A structured jet explains the extreme GRB 221009A.

Long-duration gamma-ray bursts (GRBs) are powerful cosmic explosions, signaling the death of massive stars. Among them, GRB 221009A is by far the brightest burst ever observed. Because of its enormous energy (Eiso ≈ 1055 erg) and proximity (z ≈ 0.15), GRB 221009A is an exceptionally rare event that pushes the limits of our theories. We present multiwavelength observations covering the first 3 months of its afterglow evolution. The x-ray brightness decays as a power law with slope ≈t-1.66, which is not consistent with standard predictions for jetted emission. We attribute this behavior to a shallow energy profile of the relativistic jet. A similar trend is observed in other energetic GRBs, suggesting that the most extreme explosions may be powered by structured jets launched by a common central engine.

Time-resolved, Gas-phase Kinetic and Quantum Chemical Studies of the Reaction of Germylene with Hydrogen Chloride.

Time-resolved studies of germylene, GeH2 , generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene at 193 nm and monitored by laser absorption, have been carried out to obtain rate constants for its bimolecular reaction with HCl. The reaction was studied in the gas phase, mainly at a total pressure of 10 Torr (in SF6 bath gas) at five temperatures in the range 295-558 K. Experiments at other pressures showed that these rate constants were unaffected by pressure. The second-order rate constants at 10 Torr (SF6 bath gas) fitted the Arrhenius equation: log(k/cm3 molecule-1 s-1 )=(-12.06±0.14)+(2.58±1.03 kJ mol-1 )/RTln10 where the uncertainties are single standard deviations. Quantum chemical calculations at G4 level support a mechanism in which an initial weakly bound donor-acceptor complex is formed. This can then rearrange and decompose to give H2 and HGeCl (chlorogermylene). The enthalpy barrier (36 kJ mol-1 ) is too high to allow rearrangement of the complex to GeH3 Cl (chlorogermane).

Reporting characteristics of journal infographics: a cross-sectional study.

BACKGROUND: Infographics have become an increasingly popular method to present research findings and increase the attention research receives. As many scientific journals now use infographics to boost the visibility and uptake of the research they publish, infographics have become an important tool for medical education. It is unknown whether such infographics convey the key characteristics that are needed to make useful interpretations of the data such as an adequate description of the study population, interventions, comparators and outcomes; methodological limitations; and numerical estimates of benefits and harms. This study described whether infographics published in peer-reviewed health and medical research journals contain key characteristics that are needed to make useful interpretations of clinical research. METHODS: In this cross-sectional study, we identified peer-reviewed journals listed in the top quintile of 35 unique fields of medicine and health research listed in the Journal Citation Reports database. Two researchers screened journals for the presence of infographics. We defined an infographic as a graphical visual representation of research findings. We extracted data from a sample of two of the most recent infographics from each journal. Outcomes were the proportion of infographics that reported key characteristics such as study population, interventions, comparators and outcomes, benefits, harms, effect estimates with measures of precision, between-group differences and conflicts of interest; acknowledged risk of bias, certainty of evidence and study limitations; and based their conclusions on the study's primary outcome. RESULTS: We included 129 infographics from 69 journals. Most infographics described the population (81%), intervention (96%), comparator (91%) and outcomes (94%), but fewer contained enough information on the population (26%), intervention (45%), comparator (20%) and outcomes (55%) for those components of the study to be understood without referring to the main paper. Risk of bias was acknowledged in only 2% of infographics, and none of the 69 studies that had declared a conflict of interest disclosed it in the infographics. CONCLUSIONS: Most infographics do not report sufficient information to allow readers to interpret study findings, including the study characteristics, results, and sources of bias. Our results can inform initiatives to improve the quality of the information presented in infographics.

What makes a great clinical trial in physiotherapy?

OBJECTIVE: To identify common characteristics of landmark physiotherapy clinical trials. METHODS: The Physiotherapy Evidence Database (PEDro) top five trials were compared to 91 physiotherapy trials published in top medical journals and 99 trials randomly selected from PEDro on the following characteristics: PEDro score, sample size, number of trial sites, use of prospective registration, positive or negative trial, citations, citations in guidelines, Altmetric score, impact factor, publications and citations of first and last author, and PEDro codes (sub-discipline, topic, problem, therapy, and body part). Trials were published from 2014 to 2019. One-way independent ANOVA and Chi-squared test evaluated between-group differences. RESULTS: Compared to a random sample of physiotherapy trials, the PEDro top five trials and trials in top medical journals have higher PEDro scores, larger sample sizes, more study sites, more citations (including in guidelines), higher Altmetric scores, more likely to be prospectively registered, less likely to be positive trials, and have first and last authors with more citations and publications. The problem was the only PEDro code was distributed differently across the trial groups. CONCLUSION: The PEDro top five trials and physiotherapy trials published in the top medical journals have characteristics that may inform the design, conduct, and reporting of future physiotherapy trials.

Thermochemistry of germanium and organogermanium compounds.

This article reviews the current state of thermochemistry (enthalpies of formation) of germanium and organogermanium compounds. While the emphasis is on data from experimental measurements, results of quantum chemical (ab initio) calculations are also included. We provide a set of current best values for GeX4 compounds (X = H, Me, F, Cl, Br, I) as well as mixed GeXnY4-n compounds. Also included are Ge/C/H, Ge/C/H/O and Ge/C/H/N compounds and Ge2 and Ge3 containing molecules. Further inclusions are data for species such as GeX3 (free radicals), GeX2 (germylenes) and π-bonded molecules which play prominent roles as intermediates in many thermal and photochemical reactions of germanium compounds. Bond dissociation enthalpies are derived for most commonly encountered Ge-X bonds. The evaluation of preferred values is assisted by exploiting relationships (increments, substituent effects) within specific series and between series.

Time-Resolved Gas-Phase Kinetic, Quantum Chemical, and RRKM Studies of the Reaction of Silylene with 2,5-Dihydrofuran.

Time-resolved kinetics studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, were performed to obtain rate coefficients for its bimolecular reaction with 2,5-dihydrofuran (2,5-DHF). The reaction was studied in the gas phase over the pressure range of 1-100 Torr in SF6 bath gas, at five temperatures in the range of 296-598 K. The reaction showed pressure dependences characteristic of a third body assisted association. The second-order rate coefficients obtained by Rice-Ramsperger-Kassel-Marcus (RRKM)-assisted extrapolation to the high-pressure limit at each temperature fitted the following Arrhenius equation where the error limits are single standard deviations: log(k/cm(3) molecule(-1) s(-1)) = (-9.96 ± 0.08) + (3.38 ± 0.62 kJ mol(-1))/RT ln 10. End-product analysis revealed no GC-identifiable product. Quantum chemical (ab initio) calculations indicate that reaction of SiH2 with 2,5-DHF can occur at both the double bond (to form a silirane) and the O atom (to form a donor-acceptor, zwitterionic complex) via barrierless processes. Further possible reaction steps were explored, of which the only viable one appears to be decomposition of the O-complex to give 1,3-butadiene + silanone, although isomerization of the silirane cannot be completely ruled out. The potential energy surface for SiH2 + 2,5-DHF is consistent with that of SiH2 with Me2O, and with that of SiH2 with cis-but-2-ene, the simplest reference reactions. RRKM calculations incorporating reaction at both π- and O atom sites, can be made to fit the experimental rate coefficient pressure dependence curves at 296-476 K, giving values for k(∞)(π) and k(∞)(O) that indicate the latter is larger in magnitude at all temperatures, in contrast to values from individual model reactions. This unexpected result suggests that, in 2,5-DHF with its two different reaction sites, the O atom exerts the more pronounced electrophilic attraction on the approaching silylene. Arrhenius parameters for the individual pathways were obtained. The lack of a fit at 598 K is consistent with decomposition of the O-complex to give 1,3-butadiene + silanone.

Reaction of silylene with sulfur dioxide: some gas-phase kinetic and theoretical studies.

Time-resolved kinetic studies of the reaction of silylene, SiH2, with SO2 have been carried out in the gas phase over the temperature range 297-609 K, using laser flash photolysis to generate and monitor SiH2. The second order rate coefficients at 1.3 kPa (SF6 bath gas) fitted the Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-10.10 ± 0.06) + (3.46 ± 0.45 kJ mol(-1))/RT ln 10 where the uncertainties are single standard deviations. The collisional efficiency is 71% at 298 K, and in kinetic terms the reaction most resembles those of SiH2 with CH3CHO and (CH3)2CO. Quantum chemical calculations at the G3 level suggest a mechanism occurring via addition of SiH2 to one of the S=O double bonds leading to formation of the three-membered ring, thione-siloxirane which has a low energy barrier to ring expansion to yield the four-membered ring, 3-thia-2,4-dioxasiletane, the lowest energy adduct found on the potential energy (PE) surface. RRKM calculations, however, show that, if formed, this molecule would only be partially stabilised under the reaction conditions and the rate coefficients would be pressure dependent, in contrast with experimental findings. The G3 calculations reveal the complexity of possible intermediates and end products and taken together with the RRKM calculations indicate the most likely end products to be H2SiO + SO ((3)Σ(-)). The reaction is compared and contrasted with that of SiH2 + CO2.

Time-resolved gas-phase kinetic studies of the reaction of dimethylsilylene with triethylsilane-1-d: kinetic isotope effect for the Si-H insertion process.

Time-resolved kinetic studies of the reaction of dimethylsilylene, SiMe2, generated by laser flash photolysis of 1,1-dimethyl-1-silacyclopent-3-ene, have been carried out to obtain rate coefficients for its bimolecular reactions with trimethylsilane-1-d, Me3SiD. The reaction was studied in the gas phase at five temperatures in the range 292-605 K. The rate coefficients showed no pressure dependence in the presence of up to 13 kPa of SF6. The second order rate coefficients obtained at 0.7 kPa fitted the Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-13.53 ± 0.19) + (11.29 ± 1.46) kJ mol(-1)/RT ln 10. By comparison with rate coefficients obtained previously for the reaction of SiMe2 with Me3SiH, a set of kinetic isotope effects, kH/kD, of value ca. 1.2 showing very little temperature dependence was obtained. Theoretical support for these values has been obtained by means of quantum chemical calculations used in conjunction with transition state theory. This study provides the first comprehensive set of kinetic isotope effects for the Si-H(D) insertion process of a silylene in the gas phase.

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.

Time-resolved gas-phase kinetic, quantum chemical, and RRKM studies of reactions of silylene with alcohols.

Time-resolved kinetic studies of silylene, SiH(2), generated by laser flash photolysis of 1-silacyclopent-3-ene and phenylsilane, have been carried out to obtain rate constants for its bimolecular reactions with methanol, ethanol, 1-propanol, 1-butanol, and 2-methyl-1-butanol. The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF(6) bath gas, at room temperature. In the study with methanol several buffer gases were used. All five reactions showed pressure dependences characteristic of third body assisted association reactions. The rate constant pressure dependences were modeled using RRKM theory, based on E(0) values of the association complexes obtained by ab initio calculation (G3 level). Transition state models were adjusted to fit experimental fall-off curves and extrapolated to obtain k(∞) values in the range (1.9-4.5) × 10(-10) cm(3) molecule(-1) s(-1). These numbers, corresponding to the true bimolecular rate constants, indicate efficiencies of between 16% and 67% of the collision rates for these reactions. In the reaction of SiH(2) + MeOH there is a small kinetic component to the rate which is second order in MeOH (at low total pressures). This suggests an additional catalyzed reaction pathway, which is supported by the ab initio calculations. These calculations have been used to define specific MeOH-for-H(2)O substitution effects on this catalytic pathway. Where possible our experimental and theoretical results are compared with those of previous studies.

Kinetic studies of reactions of organosilylenes: what have they taught us?

Rate constants for bimolecular reactions, obtained through time-resolved kinetic studies both in the gas and liquid phases are reviewed. Data for reactions of MeSiH, PhSiH, ClSiH, SiCl(2), SiMe(2), MeSiPh, SiPh(2) and SiMes(2) are covered. Where possible, substituent effects relative to SiH(2) have been obtained. These demonstrate widely varying effects between different types of reaction, which aids mechanistic understanding. Reactivities are high for all silylenes, but substituents can reduce them by both electronic and steric effects. The gas and liquid phase data (mainly for SiMe(2)) are compared and appear to be reasonably consistent. This review, although detailed, is not comprehensive.

Time-resolved gas-phase kinetic study of the germylene addition reaction, GeH(2) + C(2)D(2).

Time-resolved studies of germylene, GeH(2), generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene, have been carried out to obtain rate coefficients for its bimolecular reaction with C(2)D(2). The reaction was studied in the gas phase, mainly at a total pressure of 1.3 kPa (in SF(6) bath gas) at five temperatures in the range 298-558 K. Pressure variation measurements over the range 0.13-13 kPa (SF(6)) at 298, 397 and 558 K revealed a small pressure dependence but only at 558 K. After correction for this, the second-order rate coefficients gave the Arrhenius equation: log(k infinity/cm3 molecule(-1) s(-)1) = (-10.96 +/- 0.05) + (6.16 +/- 0.37 kJ mol-1)/RT ln 10. Comparison with the reaction of GeH(2) + C(2)H(2) (studied earlier) showed a similar behaviour with almost identical rate coefficients. The lack of a significant isotope effect is consistent with a rate-determining addition process and is explained by irreversible decomposition of the reaction intermediate to give Ge((3)P) + C(2)H(4). This result contrasts with that for GeH(2) + C(2)H(4)/C(2)D(4) and those for the analogous silylene reactions. The underlying reasons for this are discussed.

Gas-phase kinetics of chlorosilylene reactions II. ClSiH + C2H4: absolute rate measurements and quantum chemical and RRKM calculations for the prototype pi 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).

Time-resolved gas-phase kinetic, quantum chemical and RRKM studies of reactions of silylene with cyclic ethers.

Time-resolved kinetic studies of silylene, SiH(2), generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reactions with oxirane, oxetane, and tetrahydrofuran (THF). The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF(6) bath gas, at four or five temperatures in the range 294-605 K. All three reactions showed pressure dependences characteristic of third-body-assisted association reactions with, surprisingly, SiH(2) + oxirane showing the least and SiH(2) + THF showing the most pressure dependence. The second-order rate constants obtained by extrapolation to the high-pressure limits at each temperature fitted the Arrhenius equations where the error limits are single standard deviations: log(k(infinity)oxirane/cm3 molecule(-1)s(-1))=(-11.03+/-0.07) + (5.70+/-0.51)kJ mol(-1) ln 10, log(k(infinity)oxetane/cm3 molecule(-1)s(-1))=(-11.17+/-0.11)+(9.04+/-0.78) kJ mol(-1)/RT ln 10, log(k(infinity)THF/cm3 molecule(-1)s(-1))=(-10.59+/-0.10)+(5.76+/-0.65) kJ mol(-1)/RT ln 10. Binding-energy values of 77, 97, and 92 kJ mol-1 have been obtained for the donor-acceptor complexes of SiH2 with oxirane, oxetane, and THF, respectively, by means of quantum chemical (ab initio) calculations carried out at the G3 level. The use of these values to model the pressure dependences of these reactions, via RRKM theory, provided a good fit only in the case of SiH2 + THF. The lack of fit in the other two cases is attributed to further reaction pathways for the association complexes of SiH2 with oxirane and oxetane. The finding of ethene as a product of the SiH2 + oxirane reaction supports a pathway leading to H2Sid=O + C2H4 predicted by the theoretical calculations of Apeloig and Sklenak.

A mechanistic study of the low pressure pyrolysis of linear siloxanes.

Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of 1,1,3,3-tetramethyldisiloxane (L1), 1,1,3,3,5,5-hexamethyltrisiloxane (L2) and 3H,5H-octamethyltetrasiloxane (L3) at ca. 1000 K in a flow reactor at low pressures. The hydrocarbons CH3, CH4, C2H2, C2H4, and C2H6 were observed as prominent pyrolysis products in all three systems, and amongst the weaker features are bands arising from the methylsilanes Me2SiH2 (for L1 and L2) and Me3SiH (for L3). The fundamental of SiO was also observed very weakly. By use of quantum chemical calculations combined with earlier kinetic models, mechanisms have been proposed involving the intermediacy of silanones Me2Si=O and MeSiH=O. Model calculations on the decomposition pathways of H3SiOSiH3 and H3SiOSiH2OSiH3 show that silanone elimination is favoured over silylene extrusion.

The gas-phase reaction between silylene and 2-butyne: kinetics, isotope studies, pressure dependence studies and quantum chemical calculations.

Time-resolved kinetic studies of the reactions of silylene, SiH(2), and dideutero-silylene, SiD(2), generated by laser flash photolysis of phenylsilane and phenylsilane-d(3), respectively, have been carried out to obtain rate coefficients for their bimolecular reactions with 2-butyne, CH(3)C[triple bond, length as m-dash]CCH(3). The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF(6) bath gas at five temperatures in the range 294-612 K. The second-order rate coefficients, obtained by extrapolation to the high pressure limits at each temperature, fitted the Arrhenius equations where the error limits are single standard deviations: log(k(H)(Infinity)/cm(3) molecule(-1) s(-1) = (-9.67 +/- 0.04) + (1.71 +/- 0.33) kJ mol(1)/RTIn10log(k(D)(Infinity)/cm(3) molecule(-1) s(-1) = (-9.65 +/- 0.01) + (1.92 +/- 0.13) kJ mol(-1)/RTIn10. Additionally, pressure-dependent rate coefficients for the reaction of SiH(2) with 2-butyne in the presence of He (1-100 Torr) were obtained at 301, 429 and 613 K. Quantum chemical (ab initio) calculations of the SiC(4)H(8) reaction system at the G3 level support the formation of 2,3-dimethylsilirene [cyclo-SiH(2)C(CH(3))[double bond, length as m-dash]C(CH(3))-] as the sole end product. However, reversible formation of 2,3-dimethylvinylsilylene [CH(3)CH[double bond, length as m-dash]C(CH(3))SiH] is also an important process. The calculations also indicate the probable involvement of several other intermediates, and possible products. RRKM calculations are in reasonable agreement with the pressure dependences at an enthalpy value for 2,3-dimethylsilirene fairly close to that suggested by the ab initio calculations. The experimental isotope effects deviate significantly from those predicted by RRKM theory. The differences can be explained by an isotopic scrambling mechanism, involving H-D exchange between the hydrogens of the methyl groups and the D-atoms in the ring in 2,3-dimethylsilirene-1,1-d(2). A detailed mechanism involving several intermediate species, which is consistent with the G3 energy surface, is proposed to account for this.

Time-resolved gas-phase kinetic study of the germylene addition reaction, GeH2+CH3C[triple bond, length as m-dash]CCH3.

Time-resolved kinetic studies of the reaction of germylene, GeH2, generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reaction with 2-butyne, CH3C[triple bond, length as m-dash]CCH3. 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 300-556 K. The second order rate constants obtained by extrapolation to the high pressure limits at each temperature, fitted the Arrhenius equation: log(kinfinity/cm3 molecule(-1) s(-1)=(-10.46+/-0.06)+(5.16+/-0.47) kJ mol(-1)/RT ln 10. Calculations of the energy surface of the GeC4H8 reaction system were carried out employing the additivity principle, by combining previous quantum chemical calculations of related reaction systems. These support formation of 1,2-dimethylvinylgermylene (rather than 2,3-dimethylgermirene) as the end product. RRKM calculations of the pressure dependence of the reaction are in reasonable agreement with this finding. The reactions of GeH2 with C2H2 and with CH3C[triple bond, length as m-dash]CCH3 are compared and contrasted.

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).

A mechanistic study of cyclic siloxane pyrolyses at low pressures.

Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethyl cyclopentasiloxane (D5), and the results interpreted in the context of various kinetic models. In particular, it is shown that the significant pyrolysis products--which include CH3, CH4, C2H2, C2H4, C2H6 and SiO--may be satisfactorily accounted for by radical reactions involving dimethylsiloxane (D1), and estimates are made of the various chain lengths for the proposed reactions based on a range of ambient conditions.

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, SiD 2, generated by laser flash photolysis of phenylsilane-d 3, have been carried out to obtain rate constants for its bimolecular reaction with C 2H 2. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF 6 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 SiH 2 with C 2H 2 and C 2D 2. Additionally, pressure-dependent rate constants for the reaction of SiH 2 with C 2H 2 in the presence of He (1-100 Torr) were obtained at 300, 399, and 613 K. Quantum chemical (ab initio) calculations of the SiC 2H 4 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, SiH 2 + C 2H 2 --> Si( (3)P 1) + C 2H 4. The results are compared and contrasted with previous studies of this reaction system.