14N Nuclear Quadrupole Resonance Research Articles

Double resonance experiments in low magnetic field: Dynamic polarization of protons by 14N and measurement of low NQR frequencies

The possibilities of dynamically polarizing proton spin system via the quadrupole 14N spin system in low magnetic field are analyzed. The increase of the proton magnetization is calculated. The polarization rate of the proton spin system is related to the transition probabilities per unit time between the 14N quadrupole energy levels and proton energy levels. The experiments performed in 1,3,5-triazine confirm the results of the theoretical analysis. A new double resonance technique is proposed for the measurement of nuclear quadrupole resonance frequencies ν Q of the order of 100 kHz and lower. The technique is based on magnetic field cycling between a high and a low static magnetic field and observation of the proton NMR signal in the high magnetic field. In the low magnetic field the quadrupole nuclei and protons resonantly interact at the proton Larmor frequency ν H = ν Q/2. The quadrupole nuclei are simultaneously excited by a resonant rf magnetic field oriented along the direction of the low static magnetic field. The experimental procedure is described and the sensitivity of the new technique is estimated. Some examples of the measurement of low 14N and 2H nuclear quadrupole resonance frequencies are presented.

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 μ s 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.

14N nuclear quadrupole resonance and proton spin–lattice relaxation study of phase transition in pyridazine perchlorate

The temperature dependence of the 14N nuclear quadrupole resonance frequencies in pyridazine perchlorate has been measured by double resonance. The results show that in the low temperature phase the pyridazinium ions are static, while in the high temperature phase the ions reorient around the normal to the plane of the ring between six equivalent orientations in agreement with the X-ray data. The 14N NQR data have been obtained for the protonated nitrogen position in the pyridazine ring for the first time. Temperature and frequency dependence of the proton spin–lattice relaxation time T 1 has been measured in both crystallographic phases. The results show the ionic mobility dominates T 1 in the low temperature phase. The activation energy for the ionic mobility is 240 meV. In the high temperature phase the pyridazine ring reorientation dominates the proton spin–lattice relaxation.

Investigation of intermolecular hydrogen bond interactions in crystalline l-Cysteine by DFT calculations of the oxygen-17, nitrogen-14, and hydrogen-2 EFG tensors and AIM analysis

A systematic computational study is carried out to investigate hydrogen bond (HB) interactions in the real crystalline structures of l-Cysteine at 30 and 298 K by density functional theory (DFT) calculations of electric field gradient (EFG) tensors at the sites of O-17, N-14, and H-2 nuclei. One-molecule (monomer) and nine-molecule (cluster) models of l-Cysteine are created by available crystal coordinates at both temperatures and the EFG tensors are calculated for both models to indicate the effect of HB interactions on the tensors. The calculated EFG tensors at the level of B3LYP and B3PW91 DFT methods and 6-311++G⁎⁎ and cc-pVTZ basis sets are converted to those experimentally measurable nuclear quadrupole resonance (NQR) parameters i.e. quadrupole coupling constants (qcc) and asymmetry parameters ( η Q). The evaluated NQR parameters reveal that the EFG tensors of 17O, 14N, and 2H are influenced and show particular trends from monomer to the target molecule in the cluster due to the contribution of target molecule to classic N–H…O, and non-classic S–H…O and S–H…S types of HB interactions. On the other hand, atoms in molecules (AIM) analyses confirm the presence of HB interactions and rationalize the observed EFG trends. The results indicate different contribution of various nuclei to HB interactions in the cluster where O2 and N1 have major contributions. The EFG tensors as well as AIM analysis at the H6 site show that the N1-H6…O2 HB undergoes a significant change from 30 to 298 K where changes in other N–H…O interactions are almost negligible. There is a good agreement between the calculated 14N NQR parameters and reported experimental data.

DFT calculations of 14N, 17O, and 2H NQR parameters: Investigation of hydrogen bond interactions in sulfapyridine crystalline structure

DFT calculations of electric field gradient (EFG) tensors at the sites of 14N, 17O, and 2H nuclei are carried out to characterize the hydrogen bond (HB) interactions in the sulfapyridine crystal structure. One-molecule (monomer) and hydrogen-bonded hexameric cluster models of sulfapyridine are constructed according to available X-ray coordinates where the proton positions are optimized. Then, EFG tensors are calculated for both monomer and target molecule in the hexameric cluster of sulfapyridine to show the effect of HB interactions on the tensors. The calculated EFG tensors are converted to the experimentally measurable nuclear quadrupole resonance (NQR) parameters: quadrupole coupling constant (CQ) and asymmetry parameter (ηQ). The results reveal different contribution of various nuclei to N-H⋯N and N-H⋯O HB interactions in the cluster where the N2 and O1 have major contributions. The computations are performed with B3LYP and B3PW91 functionals DFT method and 6-311+G* and 6-311++G** standard basis sets using the Gaussian 98 package.

Special features of remote observations of weak signals from 14N nuclear quadrupole resonance

The case of excitation of nuclear quadrupole resonance (NQR) in nitrogen-containing compounds is examined for significant sample removal under conditions of strong dispersion of the registered signal. Principal limitations on the sensitivity of remote NQR observations are demonstrated. A method of registering resonant lines after fading of the main signal is suggested.

Using the Combined Approach of 35Cl NQR, 14N NQR and DFT Calculations To Study the 14N NQR Spectrum of Diazepam

The electronic structure of diazepam was analyzed by pulsed 14N nuclear quadrupole resonance (NQR) spectroscopy and density functional theory (DFT) calculations using the hybrid method B3PW91 with the 6-31G(d,p) basis set. Before searching for 14N NQR spectra, a 35Cl NQR line in diazepam was recorded to evaluate the degree of disorder in the crystal lattice and thus the chances to find much weaker NQR spectra of nitrogen-14 in this substance. In order to speed up the detection of weak NQR spectra, the spin-locked spin-echo multipulse sequence had to be applied. Despite the expected chemical inequivalence of the two nitrogen atoms in the diazepam molecule, only two resonance lines ν+ and ν− were then found for this compound at 77 K. Possible reasons for this anomaly are discussed. From the experimental frequencies, the quadrupole coupling constant, the asymmetry parameter and the principal components of the electric field gradient tensor were determined. Those parameters were compared with the results of theoretical DFT calculations showing a reasonable accuracy of the DFT model used.

Measurement of the 14N nuclear quadrupole resonance frequencies by the solid effect

1H– 14N nuclear quadrupole double resonance using magnetic field cycling between high and low magnetic field and solid effect in the low magnetic field is analyzed in details. The transition probabilities per unit time for the solid-effect transitions are calculated. The double resonance spectra are calculated in the limiting cases of fast and slow nitrogen spin–lattice relaxation. The double resonance spectra are measured in histamine and quinolinic acid. The experimental spectra are analyzed and the 14N NQR frequencies are determined.

Study of 14N NQR response to SORC pulse sequence

The behavior of nuclear quadrupole resonance (NOR) signals between RF pulses of the strong off-resonance comb (SORC) as well as the spin-locking spin-echo (SLSE) pulse sequences was studied as for 14N NOR line ν + of dimethylnitramine (CH3)2NNO2 at 77 K. The periodic variation of the signal amplitude observed by using SORC pulse sequence could be reasonably explained by the theoretical expression reported in the literature.

14N NQR study of nicotinamide and related compounds

14N nuclear quadrupole resonance (NQR) frequencies have been measured in picolinamide, nicotinamide, isonicotinamide, 2,6-pyridine dicarboxamide, and acetamide by double resonance. The 14N NQR spectra in picolinamide, nicotinamide, isonicotinamide, and 2,6-pyridine dicarboxamide show the presence of two distinct nitrogen positions: the ring position with the quadrupole coupling constant about 4,5 MHz and the amide position with the quadrupole coupling constant about 2.6 MHz. The NQR data are related to the structure of the investigated compounds and to the N--H...O hydrogen bonds.

Nuclear quadrupole resonance (NQR) enhancement by polarization transfer and its limitation due to relaxation

Aiming for polarization transfer enhancement of 14N nuclear quadrupole resonance (NQR) for the detection of explosives with low NQR frequencies, we examine the potential and limitations of this method. As illustrative sample materials two non-explosive compounds, urotropine (C6H12N4) and urea (CON2H4) with NQR frequencies of 3.3 MHz and 2.8 MHz, respectively, have been chosen. In both substances the NQR signal can be easily seen. In urotropine no signal enhancement has been detected. The reason is a 14N spin-lattice relaxation time being much shorter than the 1H–14N polarization transfer time. Although in urea the signal enhancement is significant there is, because of the long 1H polarization time, still no effective gain as compared with the pure NQR signal accumulated during the same time interval. To estimate the expected NQR signal enhancement, a polarization enhancement factor has been derived in terms of a simplified theoretical treatment, neglecting spin-lattice relaxation. The substantial influence of relaxation effects on the signal enhancement has been discussed in a qualitative manner in connection with the experiments performed for urea and urotropine.

Density functional calculations of 14N and 11B NQR parameters in the H-capped (6,0) and (4,4) single-walled BN nanotubes

Density functional theory (DFT) calculations were performed to calculate nitrogen-14 and boron-11 nuclear quadrupole resonance (NQR) spectroscopy parameters in the representative considered models of zigzag and armchair boron nitride nanotubes (BNNTs) for the first time. The considered models consisting of 1 nm length of H-capped (6,0) and (4,4) single-walled BNNT were first allowed to fully relax and then the NQR calculations were performed on the geometrically optimized models. The evaluated nuclear quadrupole coupling constants and asymmetry parameters for the mentioned nuclei reveal that the considered models can be divided into four layers of nuclei with an equivalent electrostatic environment where those nuclei at the ends of tubes have a very strong electrostatic environment compared to the other nuclei along the length of tubes. Those nuclei at the center of the tube length also have an equivalent electrostatic environment. The calculations were performed based on the B3LYP DFT method and 6-311G** and 6-311++G** standard basis sets using the Gaussian 98 package of program.

The 14N quadrupole coupling in hexamethylene triperoxide diamine (HMTD)

Using high-field NMR, we have determined the magnitude of the nuclear quadrupole interaction in hexamethylene triperoxide diamine (HMTD), the explosive allegedly used in the London bombings of July 2005. The experimental quadrupolar coupling constant, 5.334 MHz, is in good agreement with quantum chemical calculations. The predicted single zero-field transition frequency should lie in a relatively empty part of the (14)N nuclear quadrupole resonance (NQR) spectrum; the spin relaxation rate is reasonably fast.

Phase transition and temperature dependent electronic state of an organic ferroelectric,phenazine–chloranilic acid (1:1)

The isotope effect of hydrogen motion in an organic ferroelectric, phenazine (Phz)–chloranilic acid(H2ca and D2ca for normal and deuterated compounds, respectively) co-crystal, was studied by35Cl nuclear quadrupole resonance (NQR). Besides a ferroelectric transition atTc = 253 K (303 K), a neutral-to-ionic transition was found below 170 K (200 K) forPhz–H2ca (Phz–D2ca). 1H–14N nuclear quadrupole double resonance measurements were also madein order to study the temperature dependent electronic state ofPhz–(H/D)2ca. 14N NQR parameters suggested that donor orbital populations of the two nitrogen atoms in aphenazine molecule become nonequivalent (1.78 and 1.97) in the ferroelectric phase,while they are both equal to 1.89 in the paraelectric phase. In the ionic phase ofPhz–D2ca, which was obtained by cooling below 188 K, they became 1.50 and 1.95, suggesting a proton transferfrom D2ca to Phz.

A theoretical study of 17O, 14N and 2H nuclear quadrupole coupling tensors in the real crystalline structure of acetaminophen

A systematic computational investigation was carried out to characterize the 17O, 14N and 2H electric field gradient, EFG, tensors in the acetaminophen real crystalline structure. To include the hydrogen bonding effects in the calculations, the most probable interacting molecules with the target molecule in the crystalline phase were considered through the various molecular clusters. The calculations were performed with the B3LYP method and 6-311++G ∗∗ and 6-311+G ∗ standard basis sets using the Gaussian 98 suite of programs. Calculated EFG tensors were used to evaluate the 17O, 14N, and 2H nuclear quadrupole resonance, NQR, parameters in acetaminophen crystalline structure, which are in good agreement with the available experimental data. The difference between the calculated NQR parameters of the monomer and molecular clusters shows how much hydrogen bonding interactions affect the EFG tensors of each nucleus. These results indicate that both O–H⋯O and N–H⋯O hydrogen bonding have major influence on the NQR parameters. Moreover, the quantum chemical calculation indicated that the intermolecular hydrogen bonding interactions play an essential role in determining the relative orientation of quadrupole coupling principal components in the molecular frame axes.

Density Functional Theory Investigation of Hydrogen Bonding Effects on the Oxygen, Nitrogen and Hydrogen Electric Field Gradient and Chemical Shielding Tensors of Anhydrous Chitosan Crystalline Structure

A systematic computational investigation was carried out to characterize the 17O, 14N and 2H electric field gradient, EFG, as well as 17O, 15N, 13C and 1H chemical shielding tensors in the anhydrous chitosan crystalline structure. To include the hydrogen-bonding effects in the calculations, the most probable interacting molecules with the target molecule in the crystalline phase were considered through a hexameric cluster. The computations were performed with the B3LYP method and 6-311++G(d,p) and 6-31++G(d,p) standard basis sets using the Gaussian 98 suite of programs. Calculated EFG and chemical shielding tensors were used to evaluate the 17O, 14N and 2H nuclear quadrupole resonance, NQR, and 17O, 15N, 13C and 1H nuclear magnetic resonance, NMR, parameters in the hexameric cluster, which are in good agreement with the available experimental data. The difference between the calculated NQR and NMR parameters of the monomer and hexamer cluster shows how much hydrogen bonding interactions affect the EFG and chemical shielding tensors of each nucleus. These results indicate that both O(3)-H(33)...O(5-3) and N-H(22)...O(6-4) hydrogen bonding have a major influence on NQR and NMR parameters. Also, the quantum chemical calculations indicate that the intra- and intermolecular hydrogen bonding interactions play an essential role in determining the relative orientation of EFG and chemical shielding principal components in the molecular frame axes.

An ab initio quantum chemical investigation of solvent-induced effect on 14N-NQR parameters of alanine, glycine, valine, and serine using a polarizable continuum model

In this paper ab initio quantum mechanical calculations of the 14N-nuclear quadrupole resonance (NQR) parameters of alanine, glycine, valine, and serine obtained from an electric field gradient (EFG) tensor, such as quadrupole frequencies and asymmetry parameters in gas phase and different solvents, have been carried out with the Gaussian 98 software package and the solvent-induced effects on these parameters have been evaluated using density functional theory (DFT). Furthermore, direct and indirect solvent effects on asymmetry parameters have been also calculated. We determined that the NQR parameters of the nitrogen atoms of amino acids are highly sensitive to environmental effects and that the observed solvent-induced shielding variation is strongly related to the values of the dielectric constants of the solvent and whether it is protic or aprotic. For more investigation of the solvent effect, the relative energies of each amino acid in various solvents have been calculated and the graphs of the relative energies versus dielectric constants have been analyzed.

14N nuclear quadrupole resonance of picolinic, nicotinic, isonicotinic and dinicotinic acids

The 14N nuclear quadrupole resonance (NQR) quadrupole coupling tensors of picolinic, nicotinic, isonicotinic and dinicotinic acids have been determined. Two different 14N quadrupole coupling constants 1007 kHz and 4159 kHz have been observed for picolinic acid demonstrating the presence of both protonated and non-protonated nitrogen atoms in this system in the solid. Only one set of non-protonated 14N NQR lines has been observed in other pyridinecarboxylic acids demonstrating the absence of the protonated zwitter ion forms observed in picolinic acid. The non-protonated 14N quadrupole coupling constant is the highest for the non-protonated nitrogen in picolinic acid and decreases to 3774 kHz in nicotinic acid and 3570 kHz in isonicotinic acid. It is the lowest in dinicotinic acid where the corresponding 14N quadrupole coupling constant is 2794 kHz. The observed anomalous decrease in the 14N quadrupole coupling constant of dinicotinic acid with decreasing temperature is tentatively explained as reflecting the increase in the residence time of the N–H⋯O bonded proton in the potential well close to the nitrogen.

A Systematic Investigation of Hydrogen-Bonding Effects on the 17O, 14N, and 2H Nuclear Quadrupole Resonance Parameters of Anhydrous and Monohydrated Cytosine Crystalline Structures: A Density Functional Theory Study

A systematic computational study was carried out to characterize the 17O, 14N, and 2H nuclear quadrupole resonance (NQR) parameters in the anhydrous and monohydrated cytosine crystalline structures. To include the hydrogen-bonding effects in the calculations, the most probable interacting molecules with the central molecule in the crystalline phase were considered in the pentameric clusters of both structures. To calculate the parameters, couples of the methods B3LYP and B3PW91 and the basis sets 6-311++G** and CC-pVTZ were employed. The mentioned methods calculated reliable values of 17O, 14N, and 2H NQR tensors in the pentameric clusters, which are in good agreements with the experiment. The different influences of various hydrogen-bonding interactions types, N-H...N, N-H...O, and O-H...O, were observed on the 17O, 14N, and 2H NQR tensors. Lower values of quadrupole coupling constants and higher values of asymmetry parameters in the crystalline monohydrated cytosine indicate the presence of stronger hydrogen-bonding interactions in the monohydrated form rather than that of crystalline anhydrous cytosine.

14N nuclear quadrupole resonance of some sulfa drugs

The 14N nuclear quadrupole double resonance spectra of different polymorphs of sulfanilamide, sulfadiazine, sulfamerazine and sulfamethazine have been measured and the 14N quadrupole coupling tensors have been determined. The obtained 14N spectra are compared with those of other sulfa drugs like sulfathiazole. It is shown that different polymorphs can be easily discriminated. The application of this technique for non-destructive analysis, polymorph determination and quality control in the production of pharmaceuticals is stressed.