Surprisingly slow reaction of dimethylsilylene with dimethylgermane: time-resolved kinetic studies and related quantum chemical calculations

Time-resolved studies of silylene, SiH2, and dimethylsilylene, SiMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to obtain rate constants for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas phase. SiMe2 + Me2GeH2 was studied at five temperatures in the range 299-555 K. Problems of substrate UV absorption at 193 nm at temperatures above 400 K meant that only three temperatures could be used reliably for rate constant measurement. These rate constants gave the Arrhenius parameters log(A/cm3 molecule(-1) s(-1)) = -13.25 +/- 0.16 and E(a) = -(5.01 +/- 1.01) kJ mol(-1). Only room temperature studies of SiH2 were carried out. These gave values of (4.05 +/- 0.06) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + MeGeH3 at 296 K). Rate constant comparisons show the surprising result that SiMe2 reacts 12.5 times slower with Me2GeH2 than with Me2SiH2. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) of the model Si-H and Ge-H insertion processes of SiMe2 with SiH4/MeSiH3 and GeH4/MeGeH3 support these findings and show that the lower reactivity of SiMe2 with Ge-H bonds is caused by a higher secondary barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper. Other, related, comparisons of silylene reactivity are also presented.     

Proton polarizability and lamb shift in the muonic hydrogen atom

The corrections that the structure of the proton and its polarizability induce in the Lamb shift in the muonic and the conventional hydrogen atom are calculated on the basis of up-to-date experimental data on the structure functions in deep-inelastic scattering. Numerically, the contribution from proton polarizability to the 2P-2S shift in the muonic hydrogen atom is 4.4 GHz.     

Rare $$\Lambda _c\rightarrow p \ell ^+\ell ^-$$ decay in the relativistic quark model

The relativistic quark model based on the quasipotential approach with the QCD-motivate potential is employed for the calculation of the form factors of the \(\Lambda _c\rightarrow p\) rare weak transitions. Their momentum dependence is explicitly determined without additional assumptions and extrapolations in the whole kinematical range of the momentum transfer squared \(q^2\). The differential \(\Lambda _c\rightarrow p l^+l^-\) decay branching fractions and angular distributions are calculated on the basis of these form factors. Both the perturbative and effective Wilson coefficients, which include contributions of vector meson resonances, are used. The calculated branching fraction of the \(\Lambda _c\rightarrow p \mu ^+\mu ^-\) rare decay is well consistent with the experimental upper limit very recently set by the LHCb Collaboration.     

Isomerization and decomposition of germiranes: a density functional study

The formation and decomposition pathways of germiranes (germacyclopropanes), i.e., products of reactions of the GeH₂ and GeMe₂ germylenes with ethylene, tetramethylethylene, buta-1,2,3-triene, and tetramethylbuta-1,2,3-triene, were studied using the density functional approach (PBE/TZ2P approximation). The thermodynamic stabilities of the structures under consideration were evaluated by calculating the Gibbs free energies under normal conditions (ΔG°₂₉₈). Addition of germylenes to the C=C bond can proceed as a single-step process without a barrier or involve the formation of a π-complex (the barrier to this process is lower than the sum of the energies of isolated reactants). Stability of the germiranes formed is determined by their stability to retrodecomposition into the initial germylene and olefin and to the three-membered ring opening followed by simultaneous 1,2-migration of the substituent at the Ge atom and formation of the secondary germylene. Alkyl substituents can efficiently block the opening of the three-membered ring and transformation of the cyclic structure into the secondary germylene, simultaneously decreasing the germirane stability to retrodecomposition. Decomposition into germylene and olefin under normal conditions is thermally favorable for hexamethylgermirane (ΔG°₂₉₈ = −5.7 kcal mol⁻¹), being thermally forbidden for the other germiranes studied in this work (Δ G°₂₉₈ > 0). The activation energy (E _a) for the germirane ring opening depends on the substituents at the germanium atom, namely, E _a ≤ 10 kcal mol⁻¹ for unsubstituted germiranes and E _a > 30 kcal mol⁻¹ for methyl-substituted germiranes. Taking the experimentally isolated germirane as an example, it was shown how the introduction of substituents and modification of the carbon skeleton make it possible to stabilize the germacyclopropane system. Dedicated to Academician A. L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1943–1951, September, 2005.     

Chemical investigation of biomass of a culture of ginseng cells III. Polysaccharides of a callus culture of ginseng

A water-soluble polysaccharide fraction has been isolated from a callus culture of ginseng with a yield of 16–20%. It has been shown that it included starch a and acidic polysaccharides — arabinogalactans and a xyloglucan.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Branch, USSR Academy of Sciences, Vladivostok, All-Union Scientific Research Institute of Biochemical Machinery Design, USSR Ministry of the Medical Industry, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 771–772, November–December, 1989.     

Reactions between germylenes (silylenes) and phosphaalkenes: an experimental and theoretical study. Preparation of the first representative of germaphosphacyclopropanes

Reactions of a number of germylenes and dimethylsilylene with a phosphaalkene, 2,2-bis(trimethylsilyl)-1-phenyl-1-phosphaethene (1), were studied. The reaction of short-lived dimethylgermylene with 1 produced a phosphagermirane 3 (the first representative of a new class of heterocyclic compounds). Compound 3 was characterized in solution by ¹H, ¹³C, ³¹P, and ²⁹Si NMR spectroscopy. Subsequent reaction of 3 with dimethylgermylene results in 2,2,3,3-tetramethyl-4,4-bis(trimethylsilyl)-1-phenyl-2,3-digerma-1-phosphacyclobutane 4, which has not been reported so far. In order to rationalize different reactivities of germylenes towards alkenes and phosphaalkenes, the addition products of GeH₂ to ethylene and phosphaethene (HP=CH₂) were studied using the G2 computational scheme and DFT PBE technique. The adducts of GeMe₂ (GeCl₂) with HP=CH₂ and of GeMe₂ with PhP=C(SiH₃)₂ were also calculated by the DFT PBE method. According to calculations, the exothermicity, DE, of cycloaddition of GeH₂ and GeMe₂ to the phosphaalkenes HP=CH₂ and PhP=C(SiH₃)₂ (43.5—39.7 kcal mol^–1) is nearly twice as high as the exothermicity of cycloaddition of these germylenes to ethylene. In addition to the minimum corresponding to the three-membered cycle, a number of minima corresponding to quite stable donor-acceptor complexes in which the Ge atom is coordinated by the lone electron pair of the P atom in the phosphaalkene molecule were located on the potential energy surface of the germylene—phosphaalkene system. The complexation energy of the complex of GeH₂ (GeMe₂) with phosphaethene is 25.0 (16.9) kcal mol^–1. For GeCl₂, the exothermicity of cycloaddition to HP=CH₂ decreases to 7.6 kcal mol^–1 and the complexation energy decreases to 8.2 kcal mol^–1.     

The g factors of bound particles in quantum electrodynamics

A quasipotential method is formulated for calculating relativistic and radiative corrections to the magnetic moment of a two-particle bound state in the case of particles of arbitrary spin. It is shown that expressions for the g factors of bound particles contain terms of order O(α²) that depend on the spin of particles. Numerical values of the g factor of an electron in the hydrogen and deuterium atoms are obtained.     

Decay width of positronium. O(α2) correction in second-order perturbation theory

Corrections ² to the decay width of parapositronium and orthopositronium are calculated to second order in perturbation theory using the local quasipotential equation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 3–7, October, 1994.