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Faculty of Science - Department of Chemistry - Division of Quantum Chemistry and Physical Chemistry

Faculty of Science - Department of Chemistry - Division of Quantum Chemistry and Physical Chemistry. Structure-Activity Relationships - Mechanism development . Katholieke Universiteit Leuven. Luc Vereecken Research group on reaction kinetics

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Faculty of Science - Department of Chemistry - Division of Quantum Chemistry and Physical Chemistry

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  1. Faculty of Science - Department of Chemistry - Division of Quantum Chemistry and Physical Chemistry Structure-Activity Relationships - Mechanism development Katholieke Universiteit Leuven Luc Vereecken Research group on reaction kinetics Department of ChemistryQuantum Chemistry and Physical Chemistry K.U.Leuven, Belgium

  2. Introduction Structure activity relationships : WP2 Task 2.1 : alkoxy decomposition and isomerisation Task 2.2 : Site-specific NO3 and OH addition on alkenes Task 2.3 : O3 cycloaddition on alkenes Task 2.4 : H-abstraction by OH from hydrocarbons Mechanism development : WP3 - WP5 Task 3.1 : OH + -pinene Task 3.2 : O3 + -pinene, -humulene, -caryophyllene Task 5 : Oxygenates + OH : T,P-dependent mechanism

  3. Introduction - SARs Chemical mechanisms for modeling Large, explicit mechanisms (e.g. MCM)100s to 1000s of reactions/compounds  But no direct experimental or theoretical data on many of these  Use of SAR’s, predictive correlations Increasing demand for ever-better accuracy Policy-supporting predictions, what-if analyses: - Smog-episodes, chemical weather, climate - Emission control (compounds and quantities)  Need for accurate Structure Activity Relationships

  4. SAR’s and correlations Structure-Activity Relationship or Predictive Correlation: Good predictive accuracyEasy to useContinuous development Working model: Independent, additive site-specific rate coefficients ktot =  ksite (even for different types of reaction) Most rate coefficients depend primarily on local effects Inductive, hyperconjugative effects don’t carry very far H-bonds, resonances, … must be treated explicitly Linear models are easy to work with

  5. Introduction Addition of OH-radicals on (poly-)alkenes

  6. O H O H R R R . k R sec/tert C C H C C C C H C C R R resonance O H R R . C C C C H R OH-addition on (poly-)alkenes Alkenes The rate of addition depends mainly on the substituents of the radical site Cb after addition : X3 X4 kprim = 0.4510-11 cm3 s‑1 ksec = 3.010-11 cm3 s‑1 ktert = 5.510-11 cm3 s‑1 Conjugated Alkenes: some contribution from second radical site ksec/prim = 3.010-11 cm3 s‑1ksec/sec = 3.810-11 cm3 s‑1ksec/tert = 5.110-11 cm3 s‑1ktert/prim = 5.710-11 cm3 s‑1ktert/sec = 8.310-11 cm3 s‑1ktert/tert = 9.910-11 cm3 s‑1

  7. OH-addition on (poly-)alkenes Publication submitted to J. Phys. Chem. A Non-cyclic compounds: Average deviation 9% All compounds: Average deviation 13% Max. deviation 54% Can this be improved ? Yes Residual errors mostly due to H-abstraction contributions

  8. OH-addition on (poly-)alkenes Linear and mono-cyclic compounds

  9. OH-addition on (poly-)alkenes + bicyclic and (near-)conjugated compounds

  10. Introduction H-abstraction by OH-radicals

  11. H-abstraction by OH radicals

  12. H-abstraction by OH radicals Excellent correlation with bond strength Rate coefficient of abstraction determined by D(CH)Correlation is non-linear (data can be fitted by quadratic eq.)log (k298K) = -0.00328D2 + 0.3869D - 19.392 Resonance stabilization shifts curve: e.g. vinoxy stabilisationlog (k298K) = -0.00315D2 + 0.3840D – 21.860 Dependence similar for all compounds Angle and curvature similar for all resonances: Hyperconjugation, allyl, super-allyl, vinoxy. In 1st order approximation: use same value for all Different resonance stabilizations have different shift Correlation will break down for oxygenates/H-bonding at low T At room temperature: Carboxylic acids are already different

  13. Introduction Addition of NO3-radicals on (poly-)alkenes

  14. NO3-addition on (poly-)alkenes Addition of NO3 radicals: double interaction The rate of addition depends on substitution on both carbons: Radical site: factor FAddition site: factor fkadd = F  f Fprim = 1.2810-8 cm3/2 s‑1/2 fprim = 1.2810-8 cm3/2 s‑1/2 Fsec = 7.2710-7 cm3/2 s‑1/2 fsec = 3.3010-7 cm3/2 s‑1/2 Ftert = 3.8510-5 cm3/2 s‑1/2 ftert = 7.0210-7 cm3/2 s‑1/2 kadd,site = F  f  kadd,tot =  ksite Open questions: - Corrections for allyl-resonance stabilization of radical - H-abstraction (e.g. with allyl-resonance stabilization)

  15. NO3-addition on (poly-)alkenes Average deviation  1.2

  16. NO3-addition on (poly-)alkenes Average deviation  2.2

  17. NO3-addition on (poly-)alkenes Systematic underestimation

  18. NO3-addition on (poly-)alkenes

  19. NO3-addition on (poly-)alkenes Possible influence of H-abstraction: e.g. series of 1-alkenes - Could be sizable for large hydrocarbons - Affected by addition followed by HNO3 elimination ?

  20. NO3-addition on (poly-)alkenes Addition to conjugated alkadienes: Substitution effect different than for OH-addition (partial stabilisation of radical electron by allyl-resonance) Underestimation seems different for linear and cyclic Linear: underestimation by  0.3 Cyclic: underestimation by  0.1  Different addition scheme across -bonds ? Allyl-resonance Interaction across -bonds

  21. Introduction Decomposition of alkoxy radicals

  22. Alkoxy radical decomposition Decomposition barrier depends mostly on , -substituents A first version of this SAR was published as: J. Peeters, G. Fantechi, L. Vereecken, J. Atmos. Chem. 48, 59 (2004) k(T) =  × 1.8×1013 exp(-Eb/RT) s-1 Eb / kcal mol-1 = 17.5 + 2.1  n-alkyl + 3.1  n-alkyl + 8.0  n,-hydroxy + 8.0  n-oxo + 12.0  n-oxo curvature for small Eb < 7 kcal mol-1 :Eb' / kcal mol‑1 = Eb + 0.027  (9.0-Eb)2

  23. Alkoxy radical decomposition

  24. Alkoxy radical decomposition Current developments (in progress) : • More quantum chemical methods 6-31G(d,p), 6-311++G(2df,2pd), aug-cc-pVTZ MPW1K, BB1K, MPWKCIS1K, (CC, Gx, QCI) • Multi-rotamer TST with (modified) Arrhenius fit SAR for Ea, A, (n) • More substituents (preliminary) / kcal mol-1: -OR : -9.1 -OR : -9.0 -OOR : -7.5 -OOR :  =C : +21.1  =C : +4.6 -C=C : -5.0  -C=C : -9.6 -ONO2 : -3.1  -ONO2 : -2.7 -ONO : -4.2  -ONO : -6.2

  25. Alkoxy radical decomposition Future work: • Use multi-rotamer TST for alkoxy isomerisation (H-shift)L. Vereecken, J. Peeters, J. Chem. Phys. 119, 5159 (2003) • Perform URESAM calculations on these systems:  Pressure dependence SAR for Troe Parameters: Fc, k0, … O3 cycloaddition No results yet, but see literature

  26. Conclusions - I Four site-specific predictive SARs: OH-addition SAR: Very good accuracyCan only be improved by explicitly incorporating H-abstraction H-Abstraction correlation Very good correlation with bond strengthCurvature and slope similar, delocalisation shifts curve NO3 addition SAR Very good accuracy for most compounds (1.2, 2.2)Conjugated alkenes are underpredicted  delocalisation effects Alkoxy decomposition SAR: Being extended (substituents and methodology)Data serves as basis for alkoxy isomerisation SAR

  27. Introduction - Mechanism development Part II: Mechanism development Terpenes and sesquiterpenes

  28. Chemistry of -pinene + OH OH-initiated oxidation of -pinene using traditional chemistry: Experiment: acetone yields8% (Aschmann et al, 1998)2% (Orlando et al., 2000)13% (Wisthaler et al., 2001) ? Prediction of 60 % acetone formation

  29. Chemistry of unsaturated (per)oxy radicals Peroxy ringclosure in isoprene / terpenes :

  30. Ring closure in -pinene + OH

  31. -pinene + OH Nopinone: 25 %

  32. -pinene + OH Peroxy ring closure path forms dicarbonyl dihydroxy compound

  33. -pinene + OH About 4 % Chemistry with oxy ring closure finds low acetone yield comparable to experimental findings Compounds formed are highly oxygenated  cyclic esters, formates

  34. -pinene + OH Peroxy chemistry ROO + R’OO/HOO(pre- and post ring closure) Peroxy ring closuredi-OH-di-carbonyl Oxy ring closure 10ppt 100ppt 1ppb 10ppb 100ppb 1ppm [NO] Degradation mechanism depends on [NO], [HO2/RO2]

  35. -pinene + OH Minor H-abstraction channels (Klara Petrov) Mainly formation of larger (multisubstituted) oxygenates. Larger products should nearly all be reactive to OH, O3, NO3

  36. Other mechanisms -pinene + O3 Some additional theoretical verification on impact of - ring closure - low-NOx chemistry Mechanism sufficiently mature for modeling (see BIRA) Sesquiterpenes + O3 No results yet

  37. Introduction Oxygenates + OH

  38. Oxygenates + OH General mechanism:

  39. Oxygenates + OH T,P-dependences: See: Peeters and Vereecken, Int. Symp. Gas Kin. 2006 Barriers above reactants: Formation of pre-reactive complexes not too important Positive T-dependence (except at low T: tunneling) No P-dependence Barriers below reactants: Chemical activation effects Negative T-dependence at all T Pressure dependent

  40. Oxygenates + OH Specific issues for theoretical work on oxygenate+OH reactions • Calculation of tunneling contributions Small-curvature corrections most often used e.g. Masgrau et al., J. Phys. Chem. A 106, 11760 (2002) tunneling contribution 22 at 202 K for acetone+OH • Variational effects H-abstraction over H-bonds: low and broad TS Variational effects can be important (kinetic bottleneck not at energy maximum) e.g. Masgrau et al. 2002 (acetone+OH) variational effects up to order of magnitude • Specific reaction pathways (See acids)

  41. Acetone + OH The reaction of acetone + OH shows a curved Arrhenius plot: Gierczak, Gilles, Bauerle, Ravishankara, J. Phys. Chem. A 107, 5014 (2003); Talukdar et al., J. Phys. Chem. A 107, 5021 (2003) Wollenhaupt, Carl, Horowitz, Crowley, J. Phys. Chem. A 104, 2695 (2000)

  42. Acetone + OH Theoretical work shows the general features of the PES: Vandenberk, Vereecken and Peeters, PCCP 4, 461 (2002) Similar PESes by Masgrau et al., J. Phys. Chem. A 106, 11760 (2002) Vasvári et al., PCCP 3, 551 (2001)

  43. Hydroxyacetone + OH The reaction of hydroxyacetone + OH : Negative T-dependence Dillon, Horowitz, Hölscher, Crowley, Vereecken, Peeters, PCCP, 8, 236, 2006

  44. Hydroxyacetone + OH Accuracy of barrier heights did not allow for finaltheoretical kinetic predictions.

  45. Glycolaldehyde + OH The reaction of CH2OHCHO+ OH : No T-dependence • Slowdown relative to CH3CHO: due to charge distribution • Lack of T-dependence: due to specific barrier height: RRKM-MEsimulation Karunanandan, Hölscher, Dillon, Horowitz, Crowley, Vereecken, Peeters, submitted for publication

  46. Oxygenates + OH Stringent requirements for theoretical methodologies Quantum chemical methods: very high level needed Calculation of energies But also for calculation of geometries and frequencies Mechanism development Unexpected mechanisms can exist Kinetic methodologies: Important effects of Tunneling (SCT or better needed) Variational effects Anharmonicity effects Multi-conformer (multi-well) effects Multiple pathways Internal rotors

  47. Conclusions - II Mechanism development -pinene + OH Very complex reaction mechanismDepends strongly on [NOx] versus [ROO/HOO]Many fast unimolecular reaction steps  reduction of mechanism possibleIn progress Terpenoids + O3 In progress Oxygenates + OH : Very complex kineticsStringent demands on theoretical methodologyT,P-dependence of k(T) or product distribution still difficult

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