S. E. Boganov

ROOM TEMPERATURE OBSERVATION OF GEH2 AND THE FIRST TIME-RESOLVED STUDY OF SOME OF ITS REACTIONS

Abstract Using a laser flash photolysis/laser probe technique, applied to two different gaseous precursor molecules, we report absorption signals in the wavenumber region 17109–17120 cm −1 , attributable to previously unobserved rotational transitions in the vibronic spectrum of GeH 2 . By time-resolved monitoring of GeH 2 , rate constants have been obtained for its reactions with O 2 , C 2 H 2 , i-C 4 H 8 , 1,3-C 4 H 6 , C 3 H 8 and Me 3 SiH. The measurements show that GeH 2 is unreactive towards CH bonds but inserts readily in SiH bonds, as well as reacting rapidly (close to the collision rate), with π-bonds. These results represent the first direct kinetic measurements on GeH 2 . Comparisons show that it is slightly less reactive than SiH 2 .

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.

A gas-phase kinetic study of the reaction of silylene with germane: absolute rate constants, temperature dependence and mechanism

Time-resolved studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with monogermane, GeH4. The reaction was studied in the gas-phase at 10 Torr total pressure in SF6 bath gas, at five temperatures in the range 295–553 K. The second order rate constants fitted the Arrhenius equation: log(k3/cm3 molecule−1 s−1) = (−9.88 ± 0.02)+(2.13 ± 0.17 kJ mol−1)/RTln10. Experiments at other pressures showed that these rate constants were unaffected by pressure. The data are consistent with a fast association process occuring (at 298 K) close to the collision rate. Although the probable initial product is silylgermane, H3SiGeH3, thermochemical considerations show that this will decompose to GeH2 + SiH4 under experimental conditions. This silylene insertion process is compared to others as well as to the insertion processes of methylene, CH2 and germylene, GeH2.

The insertion reaction of germylene into the Si–H bond of silane: absolute rate constants, temperature dependence, RRKM modelling, and quantum chemical (ab initio and DFT) calculations

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 monosilane, SiH4. 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 295–554 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: These Arrhenius parameters are consistent with a moderately fast reaction occurring at approximately one thirtieth of the collision rate. Rice–Ramsperger–Kassel–Marcus (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 = 138 kJ mol−1, for the reverse decomposition of H3SiGeH3 , the reaction product. There is no previous experimental determination of this quantity. From it we derive ΔHf0(GeH2) = 233 ± 12 kJ mol−1, in reasonable agreement with earlier estimates. Ab initio and DFT calculations reveal the presence of two weak complexes (local energy minima) on the potential energy surface corresponding to either direct or inverted geometry of the inserting germylene fragment. As found earlier for the GeH2 + GeH4 reaction, the latter is lower in energy and has left and right handed forms. These complexes rearrange to H3SiGeH3 with low barriers. The implications of these findings and the nature of the insertion process are discussed.

A highly delocalized triplet carbene, 5-Methylhexa-1,2,4-triene-1,3-diyl: matrix IR identification, structure, and reactions.

The first representative of highly delocalized triplet carbenes bearing both vinyl and ethynyl groups at the formal carbene center, 5-methylhexa-1,2,4-triene-1,3-diyl, has been generated in a low-temperature Ar matrix upon UV photolysis of 5-ethynyl-3,3-dimethyl-3H-pyrazole and detected by FTIR spectroscopy. The transformation of 3H-pyrazole into the carbene proceeds in two stages via intermediate 3-diazo-5-methylhex-4-en-1-yne. According to DFT PBE/TZ2P calculations, 5-methylhexa-1,2,4-triene-1,3-diyl possesses an effective conjugation along the five-carbon chain and shows the same type of the bond length alternation as the HC(4m+1)H-type polyacetylenic carbenes. The carbene readily reacts with molecular oxygen, producing carbonyl oxides, which undergo further transformations typical of this type of compound upon irradiation in the UV-visible region. Two major photolytic rearrangements of 5-methylhexa-1,2,4-triene-1,3-diyl represent reactions characteristic of vinyl carbenes and resulting in the formation of 1-ethynyl-3,3-dimethylcyclopropene and 3E-2-methylhexa-1,3-dien-5-yne. A minor reaction is that typical of ethynylcarbenes; this leads to the formation of singlet 2-(2-methylpropenyl)cyclopropenylidene. Fragments of singlet and triplet potential energy surfaces of the C(7)H(8) system have been explored in DFT PBE/TZ2P calculations.

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.

Experimental and quantum-chemical study of complexation of carbene analogs with dinitrogen. Direct IR-spectroscopic observation of Cl2Si·N2 complexes in low-temperature argon-nitrogen matrices

Interaction of dichlorosilylene with dinitrogen in mixed Ar—N2 matrices at 9 - 10 K was studied by IR spectroscopy. A donor-acceptor complex Cl2Si·N2 was found and characterized by six bands of symmetric (at 511.2, 508.9, and 506.5 cm–1) and antisymmetric (at 500.1, 496.9, and 495.1 cm–1) stretching vibrations of Si—Cl bonds in the most abundant isotopomers. Two bands at 498.7 and 493.5 cm–1 observed in mixed matrices were tentatively assigned to Cl2Si·(N2)2 complex. Several stretching vibration bands of minor isotopomers of SiCl2 were detected for the first time in argon matrices. Assignment has been done for the isotopic structure of SiCl2 associates with dinitrogen observed in N2 matrices. Dimerization of SiCl2 and its complexation with one and two N2 molecules were studied by quantum-chemical DFT calculations (PBE and B3LYP functionals). The structures, energies, and vibrational frequencies of the Cl2Si·N2 and Cl2Si·(N2)2 complexes and the Si2Cl4 dimer were determined. The energies of SiCl2 complexation with one and two N2 molecules obtained from PBE and B3LYP calculations are 0.3 and 0.6 kcal mol–1, respectively. More accurate G2(MP2,SVP) calculations using the B3LYP geometries have predicted a higher stability of the Cl2Si·N2 complex (1.2 kcal mol–1). The calculated and experimental vibrational frequencies of reagents and complexes are in good agreement. A correlation has been established between the PBE calculated energies of complexation of EHal2 (E = Si, Ge, Sn, Pb) with N2 and the experimentally observed shifts of E—Hal stretching vibrations in EHal2 upon complexation. The strength of the complexes with N2 increases on going from dihalosilylenes to dihaloplumbylenes.

Transformations of benzene-argon mixture in barrier discharge

The transformation of a benzene-argon mixture in dielectric-barrier discharge (DBD) was studied. Benzene-soluble polymeric compounds (76.5 wt %), biphenyl (7.6 wt %), and phenylcyclohexadienes (9 wt %) are the major products. Monoalkylbenzenes, cyclohexadienes, and ethynylbenzene are in trace amounts in the reaction mixture. The benzene conversion per pass through the discharge zone was 5.5 wt %. The possible mechanism of the benzene transformation in DBD was suggested on the basis of experimental data and theoretical calculations.

Study of complexation between difluorostannylene and aromatics by matrix IR spectroscopy

Complexes (1:1) between difluorostannylene and benzene, chlorobenzene, and toluene as well as the (PhH)2·SnF2 complex were obtained and characterized by matrix IR spectroscopy. The capability of carbene analogs to form labile complexes with aromatics was demonstrated for the first time taking SnF2 as an example.