Henry M. Frey

The absorption cross section of the HCO radical at 614.59 nm and the rate constant for HCO+HCO → H2CO+CO

Abstract Using a combination of XeCl exciplex laser flash photolysis of gas-phase glyoxal and formaldehyde and time-resolved cw dye laser absorption at 614.59 nm, we have determined the ratio k 1 /σ for the reaction HCO+HCO → H 2 CO+CO (1) at 295 ±2 K. Similar studies involving the 308 nm photolysis of a variety of aldehydes combined with a determination of the absolute yields of the resulting hydrocarbon products have allowed us to deduce the initial yields of HCO radicals and hence the absorption cross section for HCO at the monitoring wavelength. We find σ=(2.3±0.6) × 10 −18 cm 2 , giving k 1 =(7.5±2.9)× 10 −11 cm 3 molecule −1 s −1 . Our values are compared with previous results.

The photodissociation of phenylsilane at 193 nm

Abstract Molecular hydrogen is observed to be one of the major primary products in the 193 nm photodissociation of phenylsilane. A two-channel dissociation mechanism is proposed, yielding PhSiH+H 2 and SiH 2 +PhH with the former predominant. The implications of this observation for experiments which utilise phenylsilane as a precursor for SiH 2 radicals are discussed.

Absolute rate constant and temperature dependence for the reaction of silylene with nitrous oxide

Abstract Absolute rate constants have been measured for the reaction of SiH 2 with N 2 O at temperatures between 295–747 K. The rate constants are fitted to the Arrhenius equation: log ( k /cm 3 molecule −1 s −1 ) = ( − 12.09 ± 0.04) + (2.02 ± 0.31) kJ mol− / RT ln 10. The data are plausibly consistent with a mechanism involving a short-lived H 2 SiON 2 addition complex leading to formation of H 2 SiO + N 2 . Comparisons are made with other silylene reactions. The fate and stability of H 2 SiO are briefly discussed. The significance of these measurements for the kinetic modeling of the chemical vapour deposition of silicon oxide from silane/N 2 O mixtures is also commented on.

Studies of methylene chemistry by pulsed laser-induced decomposition of ketene. Part 1.—Ketene in the presence of noble gases

Gaseous mixtures of ketene with argon (or xenon) have been photodecomposed at room temperature by 308 nm u.v. radiation from a pulsed exciplex laser. At low conversions the major products are ethylene, acetylene, hydrogen and carbon monoxide. Minor products include methane, ethane, propene and allene. Kinetic modelling based on the Gear algorithm has been used to predict the product distribution and its pressure dependence. The modelling reveals that: (a) the major product of the recombination process 23CH2→ C2H2+ H2(2H) is molecular and not atomic hydrogen; (b) the disproportionation process 23CH2→ CH3+ CH is probably occurring; (c) primary quantum yields are as follows: CH2CO +hν(308 nm)→ CH2(1A1)+ CO ϕ= 0.6 ± 0.1, → CH2(3B1)+ CO ϕ= 0.4 ∓ 0.1; (d) for the reaction 1CH2+ CH2CO → C2H4+ CO k= 2.1+0.7–0.5× 10–10 cm3 molecule–1 s–1; (e) propene and allene are formed via the intermediacy of the vinyl radical, and secondary reactions such as 3CH2+ C2H4→ C3H6, +C2H2→ C3H4 are unimportant (rate constants are < 10–15 cm3 molecule–1 s–1).These findings are compared and contrasted with previous flash-photolysis and other studies of ketene photodecomposition.

Similarities and differences in the addition reactions of silylene (SiH2, 1A1) and methylene (CH2, 1A1) to C2H4

Abstract Absolute rate constants have been measured for the reaction of SiH 2 ( 1 A 1 ) with C 2 H 4 at room temperature in highly diluted gas mixtures with Ar and SF 6 . Rate constants are pressure dependent consistent with a third-body mediated association process. This is supported by RRKM calculations on silirane (silacyclopropane). Differences with the reaction of CH 2 ( 1 A 1 )+C 2 H 4 are explained on energetic grounds.

Thermal unimolecular decomposition of β-butyrolactone (4-methyloxetan-2-one)

The thermal decomposition of β-butyrolactone has been studied in the temperature range 209–250 °C. At each temperature, runs were carried out from 10 Torr to < 0.1 Torr. In an aged reaction vessel the decomposition was homogeneous and yielded propene and carbon dioxide as the only products. All the experimental evidence indicated that the reaction was a true unimolecular decomposition which at 10 Torr was close to its high-pressure limit.The Arrhenius equation corresponding to this limit is log (k∞/s–1)= 14.39 ± 0.10–(163.4 ± 1.0) kJ mol–1/RT ln 10. Arrhenius parameters are also reported for the reaction some way into the fall-off region. The experimental fall-off curves are compared with those calculated from RRKM theory and some discussion about the deviations between theory and experiment is presented. The mechanism of the reaction is considered and the results obtained in this work compared with those reported for β-propiolactone.

Thermal unimolecular decomposition of 1,1,2,2-tetrafluorocyclobutane

The thermal decomposition of 1,1,2,2-tetrafluorocyclobutane has been studied in the gas phase in the temperature range 485–593°C and at pressures around 5–9 Torr. There are two homogeneous first order decomposition pathways, one yielding 1,1-difluoroethylene (k1) and the other ethylene and tetrafluoroethylene (k2). Arrhenius equations have been obtained for these processes viz.: log k1/s–1= 15.34 ± 0.05 –(292.0 ± 0.8 kJ mol–1)/RT ln 10 log k2/s–1= 15.27 ± 0.06 –(308.1 ± 0.9 kJ mol–1)/RT ln 10. The decomposition is almost certainly unimolecular and at the pressures studied the rate constants are close to their infinite pressure values. Rate constants were also determined from 22 to 0.01 Torr at 542.2 °C and the values obtained compared with those calculated using RRKM theory on the basis of various models. The energetics of the decomposition pathways are compared with cyclobutane and the fluorinated cyclobutanes.

Photolysis of 3-methyl-3-chlorodiazirine

The photolysis of 3-methyl-3-chlorodiazirine has been investigated in the gas phase at pressures (with added bath gases) up to 6000 Torr (800 kPa). The principal products were vinyl chloride, nitrogen, acetylene, hydrogen chloride and 1,1-dichloroethane. The experimental results indicate that the vinyl chloride is formed with a very wide energy spread; at low pressures most of it decomposes to yield acetylene and HCl. Evidence is presented that the dichloroethane results from the reaction of methylchlorodiazomethane, formed by photoisomerization from the diazirine, with the HCl produced in the system. There is no evidence that trapping by HCl of methylchlorocarbene, formed as the primary decomposition product of the electronically excited methylchlorodiazirine, can be more than a minor reaction pathway.

Thermal unimolecular reactions of vinylcyclobutane and isopropenylcyclobutane

The thermal decomposition of vinylcyclobutane proceeds by two pathways both of which are homogeneous first order processes. One reaction channel (k1) yields butadiene and ethylene, the other (k2) cyclohexene. Arrhenius equations have been obtained for both pathways from rate constants determined in the temperature range 296 to 366°C, viz: log k1/s–1= 14.87 ± 0.07 –(212,200 ± 800) J mol–1/RTln 10, log k2/s–1= 13.86 ± 0.13 –(203,500 ± 1500) J mol–1/RT ln 10. The thermal decomposition of isopropenylcyclobutane has been reinvestigated and found to be analogous to the vinyl compound. Arrhenius parameters have been obtained for its two modes of decomposition. All the reactions appear to be true unimolecular transformations. The potential energy surfaces for these decompositions and the back reactions are discussed.

Kinetics of methylene addition to cis- and trans-but-2-ene. Further evidence for the energy separation between triplet and singlet methylene

The reactions of triplet and singlet methylene with cis- and trans-but-2-ene have been studied over the temperature range 350–473 K. The results yield a value of (36.5 ± 3.2) kJ mol–1 for the energy separation between singlet and triplet methylene, and provide further confirmation of the assumption that singlet methylene reactions with hydrocarbons proceed with activation energies close to zero. Previous evidence for the similar reactivities of triplet methylene and the methyl radical receives additional support.