14N Sites Research Articles

Measurement of the 14N quadrupole coupling constants in glycine, diglycine, triglycine, and tetraglycine and a comparison with calculation

Using the techniques of nuclear quadrupole double resonance, the pure nuclear quadrupole resonance spectrum of each 14N site in glycine, diglycine, triglycine, and tetraglycine have been measured at 77 K and assigned. The 14N quadrupole coupling constants obtained are compared with the results of ab initio calculations on the same molecules. The important role of hydrogen bonding at the NH 3 + site in the crystal in determining the electric field gradient at this nitrogen site is revealed.

The Effect of pH on solid-state 13C NMR spectra of histidine

High-resolution solid-state 13C NMR spectra of histidine powder samples prepared from solutions at several pH values near the p K a of the imidazole group are presented. These spectra demonstrate several effects due to the titration of the imidazole group. The chemical shifts for all of the carbon sites change upon titration. They are in “slow exchange” in the solid state in contrast to the “fast exchange” behavior seen in solution. Changes in the quadrupole interactions in the 14N sites occur upon titration and are observed by their effects on the resonance lineshapes of the 13C sites to which they are bonded.

14N Quadrupole coupling tensors in solid pyrazole, 1H-1,2,4-triazole, and 1H-tetrazole: Theory and experiment

New 14N quadrupole double resonance measurements of pyrazole, 1H-1,2,4-triazole and 1H-tetrazole have led to complete and self-consistent sets of frequencies at all the 14N sites, from which the quadrupole coupling constants and asymmetry parameters have been deduced. The results are compared with the calculations by ab initio SCF MO methods for a trimeric molecular unit with the same geometry as found in the solid state. As a consequence, most if not all the 14N quadrupole coupling tensors can be assigned to particular nitrogen atoms within the three molecules, and their sign and orientation determined. Large changes are observed at the hydrogen bonding NH and -N = nuclei relative to their quadrupole coupling tensors measured in the gas phase by microwave spectroscopy, which are closely paralleled by the theoretically calculated differences between trimer and monomer. They can be interpreted in terms of a simple electron transfer model where almost equal charges are transferred from 2p φ to 2p σ at the nitrogen atom of the NH group and from 2 atom.

Single crystal NQR and NMR study of (NH 4) 2InBr 6·H 2O

The 81Br NQR tensors of (NH 4) 2InBr 5·H 2O have been studied on single crystals by combining a 4π Zeeman goniometer with Fourier transform NQR. AT T=293K the coupling constants are 136.27 e 2φ ZZQh −(su81Br)/Mhz ≦ 146.14 and the asymmetry parameter 0.013 ≦ η ( 81Br) ≦ 0.133. The EFG axes φ zz( 81Br) are almost parallel to the bond directions InBr. The strong changes of φ yy( 115In) and φ zz( 115In) with temperature can be explained by assuming small changes of the In(OH 2) bond distance in the complex anion (InBr 5·H 2Cl 2⊝. The D 2O molecule within the deuterated complexes flips freely in the range studied (100 ≦ T/K ≦ 298). Small EFG's at the 14N site in the cation ND ⊝ 4 are due to the EFG of the surrounding lattice charges.

Deuterium and nitrogen-14 nuclear quadrupole resonance spectra of cytosine-amino acid complexes

The 14N and 2H nuclear quadrupole resonance spectra of cytosine complexes of the acids N-formylglycine, N-benzoylglycine and N-phthaloylglutamic, and of the cytidine complex of salicylic acid have been obtained at 77 K on powdered samples, using the double resonance level crossing technique, DRLC. Both normal abundance and deuterium-enriched forms of the compounds were studied. Use of various assignment aids, discussed at length, make it possible to completely assign the electric field gradient parameters to all 14N and 2H sites in the compounds.

The 14N and 2H quadrupole coupling tensors in sodium nitroprusside

Measurements by straightforward FT NMR at room temperature in a field of 8.4 T are reported of the principal values and directions of the quadrupole coupling (QC) tensors at all nitrogen sites in sodium nitroprusside (SNP), Na 2Fe(CN) 5NO × 2H 20. The assignment of the measured QC tensors to the 14N sites in SNP is discussed. All 14N QC tensors in SNP are nearly axially symmetric. The unique axes of these tensors are parallel to the bond directions of the respective CN and NO ligands within the accuracy of this and the X-ray experiment by which the bond directions were established. The feasibility is discussed of measuring 14N QC tensors in the so-called new state of SNP discovered by U. Hauser and coworkers. The QC tensors at the hydrogen sites in SNP were measured using deuterated crystals of SNP. These measurements show that the water molecules in SNP undergo a flip process which at room temperature is fast on the time scale of deuteron quadrupole couplings (≈.5 μsec) whereas it is slow on this time scale at 77 K. The unique directions of the deuteron QC tensors are off the O-D bond directions established by X-ray scattering by as much as 11°. It is argued that the measurement of deuteron QC tensors provides much more accurate information about the orientation of water molecules in hydrates than can be inferred from X-ray scattering data.

Nitrogen and deuterium nuclear quadrupole resonance in lanthanum nicotinate dihydrate

Using nuclear double quadrupole resonance techniques the nuclear quadrupole resonance (NQR) spectra of the three inequivalent 14N sites and the four water 2D sites have been obtained at 77 K for lanthanum nicotinate dihydrate. The quadrupole coupling constants e2qQ/h in kHz and the anisotropy constant η are for nitrogen 4418 and 0.362, 4222 and 0.335, and 4220 and 0.327. For the deuterons they are 233.73 and 0.080, 177.47 and 0.153 for one water molecule and 219.73 and 0.098, 189.07 and 0.148 for the other. An attempt is made to extract structural data from these NQR results.

Temperature dependence of orientational ordering in the smectic H and smectic VI phases of TBBA

The temperature dependence of the bipolar orientational order parameter 〈cos 2φ〉 at the 14N sites has been determined for the smectic H and smectic VI phases of TBBA via a measurement of the 14N‐proton double resonance spectra. In the smectic H phase 〈cos 2φ〉 increases from 0.08 to 0.15 on cooling, whereas in the smectic VI phase 〈cos 2φ〉 varies between 0.85 at the high temperature end and 1 at the low temperature end of this phase. The results are interpreted in terms of a six‐well potential for the jump motion of the TBBA molecules around the long molecular axis.

High resolution 14N NMR in solids: Single crystals of ammonium hydrogen oxalate hemihydrate

Proton decoupled 14N NMR spectra of ammonium hydrogen oxalate (AHOX) single crystals have been employed to determine the electric field gradient tensor at the 14N sites. Despite the 99.4% abundance of 14N, the spectral lines are narrow (∼300 Hz) and show two inequivalent 14NH4+ ions with (e2qQ/h) =130 kHz, η=0.39 and (e2qQ/h) =350 kHz, η=0.27. On the basis of motional averaging arguments, we surmise that these nonvanishing field gradients arise primarily because of lattice distortions of the N–H distances. The results indicate that it should be relatively easy to obtain 14N spectra in single crystals, and thus to examine a number of phenomena which have heretofore been inaccessible.

Relative orientations of the 14N EFG tensors in hydrazine from pure nuclear quadrupole relaxation study

The description of the PNQR relaxation in the ν plane is extended to the case of two nuclei of spin I = 1 interchanging by a dynamic process. The theory is applied to hydrazine, in which the two inequivalent nitrogen atoms interchange sites. Spin lattice relaxation measurements on all six 14N NQR lines were done at 228°K. Three types of experiments were performed: (i) steady state continuous pulse experiments, (ii) two-pulse experiments on the same transitions, and (iii) cross relaxation experiments in which the first pulse was applied to a transition of one site and the second pulse to the corresponding transition of the other site. Detailed analysis of these experiments gave accurate results for the relative orientations of the EFG tensors of the two 14N sites. The angles between the corresponding directions are: φXX = 93°.7, φYY = 16°.7, and φZZ = 91°.0.

14N quadrupole coupling in paraelectric (NH4)2 SO4

Abstract The technique of pulsed nitrogen—proton double resonance in the rotating frame has been used to determine the electric field gradient (EFG) tensors at the 14N sites in paraelectric (NH4)2SO4. The results show that the NH+4-ions are highly distorted and that there are two chemically non-equivalent sets of 14N sites in the unit cell: (e2qQ/h)I = 117 kHz, ηI = 0.64 and (e2qQ/h)II = 91 kHz, ηII = 0.67. Each of these two sets further contains two physically non=equivalent NH+4-pairs.