Synthesis on the basis of electrogenerated carbenes

A series ofS _N2 reactions with halomethanes as substrates and the corresponding anions as nucleophiles were studied by the semiempirical MNDO and AM1 methods, taking into account solvent effects. Analysis of the kinetics, structures of reagents, intermediates and products, and charge distribution in them allows one to draw the conclusion that the retardation ofS _N2 reactions is stronger in solvents than in the gas phase, and the rates of reactions involving anions with a lower number of halogen atoms are higher.For Part 8 see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2148–2154, November, 1995.This work was partially financially supported by the International Science Foundation (Grant No. MHYOO).     

Kinetics of energy transfer in the condensed phase

The decay kinetics of the donor state is studied; this decay is determined by the resonance energy transfer to the surrounding donor molecules of the medium. A new element is the consideration of the strong interaction of the donor with the acceptor over short distances, thanks to which the onset of decomposition assumes an exponential character. It was shown that this circumstance does not affect the fluorescence quenching, but it can exert a considerable effect on phenomena related to radiation shielding.     

Crystal structures of bis(diethylenetriamine)cobalt(III) Heptachlorodimercurate(II)mer-[Co(Dien)2]Hg2Cl7 and (diethylenetriamine)glycylglycinato(1-) cobalt(III) hexachlorodimercurate(II) [Co(Dien)HGlygly]Hg2Cl6

An X-ray diffraction analysis was performed for crystalline [Co(Dien)₂]Hg₂Cl₇ (a = 14.452(2), b = 9.831(1), c = 15.967(2) åΒ = 97.17(1)?, space group P2₁/c, Z = 4) and [Co(Dien)HGlygly]Hg₂Cl₆ (a = 7.693(1), b = 14.439(2), c = 19.961(2) å, Β = 100.31(1)?, space group P2₁ Z = 2), where Dien is diethylenetriamine and HGlygly is the glycylglycine anion [H₂NCH₂C(OH) = NCH₂CO₂]. The anionic moiety is represented by isolated centrosymmetric ions [Hg₄Cl₁₄]_6- in the former and by infinite chains [Hg₂Cl₆] _n ^2n- in the latter (the chain is composed of two alternating vertex-sharing nonequivalent tetrahedral groups HgCl₄. In the cobalt(III) complex cations, the extreme donating atoms of the Dien and HGlygly ligands are in the trans positions of the coordination octahedron.     

DIPOLE POLARIZATION RELAXATION AND MOLECULAR STRUCTURE OF MAIN-AND SIDE-CHAIN LIQUID-CRYSTALLINE POLYMERS

Molecular mobility in thermotropic polyesters and side-chain polymers with different struc-ture of mesogens and spacers has been studied by dielectrical method in dilutesolutions. The results made it possible to establish the multiplicity of dielectric relaxationtransitions which reflects the small- and large-scale types of molecular motion. It was shownthat dielectric relaxation processes occurring in accordance with local mechanism (relaxationtimes 10~(-9)--10~(-7)s. and the activation energy 10--50kJ/mol) are due to the mobility of kineticchain elements of different length within a monomer units. It was found that the dielectricrelaxation process connected with a large-scale form of molecular motion (relaxation times10~(-5)--10~(-6)s. and the activation energy 100kJ/mol) did not depend on the molecular massbut was infiuenced by factors changing the conformational state of the macromolecule. It isestablished tha the cooperative reorientation mobility of associated mesogenic fragments isthe source of the large-scale process.     

Migrational quenching in a system of centers with cross relaxation

The concept of quenching pairs is proposed for a system of randomly distributed centers which can quench one another through an energy cross-relaxation mechanism. The formula for the quenching rate is derived for the case of an exciton migrating over the centers. Comparison is made with experimental data for cross-relaxational quenching of the vibrationally excited molecule CH₃CCl₃. The micro-interaction parameters are estimated to be C_DD = 2 × 10^?33 cm⁶/s and C_DA = 3 × 10^?35 cm⁶/s.     

Protonated Co(III) Complexes with Ethylenediaminetetraacetic and S-Proline-N-Mono-3-propionic Acids. Crystal Structures of (H3O)[Co(HEdta)(CN)] · H2O and [Co(Promp)(HPromp)] · H2O

Crystal structures of [Co(Promp)(Hpromp)] · H₂O (I) (where Promp and Hpromp are deprotonated and monoprotonated anions of S-proline-N-mono-3-propionic acid) and (H₃O)[Co(Hedta)(CN)] · H₂O (II) (where HEdta is monoprotonated anion of ethylenediaminetetraacetic acid) are determined by X-ray diffraction method. The Co coordination octahedron in compound I is formed by two N atoms in trans-positions and by four O atoms of two tridentate ligands, i.e., anions of H₂Promp acid, one of which is fully deprotonated, while the other one has protonated carboxyl group of a six-membered aminopropionate metal cycle. Neutral [Co(Promp)(Hpromp)] complexes and water molecules are united by hydrogen bonds into chains along 2₁ screw axis. Crystals II consist of the complex anions [Co(Hedta)(CN)]⁻, hydroxonium cations, and water molecules. The Co coordination octahedron includes two N atoms and three O atoms of ion of ethylenediaminetetraacetic acid, whose one acetate group is not coordinated but protonated; cyanide ion lies in the NCoN plane. Crystals II contain two types of H₃O⁺ ions that are involved in hydrogen bonds in different way.__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 8, 2005, pp. 596–605.Original Russian Text Copyright © 2005 by Poznyak, Burshtein.     

On the shape of the Q-branch of Raman scattering spectra in dense media. Comparison with experiment

The theory of the isotropic Raman line broadening is compared with gas and liquid experimental data. The effect of rotational structure narrowing with increasing density is confirmed. The vibrational relaxation (dephasing) contribution to the spectral broadening has been extracted, it being essential in liquid phase. It has been shown to be due to weak collisions and to increase proportionally to their frequency.     

Integral encounter theory of strong electron transfer

The integral encounter theory (IET) has been extended to the reactions limited by diffusion along the reaction coordinate to the level crossing points where either thermal or hot electron transfer occurs. IET describes the bimolecular ionization of the instantaneously excited electron donor D* followed by the hot geminate backward transfer which precedes the ion pair equilibration and its subsequent thermal recombination. We demonstrate that the fraction of ion pairs which avoids the hot recombination is much smaller than their initial number when the electron tunneling is strong.     

Discrimination of impact approximations for velocity and velocity sign autocorrelation functions

The velocity and velocity sign autocorrelation functions as well as the distribution of time intervals between velocity sign changes of a particle in a gas or fluid are studied within the framework of impact theory. The results of the uncorrelated successive velocity reversal model (USVRM), for which this approach provides a limiting case of infinitesimal collision time, are checked. It is shown with this simple model that the two approaches are equivalent in two special cases: either when there is no correlation between pre- and post-collision velocities, or when the only possible result of a collision is elastic back scattering. Otherwise, the USVRM leaves out the scope of its view the important correlations of velocity in course of collisions.