14N Nuclear Quadrupole Resonance Frequencies Research Articles

Molecular motion of imidazolium cations and phase transitions in (ImH)2KCo(CN)6 with double perovskite structure

Temperature dependence of 1H nuclear magnetic resonance spin–lattice relaxation time T1 of (ImH)2KCo(CN)6, ImH+: imidazolium cation, is reported. The asymmetric T1 minimum observed in intermediate-temperature phase between 198 and 112 K is analyzed assuming the Cole-Davidson distribution of correlation time. Temperature dependences of 14N nuclear quadrupole resonance frequencies and spin–lattice relaxation times are also reported. The phase transitions at 198 and 112 K are shown to be of second and first order, respectively. The 14N relaxation is shown to be governed by the electric field gradient fluctuation due to the in-plane reorientational motion of the imidazolium cations.

Tautomerism and Possible Polymorphism in Solid Hydroxypyridines and Pyridones Studied by 14N NQR

(14)N nuclear quadrupole resonance frequencies have been measured in solid 2-pyridone, 3-hydroxypyridine, and 4-pyridone by (1)H-(14)N nuclear quadrupole double resonance. Two slightly nonequivalent nitrogen positions are observed in solid 3-hydroxypyridine, whereas only one nitrogen position has been observed in 2-pyridone and 4-pyridone within the experimental resolution. Rather low (14)N quadrupole coupling constants in pyridones are the consequence of the delocalization of the nitrogen lone pair electrons in the aromatic rings. Two different compounds have been obtained by crystallization of 4-pyridone from ethanol in a normal and in a dry atmosphere. The compound obtained in the dry atmosphere is identical to the commercial sample. The compound obtained in the normal atmosphere cannot be converted to the commercial polymorph by melting. It is thus not a polymorph of anhydrous 4-pyridone. The temperature coefficient of the (14)N quadrupole coupling constant is negative in 3-hydroxypyridine and positive in 2- and 4-pyridone. Therefore, in 3-hydroxypyridine, molecular librations dominate the temperature variation of the quadrupole coupling constant, whereas in 2- and 4-pyridone, the changes in the hydrogen bonding interactions with temperature seem to give the dominant effect.

Nuclear quadrupole resonance characterization of carbamazepine cocrystals

Nuclear quadrupole resonance (NQR) is used as a method for the characterization of cocrystals and crystal polymorphs. 14N NQR spectra of several cocrystals of carbamazepine have been measured together with the 14N NQR spectra of cocrystal formers. The results show that the 14N NQR spectrum of a cocrystal and the 14N NQR spectra of cocrystal formers differ well outside the experimental resolution. It is further described how the NQDR techniques, that have been used to measure the 14N NQR frequencies, can be used to check the homogeneity of a polycrystalline sample and to monitor the stability of a metastable crystal polymorph.

New Methods for Detection of 14N NQR Frequencies

Nitrogen atoms are present in a number of solid explosives and illicit substances. The nuclear quadrupole resonance (NQR) spectra and spin–lattice relaxation of the nitrogen atomic nucleus 14N can be used to characterize these compounds and to distinguish between possible crystal polymorphs. After the characteristic 14N NQR frequencies and spin–lattice relaxation rates in a compound are determined, NQR can be used to detect this compounds and, in case of crystal polymorphs, also to determine the method of preparation. The 14N NQR frequencies and spin–lattice relaxation rates are measured either by pulse NQR or by nuclear quadrupole double resonance (NQDR) based on magnetic field cycling. Here, we discuss several 1H–14N NQDR techniques which can be used to measure the 14N NQR frequencies and spin–lattice relaxation rates under various experimental conditions. Some characteristic applications of these techniques are presented and discussed in details.

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.

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.

14N nuclear quadrupole resonance study of the structural phase transition in phenothiazine

14N nuclear quadrupole resonance frequencies have been measured in both the low-temperature and the high-temperature phase of phenothiazine by a 1H-14N nuclear quadrupole double-resonance technique. The proton spin-lattice relaxation time was measured in both crystallographic phases as well. The NQR data show the presence of large thermal librations of the phenothiazine molecules in both crystallographic phases. The NQR data are consistent with an order-disorder phase transition associated with the reorientations of the phenothiazine molecules around the orthorhombic b axis. The reorientation angle is at Tc equal to 9+or-2 degrees . The long proton spin-lattice relaxation times above and below Tc seem to rule out any large translation or butterfly motions associated with the phase transition.

1 4N nuclear quadrupole resonance and a b i n i t i o MO study of organic azides

An ab inito SCF-MO calculation of the electric field gradient (EFG) using a double zeta basis set with polarization is reported for N−3, HN3 NCN3, and H3CN3. A similar calculation using a Dunning set with polarization is also reported for N−3, HN3, and H3CN3. Excellent correlation was found between the calculated and the components obtained from gas phase microwave data. The calculation showed that the terminal N atom had a positive nuclear quadrupole coupling constant and its pincipal z axis was found to be perpendicular to the NNN direction in substituted azides. The changes in the charge distribution of HN3 and NCN3 obtained from the analysis of the atomic multipoles was shown to be in general agreement with the variations found in the N EFG components calculated for these azides. The 14N nuclear quadrupole resonance frequencies of the terminal and the first N atom of the azide group have been measured at 77 K for phenyl, benzyl, triphenylmethane, and tosyl azide. An analysis based on the Townes and Dailey approach of the measured NQR data showed that the terminal N atom have lost more σ than π orbital population. A similar analysis for the first N atom of azides showed strong inductive and resonance interactions with the substituents.

14N nuclear quadrupole resonance in the barbiturates

The 14N nuclear quadrupole resonance frequencies at 77 K are reported for several barbiturates. Differences between charge densities in the π and various σ orbitals at the nitrogen site are obtained. The charge distributions are correlated with various substituents, and are discussed with emphasis on the effects of phenyl substituents, hydrogen bonding, and formation of the sodium salt.

Effect of pressure on 14N nuclear quadrupole relaxation in hexamethylenetetramine

14N nuclear quadrupole resonance frequencies and spin-lattice relaxation time, T1, in hexamethylenetetramine (HMT) were measured as a function of hydrostatic pressure up to 6 katm at 257, 273, 294.5, 309, and 317°K. For the three higher isotherms T1 depends exponentially on pressure up to 3 katm. In this region T1 is determined mainly by molecular reorientation of the tetrahedral HMT molecule around its four threefold axes. This mechanism is discussed in terms of a simple activation process. The activation volume, ΔV*=23.6±1 cm3/mole, and activation enthalpy, ΔH*=17±1 kcal/mole were determined. From thermal expansion and compressibility data, the thermal pressure, Pt = T(∂p/∂T)v, of 6.2 katm and activation energy ΔE* = 12–15 kcal/mole were deduced. These values of ΔH*, ΔE*, and ΔV* are higher than the corresponding values reported by Anderson and Slichter for three other organic solids and might indicate the effect of the large number of hydrogen bonds in HMT.

14N Nuclear Quadrupole Resonance Frequencies of Several Nitrogen Containing Compounds

First Page

The chemical interpretation and the temperature dependence of the 14N nuclear quadrupole resonance of aniline and several derivatives

The 14N nuclear quadrupole resonance frequencies of aniline, o- and p-phenylene diamine, and p-chloro-, p-bromo-, and p-iodo-aniline were measured with a super-regenerative oscillator over a temperature range from 77 to 292 °K. The temperature dependence is analyzed. The chemical interpretation of the quadrupole coupling constants and asymmetry parameters is described.