Experimental investigation and modeling of the kinetics of CCl4 pyrolysis behind reflected shock waves using high-repetition-rate time-of-flight mass spectrometry.

The pyrolysis kinetics of CCl(4) behind reflected shock waves was studied with high-repetition-rate time-of-flight mass spectrometry. For modeling, quantum mechanical calculations were performed to evaluate the dissociation energies of CCl bonds for the different CCl(x) (x = 1 to 4) radicals. Good agreement with the JANAF thermochemical table was found. With the reaction mechanism developed for CCl(4) decomposition satisfactory agreement with experimental results was obtained. The investigations show the importance of C(2)Cl(2) formation for understanding the processes of carbon cluster growth leading to carbonaceous particle formation.

Acetone photolysis at 248 nm revisited: pressure dependence of the CO and CO2 quantum yields.

Pressure dependent CO and CO2 quantum yields in the laser pulse photolysis of acetone at 248 nm and T = 298 K have been measured directly using quantitative infrared diode laser absorption. The experiments cover the pressure range from 50 to 900 mbar. It is found that the quantum yields show a significant dependence on total pressure, with Phi(CO) decreasing from around 0.5 at 20 mbar to approximately 0.3 at 900 mbar. The corresponding CO2 yields as observed when O2 exists in the reaction mixture, exhibit exactly the opposite behaviour. For the sum of both a value of 1.05(-0.05)(+0.02) independent of pressure is obtained, showing that the sum of (Phi(CO) + Phi(CO2)) is a measure for the primary quantum yield in the photolysis of acetone. In addition, CO quantum yields and corresponding pressure dependences were measured in experiments using different bath gases including He, Ar, Kr, SF6, and O2 as third body colliders. The theoretical framework in which we discuss these data is based on our previous findings that the pressure dependence of the CO yield is a consequence of a stepwise fragmentation mechanism during which acetone decomposes initially into methyl and a vibrationally 'hot' acetyl radical, with the latter being able to decompose promptly into methyl plus CO. The pressure dependence of the CO yield then originates from the second step and is modelled quantitatively via statistical dynamical calculations using a combination of RRKM theory with a time-dependent master equation (ME) approach. From a comparison of experiment with theory the amount of excess energy in the vibrationally hot acetyl radicals (E* approximately 65 kJ mol(-1)) as well as the characteristic collision parameters for interaction of acetyl with the different bath gases were derived. Values of 90, 280, 310, 545, 550 and 1800 cm(-1) for the average energy transferred per downward collision for the bath gases He, Ar, Kr, O2, N2, and SF6, respectively, are obtained. The calculations also considered different models for the energy transfer kernel P(E,E') and best fits were obtained with a rho-weighted exponential down model.

The contribution of tunnelling to the 1,5 H-shift isomerisation reaction of alkoxyl radicals.

Theoretical models have been used to derive rate coefficients for the unimolecular reaction pathways of two prototypical alkoxyl radicals (1-butoxyl and 2-pentoxyl) which can undergo 1,5 H-shift isomerisation yielding the corresponding delta-hydroxy alkyl radical. Special emphasis has been given to the contribution of tunnelling in the isomerisation channels which has not been accounted for in previous theoretical studies. The combination of high level ab initio calculations with a fully coupled multiple channel master equation (ME) treatment resulted in a significant increase of the isomerisation rates by about a factor of 2.7 for the 1-butoxyl and 2.3 for the 2-pentoxyl radical, respectively, as compared to calculations in which tunnelling was neglected, even at 298 K. The corresponding Arrhenius energies in the temperature range from 200 up to 1000 K are significantly smaller when tunnelling is accounted for and differ from previous results which focused only on temperatures around 298 K. The rate expressions derived for the 1,5 H-shift isomerisation reactions are: k(iso,1but) = 1.58 x 10(12)(T/300 K)(-2.30) exp(-4679 K/T) s(-1) and k(iso,2pent) = 4.65 x 10(12)(T/300 K)(-3.22) exp(-4782 K/T) s(-1) valid for p = 1013 mbar and temperatures between 200 K < or = T < or = 1000 K. Our results are strongly supported by recent experiments of Cox and co-workers, (D. Johnson, P. Cassanelli, and R. A. Cox, J. Phys. Chem. A, , 2004, 108, 519 and P. Cassanelli, D. Johnson, and R. A. Cox, Phys. Chem. Chem. Phys. 2005, 7, 3702, 18) which are among the very few studies performed in a temperature regime (250 < or = T < or = 320 K) where tunnelling is thought to be effective. Since tunnelling has been neglected so far in the theoretical analysis of experimental data, the reliability of existing extrapolations to higher and lower temperatures is discussed in detail.

Pressure dependence for the CO quantum yield in the photolysis of acetone at 248 nm: a combined experimental and theoretical study.

The quantum yield of CO in the laser pulse photolysis of acetone at 248 nm and at 298 K in the pressure range 20-900 mbar (N2) has been measured directly using quantitative infrared diode laser absorption of CO. It is found that the quantum yield of CO shows a significant dependence on total pressure with Phi(CO) decreasing with pressure from around 0.45 at 20 mbar to approximately 0.25 at 900 mbar. From a combination of ab initio quantum chemical calculations on the molecular properties of the acetyl (CH3CO) radical and its unimolecular fragmentation as well as the application of statistical (RRKM) and dynamical calculations we show that CO production results from prompt secondary fragmentation (via(2a)) of the internally excited primary CH3CO* photolysis product with an excess energy of approximately 62.8 kJ mol(-1). Hence, our findings are consistent with a consecutive photochemically induced decomposition model, viz. step (1): CH3COCH3+hv--> CH3CO*+ CH3, step (2a): CH3CO*--> CH3+ CO or step (2b) CH3CO*-(+M)--> CH3CO. Formation of CO via a direct and/or concerted channel CH3COCH3+hv--> 2CH(3)+ CO (1') is considered to be unimportant.