Andrei V. Komolkin

Magnetic Resonance and Its Applications

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

Structure and molecular ordering extracted from residual dipolar couplings: A molecular dynamics simulation study

A molecular dynamics (MD) simulation, based on a realistic atom–atom interaction potential, was performed on 4-n-pentyl-4′-cyanobiphenyl (5CB) in the nematic phase. The analysis of the trajectory was focused on the determination of molecular structure and orientational ordering using nuclear dipole–dipole couplings. Three sets of couplings were calculated: 13C–13C, 13C–1H, and 1H–1H. These dipolar couplings were used for investigation of the biphenyl and the ring–chain fragments in 5CB. The models employed in the analysis were based on the rotational isomeric state (RIS) approximation and the maximum entropy (ME) approach. The main questions addressed in this article are: (i) How sensitive are the various sets of dipolar couplings to the long-range orientational order and molecular conformation? (ii) Which model predicts a molecular structure that is in best agreement with the true conformation? Computer simulation is an attractive method to address these questions since the answer is provided: we know th...

ORIENTATIONAL ORDER IN A LIQUID CRYSTALLINE MIXTURE STUDIED BY MOLECULAR DYNAMICS SIMULATION AND NMR

We have studied the orientational order in a liquid crystalline mixture consisting of benzene and 4‐n‐pentyl‐4′‐cyanobiphenyl (5CB) employing molecular dynamics simulation and NMR spectroscopy. The temperature‐dependent order parameters obtained from the NMR experiments were used to determine the average benzene–5CB and 5CB–5CB interaction parameters. It was found, using mean field theory, that the benzene–5CB interaction is ∼45% of that between the solvent particles. This analysis is based on a cascade of approximations. The validity of some of these assumptions was tested in the computer simulation. Various pair correlation functions were also calculated.

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.

Influence of temperature on the microstructure of the lithium-ion hydration shell. A molecular dynamics description

Abstract The microstructure of lithium cation hydration shell has been studied by means of molecular dynamics simulations. The aqueous LiCl solution has been analyzed in the temperature range from −30 to 120°C using the SPC and ST2 water models for a set of model Lennard-Jones parameters of intermolecular potentials. The results of simulations have been compared to NMR-relaxation data and concern the reorganization of the lithium-cation hydration shell with temperature variations. The obtained results show that it is necessary to modify the water model.

Theoretically possible spatial accuracy of geomagnetic maps used by migrating animals

Many migrating animals, belonging to different taxa, annually move across the globe and cover hundreds and thousands of kilometres. Many of them are able to show site fidelity, i.e. to return to relatively small migratory targets, from distant areas located beyond the possible range of direct sensory perception. One widely debated possibility of how they do it is the use of a magnetic map, based on the dependence of parameters of the geomagnetic field (total field intensity and inclination) on geographical coordinates. We analysed temporal fluctuations of the geomagnetic field intensity as recorded by three geomagnetic observatories located in Europe within the route of many avian migrants, to study the highest theoretically possible spatial resolution of the putative map. If migratory birds measure total field intensity perfectly and take the time of day into account, in northern Europe 81% of them may return to a strip of land of 43 km in width along one of coordinates, whereas in more southern areas such a strip may be narrower than 10 km. However, if measurements are performed with an error of 0.1%, the strip width is increased by approximately 40 km, so that in spring migrating birds are able to return to within 90 km of their intended goal. In this case, migrating birds would probably need another navigation system, e.g. an olfactory map, intermediate between the large-scale geomagnetic map and the local landscape cues, to locate their goal to within several kilometres.

Nuclear Quadrupole Resonance

Nuclear quadrupole resonance (NQR) is one of the most sensible tools to study the local structure of materials, electronic density distributions near the nuclei under study, the nature of defects in solids, the mobility of molecules, or their parts, phase transitions and so on. It is very helpful to investigate physical properties of solids, molecular crystals and polymers especially, but can be applied to metals and glasses as well. In liquids quadrupole interactions are averaged to zero and NQR is not applicable.

Nuclear Magnetic Relaxation

In Chap. 1 the concepts of the nuclear magnetic relaxation times (rates) were introduced on the basis of phenomenological considerations, as exemplified by the theory of Bloch.

Molecular Structure of Rod-Like Nematics. The Simple Simulation Accounting Intermolecular Interactions

Abstract The model accounting steric factors and intra- and intermolecular van der Waals interactions is presented. The molecular structure of 5CB is discussed.

Substitution behavior of square-planar and square-pyramidal Cu(II) complexes with bio-relevant nucleophiles

Abstract Substitution reactions of [CuCl2(en)] and [CuCl2(terpy)] complexes (where en = 1,2-diaminoethane and terpy = 2,2′:6′,2″-terpyridine) with bio-relevant nucleophiles such as inosine-5′-monophosphate (5′-IMP), guanosine-5′-monophosphate (5′-GMP), L-methionine (L-Met), glutathione (GSH) and DL-aspartic acid (DL-Asp) have been investigated at pH 7.4 in the presence of 0.010 M NaCl. Mechanism of substitution was probed via mole-ratio, kinetic, mass spectroscopic and EPR studies at pH 7.4. In the presence of an excess of chloride, the octahedral complex anion [CuCl4(en)]2− is formed rapidly while equilibrium reaction was observed for [CuCl2(terpy)]. Different order of reactivity of bio-molecules toward Cu(II) complexes was observed. Mass spectrum of [CuCl2(terpy)] in Hepes buffer has shown two new signals at m/z = 477.150 and m/z = 521.00, assigned to [CuCl(terpy)]+-Hepes fragments of coordinated Hepes buffer. These signals also appear in the mass spectra of ligand substitution reactions between [CuCl2(terpy)] and bio-molecules in molar ratio 1:1 and 1:2. According to EPR data, L-Met forms the most stable complex with [CuCl2(en)] among the ligands considered, while [CuCl2(terpy)] complex did not show significant changes in its square-pyramidal geometry in the presence of the buffer or bio-ligands.