Irina V. Krylova

An investigation of the germylene addition reaction, GeH2 + C2H2: Time-resolved gas-phase kinetic studies and quantum chemical calculations of the reaction energy surface

Time resolved studies of germylene, GeH2, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with ethylene, C2H4. The reaction was studied in the gas-phase over the pressure range 1–100 Torr, with SF6 as bath gas, at 5 temperatures in the range 293–555 K. The reaction shows the characteristic pressure dependence of a third-body assisted association reaction. The high pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k∞/cm3 molecule−1 s−1) = (−10.61 ± 0.08) + (5.37 ± 0.56 kJ mol−1)/RT ln10. These Arrhenius parameters are consistent with a fast reaction occurring at approximately half the collision rate at 298 K. RRKM modelling based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure dependent curves for a choice of the critical energy, E0 = 130 kJ mol−1. Quantum chemical calculations (both DFT and ab initio G2//QCISD) of the GeC2H6 potential energy surface (PES), show that GeH2 + C2H4 initially form a π-complex, which can either collapse to germirane or isomerise by a 1,2 H-shift to ethylgermylene with a relatively low barrier. This indicates that the observed pressure dependence must correspond the formation of two products, of which ethylgermylene is the more stable. It also shows that germiranes with 1-H substituents will thermally rearrange to ethylgermylenes with very low barriers. A detailed examination of the PES shows that other potential reaction products are unlikely to be formed. Thermochemical considerations show that germirane is less strained than silirane, and that divalent state stabilisation energies (DSSE) for germylenes are hardly greater than those for silylenes.

Absolute rate constants for the reactions of germylene and dimethylgermylene with dimethylgermane: the deactivating effect of methyl groups in heavy carbenes

Abstract Gas-phase rate constants for the title reactions have been obtained by laser flash photolysis at 297 K, by use of photoprecursors, 3,4-dimethyl-1-germacyclopent-3-ene for GeH 2 and pentamethyldigermane for GeMe 2 . The values obtained were ( k ( cm 3 molecule −1 s −1 ) ): (2.38±0.11)×10 −10 for GeH 2 , (2.26±0.10)×10 −13 for GeMe 2 . These results show that the insertion reaction of GeMe 2 is 1050 times slower than that of GeH 2 into the GeH bonds of Me 2 GeH 2 . This is explained in terms of a general mechanism involving an intermediate H-bridged complex, applicable to both silylene and germylene insertions. For the GeMe 2 insertion, reactants are in equilibrium with the complex, which rearranges to the product in the rate controlling step.

Gas-phase kinetics of chlorosilylene reactions. I. ClSiH + Me3SiH: absolute rate measurements and theoretical calculations for prototype Si-H insertion reactions.

Time-resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reaction with trimethylsilane, Me(3)SiH, in the gas phase. The reaction was studied at total pressures up to 100 torr (with and without added SF(6)) over the temperature range 297-407 K. The rate constants were found to be pressure independent and gave the following Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-13.97 +/- 0.25) + (12.57 +/- 1.64) kJ mol(-1)/RT ln 10. The Arrhenius parameters are consistent with a mechanism involving an intermediate complex, whose rearrangement is the rate-determining step. Quantum chemical calculations of the potential energy surface for this reaction and also the reactions of ClSiH with SiH(4) and the other methylsilanes support this conclusion. Comparisons of both experiment and theory with the analogous Si-H insertion processes of SiH(2) and SiMe(2) show that the main factor causing the lower reactivity of ClSiH is the secondary energy barrier. The calculations also show the existence of a novel intramolecular H-atom exchange process in the complex of ClSiH with MeSiH(3).

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.

Comparison of Effectiveness of Various Approaches to Direct Synthesis of Alkoxysilanes

The aim of the current study was to compare different approaches to the direct synthesis of alkoxysilanes, which were previously discussed in the literature, to obtain high selectivity for trimethoxysilane and satisfying silicon conversion. The possibility of obtaining alkoxysilanes by the reaction of elemental silicon with several organic precursors (first of all, alcohols) was investigated. Trialkoxysilanes and tetraalkoxysilanes were the main products of the reaction. They were identified by means of elemental analysis, NMR- and IR spectroscopies, and chemical analysis. Primary kinetic regularities were investigated as well. The direct process was carried out in 4 different reactor types: flask-like liquid-phase; tube-like gas/solid-phase; flask-like gas/solid-phase with strong UV irradiation, and an autoclave. As a result of optimization there were obtained both HSi(OAlk)3 and Si(OAlk)4 with very high selectivity (> 90 %). The data are very promising in terms of development of new chlorine-free alkoxysilanes production techniques.

Unusual Isotope Effect in the Reaction of Chlorosilylene with Trimethylsilane-1-d. Absolute Rate Studies and Quantum Chemical and Rice–Ramsperger–Kassel–Marcus Calculations Provide Strong Evidence for the Involvement of an Intermediate Complex

Time-resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reaction with trimethylsilane-1-d, Me(3)SiD, in the gas phase. The reaction was studied at total pressures up to 100 Torr (with and without added SF(6)) over the temperature range of 295-407 K. The rate constants were found to be pressure independent and gave the following Arrhenius equation: log[(k/(cm(3) molecule(-1) s(-1))] = (-13.22 ± 0.15) + [(13.20 ± 1.00) kJ mol(-1)]/(RT ln 10). When compared with previously published kinetic data for the reaction of ClSiH with Me(3)SiH, kinetic isotope effects, k(D)/k(H), in the range from 7.4 (297 K) to 6.4 (407 K) were obtained. These far exceed values of 0.4-0.5 estimated for a single-step insertion process. Quantum chemical calculations (G3MP2B3 level) confirm not only the involvement of an intermediate complex, but also the existence of a low-energy internal isomerization pathway which can scramble the D and H atom labels. By means of Rice-Ramsperger-Kassel-Marcus modeling and a necessary (but small) refinement of the energy surface, we have shown that this mechanism can reproduce closely the experimental isotope effects. These findings provide the first experimental evidence for the isomerization pathway and thereby offer the most concrete evidence to date for the existence of intermediate complexes in the insertion reactions of silylenes.

Electrochemical oxidation of tetracycline on a boron doped diamond electrode within the stability potentials of water

Electrochemical oxidation of tetracycline on a boron doped diamond electrode within the stability potentials of water was studied in order to develop an approach for the purification of waste water containing medicinal agents. Cyclic voltammetry, HPLC, and high resolution mass spectrometry were used to establish that in the electrochemical oxidation process, tetracycline adds one oxygen atom to further form organic compounds with molecular weights higher than that of tetracycline. It was found that tetracycline in this region of potentials can be almost completely deactivated without its mineralization.

Gas‐Phase Kinetics of Chlorosilylene Reactions II. ClSiH + C2H4: Absolute Rate Measurements and Quantum Chemical and RRKM Calculations for the Prototype π Addition Reaction

Time-resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, are carried out to obtain rate constants for its bimolecular reaction with ethene, C(2)H(4), in the gas-phase. The reaction is studied over the pressure range 0.13-13.3 kPa (with added SF(6)) at five temperatures in the range 296-562 K. The second order rate constants, obtained by extrapolation to the high pressure limits at each temperature, fitted the Arrhenius equation: log(k(infinity)/cm(3) molecule(-1) s(-1)) = (-10.55+/-0.10) + (3.86 +/- 0.70) kJ mol(-1)/RT ln10. The Arrhenius parameters correspond to a loose transition state and the rate constant at room temperature is 43% of that for SiH(2) + C(2)H(4), showing that the deactivating effect of Cl-for-H substitution in the silylene is not large. Quantum chemical calculations of the potential energy surface for this reaction at the G3MP2//B3LYP level show that, as well as 1-chlorosilirane, ethylchlorosilylene is a viable product. The calculations reveal how the added effect of the Cl atom on the divalent state stabilisation of ClSiH influences the course of this reaction. RRKM calculations of the reaction pressure dependence suggest that ethylchlorosilylene should be the main product. The results are compared and contrasted with those of SiH(2) and SiCl(2) with C(2)H(4).

Alkynylhalocarbenes: generation from 1,1-dihaloalk-2-ynes by base solvolysis and reaction with alkenes

A series of 1,1-dihaloalk-2-ynes 1–3 has been prepared by halogenation of the formylacetylenes 8 with PCl5 or an equimolar mixture of PCl5 and Br2. A simple, general means of access to the alkynylhalocarbenes 5 has been developed via base-initiated α-elimination of 1,1-dihaloalk-2-ynes (1–3). The carbenes 5a–i have been trapped by alkenes, to form 1-alkynyl-1-halocyclopropanes (11) in up to 90% yield. Under the same conditions compound 1g was converted into the buta-diene 12. Experimental evidence for the electrophilicity and the singlet nature of carbenes 5 has been obtained.

Direct electrochemical synthesis of germanium alkoxides

Anodic galvanostatic dissolution of germanium in alcohols in an undivided cell in the presence of trace amounts of NaOAc as an electrolyte afforded germanium alkoxides with a current efficiency of 88—95%.