P. H. Wine

Thermochemistry and Kinetics of the Cl+O2 Association Reaction

Abstract Laser flash photolysis of Cl 2 /O 2 mixtures has been employed in conjunction with Cl( 2 P 3 2 ) detection by time-resolved resonance fluorescence spectroscopy to investigate equilibration kinetics for the reactions Cl + O 2 + O 2 ⇌ ClOO + O 2 at temperatures of 181–200 K and O 2 pressures of 15–40 Torr. The third-order rate coefficient for the association reaction at 186.5 ± 5.5 K is (8.9 ± 2.9) × 10 −33 cm 6 molecule −2 s −1 and the equilibrium constant ( K p ) at 185.4 K is 18.9 atm −1 (factor of 1.7 uncertainty). A third law analysis of our data leads to a value for the ClOO bond dissociation energy of 4.76 ± 0.49 kcal mol −1 .

A temperature-dependent kinetics study of the aqueous phase reactions OH + SCN− → SCNOH− and SCN + SCN− ⇆(SCN)2−

Abstract A laser flash photolysis—long-path absorption technique was employed to investigate the kinetics of the aqueous phase reactions as a function of temperature in the range 277–321 K. On the basis of analyses of double-exponential (SCN)2− appearance temporal profiles, the following temperature-dependent rate coefficients were determined: ln k1(T) = (29.614 ± 0.636) − (1900 ± 190)/T M−1 s−1, ln k3(T) = (23.459 ± 1.259) − (271 ± 374)/T M−1 s−1 and ln k−3(T) = (29.182 − 0.534) - (5580 − 160)/T s−1; the uncertainties in the above Arrhenius expressions are 1σ and represent precision only. The enthalpy change associated with reaction (3) was evaluated on the basis of kinetic data and measurements of the equilibrium concentration of (SCN)2− as a function of [SCN−] and T; a value ΔH= −8.8 ± 2.8 kcal mol−1 was found to be consistent with all data.

Kinetics of the Br2CH3CHO photochemical chain reaction

Abstract Time-resolved resonance fluorescence spectroscopy was employed in conjunction with laser flash photolysis of Br2 to study the kinetics of the two elementary steps in the photochemical chain reaction nBr2 + nCH3CHO + hv → nCH3CBrO + nHBr. In the temperature range 255 – 400 K, the rate coefficient for the reaction Br(2P 3 2 ) + CH3CHO → CH3CO + HBr is given by the Arrhenius expression k6(T) = (1.51 ± 0.20) × 10−11 exp {−(364 ± 41)/T} cm3 molecule−1 s−1. At 298 K, the reaction CH3CO + Br2 → CH3CBrO + Br proceeds at a near gas kinetic rate, k7 (298 K) = (1.08 ± 0.38) × 10−10 cm3 molecule−1 s−1.

Laser flash photolysis studies of atmospheric free-radical chemistry using optical diagnostic techniques

Some recent studies carried out in our laboratory are described where laser flash photolytic production of reactant free radicals has been combined with reactant and/or product detection using time-resolved optical techniques to investigate the kinetics and mechanisms of important atmospheric chemical reactions. Discussed are (1) a study of the radical-radical reaction O + BrO yields Br + O2 where two photolysis lasers are employed to prepare the reaction mixture and where the reactants O and BrO are monitored simultaneously using atomic resonance fluorescence to detect O and multipass UV absorption to detect BrO; (2) a study of the reaction of atomic chlorine with dimethylsulfide (CH3SCH3) where atomic resonance fluorescence detection of Cl is employed to elucidate the kinetics and tunable diode laser absorption spectroscopy is employed to investigate the HCl product yield; and (3) a study of the aqueous phase chemistry of Cl2- radicals where longpath UV absorption spectroscopy is employed to investigate the kinetics of the Cl2- + H2O reaction.