The branching ratios for the reaction of the OH radical with the primary and secondary alkylamines: methylamine (MA), dimethylamine (DMA), and ethylamine (EA), have been determined using the technique of pulsed laser photolysis–laser-induced fluorescence. Titration of the carbon-centered radical, formed following the initial OH abstraction, with oxygen to give HO2 and an imine, followed by conversion of HO2 to OH by reaction with NO, resulted in biexponential OH decay traces on a millisecond time scale. Analysis of the biexponential curves gave the HO2 yield, which equaled the branching ratio for abstraction at αC–H position, rαC–H. The technique was validated by reproducing known branching ratios for OH abstraction for methanol and ethanol. For the amines studied in this work (all at 298 K): rαC–H,MA = 0.76 ± 0.08, rαC–H,DMA = 0.59 ± 0.07, and rαC–H,EA = 0.49 ± 0.06 where the errors are a combination in quadrature of statistical errors at the 2σ level and an estimated 10% systematic error. The branching ratios rαC–H for OH reacting with (CH3)2NH and CH3CH2NH2 are in agreement with those obtained for the OD reaction with (CH3)2ND (d-DMA) and CH3CH2ND2 (d-EA): rαC–H,d-DMA = 0.71 ± 0.12 and rαC–H,d-EA = 0.54 ± 0.07. A master equation analysis (using the MESMER package) based on potential energy surfaces from G4 theory was used to demonstrate that the experimental determinations are unaffected by formation of stabilized peroxy radicals and to estimate atmospheric pressure yields. The branching ratio for imine formation through the reaction of O2 with α carbon-centered radicals at 1 atm of N2 are estimated as rCH2NH2 = 0.79 ± 0.15, rCH2NHCH3 = 0.72 ± 0.19, and rCH3CHNH2 = 0.50 ± 0.18. The implications of this work on the potential formation of nitrosamines and nitramines are briefly discussed.

The methoxymethyl radical, CH3OCH2, is an important intermediate in the low temperature combustion of dimethyl ether. The kinetics and yields of OH from the reaction of the methoxymethyl radical with O2 have been measured over the temperature and pressure ranges of 195–650 K and 5–500 Torr by detecting the hydroxyl radical using laser-induced fluorescence following the excimer laser photolysis (248 nm) of CH3OCH2Br. The reaction proceeds via the formation of an energized CH3OCH2O2 adduct, which either dissociates to OH + 2 H2CO or is collisionally stabilized by the buffer gas. At temperatures above 550 K, a secondary source of OH was observed consistent with thermal decomposition of stabilized CH3OCH2O2 radicals. In order to quantify OH production from the CH3OCH2 + O2 reaction, extensive relative and absolute OH yield measurements were performed over the same (T, P) conditions as the kinetic experiments. The reaction was studied at sufficiently low radical concentrations (∼1011 cm–3) that secondary (radical + radical) reactions were unimportant and the rate coefficients could be extracted from simple bi- or triexponential analysis. Ab initio (CBS-GB3)/master equation calculations (using the program MESMER) of the CH3OCH2 + O2 system were also performed to better understand this combustion-related reaction as well as be able to extrapolate experimental results to higher temperatures and pressures. To obtain agreement with experimental results (both kinetics and yield data), energies of the key transition states were substantially reduced (by 20–40 kJ mol–1) from their ab initio values and the effect of hindered rotations in the CH3OCH2 and CH3OCH2OO intermediates were taken into account. The optimized master equation model was used to generate a set of pressure and temperature dependent rate coefficients for the component nine phenomenological reactions that describe the CH3OCH2 + O2 system, including four well-skipping reactions. The rate coefficients were fitted to Chebyshev polynomials over the temperature and density ranges 200 to 1000 K and 1 × 1017 to 1 × 1023 molecules cm–3 respectively for both N2 and He bath gases. Comparisons with an existing autoignition mechanism show that the well-skipping reactions are important at a pressure of 1 bar but are not significant at 10 bar. The main differences derive from the calculated rate coefficient for the CH3OCH2OO → CH2OCH2OOH reaction, which leads to a faster rate of formation of O2CH2OCH2OOH.

The reaction of OH with dimethyl ether (CH3OCH3) has been studied from 195 to 850 K using laser flash photolysis coupled to laser induced fluorescence detection of OH radicals. The rate coefficient from this work can be parametrized by the modified Arrhenius expression k = (1.23 ± 0.46) × 10–12 (T/298)2.05±0.23 exp((257 ± 107)/T) cm3 molecule–1 s–1. Including other recent literature data (923–1423 K) gives a modified Arrhenius expression of k1 = (1.54 ± 0.48) × 10–12 (T/298 K)1.89±0.16 exp((184 ± 112)/T) cm3 molecule–1 s–1 over the range 195–1423 K. Various isotopomeric combinations of the reaction have also been investigated with deuteration of dimethyl ether leading to a normal isotope effect. Deuteration of the hydroxyl group leads to a small inverse isotope effect. To gain insight into the reaction mechanisms and to support the experimental work, theoretical studies have also been undertaken calculating the energies and structures of the transition states and complexes using high level ab initio methods. The calculations also identify pre- and post-reaction complexes. The calculations show that the pre-reaction complex has a binding energy of ∼22 kJ mol–1. Stabilization into the complex could influence the kinetics of the reaction, especially at low temperatures (<300 K), but there is no direct evidence of this occurring under the experimental conditions of this study. The experimental data have been modeled using the recently developed MESMER (master equation solver for multi energy well reactions) code; the calculated rate coefficients lie within 16% of the experimental values over the temperature range 200–1400 K with a model based on a single transition state. This model also qualitatively reproduces the observed isotope effects, agreeing closely above ∼600 K but overestimating them at low temperatures. The low temperature differences may derive from an inadequate treatment of tunnelling and/or from an enhanced role of an outer transition state leading to the pre-reaction complex.

The rate coefficients for reactions of OH with ethanol and partially deuterated ethanols have been measured by laser flash photolysis/laser-induced fluorescence over the temperature range 298−523 K and 5−100 Torr of helium bath gas. The rate coefficient, k1.1, for reaction of OH with C2H5OH is given by the expression k1.1 = 1.06 × 10−22T3.58 exp(1126/T) cm3 molecule−1 s−1, and the values are in good agreement with previous literature. Site-specific rate coefficients were determined from the measured kinetic isotope effects. Over the temperature region 298−523 K abstraction from the hydroxyl site is a minor channel. The reaction is dominated by abstraction of the α hydrogens (92 ± 8)% at 298 K decreasing to (76 ± 9)% with the balance being abstraction at the β position where the errors are 2σ. At higher temperatures decomposition of the CH2CH2OH product from β abstraction complicates the kinetics. From 575 to 650 K, biexponential decays were observed, allowing estimates to be made for k1.1 and the fractional production of CH2CH2OH. Above 650 K, decomposition of the CH2CH2OH product was fast on the time scale of the measured kinetics and removal of OH corresponds to reaction at the α and OH sites. The kinetics agree (within ±20%) with previous measurements. Evidence suggests that reaction at the OH site is significant at our higher temperatures: 47−53% at 865 K.

The photolysis of acetylene at 193 nm has been investigated as a source of the ethynyl radical, C2H, for product branching ratio studies, particularly the formation of H atom product as the photolysis, producing a 1:1 ratio of C2H and H, provides an internal calibration. Previous literature had suggested that C2H and H may only be a minor component of acetylene photolysis at 193 nm. Acetylene was photolyzed at low laser energy densities (<7 mJ cm−2), with H atoms being observed as a function of time by VUV laser induced fluorescence. When C2H was reacted with C2H2, a reaction that is known to produce H atoms with unit yield, the ratio of photolytic H atom production to chemical production was 0.96 ± 0.03. The rate coefficient for the reaction of C2H with C2H2 could accurately be retrieved from the time evolution of the H atom signal. The results suggest that acetylene photolysis at low laser energies is a good source of C2H for product branching studies, and the technique has been applied to the reactions of C2H with ethene and propene. For the reaction with ethene between 23 and 81 Torr, the yield of H is 0.94 ± 0.06, suggesting that an addition elimination mechanism dominates with the formation of vinylacetylene and H atoms. For the reaction of C2H with propene, no H atom product was observed, putting a lower limit of <5% for H atom production. Possible explanations for the low H atom yield are discussed. The implications of these results in combustion and planetary atmospheres are briefly considered.

The rate coefficients for the removal of the excited state of methylene, 1CH2 (a1A1), by acetylene, ethene, and propene have been studied over the temperature range 195−798 K by laser flash photolysis, with 1CH2 being monitored by laser-induced fluorescence. The rate coefficients of all three reactions exhibit a negative temperature dependence that can be parametrized as k1CH2+C2H2 = (3.06 ± 0.11) × 10−10 T (−0.39±0.07) cm3 molecule−1 s−1, k1CH2+C2H4 = (2.10 ± 0.18) × 10−10 T (−0.84±0.18) cm3 molecule−1 s−1, k1CH2+C3H6 = (3.21 ± 0.02) × 10−10 T (−0.13±0.01) cm3 molecule−1 s−1, where the errors are statistical at the 2σ level. Removal of 1CH2 occurs by chemical reaction and electronic relaxation to ground state triplet methylene. The H atom yields from the reactions of 1CH2 with acetylene, ethene, and propene have been determined by laser-induced fluorescence over the temperature range 298−498 K. For the reaction with propene, H atom yields are close to the detection limit, but for acetylene and ethene, the fraction of H atom production is approximately 0.88 and 0.71, respectively, at 298 K, rising to unity by 398 K, with the balance of the reaction with acetylene presumed to be electronic relaxation. Experimental constraints limit studies to a maximum of 1 Torr of bath gas; master equation calculations using an approach that allows treatment of intermediates with deep energy wells have been carried out to explore the role of collisional stabilization for the reaction of 1CH2 with acetylene. Stabilization is calculated to be insignificant under the experimental conditions, but does become significant at higher pressures. Between pressures of 100 and 1000 Torr, propyne and allene are formed in similar amounts with a slight preference for propyne. At higher pressures propyne formation becomes about a factor two greater than that of allene, and above 105 Torr (300 < T (K) < 600) cyclopropene formation starts to become significant. The implications of temperature-dependent 1CH2 relaxation on the roles of 1CH2 in chemical mechanisms for soot formation are discussed.

The reaction of chlorine atoms with alkyl iodides can play a role in the chemistry of the marine boundary layer. Previous studies have shown that at room temperature the reaction takes place via a complex mechanism including adduct formation. For the Cl + ethyl iodide reaction results on the thermodynamics of adduct formation and on the product yields are inconsistent. The kinetics of the reaction Cl + C2H5I have been studied by the direct observation of the HCl product in real time flash photolysis/IR absorption experiments as a function of temperature from 273 to 450 K. At temperatures above 375 K kinetic measurements confirm a direct process and the rate coefficient determined (4.85 ± 0.55) × 10−11 exp((−363 ± 51)/T) cm3 molecule−1 s−1 is in good agreement with previous direct determinations. Product yield studies have also been undertaken by comparing the HCl signal from Cl + C2H5I with that from a calibration reaction which shows that HCl is the sole product of the reaction at these temperatures. Yield studies with selectively deuterated ethyl iodide demonstrate that abstraction occurs predominantly from the α site, with the selectivity decreasing with temperature. Extrapolation of the yield data to 298 K predicts an α : β ratio of 0.68 : 0.32. At temperatures between 273 and 325 K a biexponential growth was observed for the HCl signal consistent with adduct formation. Analysis of the HCl time profiles allowed the extractions of the forward and reverse rate coefficients for adduct formation and hence the calculations of the thermodynamic properties of adduct formation. A third law analysis yields a value of ΔrH = (−54 ± 4) kJ mol−1. The value of ΔrH is in good agreement with a previous third law determination (J. J. Orlando, C. A. Piety, J. M. Nicovich, M. L. McKee, P. H. Wine, J. Phys. Chem. A, 2005, 109, 6659).

The reaction of chlorine atoms with alkyl iodides can play a role in the chemistry of the marine boundary layer. Previous studies have shown that at room temperature the reaction takes place via a complex mechanism including adduct formation. For the Cl + ethyl iodide reaction results on the thermodynamics of adduct formation and on the product yields are inconsistent. The kinetics of the reaction Cl + C2H5I have been studied by the direct observation of the HCl product in real time flash photolysis/IR absorption experiments as a function of temperature from 273 to 450 K. At temperatures above 375 K kinetic measurements confirm a direct process and the rate coefficient determined (4.85 ± 0.55) × 10−11 exp((−363 ± 51)/T) cm3 molecule−1 s−1 is in good agreement with previous direct determinations. Product yield studies have also been undertaken by comparing the HCl signal from Cl + C2H5I with that from a calibration reaction which shows that HCl is the sole product of the reaction at these temperatures. Yield studies with selectively deuterated ethyl iodide demonstrate that abstraction occurs predominantly from the α site, with the selectivity decreasing with temperature. Extrapolation of the yield data to 298 K predicts an α : β ratio of 0.68 : 0.32. At temperatures between 273 and 325 K a biexponential growth was observed for the HCl signal consistent with adduct formation. Analysis of the HCl time profiles allowed the extractions of the forward and reverse rate coefficients for adduct formation and hence the calculations of the thermodynamic properties of adduct formation. A third law analysis yields a value of ΔrH = (−54 ± 4) kJ mol−1. The value of ΔrH is in good agreement with a previous third law determination (J. J. Orlando, C. A. Piety, J. M. Nicovich, M. L. McKee, P. H. Wine, J. Phys. Chem. A, 2005, 109, 6659).