Effective Quadrupole Coupling Constants Research Articles

Nuclear quadrupole resonance in the gamma-resonance spectra of soft matter

It is shown that the harmonic librations (oscillations) of the principal axis of the electric field gradient tensor in “cages” of liquids, glasses, ferroliquids, and other “soft” systems qualitatively change the shape of the Mossbauer spectra of the quadrupole hyperfine structure. In addition to an effective decrease in the quadrupole coupling constant in the fast-libration limit, nuclear quadrupole resonance is predicted, which must be manifested in the Mossbauer spectra at the libration frequency that is approximately equal to the quadrupole splitting of spectral lines. By analogy with nuclear magnetic resonance, simple analytical expressions are derived, which describe resonance Mossbauer spectra in terms of the effective quadrupole coupling constant and the resonance splitting constant for the main lines. The observed features of the formation of quadrupole hyperfine structure spectra can be manifested in the Mossbauer spectra of soft matter and must be taken into account in analysis of experimental data.

Mössbauer Spectra of the Quadrupole Hyperfine Structure in the Presence of the Rotation of the Principal Axis of the Electric Field Gradient Tensor

It is found that the rotation of the principal axis of the electric field gradient tensor about a certain axis radically changes the shape of the Mossbauer spectra of the quadrupole hyperfine structure. This change may be a manifestation of a qualitatively different physical origin of rotational dynamics in liquids. In particular, the absorption spectra of 57Fe nuclei may contain the triplet, quartet, and octet of lines instead of the standard quadrupole doublet. Simple analytical expressions derived for a number of limiting cases describe the spectra in terms of the effective quadrupole coupling constant and the rotational splitting constant for the main lines. The observed features of the formation of quadrupole hyperfine structure spectra must be taken into account in analysis of experimental Mossbauer spectra in liquids.

Nutation Magic-Angle-Spinning Spectra of Half-Integer Quadrupole Spin Systems

The nutation spectra of spin-32 and spin-52 nuclei under magic-angle spinning (MAS) were investigated in detail. When the nutation time is larger than a spinning period, the lineshape of a nutation spectrum is evidently changed by MAS. The center of gravity of the nutation MAS spectrum is generally shifted to lower frequency compared with the static spectrum. The shift is approximately inversely proportional to the spinning speed. When the effective quadrupole coupling constant is much larger than the irradiation field amplitude, the nutation spectra are broadened by MAS, while if the effective quadrupole coupling constant is much smaller than the RF field amplitude, the nutation spectra are generally narrowed by MAS. For certain values of the effective quadrupole coupling constant, the nutation spectrum may exhibit a minor dispersion. For a nutation time shorter than one spinning period, the lineshape is similar to that in the static case. A natural and exact computation approach for theoretical spectral simulation is presented, and the necessity for phase correction of theoretical spectra is discussed. The experimental results are in good agreement with theoretical simulations.

Deuteron NMR spectra of ND 4ClO 4 single crystal at low temperatures

2H NMR spectra of ND 4ClO 4 single crystal were obtained at ν 0 = 44 MHz. Orientation and temperature (1.9–75 K) dependences were measured. Fitting the spectra gives the effective quadrupole coupling constants for all deuterons and the ground torsional level structure. The isotope reduction of the (A-T) and (A-E) tunnelling splittings, i.e., the ratios of the respective splittings for NH 4 + and ND 4 +, were found to be different. The splittings at T = 24 K are about 60% of the helium temperature values. The spectrum undergoes intermediate narrowing by reorientations between 26 and 34 K and tunnelling related features in the spectra are eradicated. After reaching the extreme narrowing limit, a doublet with gradually decreasing separation was observed, what was attributed to averaging by torsional oscillations of increasing amplitude. At high temperatures ( T > 75 K), the narrow spectrum reflects fast multiaxial reorientation of the ammonium ion.

Nitrogen Nuclear Quadrupole Resonance Dips in the Proton Spin Relaxation Dispersion of Nematic and Smectic Thermotropic Liquid Crystals

Abstract The Larmor frequency dependence of the proton spin relaxation time, obtained by means of the fast field-cycling NMR technique, has been used to study the 14N quadrupole coupling constant K and its asymmetry parameter η in the nematic and smectic phases of some liquid crystalline azoxybenzenes (PAA, BAB, HAB, HpAB), cyanobiphenyls (8CB, 9CB, 11CB) and oxycyanobiphenyls (9 OCB). Due to fast molecular reorientations, the effective quadrupole coupling constants are relatively small, whereas surprisingly the asymmetry parameters are rather large. The temperature dependence of both K and η within the mesophases, as well as their discontinuities at the different mesophase transitions, can be interpreted by the anisotropy of molecular rotations. It is found that temperature effects are significantly more pronounced for the (biaxial) smectic-C phase of the heptyloxyazoxybenzene (HpAB) than for the (uniaxial) smectic-A phase of the various investigated cyano- and oxycyanobiphenyls. As a rule, η turned out smaller in the smectic than in the nematic state, whereas K has similar values in both phases

NMR relaxation behavior and quadrupole coupling constants of 39K and 23Na ions in glycerol. Comparisons with 39K tissue data

The quadrupole coupling constants (qcc) for 39K and 23Na ions in glycerol have been calculated from linewidths measured as a function of temperature (which in turn results in changes in solution viscosity). The qcc of 39K in glycerol is found to be 1.7 MHz, and that of 23Na is 1.6 MHz. The relaxation behavior of 39K and 23Na ions in glycerol shows magnetic field and temperature dependence consistent with the equations for transverse relaxation more commonly used to describe the reorientation of nuclei in a molecular framework with intramolecular field gradients. It is shown, however, that τ c is not simply proportional to the ratio of viscosity/temperature (η T). The 39K qcc in glycerol and the value of 1.3 MHz estimated for this nucleus in aqueous solution are much greater than values of 0.075 to 0.12 MHz calculated from T 2 measurements of 39K in freshly excised rat tissues. This indicates that, in biological samples, processes such as exchange of potassium between intracellular compartments or diffusion of ions through locally ordered regions play a significant role in determining the effective quadrupole coupling constant and correlation time governing 39K relaxation. T 1 and T 2 measurements of rat muscle at two magnetic fields also indicate that a more complex correlation function may be required to describe the relaxation of 39K in tissue. Similar results and conclusions are found for 23Na.

Theoretical17O nuclear quadrupole coupling surface for the hydroxonium ion

A property surface has been calculated by SCF theory for the variation with symmetric stretch and inversion motion of the electric field gradient at the oxygen centre in the hydroxonium ion. Using this surface and semiempirical non-rigid-invertor eigenfunctions, effective quadrupole coupling constants are obtained for the lowest inversion states of H173O+, H2D17O+, HD172O+ and H2Mu17O+. A test calculation performed on H172O indicates that the predicted constants for the isotopomers of the hydroxonium ion represent useful estimates of the true values.

Orientational ordering of hydrogen molecules adsorbed on graphite

The orientational behaviour of ortho-H2 and para-D2 molecules adsorbed on graphite has been studied by NMR from 0.3 to 12 K at coverages where the molecules form a [Formula: see text] registered lattice. We have observed that for 90% ortho-H2, the splitting of the high-temperature NMR doublet increases rapidly near 0.6 K and additional structure appears. We have identified this as a transition to the orientationally ordered pinwheel phase: one of the phases predicted by the mean field theory of Harris and Berlinsky. In contrast, the spectrum of 90% para-D2 evolves smoothly to a broad, weak structure as the temperature decreases, showing no evidence of ordering down to 0.3 K. This very different behaviour is unexpected, given that in the bulk solid both isotopes order at comparable temperatures. From the splitting of the high-temperature NMR doublets, we have deduced the crystal fields Vc and the effective quadrupole coupling constants Γ. For D2, Γ is somewhat less than the rigid lattice value, as expected in the presence of translational zero-point motion. However, for H2, Γ is anomalously small. The ortho–para conversion rates of H2 and D2 on our Grafoil substrate have also been measured.

The υ2=1 inversional dependence of the quadrupole coupling in14NH3

We have investigated the hyperfine structure of the υ2=1, (J, K)=(1, 1)-→(2, 1)+ mm wave transition and of the ground state (8, 7) inversion transition of 14NH3 in Doppler-limited resolution. Using additional spectroscopic information from different sources it was possible to determine the υ2=1 effective quadrupole coupling constants. Their values are = - (4429 ± 12) kHz and = - (4263 ± 18) kHz.

Molecular Reorientation in Liquids. Deuteron Quadrupole Relaxation in Liquid Deuterium Oxide and Perdeuterobenzene

Temperature-dependence measurements have been made on the deuteron spin—lattice relaxation times for liquid deuterium oxide and perdeuterobenzene. The quadrupole spin—lattice relaxation is expressed in terms of motional models previously developed for magnetic dipole—dipole relaxation. The deuterium oxide data are discussed in terms of microwave, dielectric relaxation, and hydrogen-bonding phenomena. The T1 values follow rather closely a proportionality to absolute temperature divided by viscosity. The effective quadrupole coupling constant is only ⅔ to ¾ of the value derived from microwave data. For perdeuterobenzene, the 1.86-kcal/mole activation energy rules out a dominance of the rotational-inertia effect proposed recently by Steele. The temperature dependence is much lower than that of the absolute temperature divided by viscosity. Possible reasons for this are discussed. By use of proton-relaxation data, a quadrupole coupling constant value of 138 kc/sec is estimated, assuming axial symmetry of the electric-field gradient about the C—D bond.