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/cm(3) 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) cm(3) molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm(3) 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.

Direct time-resolved study of the gas-phase reactions of germylene with ethyl- and diethylgermane: absolute rate constants, temperature dependences, and mechanism

Time-resolved studies of germylene, GeH2, generated by the 193 nm laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reactions with ethyl- and diethylgermanes in the gas phase. The reactions were studied over the pressure range 1-100 Torr with SF6 as bath gas and at five temperatures in the range 297-564 K. Only slight pressure dependences were found for GeH2 + EtGeH3 (399, 486, and 564 K). The high pressure rate constants gave the following Arrhenius parameters: for GeH2 + EtGeH3, log A = -10.75 +/- 0.08 and E-a = -6.7 +/- 0.6 kJ mol(-1); for GeH2 + Et2GeH2, log A = -10.68 +/- 0.11 and E-a = -6.95 +/- 0.80 kJ mol(-1). These are consistent with fast, near collision-controlled, association processes at 298 K. RRKM modeling calculations are, for the most part, consistent with the observed pressure dependence of GeH2 + EtGeH3. The ethyl substituent effects have been extracted from these results and are much larger than the analogous methyl substituent effects in the SiH2 + methylsilane reaction series. This is consistent with a mechanistic model for Ge-H insertion in which the intermediate complex has a sizable secondary barrier to rearrangement.

Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with nitric oxide

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with NO. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas at five temperatures in the range 299-592 K. The second-order rate constants at 10 Torr fitted the Arrhenius equation log(k/cm(3) molecule(-1) s(-1)) = (- 11.66 +/- 0.01) + (6.20 +/- 0.10 kJ mol(-1))IRT In 10 The rate constants showed a variation with pressure of a factor of ca. 2 over the available range, almost independent of temperature. The data could not be fitted by RRKM calculations to a simple third body assisted association reaction alone. However, a mechanistic model with an additional (pressure independent) side channel gave a reasonable fit to the data. Ab initio calculations at the G3 level supported a mechanism in which the initial adduct, bent H2SiNO, can ring close to form cyclo-H2SiNO, which is partially collisionally stabilized. In addition, bent H2SiNO can undergo a low barrier isomerization reaction leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are NH2 + SiO. The rate controlling barrier for this latter pathway is only 16 kJ mol(-1) below the energy of SiH2 + NO. This is consistent with the kinetic findings. A particular outcome of this work is that, despite the pressure dependence and the effects of the secondary barrier (in the side reaction), the initial encounter of SiH2 with NO occurs at the collision rate. Thus, silylene can be as reactive with odd electron molecules as with many even electron species. Some comparisons are drawn with the reactions of CH2 + NO and SiCl2 + NO.

The addition reaction between silylene and ethyne: further isotope studies, pressure dependence studies, and quantum chemical calculations

Time-resolved kinetic studies of the reaction of dideutero-silylene, SiD2, generated by laser flash photolysis of phenylsilane-d(3), have been carried out to obtain rate constants for its bimolecular reaction with C2H2. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at five temperatures in the range 297-600 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)) = (-10.05 +/- 0.05) + (3.43 +/- 0.36 kJ mol(-1))/RT ln 10. The rate constants were used to obtain a comprehensive set of isotope effects by comparison with earlier obtained rate constants for the reactions of SiH2 with C2H2 and C2D2. Additionally, pressure-dependent rate constants for the reaction of SiH2 with C2H2 in the presence of He (1-100 Tort) were obtained at 300, 399, and 613 K. Quantum chemical (ab initio) calculations of the SiC2H4 reaction system at the G3 level support the initial formation of silirene, which rapidly isomerizes to ethynylsilane as the major pathway. Reversible formation of vinylsilylene is also an important process. The calculations also indicate the involvement of several other intermediates, not previously suggested in the mechanism. RRKM calculations are in semiquantitative agreement with the pressure dependences and isotope effects suggested by the ab initio calculations, but residual discrepancies suggest the possible involvement of the minor reaction channel, SiH2 + C2H2 - SWPO + C2H4. The results are compared and contrasted with previous studies of this reaction system.

Investigation of the prototype silylene reaction, SiH2+H2O(and D2O): time-resolved gas-phase kinetic studies, isotope effects, RRKM calculations, and quantum chemical calculations of the reaction energy surface

Time-resolved kinetic studies of the reaction of silylene, SiH2, with H2O and with D2O have been carried out in the gas phase at 296 and at 339 K, using laser flash photolysis to generate and monitor SiH2. The reaction was studied over the pressure range 10-200 Torr with SF6 as bath gas. The second-order rate constants obtained were pressure dependent, indicating that the reaction is a third-body assisted association process. Rate constants at 339 K were about half those at 296 K. Isotope effects, k(H)/k(D), were small averaging 1.076 0.080, suggesting no involvement of H- (or D-) atom transfer in the rate determining step. RRKM modeling was undertaken based on a transition state appropriate to formation of the expected zwitterionic donoracceptor complex, H2Si...OH2. Because the reaction is close to the low pressure (third order) region, it is difficult to be definitive about the activated complex structure. Various structures were tried, both with and without the incorporation of rotational modes, leading to values for the high-pressure limiting (i.e., true secondorder) rate constant in the range 9.5 x 10(-11) to 5 x 10(-10) cm(3) molecule' s(-1). The RRKM modeling and mechanistic interpretation is supported by ab initio quantum calculations carried out at the G2 and G3 levels. The results are compared and contrasted with the previous studies.

Reactions of silylene with unreactive molecules. 2. nitrogen: gas-phase kinetic and theoretical studies

Time-resolved studies of the reaction of silylene, SiH2, with N-2 have been attempted at 296, 417, and 484 K, using laser flash photolysis to generate and monitor SiH2. No conclusive evidence for reaction could be found even with pressures of N-2 of 500 Torr. This enables us to set upper limits of ca. 3 x 10(-15) cm(3) molecule(-1) s(-1) for the second-order rate constants. A lower limit for the activation energy, E-a, of ca. 47 kJ mol(-1) is also derived. Ab initio calculations at the G3 level indicate that the only SiH2N2 species of lower energy than the separated reactants is the H2Si...N-2 donor-acceptor (ylid) species with a relative enthalpy of -26 kJ mol(-1), insufficient for observation of reaction under the experimental conditions. Ten bound species on the SiH2N2 surface were found and their energies calculated as well as those of the potential dissociation products: HSiN + NH((3)Sigma(-)) and HNSi + NH((3)Sigma(-)). Additionally two of the transition states involving cyclic-SiH2N2 (siladiazirine) were explored. It appears that siladiazirine is neither thermodynamically nor kinetically stable. The findings indicate that Si-N-d bonds (where N-d is double-bonded nitrogen) are not particularly strong. An unexpected cyclic intermediate was found in the isomerization of silaisocyanamide to silacyanamide.

Time-resolved gas-phase kinetic and quantum chemical studies of reactions of silylene with chlorine-containing species. 1. HCl

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with HCL The reaction was studied in the gas phase at 10 Torr total pressure in SF6 bath gas, at five temperatures in the range of 296-611 K. The second-order rate constants fitted the Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-11.51 +/- 0.06) + (1.92 +/- 0.47 kJ mol(-1))/RTIn10 Experiments at other pressures showed that these rate constants were unaffected by pressure in the range of 10-100 Torr, but showed small decreases in value of no more than 20% ( +/- 10%) at I Toff, at both the highest and lowest temperatures. The data are consistent with formation of an initial weakly bound donor-acceptor complex, which reacts by two parallel pathways. The first is by chlorine-to-silicon H-shift to make vibrationally excited chlorosilane, SiH3Cl*, which yields HSiCl by H-2 elimination from silicon. In the second pathway, the complex proceeds via H-2 elimination (4-center process) to make chlorosilylene, HSiCl, directly. This interpretation is supported by ab initio quantum calculations carried out at the G3 level which reveal the direct H-2 elimination route for the first time. RRKM modeling predicts the approximate magnitude of the pressure effect but is unable to determine the proportions of each pathway. The experimental data agree with the only previous measurements at room temperature. Comparisons with other reactions of SiH2 are also drawn.

Time-resolved gas-phase kinetic and quantum chemical studies of reactions of silylene with chlorine-containing species. 2. CH3Cl

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of both silacyclopent-3-ene and phenylsilane, have been carried out to obtain second-order rate constants for its reaction with CH3Cl. The reaction was studied in the gas phase at six temperatures in the range 294-606 K. The second-order rate constants gave a curved Arrhenius plot with a minimum value at T approximate to 370 K. The reaction showed no pressure dependence in the presence of up to 100 Torr SF6. The rate constants, however, showed a weak dependence on laser pulse energy. This suggests an interpretation requiring more than one contributing reaction pathway to SiH2 removal. Apart from a direct reaction of SiH2 with CH3Cl, reaction of SiH2 with CH3 (formed by photodissociation of CH3Cl) seems probable, with contributions of up to 30% to the rates. Ab initio calculations (G3 level) show that the initial step of reaction of SiH2 with CH3Cl is formation of a zwitterionic complex (ylid), but a high-energy barrier rules out the subsequent insertion step. On the other hand, the Cl-abstraction reaction leading to CH3 + ClSiH2 has a low barrier, and therefore, this seems the most likely candidate for the main reaction pathway of SiH2 with CH3Cl. RRKM calculations on the abstraction pathway show that this process alone cannot account for the observed temperature dependence of the rate constants. The data are discussed in light of studies of other silylene reactions with haloalkanes.

Argon predissociation spectroscopy of the OH-center dot H2O and Cl-center dot H2O complexes in the 1000-1900 cm(-1) region: Intramolecular bending transitions and the search for the shared-proton fundamental in the hydroxide monohydrate

We present argon predissociation vibrational spectra of the OH-.H2O and Cl-.H2O complexes in the 1000-1900 cm(-1) energy range, far below the OH stretching region reported in previous studies. This extension allows us to explore the fundamental transitions of the intramolecular bending vibrations associated with the water molecule, as well as that of the shared proton inferred from previous assignments of overtones in the higher energy region. Although the water bending fundamental in the Cl-.H2O spectrum is in very good agreement with expectations, the OH-.H2O spectrum is quite different than anticipated, being dominated by a strong feature at 1090 cm(-1). New full-diniensionality calculations of the OH-.H2O vibrational level structure using diffusion Monte Carlo and the VSCF/CI methods indicate this band arises from excitation of the shared proton.

Evaluation of data for atmospheric models: master equation/RRKM calculations on the combination reaction, BrO+NO2 -> BrONO2, a conundrum

Experimental data for the title reaction were modeled using master equation (ME)/RRKM methods based on the Multiwell suite of programs. The starting point for the exercise was the empirical fitting provided by the NASA (Sander, S. P.; Finlayson-Pitts, B. J.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Molina, M. J.; Moortgat, G. K.; Orkin, V. L.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 15; Jet Propulsion Laboratory: Pasadena, California, 2006)(1) and IUPAC (Atkinson, R.; Baulch, D. L.; Cox, R. A.: R. F. Hampson, J.; Kerr, J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data. 2000, 29, 167) 2 data evaluation panels, which represents the data in the experimental pressure ranges rather well. Despite the availability of quite reliable parameters for these calculations (molecular vibrational frequencies (Parthiban, S.; Lee, T. J. J. Chem. Phys. 2000, 113, 145)3 and a. value (Orlando, J. J.; Tyndall, G. S. J. Phys. Chem. 1996, 100,. 19398)4 of the bond dissociation energy, D-298(BrO-NO2) = 118 kJ mol(-1), corresponding to Delta H-0(circle) = 114.3 kJ mol(-1) at 0 K) and the use of RRKM/ME methods, fitting calculations to the reported data or the empirical equations was anything but straightforward. Using these molecular parameters resulted in a discrepancy between the calculations and the database of rate constants of a factor of ca. 4 at, or close to, the low-pressure limit. Agreement between calculation and experiment could be achieved in two ways, either by increasing Delta H-0(circle) to an unrealistically high value (149.3 kJ mol(-1)) or by increasing , the average energy transferred in a downward collision, to an unusually large value (> 5000 cm(-1)). The discrepancy could also be reduced by making all overall rotations fully active. The system was relatively insensitive to changing the moments of inertia in the transition state to increase the centrifugal effect. The possibility of involvement of BrOONO was tested and cannot account for the difficulties of fitting the data.

Do mitochondriotropic antioxidants prevent chlorinative stress-induced mitochondrial and cellular injury?

Reactive chlorine species such as hypochlorous acid ( HOCl) are cytotoxic oxidants generated by activated neutrophils at the sites of chronic inflammation. Since mitochondria are key mediators of apoptosis and necrosis, we hypothesized that mitochondriotropic antioxidants could limit HOCl-mediated intracellular oxidative injury to human fetal liver cells, preserve mitochondrial function, and prevent cell death. In this current study, we show that recently developed mitochondria-targeted antioxidants ( MitoQ and SS31) significantly protected against HOCl-induced mitochondrial damage and cell death at concentrations >= 25 nM. Our study highlights the potential application of mitochondria-specific targeted antioxidants for the prevention of cellular dysfunction and cell death under conditions of chlorinative stress, as occurs during chronic inflammation.

Effect of oxidized low-density lipoprotein on differential gene expression in primary human endothelial cells

Oxidative modification of low-density lipoprotein (LDL) plays an important role in the initiation and progression of atherosclerosis. It has been proposed that the biological action of oxidized LDL (ox-LDL) may be partially attributed to its effect on a shift of the pattern of gene expression in endothelial cells. To examine the transcriptional response to ox-LDL, we applied cDNA array technology to cultured primary human endothelial cells challenged with oxidized human LDL. A twofold or greater difference in the expression of a particular gene was considered a significant difference in transcript abundance. Seventy-eight of the 588 genes analyzed were differentially expressed in response to the treatment. Ox-LDL significantly affected the expression of genes encoding for transcription factors, cell receptors, growth factors, adhesion molecules, extracellular matrix proteins, and enzymes involved in cholesterol metabolism. The alteration of the expression pattern of several genes was substantiated post hoc using RT-PCR. The experimental strategy identified several novel ox-LDL-sensitive genes associated with a "response to injury" providing a conceptual background to be utilized for future studies addressing the molecular basis of the early stages of atherogenesis.

Impact of saturated, polyunsaturated and monounsaturated fatty acid-rich micelles on lipoprotein synthesis and secretion in Caco-2 cells

Meal fatty acids have been shown to modulate the size and composition of triacylglycerol (TAG)-rich lipoproteins influencing the magnitude and duration of the postprandial plasma TAG response. As a result there is considerable interest in the origin of these meal fatty-acid induced differences in particle composition. Caco-2 cells were incubated over 4 days with fatty acid mixtures resembling the composition of saturated (SFA), monounsaturated (MUFA) and polyunsaturated fatty acid (PUFA)-rich meals fed in a previous postprandial study to determine their impact on lipoprotein synthesis and secretion. The MUFA- and PUFA-rich mixtures supported greater intracellular TAG, but not cholesterol accumulation compared with the SFA-rich mixture (P < 0.001). The MUFA-rich mixture promoted significantly greater TAG and cholesterol secretion than the other mixtures and significantly more apolipoprotein B-100 secretion than the PUFA-rich mixture (P < 0.05). Electron microscopy revealed the SFA-rich mixture had led to unfavourable effects on cellular morphology, compared with the unsaturated fatty acid-rich mixtures. Our findings suggest the MUFA-rich mixture, may support the formation of a greater number of TAG-rich lipoproteins, which is consistent with indirect observations from our human study. Our electron micrographs are suggestive that some endocytotic uptake of MUFA-rich taurocholate micelles may promote greater lipoprotein synthesis and secretion in Caco-2 cells.

Artichoke leaf extract reduces symptoms of irritable bowel syndrome and improves quality of life in otherwise healthy volunteers suffering from concomitant dyspepsia: a subset analysis

Objectives: Does artichoke leaf extract (ALE) ameliorate symptoms of Irritable bowel syndrome (IBS) in otherwise healthy volunteers suffering concomitant dyspepsia? Methods: A subset analysis of a previous dose-ranging, open, postal study, in adults suffering dyspepsia. Two hundred and eight (208) adults were identified post hoc as suffering with IBS. IBS incidence, self-reported usual bowel pattern, and the Nepean Dyspepsia Index (NDI) were compared before and after a 2-month intervention period. Results: There was a significant fall in IBS incidence of 26.4% (p<0.001) after treatment. A significant shift in self-reported usual bowel pattern away from "alternating constipation/diarrhea" toward "normal" (p<0.001) was observed. NDI total symptom score significantly decreased by 41% (p<0.001) after treatment. Similarly, there was a significant 20% improvement in the NDI total quality-of-life (QOL) score in the subset after treatment. Conclusion: This report supports previous findings that ALE ameliorates symptoms of IBS, plus improves health-related QOL.

Turmeric extract may improve irritable bowel syndrome symptomology in otherwise healthy adults: a pilot study

Objectives: To assess the effects of turmeric (Curcuma longa) extract on irritable bowel syndrome (IBS) symptomology in otherwise healthy adults. Design: Partially blinded, randomized, two-dose, pilot study. Subjects: Five hundred (500) volunteers were screened for IBS using the Rome II criteria. Two hundred and seven (207) suitable volunteers were randomized. Interventions: One or two tablets of a standardized turmeric extract taken daily for 8 weeks. Outcomes measures: IBS prevalence, symptom-related quality of life (IBSQOL) and self-reported effectiveness. Results: IBS prevalence decreased significantly in both groups between screening and baseline (41% and 57%), with a further significant drop of 53% and 60% between baseline and after treatment, in the one- and two-tablet groups respectively (p < 0.001). A post-study analysis revealed abdominal pain/discomfort score reduced significantly by 22% and 25% in the one- and two-tablet group respectively, the difference tending toward significance (p = 0.071). There were significant improvements in all bar one of the IBSQOL scales of between 5% and 36% in both groups, approximately two thirds of all subjects reported an improvement in symptoms after treatment, and there was a favorable shift in self-reported bowel pattern. There were no significant differences between groups. Conclusions: Turmeric may help reduce IBS symptomology. Placebo controlled trials are now warranted to confirm these findings.