14N Quadrupole Resonance Research Articles

Detection of plastic explosives in luggage with14N nuclear quadrupole resonance spectroscopy

Nuclear quadrupole resonance (NQR) of14N nuclei has many advantages as a method for detecting nitrogen-containing explosives, the most important are very high chemical specificity, true noninvasive operation and detection of bulk explosive in situ only (no vapor or particular capture needed). One of the most high explosives is hexogen (RDX) often used by terrorists in plasticized forms. The ring nitrogen nuclei in an RDX molecule generate three sets of NQR frequencies corresponding to three physically nonequivalent positions of the molecule in the crystal lattice. The prototype device we have constructed is intended for inspection of suitcases for the presence of plastic explosives containing RDX or octogen by14N quadrupole resonance. It is essentially a fully automated PC-controlled pulsed FT NQR spectrometer equipped with a large volume (70 1) radio-frequency (RF) sample coil to accommodate a typical suitcase. The device consists of a measure chamber with an RF coil, tuning and matching box, an RF pulse transmitter and a control PC with dedicated cards like digital receiver, frequency synthesizer, pulse programmer and probe-tuning controller. The control software finds the NQR lines and measures their frequencies. An alarm is produced if any of these frequencies matches the characteristic NQR frequency of the explosive and the signal-tonoise ratio exceeds the preset threshold. Multipulse sequences of the type SORC (strong off-resonance comb) or SLSE (spin-locked spin echo) were used in order to increase the allowed data acquisition rate. We could detect 230 g of PMW-8, a plastic explosive (containing 81% of RDX) in 10 s or 100 g in 30 s. Detection probability was not less than 90%.

The two-frequency nuclear quadrupole resonance for explosives detection

The two-frequency nuclear quadrupole resonance (NQR) of14N nuclei is described for purposes of explosives detection. Two applications are known: two-frequency NQR for increasing the signal intensity, two-frequency NQR for improved reliability of explosives detection. The two-frequency experiments were carried out in hexahydro-1,3,5-trinitro-s-triazine C3H6N6O6 and sodium nitrite NaNO2 as a substitute for octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocycine C4H8N8O8. The two-frequency sequences for NQR are proposed for increasing the amplitude of NQR signal and improvement of detection condition.

SQUID-NQR of nitrogen-14 in amino acids and small peptides

We present a technique for the study of nuclear quadrupole resonance of 14N interacting with low electric field gradients as found, for example, in amino acids. A superconducting quantum interference device (SQUID) is used to detect directly small NMR signals via cross relaxation of the 14N polarization to adjacent proton spins, eliminating the need for field cycling. When one nitrogen quadrupolar transition matches the proton Zeeman splitting, the remaining two quadrupolar transitions can be observed by sweeping a saturating rf field through resonance. In addition, signal enhancement by simultaneous excitation of two nitrogen resonances helps to identify connected transitions.

Nuclear quadrupole resonance of 14N and 2H in pyrimidines, purines, and their nucleosides

Using nuclear quadrupole double-resonance techniques, nitrogen-14 and deuterium nuclear quadrupole coupling constants and asymmetry parameters have been measured in uracil, 5-bromouracil, cytosine, adenine, xanthine, hypoxanthine, their nucleosides, 2-aminopyrimidine, and benzimidazole. Zeeman studies and the detection of the simultaneous transitions of neighboring nuclei allowed in many cases a complete assignment of the observed spectral lines to particular 14N and 2D sites.

Nuclear Quadrupole Resonance of Nitrogen-14 in Potassium Hexathiocyanatoplatinate(IV) and Tetrathiocyanatomercurate(II)

The nuclear quadrupole resonance of 14N was observed in K2Pt(SCN)6 and K2Hg(SCN)4 at various temperatures. Both compounds yield large asymmetry parameters indicating the existence of asymmetric π-electron distribution about a CN bond axis. The ionic character of metal-ligand bonds in the platinum and mercury complexes is estimated to be 28% and 58%, respectively. The temperature dependence of each component of quadrupole coupling can practically be interpreted in terms of a simple Bayer theory. Both complexes show the quadrupole coupling constant versus temperature curve faintly convex to the abscissa. This anomaly is attributable to vibronic interaction in a thiocyanate group

Lineshape and temperature dependence of the nuclear quadrupole resonance of 14N 2 in β-quinone clathrate

Nuclear quadrupole resonance of 14N 2 molecules entrapped in β hydroquinone clathrate has been studied using Fourier transform spectroscopy. The spectrum at 4.2 K spans the frequency range 3.577 to 3.596 MHz and consists of three well-solved major components, each showing partially resolved fine structure. A consistent interpretation of intensities and frequencies is given by postulating that an N 2 molecule introduced into a clathrate cavity produces a slight lattice distortion extending to nearest-neighbor cavities arrayed along the crystallographic c axis, thereby affecting the electric field gradient. A one-to-one correlation is established among the three major components of the spectrum and the possible configurations for occupancy of nearest-neighbor cavities. Fine structure of the low- and high-frequency components is due to intramolecular dipolar interaction, whereas the middle component additionally exhibits a Stark splitting due to an asymmetric cavity environment. From the temperature dependence of the resonance frequency the molecular libration frequency in the cavity is deduced to be 50.7 cm −1 at 0 K, the zero-point rms amplitude is 11.3°, and the static resonance frequency is 4.025 MHz. The libration frequency decreases by about 4% on warming to 80 K.

Nuclear quadrupole resonance of 14N in imidazole and related compounds

Using the technique of nuclear double resonance with spin mixing by level crossing, the nuclear quadrupole resonance of 14N has been measured in L-histidine, D-L-histidine, L-histidine hydrochloride monohydrate, sodium urocanate and urocanic acid at 77 K. The nuclear quadrupole resonance of both 14N and 2D has been measured in deuterated imidazole at 77 K. The previously measured spectra for normal imidazole have been positively assigned with the electric field gradient parameters for the tri-coordinated nitrogen site given by e 2 qQ/h = 1424 kHz and η = 0.980, and for the di-coordinated nitrogen site by e 2 qQ/h = 3267 kHz and η = 0.129.

Nuclear Quadrupole resonance of 14N in potassium tetracy anoquinodimethanide

The nitrogen-14 nuclear quadrupole resonance spectrum of the potassium salt of 7, 7′, 8, 8′-tetracyanoquinodimethan (K-TCNQ) as found at liquid nitrogen temperature is presented. The data are interpreted in terms of the Townes and Dailey theory and compared with those available for neutral TCNQ 0. The results indicate an excess charge density of 0.13 electron per nitrogen in general agreement with the charge distribution calculated for the TCNQ − ion. The 16 line spectrum demonstrates the existence of eight nonequivalent nitrogen sites in the K-TCNQ crystal. The asymmetry parameters range from 45.3 to 58.7%.

Pure quadrupole resonance of 14N in a tetrahedral environment

The pure quadrupole resonance of 14N has been measured in several compounds with nitrogen in a tetrahedrally bonded environment. In l-proline, dimethyl- and diethylammonium chloride the value of the anisotropy constant η is close to its maximum possible value of unity, whilst in methyl- and ethylammonium chloride the value of η is close to zero. These results are in good agreement with those predicted by a simple theoretical model. The model is used to discuss the structure of glycine and l-serine from their measured quadrupole spectra. Satellite lines are observed at each side of a quadrupole resonance line due to a simultaneous flip of a nitrogen nucleus and an adjacent proton spin as in the solid state effect. The derivation of structural information from such satellite lines is discussed.

Nuclear quadrupole resonance of nitrogen-14 in some primary aliphatic amines

The nuclear quadrupole resonance of 14N in ethylamine, n-propylamine, isopropylamine, n-butylamine, and tert-butylamine were observed at liquid nitrogen temperature. For ethylamine, the temperature dependence of resonance frequencies was measured, and a phase transition was found at 184°K. The observed frequencies, ν I and ν II, yield almost the same quadrupole coupling constant and asymmetry parameter regardless of the kind of alkyl groups. The ionic character of NH and NC bonds was estimated as 0.42 and 0.27, respectively.

Nuclear quadrupole resonance of 14N in pyrimidines, purines and their nucleosides

Pure quadrupole resonance of 14N has been observed in uracil, 5-bromouracil, cytosine, adenine, xanthine, hypoxanthine, their corresponding nucleosides, and some related model compounds. A double nuclear resonance technique was employed. The values of the electric field gradient and the anisotropy constant have been calculated for five distinct nitrogen sites in the series and the results obtained are shown to be consistent with molecular orbital calculations on these compounds. The tautomeric form is easily deduced from the data and is invariably amino and keto.

Temperature dependence of dipolar spin-echo envelope modulation in solid α-nitrogen

The temperature dependence of the intramolecular dipolar splitting of the nuclear quadrupole resonance of 14N has been studied in solid αnitrogen from 4.2 to 35.6°K by observing the modulation of the envelope of the spin echoes. Over this temperature range the modulation period increases by 20% and this is ascribed to progressive motional averaging of the dipolar interaction by the molecular librations. The averaging factor is identical within 2 % with that for the quadrupole resonance frequency and is satisfactorily explained theoretically using published Raman data on the libration frequencies.

Stark effect on the nuclear quadrupole resonance of 14n in an asymmetric field gradient : I. Line shape

Nuclear quadrupole resonance line shapes in the presence of the Stark effect were studied in the case of asymmetric field gradients, where the exciting magnetic field forms an arbitrary angle with the electric field. This study was performed in order to interpret 14N resonance results but it can also be extended to other nuclei.

Stark effect on the nuclear quadrupole resonance of 14N in an asymmetric field gradient : II. Observations

The Stark effect on the nuclear quadrupole resonance of 14N was observed and measured. The materials studied were acetonitrile, pyridine, 2-, 3- and 4-methylpyridine, 3,5-dimethylpyridine and 2-chloropyridine. The experimental apparatus and high voltage cell are described.

Stark effect on the nuclear quadrupole resonance of 14N and the ferroelectricity of thiourea

The increase in the nuclear quadrupole coupling constant and in the asymmetry parameter of 14N brought about by the electronic polarization in the ferroelectric phase of thiourea is discussed, as well as the influence of the local electric field upon the splitting of the transition and their temperature dependence.

Nuclear quadrupole resonance of 14N in hydrazine derivatives

The nuclear quadrupole resonance of nitrogen in some hydrazine derivatives was observed at liquid nitrogen temperature. From the observed NQR frequencies, the quadrupole coupling constants and the asymmetry parameters were calculated for chemically nonequivalent nitrogen atoms. The ionic character of NH and NC bonds was estimated. The atomic quadrupole coupling constant of nitrogen was evaluated as 11.3 MHz. Very large asymmetry parameters were explained in terms of the nature of NN bonds.

Erratum: Nuclear Quadrupole Resonance of 14N of the CN Group in Some para-Substituted Benzonitriles

The NQR of 14N of the –CN group in some benzonitriles with electron‐releasing as well as electron‐withdrawing para‐substituent groups, has been studied. The experimentally obtained values of (e2qQ)z, the quadrupole coupling constant, and of Γ, the asymmetry parameter of the electric field gradient, are discussed on the basis of the Townes and Dailey theory. The discussion may be developed further, with some assumptions, to obtain the electron populations of the two p‐type orbitals describing the π bonds between the nitrogen and the carbon atoms of the cyano group. The variations of such electron populations when changing the para substituent is obtained. A good correlation was also found between the values of the electron populations of the p‐type orbital orthogonal to the benzene ring and the variations of the electron distribution on the ring, as described by the Hammett σ values for the corresponding para‐substituent groups.

Nuclear Quadrupole Resonance of 14N of the –CN Group in Some para-Substituted Benzonitriles

The NQR of 14N of the –CN group in some benzonitriles with electron-releasing as well as electron-withdrawing para-substituent groups, has been studied. The experimentally obtained values of (e2qQ)z, the quadrupole coupling constant, and of Γ, the asymmetry parameter of the electric field gradient, are discussed on the basis of the Townes and Dailey theory. The discussion may be developed further, with some assumptions, to obtain the electron populations of the two p-type orbitals describing the π bonds between the nitrogen and the carbon atoms of the cyano group. The variations of such electron populations when changing the para substituent is obtained. A good correlation was also found between the values of the electron populations of the p-type orbital orthogonal to the benzene ring and the variations of the electron distribution on the ring, as described by the Hammett σ values for the corresponding para-substituent groups.

Resolved dipolar splittings of the nuclear quadrupole resonance of 14N in Solid nitrogen

Dipolar splittings of the NQR of 14N in 14N- 14N and 14N- 15N molecules have been resolved in carefully annealed samples of α-N 2. The quadrupole coupling constant is negative. Values for the isotope splitting and motional averaging are reported.

Relaxation Times and Saturation of Nuclear Quadrupole Resonance of 14N in Asymmetric Field Gradients

Relaxation Times and Saturation of Nuclear Quadrupole Resonance of 14N in Asymmetric Field Gradients