J. Seliger

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.

Proton NMR study of the structural phase transitions in perovskite layer compounds: (CnH2n+1NH3)2CdCl4 and (NH3–(CH2)n–NH3) CdCl4

The temperature dependences of the second moments M2 of the proton magnetic resonance absorption spectra and of the proton spin–lattice relaxation times T1 of perovskite layer compounds (CnH2n+1NH3)2CdCl4 with n=1–3 and (NH3– (CH2)n–NH3) CdCl4 with n=2−5 have been studied together with the frequency dispersion of T1. The structural phase transitions in these compounds were found to be connected with a change in the state of motion of the alkyl or alkylene groups, i.e., by an interplay of transitions between different N–H−−−Cl hydrogen bonding schemes and the excitation of hindered rotations of the hydrocarbon chains.

Proton-14N double resonance study of the structural phase transitions in the perovskite type layer compound (CH3NH3)2CdCl4

Using a proton-nitrogen double resonance technique we have determined the quadrupole coupling of14N in the room temperature orthorhombic (Cmca), the low temperature tetragonal (P42/ncm), and the monoclinic low temperature (P21/c) phases of (CH3NH3)2CdCl4. In all these phases all nitrogens are chemically equivalent demonstrating that the disorder in the orientations and H-bonding arrangements of the CH3-NH3 groups in theC m c a andP42/ncm phases is indeed dynamic and not static. In the monoclinic phase the14N quadrupole coupling constant equalse2qQ/h=880 kHz and the asymmetry parameter isη=0.20, wherease2qQ/h=790 kHz,η=0.1 in the tetragonal low temperature phase ande2qQ/h=726 kHz,η=0.21 in the room temperature orthorhombic phase. The observed increase in the14N quadrupole coupling constant on going from the orthorhombic phase to the tetragonal low temperature phase which is coupled with a simultaneous decrease in the asymmetry parameter can be understood in terms of a partial freezing in of the dynamic disorder in the C-N bond directions whereas the14N quadrupole coupling tensor in the monoclinic phase is characteristic of a frozen in C-N bond in a deformed lattice, where the N-H — Cl bonds are of different length.

13C NMR in ferroelectric smectic liquid crystals

Abstract The temperature dependence of the 13C NMR chemical shifts has been measured in the isotropic, the smectic A and the smectic C* phases of chiral DOBAMBC (p-decyloxybenzylidene p′-amino 2-methyl butyl cinnamate) and HOBACPC (hexyloxybenzylidene p′-amino 2-chloro α propyl cinnamate). The measurements allowed for a microscopic determination of the temperature dependence of the molecular tilt angle and the dipolar ordering of the C=O groups-i.e. the in-plane spontaneous polarization-without destroying the helical structure.

Spin–lattice relaxation mechanisms in the smectic phases of TBBA

The observed frequency dispersions of the proton spin–lattice relaxation rate T−11 in the smectic phases of TBBA in the 105–108 Hz region have been analyzed in terms of the order fluctuation, self‐diffusion, and rotational contributions to T−11. In the smectic A and smectic C phases the main rate determining contributions are order fluctuations and fast self‐diffusion, whereas in the smectic H and smectic VI phases fast rotations and slow translation self‐diffusion determine T−11.

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

(1)H-(14)N 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 (14)N NQR frequencies are determined.

Nuclear Quadrupole Double Resonance Techniques for the Detection of Explosives and Drugs

Recent work of the Ljubljana group in the nuclear quadrupole double resonance detection of explosives, mines, narcotics and drugs is reviewed with particular emphasis on enhancing low-frequency nuclear quadrupole resonance signals.

13C N.M.R. and 14N N.Q.R. in ferroelectric liquid crystals Polar versus quadrupolar ordering

Abstract 13C nuclear magnetic resonance and 14N nuclear quadrupole resonance spectra of ferroelectric smectic C∗liquid crystals and their non-chiral analogues allow for a microscopic determination of the polar and quadrupolar (or bipolar) biasing of rotation around the long molecular axis as well as for a determination of the anisotropy in the fluctuations of this axis. The results show that the microscopic origin of the biquadratic coupling between the polarization and the tilt, which has been recently introduced into the extended Landau model of the SA–S∗C transition, is the quadrupolar (or bipolar) rotational bias induced by the anisotropy in the fluctuations of the long molecular axis. The tilt induced anisotropy in the fluctuations is practically identical in chiral and non-chiral smectic C phases.

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.