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Predicting Gas Phase Kinetic Data for Organic Compounds

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

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Predicting Gas Phase Kinetic Data for Organic Compounds

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

  2. Background • OH is the dominant loss process for VOC’s in atmosphere • Knowing rates at which OH reacts with VOC’s is important – affects O3 formation • Some reactions are difficult to measure experimentally

  3. Methane + OH CH4 + OH  ·CH3 + H2O (7) CH3 + O2 + M  CH3O2· + M (8) CH3O2· + NO  CH3O· + NO2 (9) CH3O· + O2 HCHO + HO2 (10) HO2 + NO  OH + NO2 (11) Net: CH4 + 2NO + 2O2  HCHO + H2O + 2NO2

  4. Methodology • Look at simplest case – alkanes • Identify the types of Hydrogens ((1o)CH3-R, (2o)RCH2-R, (3o)RCHRR) present • Assign appropriate values for each type • Use up to date experimentally determined rate constants • Apply to larger species

  5. Deriving a k_1o value • Use ethane and 2,2 dimethylpropane to get an average value for contribution of primary C-H to overall rate constant CH3 H3C – C – CH3 CH3 H3C – CH3 k_1o = 0.165 x 1012 cm3 molec-1 sec-1

  6. Deriving a k_2o value k_2o = 0.725 x 1012 cm3 molec-1 sec-1 Alkanes used: n-propane, n-butane, n-pentane, n-hexane

  7. Deriving a k_3o value • Use previous values for 1o and 2o and compare to experimental k to deduce the value for contribution from 3o C-H • Using 2-methylbutane, 2-methylpentane, 2,4-dimethylpentane, 2,3,4-trimethylpentane an average value of 2.2 x 1012 cm3 molec-1 sec-1 is found for each 3o C-H

  8. Apply to larger OC 3,4 diethylhexane: C10H22 4 - 1o, 4 - 2o, 2 - 3o 4*(.165) + 4*(.725) + 2 * (2.2) = 7.96 Exp: 6.92; % Error: 13% CH3 CH2 H3C-CH2-CH-CH-CH2-CH3 CH2 CH3 3,3 diethylpentane: C9H20 4 - 1o, 4 - 2o 4*(.165) + 4*(.725) = 3.56 Exp: 4.8; % Error: 25% CH3 CH2 H3C-CH2-C-CH2-CH3 CH2 CH3

  9. 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. 4605-4638.

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