Rate Coefficients for the Cl Atom Gas‐Phase Reaction With Permethylsiloxanes (PMS): L₂, L₃, L₄, L₅, D₃, D₄, D₅, and D₆

Abstract

Rate coefficients, k(T), for the gas‐phase Cl atom reaction with hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, L2), k1; octamethyltrisiloxane ([(CH3)3SiO]2Si(CH3)2, L3), k2; decamethyltetrasiloxane ((CH3)3SiO[Si(CH3)2O]2Si(CH3)3, L4, k3; dodecamethylpentasiloxane ((CH3)3SiO[Si(CH3)2O]3Si(CH3)3, L5, k4; hexamethylcyclotrisiloxane ([‐Si(CH3)2O‐]3, D3), k5; octamethylcyclotetrasiloxane ([‐Si(CH3)2O‐]4, D4), k6; decamethylcyclopentasiloxane ([‐Si(CH3)2O‐]5, D5, k7), and dodecamethylcyclohexasiloxane ([‐Si(CH3)2O‐]6, D6, k8) were measured over a range of temperature (273–363 K) using a pulsed laser photolysis (PLP) – resonance fluorescence (RF) technique. The obtained k(296 K) and Arrhenius expressions with 2σ uncertainties including estimated systematic errors are (in units of 10−10 cm3 molecule−1 s−1): L2: k1(296 K) = (1.58 ± 0.07) k1(273–363 K) = (1.36 ± 0.10) exp((43 ± 249)/T) L3: k2(296 K) = (1.93 ± 0.09) k2(273–353 K) = (1.95 ± 0.18) exp((1 ± 632)/T) L4: k3(296 K) = (2.36 ± 0.12) k3(273–353 K) = (2.09 ± 0.16) exp((43± 522)/T) L5: k4(296 K) = (2.84 ± 0.19) k4(296–353 K) = (2.70 ± 0.18) exp((15 ± 490)/T) D3: k5(296 K) = (0.588 ± 0.028) k5(273–353 K) = (0.47 ± 0.03) exp((78 ± 470)/T) D4: k6(296 K) = (1.13 ± 0.06) k6(273–353 K) = (1.14 ± 0.07) exp((4 ± 432)/T) D5: k7(296 K) = (1.72 ± 0.09) k7(273–353 K) = (1.60 ± 0.10) exp((22 ± 432)/T) D6: k8(296 K) = (2.16 ± 0.14) k8(296–353 K) = (1.73 ± 0.14) exp((71 ± 562)/T) The cyclic permethyl siloxanes (cyclic PMS) were found to be less reactive than the analogous linear permethyl siloxane (linear PMS) with an equal number of CH3‐ groups. Both linear and cyclic compounds show a linear relationship between the measured rate coefficient and the number of CH3‐ groups in the molecule. A structure–activity relationship (SAR) is presented that reproduces the experimental data to within ∼10% at all temperatures. For [Cl] ≈ 104 atom cm−3, an approximate free troposphere abundance, the PMS loss due to Cl atom reaction leads to relatively short estimated lifetimes of 7, 6, 5, 4, 20, 10, 7, and 5 days for L2, L3, L4, L5, D3, D4, D5, and D6, respectively. Therefore, the PMSs included in this study are classified as atmospherically very short‐lived substances and Cl atom reaction represents a significant loss process.