Measurements of the kinetics of the OH + α-pinene and OH + β-pinene reactions at low pressure

Abstract

The rate constants for the OH + α-pinene and OH + β-pinene reactions have been measured in 5 Torr of He using discharge-flow systems coupled with resonance fluorescence and laser-induced fluorescence detection of the OH radical. At room temperature, the measured effective bimolecular rate constant for the OH + α-pinene reaction was (6.08 ± 0.24) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. These results are in excellent agreement with previous absolute measurements of this rate constant, but are approximately 13% greater than the value currently recommended for atmospheric modeling. The measured effective bimolecular rate constant for the OH + β-pinene reaction at room temperature was (7.72 ± 0.44) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, in excellent agreement with previous measurements and current recommendations. Above 300 K, the effective bimolecular rate constants for these reactions display a negative temperature dependence suggesting that OH addition dominates the reaction mechanisms under these conditions. This negative temperature dependence is larger than that observed at higher pressures. The measured rate constants for the OH + α-pinene and OH + β-pinene reactions are in good agreement with established reactivity trends relating the rate constant for OH + alkene reactions with the ionization potential of the alkene when ab initio calculated energies for the highest occupied molecular orbital are used as surrogates for the ionization potentials for α- and β-pinene. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 300–308, 2002