Electron spin relaxation times and internal motions of radicals in the solid state investigated by ENDOR and pulsed EPR

The angular dependent ENDOR spectra of the radical formed by ψ-irradiated single crystal of 4-methyl-2,6-di-t-butylphenol have been studied and the full hyperfine tensors of all the t-butyl and the ring protons have been obtained atT=190 K. We found six different tensors for the t-butyl protons. This result shows that the t-butyl groups are slowly rotating on the ENDOR time scale, whereas each methyl group rotates fastly. The dynamical parameters of the motions have been determined by pulsed EPR experiments. The longitudinal relaxation and the phase memory times have been measured in the temperature range 130–290 K. Three different kinds of motions have been detected and the resulting values of the dynamical parameters have been compared with those obtained for the undamaged molecule by previous NMR studies.     

Rotational state effects in the dissociative recombination of H2+

We have studied the dissociative recombination (DR) of molecular hydrogen ions with slow electrons over a range of collision energies from 0 to 400 meV. By employing a pulsed expansion source for rotational cooling and by exploiting super elastic collisions with near-0-eV electrons in a heavy ion storage ring for vibrational cooling, we observe a highly structured DR cross section, comparable to that reported for HD+. Using para-hydrogen-enriched ion beams, we identify for the first time features in the DR cross sections attributed to nu=0, J=even molecules (para-H2) and nu=0, J=odd (ortho-H2) molecules, separately. Indications are given that para levels have different DR rate coefficients from ortho levels for the first four vibrational levels at near-0-eV collisions.     

Determination of the ground state energies of the H 2 + , D 2 + and H 2 + molecular ions taking into account relativistic corrections

On the basis of determination of the asymptotic behavior of correlation functions of the corresponding field currents with the corresponding quantum numbers an analytic method for determination of the energy spectrum of three-body Coulomb system is suggested. Our results show that the constituent masses of particles, which we have defined as masses of particles in a bound state, differ from masses of particles in a free-state. The constituent mass to the free state mass relation for the electron is greater than the same mass relation for the proton, deuteron and triton. It was also found that this constituent electron mass has different values in each systems, i.e. in H ₂ ⁺ , D ₂ ⁺ and T ₂ ⁺ hydrogen molecular ions. The contributions of exchange and self-energy diagrams were taken into account in the determination of the energy spectrum of the three-body Coulomb system. Our results show that the self-energy diagram contribution is inversely proportional to the square of the constituent mass of particles. This contribution is sufficient for the electron and is negligible for the proton, deuteron and triton. When defining the energy and the wave function (WF), it is necessary to take into account the contributions of both the exchange and self-energy diagrams.     

CARS investigations of quenching and photochemical reactions in the Na+H2 collision system

Summary We have carried out parallel studies of the quenching process in Na(3p)+H₂ collisions and the possible reactive process in Na(3p)+H₂ (v ^″=1,2,3) collisions. Rich CARS spectra which were obtained at H₂ pressure of 100 mbar and oven temperature of 600 K indicate the presence of vibrationally excited H₂ and photochemically produced NaH molecules. Temporal resolution of NaH CARS lines was employed in order to rule out competing collisional processes. We make use of resonantly enhanced CARS methods which enabled us to achieve very high sensitivity for NaH detection. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

Ionic conduction and dielectric relaxation in polycrystalline Na2SO4

In the present paper, the ionic conductivity and the dielectric relaxation properties of Na₂SO₄ have been investigated by means of impedance spectroscopy measurements over wide ranges of frequencies and temperatures. The frequency dependent impedance data has been modeled by appropriate equivalent circuit representing the bulk and grain boundary properties of the sample. Phase transition temperatures and the activation energies of conduction in different phases have been determined from the temperature dependence of the dc conductivity. Dielectric relaxation has been studied using the complex electric modulus and permittivity formalisms. From the electric modulus formalism it is concluded that the relaxation mechanism is independent of temperature. The dielectric permittivity spectra were analyzed by the Cole–Cole formula where different parameters are determined.     

Valence state alternation of copper species doped in HY zeolite as revealed by paramagnetic relaxation enhancement NMR spectroscopy

Paramagnetic relaxation enhancement (PRE) solid-state NMR (ssNMR) was used to monitor the valence state alternation of copper species doped in HY zeolite during catalytic reaction processes. The combination of PRE ssNMR and in-situ NMR spectroscopy facilitates the detection of copper species as well as the monitoring of evolution from reactants, intermediates to products in heterogeneously catalyzed processes, which is of great importance for elucidating the detailed catalytic reaction mechanism.     

A difference in the electromagnetic properties of D2O and H2O ice revealed by resonance measurements

The dielectric parameters of H₂O and D₂O ice samples were studied by a resonance technique in the region of 6 GHz. The ratio of the resonance curve halfwidths at half maximum was measured for light and heavy ice in the temperature range from 0 to ?130°C. The temperature dependences of the resonance peak asymmetry of the two types of ice are qualitatively different. The ratio of the higher-frequency halfwidths at half maximum to the lower-frequency one plotted versus temperature exhibits a positive slope for light ice and a negative slope for heavy ice. It is suggested that the difference reflects the different quantum statistics of the hydrogen and deuterium nuclei in ice.     

1H spin–lattice relaxation in water solution of 209Bi counterparts of Gd3+contrast agents

ABSTRACT

¹H spin–lattice relaxation studies of water solutions of Bismuth-ethylenediamine-tetraacetic acid (Bi-EDTA), Bismuth-ethylenediamine-tetrakis(methylenephosphonic) acid (Bi-EDTP), Bismuth-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (Bi-DOTA), Bismuth-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylenephosphonic acid) (Bi-DOTP) and Bismuth-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (Bi-DO3A) have been performed in order to compare Quadrupole Relaxation Enhancement (QRE) effects with Paramagnetic Relaxation Enhancement (PRE) from the perspective of exploiting the first one as a novel contrast mechanism for Magnetic Resonance Imaging (MRI). The selected compounds can be considered as ²⁰⁹Bi counterparts of Gd³⁺ complexes. The relaxation experiments have been performed in a broad frequency range of 5?kHz–30?MHz. The relaxation contribution associated with QRE has been extracted from the data and compared with PRE. Similarities and differences between the two effects have been discussed.     

Dynamics of NaHSeO3 and NaHSeO4 single crystals by observation of 1H and 23Na spin-lattice relaxation

The 1H and 23Na spin-lattice relaxation times of NaHSeO3 and NaHSeO4 single crystals grown by the slow evaporation method were measured. The phase transitions of the two crystals were studied using differential scanning calorimetry (DSC), and the environments of the 1H and 23Na nuclei in these crystals were investigated by means of 1H and 23Na NMR spectroscopy. The DSC results show that NaHSeO3 crystals do not undergo phase transitions, whereas for NaHSeO4 crystals there is a small endothermic peak at 419 K. The dimers in NaHSeO3 stabilize its structure to the extent that there are no phase transitions even in the presence of disordered hydrogen bonds. In addition, the NMR results indicate that the temperature dependencies of T1 for the 1H and 23Na nuclei in the two hydrogen-bonded crystals-the NaHSeO3 crystal with 3-coordinated SeO3 and the NaHSeO4 crystal with 4-coordinated SeO4--are significantly different. The different trends in T1 for Na in the two crystals are accompanied by different shifts of the oxygen atoms from the 3-coordinated SeO3 and the 4-coordinated SeO4 species around the Na+ ions. Thus, although the two crystals have hydrogen-bonded structures, the differences between the hydrogen bonding of the 3-coordinated SeO3 in NaHSeO3 and the hydrogen bonding of the 4-coordinated SeO4 in NaHSeO4 result in different T1 temperature dependences and different phase transitions.     

A study of the ground states of CaC2H2+,CaC2D2+ and CaC2H4+

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Solid state 13C NMR of 30-crown-10 ether and 30-crown-10.4H2O

The ¹³C NMR solution spectra of 30-crown-10 ether and its tetrahydrate show only one resonance at all accessible temperatures. In contrast, the solid state ¹³C NMR spectrum of the 30-crown-10.4H₂O shows two resonances in the ratio of 4:1, separated by 1.2 ppm. In the case of 30-crown-10 itself, six resolvable ¹³C resonances in the ratio of 4:1:1:2:1:1 are observed in the solid with an overall chemical shift dispersion of 5 ppm. The remarkably different spectral behavior of these two systems in the solid state is discussed in terms of the torsional environments of the crystallographically unique carbons and the results of GIAO calculations of isotropic ¹³C shieldings for simpler model compounds. Results of dipolar dephased ¹³C CPMAS spectra indicate that 30-crown-10 does not undergo a large amplitude molecular motion, in contrast to earlier results for 18-crown-6. Only a small amount of residual intensity is found in the dipolar dephased spectrum of 30-crown-10.4H₂O, indicating that it also is relatively rigid in the solid.     

Osmotic and aging effects in caviar oocytes throughout water and lipid changes assessed by 1H NMR T1 and T2 relaxation and MRI

By combining NMR relaxation spectroscopy and magnetic resonance imaging techniques, unsalted (us) and salted (s) caviar (Acipenser transmontanus) oocytes were characterized over a storage period of up to 90 days. The aging and the salting effects on the two major cell constituents, water and lipids, were separately assessed. T1 and T2 decays were interpreted by assuming a two-site exchange model. At Day 0, two water compartments that were not in fast exchange were identified by the T1 relaxation measurements on the us oocytes. In the s samples, T1 decay was monoexponential. During the time of storage, an increment of the free water amount was found for the us oocytes, ascribed to an increased metabolism. T1 and T2 of the s oocytes shortened as a consequence of the osmotic stress produced by salting. Selective images showed the presence of water endowed with different regional mobility that severely changed during the storage. Lipid T1 relaxation decays collected on us and s samples were found to be biexponential, and the T1 values lengthened during storage. In us and s oocytes, the increased lipid mobility with the storage was ascribed to lipolysis. Selective images of us samples showed lipids that were confined to the cytoplasm for up to 60 days of storage.