The rate constants k(d) for the unimolecular decomposition of two acyl (RCO) radicals: 2,2-dimethylpropionyl (CH3)(3)CCO and 2-methylpropionyl (CH3)(2)CHCO (pivaloyl and isobutyryl, respectively in what follows) have been directly measured via the time-resolved kinetics of CO formation as a function of pressure and temperature. The acyl radicals are generated by H-atom abstraction of the aldehydic hydrogen from the corresponding substituted propanals using pulsed laser photolysis of aldehyde/Cl-2/N-2 mixtures. The buildup of CO concentration is monitored by time-resolved infrared absorption at congruent to 2177 cm(-1). The extent of the chain reactions triggered by the reactions of Cl-2 with alkyl and acyl radicals (thus generating further Cl atoms after the initial laser pulse) has been limited by adding variable known concentrations of O-2, a radical scavenger which is sequestering all radicals to form less reactive RO2 radicals, Besides k(d), we have also determined for the first time the rate constants of the reactions of the two acyl radicals with O-2; since the latter do not exhibit any clear trend with either temperature or pressure, we thus adopt the average of all experimental values: i.e. for pivaloyl : kO(2) = (3.5 +/- 1.0) x 10(-12) cm(3) molecule(-1) s(-1), and for isobutyryl : kO(2) = (3.2 +/- 1.0) x 10(-12) cm(3) molecule(-1) s(-1). our values of kd, which are in the falloff range, are compared to those obtained from theoretical calculations using the Troe formalism [5]. For pivaloyl at 298 K, we obtain the following limiting rate constants : k(0) = 2.1 x 10(-13) cm(3) molecule(-1) s(-1), k(infinity) = 4.3 x 10(5) s(-1), F-C = 0.515, which are at variance with the theoretical predictions of Tomas et al. (Phys Chem Chem Phys 2000, 2, 1165-1174). For isobutyryl, our experimental data are entirely consistent with the predictions of Tomas et al. Extrapolation of our Troe parameters to tropospheric conditions (298 K, atmospheric pressure) shows, in agreement with previous authors, that for both radicals the reaction with O-2 is several orders of magnitude faster than the unimolecular decomposition. (c) 2005 Wiley Periodicals, Inc.

Recent studies have shown that the reaction between acetyl radicals and O2 at low pressures leads to the direct fast formation of OH radical, but the nature of co-products is controversial. Laser photolysis coupled to TDLAS (10–200Torr, 298K) has been employed to directly monitor two possible co-products of this reaction, CO and formaldehyde. Only CH2O has been detected in yield of 0.29±0.13, but with time constants much slow than the OH formation under these conditions; the observed CH2O-time profiles are compatible with the known mechanism of peroxyacetyl secondary reactions.

The reactivity of 2-butoxy radicals has been investigated using the laser photolysis/laser induced fluorescence technique. Three reactions have been studied: (i) The rate constants for the reaction with NO have been measured by the same technique at total pressures between 0.03 < p < 0.4 bar of helium and at five temperatures between 295 - 348 K. No pressure dependence has been found within the experimental error, and a small negative temperature dependence has been found, in agreement with earlier studies: k(1) = (4.4 +/- 0.6) x 10(-12) exp((4.9 +/- 0.3) kJ mol(-1)/RT) cm(3) s(-1). (ii) The rate constant with O-2 has been measured at room temperature and 0.131 bar of helium: k(2) = (9 +/- 2) x 10(-15) cm(3) s(-1). Significant quenching of 2-butoxy fluorescence by O2 prevented experiments in a larger temperature range: k(q),(O2) = (4 +/- 1) x 10(-11) cm(3) s(-1). (iii) The temperature and pressure dependence of the unimolecular decomposition at total pressures between 0.01 < p < 0.8 bar of helium and at four temperatures between 291 - 348 K. The low and the high pressure limiting rate constants as well as the broadening factor F-cent have been extracted from a falloff analysis of the experimental results: k(3,0,He) = 3.2 x 10(-8) exp(-35.9 kJ mol(-1)/RT) cm(3) s(-1), k(3,infinity) = 1.1 x 10(14) exp(-53.6 kJ mol(-1)/RT) s(-1), and F-3,F-c = 0.87 - T/870 K. We anticipate an uncertainty of +/-30% for these rate constants. These results are in excellent agreement with earlier predictions (C. Fittschen, H. Hippler and B. Viskolcz, Phys. Chem. Chem. Phys., 2000, 2, 1677 - 1683 (ref. 1); R. Mereau, M.- T. Rayez, F. Caralp and J.-C. Rayez, Phys. Chem. Chem. Phys., 2000, 2, 3765 ( ref. 2)).

The reactions of vinoxy, CH2CHO and 1-methylvinoxy (acetonyl) CH2C(CH3)O radicals with NO and He as a bath gas have been investigated by experimental and theoretical methods as a function of temperature and pressure (0.8-920 mbar). Two classical techniques have been used for the experiments: the discharge flow (DF) and the laser ash photolysis (LFP) techniques, both associated with a monitoring of the radicals by laser induced fluorescence. Calculations of the potential energy surface were performed using ab initio molecular orbital theory at the G2 level. Derived molecular properties of the characteristic points of the potential energy surface were used to describe the mechanism and kinetics of the reactions under investigation. The nitroso-adducts, ON-CH2CHO and ON-CH2C(CH3)O are predicted as the major addition products. Fall-off behavior of the reaction system was analyzed within the Troe formalism. The constructed fall-off curves allow a description of the reaction kinetics in wide ranges of pressure and temperature.The following high and low pressure limiting rate constants have been obtained:[GRAPHICS]

A new method for measuring trace concentrations of atmospheric pollutants by infrared diode laser spectroscopy has been devised. This method relies on the increase of the signal as the pressure inside the cell increases, while the frequency of the diode is stabilized on the line, even if it is unresolved. Performances of this method were tested with N2O and with 1,3-butadiene. As an example of application, we measured the butadiene emitted by car exhausts. Sensitivity and rapidity of this method are equivalent to the usual scanning method in which the whole line is described, but this new method benefits from its simplicity and robustness.

First kinetic measurements for CH3O reactions have been obtained for three cyclohydrocarbons using the discharge flow reactor combined with the laser induced fluorescence technique to detect CH3O radicals over the pressure and temperature ranges 1-7 Torr of helium and 300-513 K. Measurements have been performed for the cyclohydrocarbons c-C6H12 (k(1)), c-C6H10 (k(2)), and 1,4-c-C6H8 (k(3)). In addition to the experimental work, we have performed ab initio molecular orbital computations to get an insight into the mechanism of the three reactions, using PMP2, HF-DFT (B3LYP), and CASPT2 methods. The rate constant k(1) has been calculated using the Transition State Theory. Measured rate constants are pressure independent in our experimental range. Arrhenius expressions are (k(1) in cm(3) s(-1)) k(1) = 8.8((-5.0) (+11.0)) x 10(-12) exp[-(24.5 +/- 3.0) Kj mol(-1)/RT] (403-513 K), k(2) (3.1 +/- 0.8) x 10(-12) exp[-(15.3 +/- 0.8) U mol(-1)/RT] (300-503 K), and k(3) = 1.9((+1.6)(-0.9)) x 10(-12) exp[-(7.6 +/- 1.9) Kj mol(-1)/RT] (300-513 K). A good agreement between the experimental -0.9 and theoretical rate constants k(1) has been found. The comparison between the computed and the experimentally determined barrier heights serves as an endorsement of the increasing reactivity in the series from cyclohexane system to the 1,4-cyclohexadiene system.

The reactions of formyl (HCO) and hydroxymethyl (CH2OH) radicals with O2 have been studied at room temperature (294±2K) using pulsed laser photolysis (PLP)/tunable diode laser (TDL) absorption spectroscopy. The formation of the stable molecular product, CO or HCH(O), has been monitored. The derived values, k1=(5.0±0.7)×10−12cm3molecule−1s−1 and k2=(10±1.4)×10−12cm3molecule−1s−1, are in excellent agreement with previously published values obtained with different techniques.

We experimentally determined complete falloff curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two different techniques, laser flash photolysis and fast flow reactor were used both coupled to a detection of C2H5O radicals by laser induced fluorescence. Experiments were performed at total pressures between 0.001 and 60 bar of helium and in the temperature range of 391–471 K. Under these conditions the β-C–C scission (1a) CH3CH2O+M→CH2O+CH3+M is the dominating decomposition channel. From a complete analysis of the experimental falloff curves the low and the high pressure limiting rate constants of k1a,0=[He] 3.3×10-8 exp(-58.5 kJ mol-1/RT) cm3 s-1 and k1a,∞=1.1×1013 exp(-70.3 kJ mol-1/RT) s-1 were extracted. We estimate an uncertainty for the absolute values of these rate constants of ±30%. Preexponential factor and activation energy are significantly lower than previous estimations. The rate constants are discussed in terms of statistical unimolecular rate theory. Excellent agreement between the experimental and the statistically calculated rate constants has been found. BAC-MP4, QCISD(T), or higher level of theory provide a reliable picture of the energy and the structure of the transition state of this radical bond dissociation reaction. On the same theoretical basis we predict the high pressure limiting rate constant for the β-C–H scission (1b) CH3CH2O+M→CH3CHO+H+M of k1b,∞=1.3×1013 exp(-84 kJ mol-1/RT) s-1. Atmospheric implications are discussed.

The rate constants of the reactions of ethoxy (C2H5O), i-propoxy (i-C3H7O) and n-propoxy (n-C3H7O) radicals with O-2 and NO have been measured as a function of temperature. Radicals have been generated by laser photolysis from the appropriate alkyl nitrite and have been detected by laser-induced fluorescence. The following Arrhenius expressions have been determined:(R-1) C2H5O + O-2 --> products k(1) = (2.4 +/- 0.9) X 10(-14) exp(-2.7 +/- 1.0 kJmol(-1)/RT) cm(3) s(-1) 295K < T < 354K p = 100 Torr(R-2) i-C3H7O + O-2 --> products k(2) = (1.6 +/- 0.2) X 10(-14) exp(-2.2 +/- 0.2 kJmol(-1)/RT) cm(3) s(-1) 288K < T < 364K p = 50-200 Torr(R-3) n-C3H7O + O-2 --> products k(3) = (2.5 +/- 0.5) X 10(-14) exp(-2.0 +/- 0.5 kJmol(-1)/RT) cm(3) s(-1) 289K < T < 381K p = 30-100 Torr(R-4) C2H5O + NO --> products k(4) = (2.0 +/- 0.7) X 10(-11) exp(0.6 +/- 0.4 kJmol(-1)/RT) cm(3) s(-1) 286K < T < 388K p = 30-500 Torr(R-5) i-C3H7O + NO --> products k(5) = (8.9 +/- 0.21 x 10(-12) exp(3.3 +/- 0.5 kJmol(-1)/RT) cm(3) s(-1) 286K < T < 389K p = 30-500 Torr(R-6) n-C3H7O + NO --> products k(6) = (1.2 +/- 0.2) X 10(-11) exp(2.9 +/- 0.4 kJmol(-1)/RT) cm(3)s(-1) 289K < T < 380K p = 30-100 TorrAll reactions have been found independent of total pressure between 30 and 500 Torr within the experimental error. (C) 1999 John Wiley & Sons, Inc.

The temperature and pressure dependence of the rate constant for the unimolecular decomposition of i-propoxy radicals has been determined using the laser photolysis/laser induced fluorescence technique. Important features of the potential energy surface have been calculated by nb initio methods. Experiments have been performed at total pressures between 0.01 and 60 bar of helium and in the temperature range 330-408 K. The low and the high pressure limiting rate constants have been extracted from a complete falloff analysis: k(0) = [He] x 1.0 x 10(-8) exp(-43.8 kJ mol(-1)/RT) cm(3) s(-1) and k(infinity) = 1.2 x 10(14) exp(- 63.7 kJ mol(-1)/RT) s(-1). We estimate an uncertainty for these rate constants of +/-30%. Both rate constants have been discussed in terms of statistical unimolecular rate theory. Very good agreement between the calculated and the experimental rate constants has been found.

The rate constants for the reactions of CH3O radicals with 3 reactants have been measured by two different techniques : laser photolysis (LP) and fast flow reactor (FF), both coupled with a detection of CH3O radicals by laser induced fluorescence. The reaction with formaldehyde CH2O has been measured in the temperature range 295 - 450 K. Both sets of results (LP and FF) are in excellent agreement and lead to a pressure independent rate constant of k(1) (1.1+/-0.3) x 10(-12) exp(-9.6+/-1.0 kJ mol(-1)/ RT) cm(3) s(-1). The reaction with acetaldehyde CH3CHO has been measured between 286 and 493 K. Both sets of experiments are again in excellent agreement and lead to a pressure independent rate constant of k(2) = (5.7+/-1.3) x 10(-13) exp(-5.2+/-0.7 kJ mol(-1)/RT) cm(3) s(-1). No reaction could be detected with isobutane i-C4H10 at temperature up to 356 K, leading to an upper limit for k(3) less than or equal to 2.5 x 10(-15) cm(3) s(-1).

New kinetic measurements for the CH3O + NO reaction have been performed using two different techniques. The discharge flow (DF) technique has been used to investigate the 0.5–5 Torr and 248–473 K pressure and temperature ranges and pulsed laser photolysis (PLP) has been used for the 30–500 Torr and 284–364 K ranges. These new results represent an extension of the pressure and temperature ranges investigated previously. This reaction is known to present two reaction pathways, the association pathway yielding CH3ONO and the disproportionation pathway yielding CH2O+HNO. Based on literature and present experimental data, using the results of ab initio calculations, a multichannel RRKM analysis was developed to interpret the experimental results. This analysis has shown that the disproportionation reaction occurs simultaneously by both a direct hydrogen abstraction reaction, and via the formation of energized CH3ONO* complex in competition with the association reaction. The RRKM analysis, fitted to present and previous data, has yielded a second-order limiting low-pressure value of 2.5 × 10−12 cm3 molecule−1 s−1 at 298 K, with a complex temperature dependence. The limiting high-pressure rate constant derived in the same way is k∞ = (3.4 ± 0.4) × 10−11(T/298)−0.75. The model allows the prediction of CH3O loss rate constants and of the branching ratios in the 1–760 Torr and 220–600 K ranges. For a convenient presentation of the overall rate constant, an analytical expression using the conventional Troe expression with a temperature-dependent addition constant, has been fitted to the results of the RRKM analysis.

The kinetics of the reaction of the CF3O radical with NO2 has been studied at 298 K at low pressure (0.5-9 Ton of helium) by fast-low-laser-induced fluorescence (LIF), pyrolysis of a dilute mixture of CF3OOCF3 and He being used as the source of the CF3O radical. In good agreement with the results of Zellner and co-workers (obtained in the pressure range 5-100 Ton), the rate constant has been found to be pressure dependent. All the experimental data were analysed by a multichannel RRKM procedure using the results of ab initio calculations as input data. This revealed that two reaction channels, the association CF3O +/- NO2 --> CF3ONO2 (1a) and the disproportionation CF3O + NO2 --> CF2O + FNO2 (1b), must be invoked and they proceed via a common energized adduct CF3ONO2*. The pressure dependence of the branching ratio was predicted by this calculation: the disproportionation channel would be negligible near atmospheric pressure and becomes the major channel at pressure below ca. 0.3 Torr, with a calculated second-order limiting low-pressure value of the rate equal to 3.2 x 10(-12) cm(3) molecule(-1) s(-1). The high-pressure limit rate constant obtained is k(infinity) = (1.65 +/- 0.2) x 10(-11) cm(3) molecule(-1) s(-1). An analytical representation of the pressure dependence of the rate constant at 298 K is proposed using the conventional Tree expression with an added constant.

The rate constant k(5) of the reaction R(5) : CF3O + O-3 --> products has been measured at room temperature with the absolute Fast Flow tube technique in the pressure range 2-9 Torr. The CF3O radicals are generated by pyrolysis of the dimer CF3OOCF3 and their relative concentration is followed by Laser Induced Fluorescence. The CF3O/He mixture is flowed through the movable central injector whereas the reactant (ozone) is flowed into the main flow tube; this protocol allows a better control of the side reactions which compete with R(?)5. The average of our measurements leads to: k(5) = (1.3 +/- 0.5) x 10(-14) cm(3) molec(-1) s(-1) which is in reasonable agreement with the more recent determinations.