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.

A comparison of the reactivity of germylene and dimethylgermylene with some methylgermanes. Direct kinetic and quantum chemical studies.

Time-resolved studies of germylene, GeH(2), and dimethygermylene, GeMe(2), generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to try to obtain rate coefficients for their bimolecular reactions with dimethylgermane, Me(2)GeH(2), in the gas-phase. GeH(2) + Me(2)GeH(2) was studied over the pressure range 1-100 Torr with SF(6) as bath gas and at five temperatures in the range 296-553 K. Only slight pressure dependences were found (at 386, 447 and 553 K). RRKM modelling was carried out to fit these pressure dependences. The high pressure rate coefficients gave the Arrhenius parameters: log(A/cm(3) molecule(-1) s(-1)) = -10.99 +/- 0.07 and E(a) =-(7.35 +/- 0.48) kJ mol(-1). No reaction could be found between GeMe(2) + Me(2)GeH(2) at any temperature up to 549 K, and upper limits of ca. 10(-14) cm(3) molecule(-1) s(-1) were set for the rate coefficients. A rate coefficient of (1.33 +/- 0.04) x 10(-10) cm(3) molecule(-1) s(-1) was also obtained for GeH(2) + MeGeH(3) at 296 K. No reaction was found between GeMe(2) and MeGeH(3). Rate coefficient comparisons showed, inter alia, that in the substrate germane Me-for-H substitution increased the magnitudes of rate coefficients significantly, while in the germylene Me-for-H substitution decreased the magnitudes of rate coefficients by at least four orders of magnitude. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) supported these findings and showed that the lack of reactivity of GeMe(2) is caused by a positive energy 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.

Structure and stereodynamics of N,N-bis(silyloxy)enamines.

The structure and stereodynamics of N,N-bis(silyloxy)enamines (1), a new class of enamines with extraordinary reactivity, have been simulated by the DFT PBE/TZP method. The computed pattern of dynamic behavior and structural peculiarities of 1 was shown to reflect adequately the results of the studies by a series of physical methods including X-ray analysis and dynamic NMR and UV spectroscopies, which provided evidence of a rather low barrier for rotation around the C,N single bond, a negligible contribution of the n-pi-conjugation, a high barrier of inversion, and high pyramidality of the nitrogen atom.

First gas-phase detection of dimethylstannylene and time-resolved study of some of its reactions.

Using a laser flash photolysis/laser probe technique, we report the observation of strong absorption signals in the wavelength region 450-520 nm (highest intensity at 514.5 nm) from four potential precursors of dimethylstannylene, SnMe(2), subjected to 193 nm UV pulses. From GC analyses of the gaseous products, combined with quantum chemical excited state CIS and TD calculations, we can attribute these absorptions largely to SnMe(2), with SnMe(4) as the cleanest source of the species. Kinetic studies have been carried out by time-resolved monitoring of SnMe(2). Rate constants have been measured for its reactions with 1,3-C(4)H(6), MeC[triple bond]CMe, MeOH, 1-C(4)H(9)Br, HCl, and SO(2). No evidence could be found for reaction of SnMe(2) with C(2)H(4), C(3)H(8), Me(3)SiH, GeH(4), Me(2)GeH(2), or N(2)O. Limits of less than 10(-13) cm(3) molecule(-1) s(-1) were set for the rate constants for these latter reactions. These measurements showed that SnMe(2) does not insert readily into C-H, Si-H, Ge-H, C-C, Si-C, or Ge-C bonds. It is also unreactive with alkenes although not with dienes or alkynes. It is selectively reactive with lone pair donor molecules. The possible mechanisms of these reactions are discussed. These results represent the first visible absorption spectrum and rate constants for any organo-stannylene in the gas phase.