Vladimir I. Chizhik

Magnetic Resonance and Its Applications

Magnetic resonance and its applications , Magnetic resonance and its applications , کتابخانه دیجیتال جندی شاپور اهواز

Microstructure and dynamics of electrolyte solutions containing polyatomic ions by NMR relaxation and molecular dynamics simulation

The NMR relaxation method has been used for determing coordination numbers of oxygen-containing polyatomic ions in diamagnetic aqueous solutions. The results, obtained for 7 ions, make it possible to define the rules which govern the formation of ion hydration shells. Probably the developed model can be used for the estimation of results of quantum-chemical calculations and computer simulations. As an example, the molecular dynamics simulation of the sulfate dianion hydration shell has been analyzed.

Low rf power high resolution 1H–13C–14N separated local field spectroscopy in lyotropic mesophases

Efficiency of the proton detected (encoded) (1)H-(13)C-(14)N local field spectroscopy technique at low radiofrequency power is examined when applied to concentrated ionic lyotropic mesophases exhibiting heteronuclear and homonuclear dipolar couplings in kHz range. We demonstrate that highly resolved heteronuclear dipolar spectra can be obtained with limited radiofrequency power corresponding to decoupling B(1) field of 20 kHz and with superior spectral resolution and sensitivity provided by standard solution state probes. To maintain sufficiently large spectral window in indirect dimension, power level alternation scheme for homonuclear decoupling was introduced. In anisotropic mesophases of an ionic surfactant, pair-wise coupling constants in the three-spin system (1)H-(13)C-(14)N were accurately determined. Relative signs of heteronuclear dipolar couplings to nitrogen were obtained employing frequency selective decoupling.

Cross-Polarization with Radio-Frequency Field Phase and Amplitude Modulation under Magic-Angle Spinning Conditions

Nucleus cross-polarization technique in a rotating frame of reference is analyzed as applied to NMR experiments with sample magic-angle spinning. The concept of simultaneous phase and amplitude modulation is suggested. According to this suggestion, the form of the Hamiltonian of recoupled dipolar interaction remains unchanged if phase inversion is accompanied by inversion of the difference of radio-frequency (RF) field amplitudes. A theoretical treatment is given in terms of the average Hamiltonian theory. The concept is demonstrated experimentally and by numerical analysis for several particular cases. Periodic phase inversion in cross-polarization experiments was shown to have the practically important advantage of suppressing chemical shift interactions and the effect of inaccurate tuning of RF field parameters.

Spin-lattice relaxation and mobility of protons in the lattice of the TiV0.8Cr1.2 alloy

Titanium-vanadium-chromium alloys are promising materials for hydrogen storage. They can absorb up to 3.8 wt % of hydrogen with a variable (depending on the composition) temperature of hydrogen release in a convenient range. This paper reports on the results of investigations of the TiV0.80Cr1.20H5.29 hydride by continuous-wave (cw) and pulsed 1H nuclear magnetic resonance spectroscopy. It has been revealed that the hydrogen atoms occupy tetrahedral positions of the face-centered cubic lattice. A model that takes into account the exchange between two states of hydrogen, i.e., mobile hydrogen and hydrogen bound to the lattice, has been proposed for interpreting the temperature dependences of the relaxation times T1 and T2 of 1H nuclei. The assumption that the exchange occurs in these alloys has made it possible, in particular, to explain the strong difference between the relaxation times T1 and T2 in the high-temperature range.

The temperature dependences and methods for the functional representation of the rates of proton magnetic relaxation in aqueous solutions of electrolytes

The paper presents the results of temperature dependence measurements for the rate of proton relaxation (1/T1) in sea water with salinity 35‰ over the temperature range −22−+120°C at atmospheric pressure and in some salt solutions at two concentrations (0.5 and 1 mol/l). The possibility of approximating the temperature dependences of magnetic relaxation rates by various functions in pure water, sea water, and solutions of salts with various concentrations was studied. The parameters of these dependences and trends of their variations under the influence of salt components are reported. The most well grounded method for the functional representation of the temperature dependences of 1/T1 is the use of the sum of exponents with the number of terms depending on solution concentration. This representation takes into account structural changes in solutions as the concentration grows and corresponds to the Frenkel model of the thermal motion of molecules in aqueous solutions of electrolytes. The combined use of the parameters of the temperature dependences of the rate of relaxation represented by various functions can be a mutually augmenting method for studying the dynamic properties of aqueous solutions of electrolytes with low and moderate concentrations.

Unexpected Temperature Behavior of Polyethylene Glycol Spacers in Copolymer Dendrimers in Chloroform.

We have studied copolymer dendrimer structure: carbosilane dendrimers with terminal phenylbenzoate mesogenic groups attached by poly(ethylene) glycol (PEG) spacers. In this system PEG spacers are additional tuning to usual copolymer structure: dendrimer with terminal mesogenic groups. The dendrimer macromolecules were investigated in a dilute chloroform solution by 1H NMR methods (spectra and relaxations). It was found that the PEG layer in G = 5 generations dendrimer is “frozen” at high temperatures (above 260 K), but it unexpectedly becomes “unfrozen” at temperatures below 250 K (i.e., melting when cooling). The transition between these two states occurs within a small temperature range (~10 K). Such a behavior is not observed for smaller dendrimer generations (G = 1 and 3). This effect is likely related to the low critical solution temperature (LCST) of PEG and is caused by dendrimer conformations, in which the PEG group concentration in the layer increases with growing G. We suppose that the unusual behavior of PEG fragments in dendrimers will be interesting for practical applications such as nanocontainers or nanoreactors.

Molecular mobility of counterion functional groups in ionic liquid 1-ethyl-3-methylimidazolium acetate according to 1H and 13C NMR relaxation data

Abstract1-Ethyl-3-methylimidazolium acetate was studied by NMR relaxation. The temperature dependences of the spin-lattice relaxation rates (1/T1) for 1H and 13C were obtained. The curves with maxima were observed for the majority of the temperature dependences 1/T1, which provided a reliable temperature dependence of the correlation times (τc). In the low-temperature range, the proton relaxation rates tend to an asymptotic value, which is related, most likely, to spin diffusion manifested in the studied samples. The values of correlation times τc calculated for 1H and 13C of the same functional group almost coincide at high temperatures, which confirms that the used approach is adequate for the determination of characteristic times of rotational reorientation of counterions in the studied ionic liquid.

Hydrate shell models of acetate ions according to nuclear magnetic relaxation data and quantum-chemical calculations

Different ion-water clusters for the CH3COO− ion that simulate the structure of its hydrate shells in solution are considered on the basis of the quantum mechanical approach. The possible coordination of water molecules around the ion is studied and the calculation results are compared with the experimental data obtained by nuclear magnetic relaxation.

Phase state of the gellan gum-SDS-water system

A study of macroscopic properties and microstructure of the 1 wt.% gellan gum-5 wt.% SDS-water system is presented. The complementary data obtained by DSC, NMR, rheological and CD studies allow us to conclude that the system has thermoreversible sol-I, sol-II (double helix) and gel states which are similar to those for aqueous gellan gum solutions. The micelle state of SDS molecules is not perturbed by the gellan network for the surfactant concentration up to 5 wt.%. The distinctive property of the aqueous gellan gum solutions phase diagram in the presence of SDS is the increase of the sol-helix transition temperature. A NMR diffusion and relaxation study of the SDS micelle dynamics in gellan network yields the most probable model for the sol-helix transition.