George Marston

Kinetics of the reactions of the nitrate radical with a series of halogenobutenes. A study of the effect of substituents on the rate of addition of NO3 to alkenes

Rate coefficients for the reaction of NO3 with a series of halogenobutenes have been measured using the discharge-flow technique coupled to optical-absorption detection of NO3. The following room-temperature rate coefficients and Arrhenius parameters (cm3 molecule–1 s–1) were measured: 1-chlorobut-1-ene, 1.2 × 10–14; 2-chlorobut-1-ene, 7.0 × 10–14; 2-chlorobut-2-ene, 11.0 × 10–14; 1-chloromethylpropene, 9.0 × 10–14; 3-bromobut-1-ene, 0.4 × 10–14; 4-bromobut-1-ene, 0.5 × 10–14; 2-bromobut-2-ene, 13.4 × 10–14; 3-chlorobut-1-ene, 2.4 × 10–12 exp(–1992/T); 1-chlorobut-2-ene, 6.0 × 10–13 exp(–981/T); 3-chloromethylpropene, 1.7 × 10–12 exp(–1277/T). Trends in the reactivities of the compounds towards NO3 are discussed in terms of the relative energies of the interacting orbitals, and the data are used to calculate group reactivity factors. These factors can be used to estimate rate constants which have not, as yet, been measured.

Correlations between rate parameters and calculated molecular properties in the reactions of the nitrate radical with alkenes

A study of the reactivity of the No3 radical towards a series of alkenes and halogenoalkenes has been carried out. For those compounds not containing a vinylic chlorine atom, the activation energy, the logarithm of the pre-exponential A factor and the logarithm of the room-temperature rate coefficient (k) correlate strongly with the calculated ionization potential (Ei) of the organic reactant. In the case of alkenes containing vinylic chlorine atoms, the correlations of k and activation barriers with Ei break down, although the correlation between log A and Ei still holds. On the basis of the extent of interaction between the chlorine atom lone-pair orbitals and the carbon–carbon π bond, corrections to the Ei s can be made and the activation barriers and log ks then correlate well with these corrected Ei s. Using these correlations, Arrhenius parameters can be estimated and used to calculate ks, which are compared with measured values. Excellent agreement is observed, providing support for this method of estimating Arrhenius parameters. The observations are rationalized in terms of frontier orbital theory.

Correlations between rate parameters and calculated molecular properties in the reactions of the hydroxyl radical with hydrofluorocarbons

Abstract A study of the reactivity of the hydroxyl radical towards a series of hydrocarbons and fluorinated hydrocarbons has been carried out. The activation energy (Ea), the logarithm of the pre-exponential A factor, and the logarithm of the room temperature rate coefficient (k) all correlate strongly with the calculated ionization potential (IP) of the organic reactant. Using these correlations, the accuracy of the experimental database of reaction-rate parameters is examined, and predictions are made of rate parameters for related reactions that have not, as yet, been measured.

Kinetics and mechanism of the reaction of CH3 and CH3O with ClO and OClO at 298 K

A discharge-flow system equipped with a laser-induced fluorescence cell to detect the methoxyl radical and a quadrupole mass spectrometer to detect both the chlorine monoxide and chlorine dioxide radicals has been used to measure the rate constants for the reactions CH3+ ClO → products (1), CH3O + ClO → products (2), CH3+ OClO → products (3), CH3+ OClO → products (4), at T= 298 K and P= 1–3 Torr. The observed products of these reactions are CH3O for reaction (1), HOCl for reaction (2), and CH3O and ClO for reaction (3). For reaction (4), CH3OCl is a possible product. The rate constants derived for reactions (1)–(4) are: k1=(1.3 ± 0.4)× 10–10 cm3 molecule–1 s–1; k2=(1.3 ± 0.3)× 10–11 cm3 molecule–1 s–1; k3=(1.6 ± 0.3)× 10–11 cm3 molecule–1 s–1; k4=(1.5 ± 0.5)× 10–12 cm3 molecule–1 s–1. The likely mechanisms for reactions (1)–(4) are briefly discussed.

Kinetics and chemiluminescence in the reaction of N atoms with O2 and O3

Room-temperature rate constants for the reactions N + O2→ NO + O (1), N + O3→ NO + O2(2), have been measured using the discharge flow chemiluminescence technique. Values obtained are, k1=(8.8 ± 0.4)× 10–17 cm3 molecule–1 s–1, k2=(1.0 ± 0.2)× 10–16 cm3 molecule–1 s–1. Visible and near-i.r. emissions were observed on adding O2 or O3 to a flow of N atoms, but neither reaction (1) nor reaction (2) appears to be the source of the chemiluminescence. Further experiments were conducted to elucidate the excitation mechanisms. Chemiluminescence from electronically excited NO and O2 was observed and mechanisms for the formation of these species are proposed.

Formation of O2(a1Δg) and vibrationally excited OH in the reaction between O atoms and HOx species

The first excited state of molecular oxygen [O2(a1Δg)] has been observed in the reaction between H atoms and O3. It is shown that the most likely mechanism for the generation of the excited species is a branch of the reaction O + OH(v 1)→ O2+ H (1a) that leads to O2(a1Δg) O + OH(v 1)→ O2(a1Δg)+ H. (1b) It is estimated that reaction (1b) contributes ca. 2.5% to the overall reaction (1a). Chemiluminescence from OH(v⩽6) has been observed in the reaction between O atoms and HO2. Implications for atmospheric airglow chemistry are discussed.

Arrhenius parameters for the reaction of the nitrate radical with 1,1-dichloroethene and (E)-1,2-dichloroethene

The kinetics of the reactions of the nitrate radical, NO3, with 1,1-dichloroethene and (E)-1,2-dichloroethene have been measured using the discharge-flow technique coupled to detection of NO3 by optical absorption. The values obtained for the rate coefficients at room temperature are (1.2 ± 0.3)× 10–15 and (1.2 ± 0.5)× 10–16 cm3 molecule–1 s–1(95% confidence with 15% systematic errors included) and the derived Arrhenius expressions are k2(T)=(4.7 ± 2.4)× 10–13 exp[–(1800 ± 240)/T] and k3(T)=(4.5 ± 0.5)× 10–13 exp [–(2400 ± 40)/T] cm3 molecule–1 s–1(errors quoted at 1σ). Comparison of the results with those of previous studies indicates that Cl-atom elimination is not important for either reaction. The dependence of the kinetic parameters on the molecular structure of the dichloroethenes is discussed.

Fourier-transform detection of singlet oxygen and fluorescence from cell membrane bound porphyrins

A weak luminescence due to singlet oxygen O2(a 1Δg) and a stronger fluorescence emission from a porphyrin bound to the surface of living cells have been obtained using a Fourier-transform interferometric technique. The porphyrin fluorescence shows a marked wavelength shift compared with that observed when the cells are not present. The singlet oxygen emission is accompanied by cell membrane destruction as shown by the Trypan Blue exclusion test.

Phosphorescence of ?-carotene

The lowest triplet energy level of all-trans β-carotene has been directly established as 88 ± 3 kJ mol–1 from the weak phosphorescence observed using a sensitive Fourier transform based interferometer.