R. Osredkar

14N NQR Spectroscopy of Some Amino Acids and Nucleic Bases via Double Resonance in the Laboratory Frame

The 14N nuclear quadrupole resonance spectra and spin‐lattice relaxation times of the amino acids histidine, methionine, cystine, cysteine, and tyrosine as well as of the nucleic bases uracil, thymine, cytosine, and guanine have been determined by a 14N‐proton double resonance technique in the laboratory frame. The experiments were performed on polycrystalline samples at 77°K or above this temperature. A theoretical estimate of the sensitivity of this method for a variety of experimental conditions is as well presented. It is shown that in contrast to usual double resonance techniques the method works even in the case of a short proton spin‐lattice relaxation time if only the nitrogen relaxation time is long.

Pulsed Nitrogen—Proton Double Resonance Study of the Ferroelectric Transition in Triglycine Sulfate

The technique of nuclear magnetic pulsed double resonance in the rotating frame has been used to study the changes in the 14N quadrupole interactions in a triglycine sulfate (TGS) single crystal on going from the paraelectric to the ferroelectric phase. The 14N electric field gradient tensors have been determined above as well as below Tc. The most significant effect of the ferroelectric transition is a large change in the time averaged value of the C–N bond direction of glycine I and the onset of chemical nonequivalence of glycines II and III due to an ordering of the protons in the short O–H ··· O bond. The results show that the phase transition in TGS is of the order—disorder type as far as the glycine groups are concerned.

14N quadrupole resonance of some liquid crystalline compounds in the solid

Using proton–nitrogen double resonance and cross relaxation in the laboratory frame, the 14N NQR spectra of solid p‐azoxyanisole (PAA), diheptyloxyazoxybenzene (HOAB), ethoxybenzilidene butylaniline (EBBA), and p‐anisalazine have been measured. The room temperature 14N quadrupole coupling constant of p‐anisalazine is e2qQ/h=4410 kHz whereas the asymmetry parameter is η=0.213. In EBBA, e2qQ/h=3990 kHz and η=0.186. In PAA and HOAB there are more than two chemically nonequivalent nitrogens in the unit cell. The 14N quadrupole coupling constants lie in the range between 4320–4370 kHz (PAA) and 4160–4400 kHz (HOAB) whereas the asymmetry parameters lie between 0.37 and 0.5. A comparison of the results for solid and nematic PAA shows that the largest principal axis of the 14N electric field gradient tensor is very nearly parallel to the N=N bond and that the central part of the PAA molecules containing the N=N bonds is rigid in the solid whereas it rotates around the long molecular axis in the nematic phase.

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.

14N quadrupole resonance in polyglycine

Abstract Using proton-nitrogen double resonance in the laboratory frame, the quadrupole coupling of the peptide nitrogens in polyglycine has been determined. At 77 K, e2qQ/h = 3.097 MHz and the asymmetry parameter equals η = 0.76.

Nitrogen and chlorine NQR in paraelectric and ferroelectric tris‐sarcosine calcium chloride

The pure nuclear quadrupole resonance spectra of 14N have been measured in tris‐sarcosine calcium chloride (TSCC), deuterated TSCC, and sarcosine using nuclear double resonance in the laboratory frame. In TSCC two chemically nonequivalent nitrogen sites have been found at room temperature, one with a quadrupole coupling constant e2Qq/h =1040 kHz and an asymmetry parameter η=1 and another one with e2Qq/h = 940 kHz, η=1. The 14N results in deuterated TSCC differ from those in undeuterated TSCC by less than 10%. The corresponding 14N data for sarcosine are e2Qq/h = 1440 kHz, η=1 and e2Qq/h = 1470 kHz, η=0.79. In the ferroelectric phase at 77 °K only one nitrogen site has been found in TSCC with e2Qq/h = 1010 kHz and η=0.89. In TSCC, ν (35Cl) =2160 kHz and ν (37Cl) =1710 kHz, and deuterated TSCC 35Cl and 37Cl quadrupole resonance lines were found in the paraelectric phase, whereas in the ferroelectric phase no chlorine lines could be observed.

17O quadrupole resonance study of the ferroelectric phase transition in KH2PO4

Abstract The temperature dependence of the 17 O NQR spectra in KH 2 PO 4 has been measured using a proton- 17 O double resonance technique in the laboratory frame. The spectra are consistent with a model where the protons rapidly move above T c between the two equilibrium sites in the O-H--O bonds, whereas the motion freezes in below T c .

87Rb NMR Study of the incommensurate phase transition in Rb2ZnCl4

Abstract The temperature dependence of the 87Rb quadrupole perturbed NMR spectra and spin-lattice relaxation rates has been measured in the high temperature paraelectric (P), the intermediate incommensurate (I) and the low temperature commensurate (C) ferroelectric phase. The obtained data show the presence of commensurate domains separated by broad incommensurate domain walls in the I phase. The spin-lattice relaxation rates of the 87Rb 1/2 → -1/2 transitions are dominated by the soft mode in the P phase whereas an anomalously strong, nearly temperature independent contribution dominates T−1 1 in the I phase. This contribution which disappears on going to the ferroelectric C phase seems to represent the first direct observation of a phason in an incommensurate ferroelectric.

Double resonance study of the 17O quadrupole coupling in paraelectric and ferroelectric KH2PO4

Abstract The pure quadrupole resonance spectrum of 17 O has been measured in a 10% enriched KH 2 PO 4 sample at 190°K at at 77°K. In the paraelectric phase all oxygen sites are chemically equivalent with e 2 qQ/h = 5.16 MHz and η = 0.55, whereas there are two chemically non-equivalent 17 O sites in the ferroelectric phase: ( e 2 q Q/h) 1 = 5.96 MHz, η t = 0.72 and ( e 2 qQ/h) II = 4.85 MHz, η II = 0.18. The results can be interpreted in terms of a double minimum type O-H—O hydrogen bond potential, where the protons above T c rapidly fluctuate between the two equilibrium sites. In contrast with the neutron scattering results in KD 2 PO 4 , the present data show no evidence for any anomalous oxygen ferroelectric mode displacements in KH 2 PO 4 , which would not condense out below T c .

Pulsed double resonance study of the 14N nuclear quadrupole coupling in glycine

Abstract The technique of nuclear magnetic pulsed double resonance in the rotating frame has been used to determine the quadrupole interactions of 14N in a single crystal of glycine at 140°C. The 14N quadrupole coupling constant is found to be very small e2qQ/h = 745 kHz, whereas the asymmetry parameter is relatively large η = 0.61. The direction of the largest principal axis of the electric field gradient tensor does not coincide with the CN direction as determined by X-ray data at room temperature.