Predicting Gas Phase Kinetic Data for Organic Compounds Patrick Laine Atmospheric Chemistry Fall 2008 Term Paper Presentation

Background  OH is the dominant loss process for VOC’s in atmosphere  Knowing rates at which OH reacts with VOC’s is important – affects O 3 formation  Some reactions are difficult to measure experimentally

CH 4 + OH  ·CH 3 + H 2 O(7) CH 3 + O 2 + M  CH 3 O 2 · + M (8) CH 3 O 2 · + NO  CH 3 O· + NO 2 (9) CH 3 O· + O 2  HCHO + HO 2 (10) HO 2 + NO  OH + NO 2 (11) Net: CH 4 + 2NO + 2O 2  HCHO + H 2 O + 2NO 2 Methane + OH

Methodology  Look at simplest case – alkanes  Identify the types of Hydrogens ((1 o )CH 3 -R, (2 o )RCH 2 -R, (3 o )RCHRR) present  Assign appropriate values for each type  Use up to date experimentally determined rate constants  Apply to larger species

VOCC-H typesk_x10 12 cm 3 molec -1 s -1 ethane2 1 o propane2 1 o, 1 2 o 1.09 n-butane2 1 o, 2 2 o methylprop3 1 o, 1 3 o 2.12 n-pentane2 1 o, 3 2 o methylbut3 1 o, 1 2 o, 1 3 o 3.6 2,2-dimethylprop4 1 o n-hexane2 1 o, 4 2 o 5.2

Deriving a k_1 o value   Use ethane and 2,2 dimethylpropane to get an average value for contribution of primary C-H to overall rate constant 2,2-dimethylprop4 1 o ethane2 1 o k_1 o = x cm 3 molec -1 sec -1 CH 3 H 3 C – C – CH 3 CH 3 H 3 C – CH 3

Deriving a k_2 o value k_2 o = x cm 3 molec -1 sec -1 Alkanes used: n-propane, n-butane, n-pentane, n-hexane

Deriving a k_3 o value  1 o 2 o 3 o  Use previous values for 1 o and 2 o and compare to experimental k to deduce the value for contribution from 3 o C-H   Using 2-methylbutane, 2-methylpentane, 2,4-dimethylpentane, 2,3,4-trimethylpentane an average value of 2.2 x cm 3 molec -1 sec -1 is found for each 3 o C-H

VOCCalc =Σ kn o ; n=1-3 k_x10 12 cm 3 molec - 1 s -1 ethane propane n-butane methylprop n-pentane methylbut ,2-dimethylprop n-hexane

Apply to larger OC H3C-CH2-CH-CH-CH2-CH3 CH2 CH3 CH2 3,4 diethylhexane: C 10 H o, o, o 4*(.165) + 4*(.725) + 2 * (2.2) = 7.96 Exp: 6.92; % Error: 13% 3,3 diethylpentane: C 9 H 20 H3C-CH2-C-CH2-CH3 CH2 CH3 CH o, o 4*(.165) + 4*(.725) = 3.56 Exp: 4.8; % Error: 25%

Conclusions/Limitations  Analysis of rate constant trends can lead to reasonable estimations  Alkenes, cyclic hydrocarbons, halogen containing, aromatic compounds each present unique factors All experimentally determined rate constants from: Atkinson and Arey, Chemical Reviews, 2003 Vol. 103, No. 12 pp