Noncovalent Hydrogen Isotope Effects

Zero-point energies (ZPE) and isotope effects, induced by intermolecular, noncovalent vibrations, are computed and tested by experimental data. The ZPE differences of H- and D-complexes of water with hydrogen, methane, and water molecules are about 100–300 cal/mol; they result to isotope effects IE of 1.20–1.70. Semi-ionic bonds between metal ions and water ligands in M(H₂O) ₆ ²⁺ complexes are much stronger; their ZPEs are about 12–14 kcal/mol per molecule and result to IE of 1.9–2.1 at 300 K. Protonated (deuterated) water and biwater exhibit the largest ZPE differences and isotope effects; the latter are 25–28 and 12–13 for water and biwater, respectively. Noncovalent IEs contribute markedly into the experimentally measured effects and explain many anomalous and even magic properties of the effects, such as the dependence of IE on the solvents and on the presence of the third substances, enormously large isotope effects at the mild conditions, the difference between IEs measured in the reactions of individual protiated and deuterated compounds and those measured in their mixture. Noncovalent IEs are not negligible and should be taken into account to make correct and substantiated conclusions on the reaction mechanisms. The kinetic equations are derived for the total isotope effects, which include noncovalent IEs as additive factors.     

Bond energies in polyunsaturated acids and kinetics of co-oxidation of protiated and deuterated acids

A computational program specially designed to analyze co-oxidation of substances in mixtures is suggested. The rigorous kinetic scheme of 32 reactions describing co-oxidation of isotope differing polyunsaturated fatty acids was computed to enlighten experimentally detected enormously large H/D isotope effects. The latter were shown to depend on the kinetic chain length and exhibit two extreme regimes of short and long chains which characterize isotope effects on the initiation and propagation chain reactions of hydrogen (deuterium) atom abstraction. No protective effect of deuterated polyunsaturated acids on the oxidation of protiated acids was detected. Protective effect of the deuterated compounds on the biologically important processes seems to be induced by the low yield of products formed in the chain termination reactions due to the low rate of initiation by deuterated compounds.     

Photoelectron spectroscopy of the Cl(-)...H(2)D(2) anions: a model beyond the rotationless and Franck-Condon approximations

A model for simulating photoelectron spectra of the triatomic van der Waals complexes containing stable atomic anion and diatomic molecule is proposed and applied to the Cl(-)...H(2) and Cl(-)...D(2) anions. The model assumes adiabatic separation of the electronic and nuclear motions and localization of the photodetachment act at the atomic chromophore. Under these approximations, the electronic transition dipole moment matrix elements are evaluated using the atoms-in-molecule approach and explicit expressions for the rovibrational line strength factors are derived. The energies and intensities of a number of rovibronic photoelectron transitions are calculated for the Cl(-)...H(2) and Cl(-)...D(2) anions within the adiabatic bender model, i.e., with the full separation of the vibrational motions, whereas the simulations of the broad spectral envelopes are performed using the equilibrium conditions, asymmetric line shape function, and two choices of the relative abundances of the para- and ortho-forms of the complex. The simulations reproduce experimental spectra reasonably well allowing for their unambiguous assignment in terms of vibronic transitions fully consistent with the previous time-dependent calculations. Agreement with the previous theoretical works, manifestations of non-Franck-Condon effects, and implications to the assessment of the neutral potential energy surfaces are discussed.     

Magnetic organometallosiloxanes

The generalized analysis of a change in magnetic properties of organometallosiloxanes as a function of the number of metal atoms in their molecules (on going from mono- and binuclear to polynuclear crystalline and polymeric compounds) is presented. The relation between the magnetic characteristics and structural parameters of metal-containing fragments is studied. The factors affecting the formation of exchange-coupled ionic pairs and clusters in polymeric systems are analyzed. The ways to the organization of magnetic phases in polymeric systems are considered: the choice of conditions of chemical synthesis, the organization of metal atoms in a common spin system upon incorporation of conducting coordinating additives, and thermocondensation and reduction processes leading to the formation of a metallic phase dispersed in a silicon dioxide matrix. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1432–1442, August, 1997.     

Nuclear relaxation of H2 and H2@C60 in organic solvents

The 1H nuclear spin-lattice relaxation time (T1) of H2 and H2@C60 in organic solvents varies with solvent, and it varies proportionally for H2 and for H2@C60. Since intermolecular magnetic interactions are ruled out, the solvent must influence the modulating processes of the relaxation mechanisms of H2 both in the solvent cage and inside C60. The temperature dependence of T1 also is very similar for H2 and H2@C60, T1 going through a maximum by varying the temperature in solvents which allow a wide range of temperatures to be explored. This behavior is attributed to the presence of dipolar and spin-rotation mechanisms which have an opposite dependence on temperature.     

Magnetoplasticity and magnetic memory in diamagnetic solids

A mechanism for the depinning of dislocations pinned by a stopper is formulated. This mechanism includes the transfer of an electron from a dislocation to the stopper and the appearance of a spin two-electron nanoreactor that has no Coulomb interaction that would hold the dislocation at the stopper in the initial state. The spin dynamics in the nanoreactor is controlled by a magnetic field; therefore, it causes magnetoplasticity and short-term magnetic memory. Another origin of magnetoplasticity is the aggregation of diffusing paramagnetic ions (stoppers) into dimers, trimers, and clusters; this aggregation is also spin-selective and magnetically sensitive. The magnetic-field dependence of the structural evolution of the stoppers provides long-term magnetic memory in diamagnetic solids. Both mechanisms of magnetoplasticity and magnetic memory can coexist and be independent of or dependent on each other.     

The study of rotational mobility of stable nitroxyl radicals in polyvinylacetate

Analyses of the rotational diffusion characters of free and bonded nitroxyl radicals in polyvinylacetate were carried out. The radical rotational character essentially depends on the molecular sizes of the radicals. The movement of the “small” radical is matched by the arbitrary jump tumbling model. The rotation of the “large” radical (both probe and label) occurs by the Brownian rotational diffusion mechanism. The correlation times in the slow-motion region are calculated by taking into account the radical rotation mechanism. Comparison of the τ_c values for the free and bonded radicals with those obtained by the NMR technique shows that movements of the spin probes and labels depend not only on the short polymer segments but on other factors also.     

Hyperfine coupling constants of Mg-centered radicals and radical ions

The energies, geometric parameters, spin densities, and HFC constants of the Mg-centered radicals ^•MgH, ^•MgOH, ^•MgMe, and monovalent magnesium aqua complexes Mg(H₂O)_n ⁺ (n = 0–3) were calculated using the density functional theory. As n increases, the HFC constants decrease for Mg atoms and increase for O atoms. The theoretical HFC constants in the Mg-centered radicals are in good agreement with experimental data. Published In Russian In Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, Pp. 861–865, May, 2007.     

Effect of magnetic field on mechanics of nonmagnetic crystals: The nature of magnetoplasticity

The magnetoplastic effect in mechanics of nonmagnetic crystals is attributed to spin evolution in the spin-selective nanoscale reactor created by electron transfer from a dislocation to a stopper. In this “dislocation + stopper” system, dislocation depinning is facilitated because the Coulomb attraction between the dislocation and the stopper is switched off. Since magnetic field stimulates the singlet-to-triplet conversion of the nanoscale reactor (the reverse electron transfer is forbidden), the nanoscale reactor with switched-off Coulomb interaction has a longer lifetime. The resulting increase in depinning rate and dislocation mobility provides a physical explanation for magnetoplasticity.