detection frequencies and a 2D sensitivity map was obtained. Our analysis suggests, that sensitivity is maximum and practically Q-independent when 400

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.

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.

The resonance frequency-space and the frequency gradient-space relations are evaluated analytically for the static fringe magnetic field of superconducting magnets used in the NMR diffusion measurements. The model takes into account the actual design of the high-homogeneity magnet coil system that consists of the main coil and the cryoshim coils and enables a precise calibration of the on-axis magnetic field gradient and the resonance frequency inside and outside of the superconducting coil., (Copyright 2001 Academic Press.)

Liquid crystals confined into small cavities are known to have a weak orientational order even above the nematic-isotropic transition temperature. The surface-induced order and molecular dynamics in this temperature range are studied with the aid of deuteron NMR spectra, spin relaxation times T(1) and T(2,) proton dipolar-correlation effect, and direct measurements of the effective diffusion coefficient for the liquid crystal 5CB confined to controlled-pore glasses. Our results show that an arrangement of molecules parallel to the wall is induced by local molecular interactions between the liquid crystal and solid, resulting in a weak and temperature independent surface order parameter, S(0) approximately 0.02 +/- 0.01. There is no indication of a significant slowing-down of molecular diffusion at the wall, neither rotational nor translational. In cavities of nanometer size, where the nematic order evolves gradually upon cooling, a broadening of the NMR linewidths due to dynamic effects should be taken into account.