Rate coefficients for the gas-phase reaction of OH radicals with the L4, L5, D5, and D6 permethylsiloxanes

Rate coefficients, k(T), for the gas-phase OH radical reaction with decamethyltetrasiloxane ((CH₃)₃SiO[Si(CH₃)₂O]₂Si(CH₃)₃, L₄, k₁), dodecamethylpentasiloxane ((CH₃)₃SiO[Si(CH₃)₂O]₃Si(CH₃)₃, L₅, k₂), and decamethylcyclopentasiloxane ([–Si(CH₃)₂O–]₅, D₅, k₃), and dodecamethylcyclohexasiloxane ([–Si(CH₃)₂O–]₆, D₆, k₄) were measured using a pulsed laser photolysis—laser induced fluorescence absolute method over the temperature range 270–370 K. The obtained room temperature rate coefficients, with quoted 2σ absolute uncertainties, and fitted temperature dependence are (cm⁻³ molecule⁻¹ s⁻¹):

The 2σ absolute rate coefficient uncertainty, for all compounds included in this study, is conservatively estimated to be ∼10% over the entire temperature range. The cyclic permethylsiloxanes were found to be less reactive than the analogous linear compound, while both linear and cyclic compounds show increasing reactivity with increasing number of CH₃- groups. A structure activity relationship (SAR) parameterization for the permethylsiloxanes is presented. The estimated atmospheric lifetimes due to OH reaction for L₄, L₅, D₅, and D₆ are 5.2, 4.4, 6.8, and 5.2 days, respectively.