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Faculty of Science - Department of Chemistry - Division for Quantum chemistry and physical chemistry

Faculty of Science - Department of Chemistry - Division for Quantum chemistry and physical chemistry. OH-initiated oxidation of oxygenates at low temperatures. Katholieke Universiteit Leuven. Computational difficulties in a priori predictions. Luc Vereecken

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Faculty of Science - Department of Chemistry - Division for Quantum chemistry and physical chemistry

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  1. Faculty of Science - Department of Chemistry - Division for Quantum chemistry and physical chemistry OH-initiated oxidation of oxygenates at low temperatures Katholieke Universiteit Leuven Computational difficulties in a priori predictions Luc Vereecken Department of ChemistryDivision for Quantum chemistry and physical chemistry K.U.Leuven, Belgium Expert Discussion meeting, Alpe D’Huez, 2006

  2. Introduction A priori predictions of rate coefficients and product distributions General methodology: 1) Quantum chemical characterization of PES DFT, MBPT, CAS, CC, QCI, Gx, CBS, … Identification of pathways Relative energies, barrier heights Rovibrational characteristics 2) Application of rate theories TST, RRKM-ME, Direct Dynamics, Trajectories Absolute rate coefficients Relative rates / product distributions

  3. Introduction Considerations for quantum chemical calculations at low T Calculation of energies - A trade-off between calculation time and accuracy - DFT, MP2 - Explicit correlation: CC, QCI, MP4 - Multi-reference methods: CAS - Extrapolation methods: G2M, G3, CBS - Intrinsic reliability on relative energies never below 0.5 to 1 kcal/mol But: very accurate energies are needed: Uncertainty E of 1 kcal/mol: factor 1.4 at 1500K factor 5.3 at 300K factor 12.4 at 200K

  4. Introduction Considerations for quantum chemical calculations at low T Calculation of molecular properties - Rotational constants: usually not a problem - Vibrational wavenumbers Low internal energies  lowest  excited Lowest  : molecular bendings, torsions, internal rotations - Least accurately predicted - Most anharmonic - Low-barrier and barrierless reactions are most important Broad, flat transition states: harder to optimize

  5. Introduction Considerations for theoretical-kinetic calculations at low T Low internal energies  State densities become more quantized  Some approximations become less applicable Internal rotations  Transition from free rotor to hindered rotor to anharmonic vibration Increasing importance of tunneling (Variational effects)

  6. Oxygenates + OH General view of the reactions of oxygenates + OH “The OH radical has a rather large electric dipole moment (1.668 D) and is clearly capable of forming strong hydrogen bonds.” Role of H-bonded intermediates in the bimolecular reactions of the hydroxyl radical, Ian Smith and A. Ravishankara, J. Phys. Chem. A 106, 4798 (2002)

  7. Oxygenates + OH Ian Smith and A. Ravishankara, J. Phys. Chem. A 106, 4798 (2002)

  8. Oxygenates + OH General mechanism: Formation of prereactive complexes (usually barrierless) Multiple complexes are possible Complexes can interconvert Possible assumptions: - Rate of interconvertion is fast/slow - Rate of formation/dissociation faster/slower than reaction

  9. Oxygenates + OH General mechanism: H-abstraction transition states Multiple channels are possible Barrier height depends on abstracted hydrogen Barrier can be above/below energy of initial reactants TS can have similar/different H-bonding Other reactions e.g. OH-addition on >C=O Usually not important

  10. Oxygenates + OH General mechanism: H-bonded productcomplexes Usually not important Separated products as measured

  11. Oxygenates + OH General mechanism: “High” temperatures Formation of pre-reactive complexes not important  Positive T-dependence (if barriers above reactants) “Low” temperatures Reactants trapped as complexes  more reactants near TS Tunneling through barriers  Negative T-dependence (more tunneling probability)

  12. Oxygenates + OH General mechanism: “Intermediate” temperatures Transition from one regime to the other  T-dependence flattens out, changes sign Transition shifts to higher temperatures with: - Stronger H-bonding and/or more complex conformers - faster tunneling (lower or thinner barriers)

  13. Oxygenates + OH General mechanism: Special case very low-lying TS (e.g. below free reactant)  barrierless reaction (or governed by complex formation)  k(T) intrinsically has negative T-dependence Equilibrium between complexes and free reactants not obvious

  14. Oxygenates + OH Specific issues for theoretical work on oxygenate+OH reactions • Calculation of relative energies - H-bonds: Longer-range interactions  Choice of basis set, QC method - Cyclic/strained H-bonded complexes and structures:  angular dependence of H-bond strength - Changes in H-bonding (reactants)  pre-reactive complexes  TSs  post-reactive complexes • - Calculation of “rigidity” Effect of H-bonding on vibrational wavenumbers on internal rotations Anharmonicity effects

  15. 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)

  16. Examples Some specific examples of theoretical work Acetone + OHHydroxyacetone + OHGlycolaldehyde + OH Formic acid + OHAcetic acid + OH

  17. Acetone + OH The reaction of acetone + OH shows a curved Arrhenius plot: graph from wollenhaupt et al. + reference Ravishankara + reference 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)

  18. 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)

  19. Acetone + OH • Two issues needed to be addressed: • Product distribution: addition vrs. H-abstraction Rate calculations showed small contribution for addition High barrier of  6 kcal/mol Tight TS  A  410-14 cm3 molec-1 s-1 • - Curvature in Arrhenius plot “The negative T-dependence observed by Wollenhaupt et al. might be explained by the pre-reactive complex …/…though no quantitative theoretical calculations could be made due to the sensitivity of these calculations to the uncertainties on the barrier heights” - Vandenberk, Vereecken and Peeters, PCCP 4, 461 (2002)

  20. Acetone + OH “…its value of -0.44 kcal/mol is not negative enough to reproduce the observed negative activation energy. An accurate treatment of a competitive mechanism based on reactions with such small barrier heights but also with tunneling and variational contributions is probably still a challenge for computational chemists.” - Masgrau, González-Lafont, Lluch, J. Phys. Chem. A 106, 11760 (2002)

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

  22. Hydroxyacetone + OH Issues for hydroxyacetone+OH: - Why is it faster than acetone + OH - Why is the T-dependence negative for T = 233-363 K - Which absolute k is right: the older measurements (15x faster then acetone) or Crowley et al. (30 x faster) Theoretical work: Dillon et al., PCCP, 8, 236, 2006 - reaction faster because : - C-H bond is weaker with OH substituent - Entropically more favorable (more internal rotors in TS) - Negative T-dependence: more H-bonded complexes, more TS, lower TS  transition region shifts to higher T - Barrier decrease + entropical differences: 24x faster than acetone (if everything else stays the same)

  23. Hydroxyacetone + OH “…the calculated absolute barrier heights are not expected to be accurate within 1 kcal/mol and are most likely overestimations of the barriers as the predicted absolute barrier heights of about 3 kcal/mol are too high to reproduce the experimental rate coefficients and temperature dependence”

  24. Hydroxyacetone + OH Because of inaccurate barrier heights, inability to model tunneling, complex formation, etc.: - No differentiation between 2 groups of experiments - No theoretical extension to higher/lower T  Is there upward curvature at higher T? Note: this paper only examined relative differences with acetone

  25. Glycolaldehyde + OH The reaction of hydroxyacetone + OH: a priori expectations: - Increase in rate of abstraction relative to acetaldehyde? Ethane < ethanol Acetone < Hydroxyacetone Acetaldehyde < glycolaldehyde ? But: Dominant abstraction is not –CH3 group but –CHO (Butkovskaya et al. J. Phys. Chem. A 108, 1160 (2004)) -C(O)H Bond strength in glycolaldehyde 2 kcal/mol higher  abstraction slower in glycolaldehyde  H-abstraction correlation predicts 7.410-12 cm3 molec-1 s-1 at 298K As barrier is higher  less negative T-dependence

  26. Glycolaldehyde + OH The reaction of glycolaldehyde+ OH : 810-12 cm3 molec-1 s-1, no T-dependence for T=240-362K Crowley et al, in preparation, 2006

  27. Glycolaldehyde + OH The reaction of Glycolaldehyde + OH: theoretical work Most of the expectations are qualitatively correct But: Why is there no T-dependence ? Galano, Alvarez-Idaboy, Ruiz-Santoyo, Vivier-Bunge, J. Phys. Chem. A 109, 169 (2005) • Quantum chemical calculations with CVT/SCT kinetics- 2 channels, diff. T-dependence cis, trans HO-C-C=O • Still negative T-dependence • Barriers below reactants intrinsic neg. T-dep.  interconvertion of complexes is too slow

  28. HCOOH + OH The reaction of formic acid + OH Most intensively studied by Anglada et al. Torrent-Sucarrat, Anglada, ChemPhysChem 5, 183 (2004)Olivella, Anglada, Solé, Bofill, Chem. Eur. J. 10, 3404 (2004)Anglada, J. Am. Chem. Soc. 126, 9809 (2004) Formic acid + OH: 15 pre-reactive complexes syn- and anti-formic acid in-plane or out-of-plane radical orbital (A’, A”) C=O---HO C(HO)---HO C(OH)---OH cyclic

  29. HCOOH + OH Unexpected reaction pathways: 2 mechanisms possible: - Regular radical H-transfer (H-radical jumps to radical orbital) - Proton coupled electron transfer (H+ jumps to LP, LP gives e-)

  30. HCOOH + OH Lowest barrier is coupled mechanism (0.51 kcal/mol)Traditional acidic H-abstraction higher barrier (3.40 kcal/mol)HOCO formation: barriers of 1.17, 1.61 kcal/mol

  31. HCOOH + OH Predicted total rate coefficient at 298K (6.2410-13 cm3 s-1)close to experimental data (3.2 - 4.9 10-13 cm3 s-1). But: Small temperature dependence predicted, whereas experiment shows near T-independent k(T) TS2 : Two imaginary frequencies using B3LYP, QCISD One imaginary frequency at MP2, CASSCF “…Thus, the CASSCF and MP2 results make us confidentthat TS2 is a true transition state.” - Anglada, JACS 126, 9809 “…Preliminary calculations … computed at B3LYP and MP2..result in values of the rate constant that differ in a factor of about 100…very large differences in the computed value of theimaginary frequency…” - Anglada, JACS 126, 9809 (2004) Final results are based on QCISD(T) energies, B3LYP partitionfunctions Q for complexes, QCISD(T) and MP2 Q for TSs

  32. CH3COOH + OH The reaction of acetic acid + OH Experimental studies: 65% acidic H-abstraction Butkovskaya et al., J. Phys. Chem. A 108, 7021 (2004) Desmedt et al., J. Phys. Chem. A, 109, 2401 (2005) Negative T-dependence for k(T) Theoretical work proved difficult: - Mechanism conforms to general layout - Strong H-bonding and lowered TS lead to negative T-dep.

  33. CH3COOH + OH “However, a discrepancy up to 1.6 kcal/mol between the G2M//MP2 and G2M//B3LYP was found for TS3…branching fractions of 10-90% and beyond can be obtained depending onthe set of input parameters used.” “An optimal fit… reverses the relative ordering of the G2M//MP2 versus G2M//B3LYP TS3 energies” “The B3LYP imaginary wavenumbers differ strongly (700i vrs1300i cm-1)… altering the predicted branching by a factor 3.5.the MP2 imaginary wavenumbers are much higher but closer together (1900-2200i cm-1)… hardly affecting the branching…” “…suggesting that the high imaginary wavenumbers at MP2 result in an overcorrection for tunneling…” - Desmedt et al., J. Phys. Chem. A 109, 2401 (2005)

  34. CH3COOH + OH If the general mechanism is correct: - Barriers above reactants - Less influence of H-bonded complexes as T increases  Positive T-dependence at high temperatures

  35. CH3COOH + OH Positive T-dep. section: A = 5.410-9 cm3 molec-1 s-1 Ea = 13 kcal/mol Khamaganov, Peeters, 2006, unpublished A different channel ? Addition ? But barrier not right, pre-exponential factor not right Effect of higher-energy increasing state densities ?  Transition to “direct” H-abstraction mechanism ? Product measurements in progress to verify…

  36. Conclusions 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

  37. Acknowledgments The lab: Prof. Dr. J. Peeters L. Vereecken V. KhamaganovT.L. Nguyen F. DesmedtX.V. Bui S. Vandenberk Many of the results come from the FP5 UTOPIHAN-ACT project (inc. group of J. Crowley, G. Lebras) Financial support FWO-VlaanderenKULeuven Research council IWTDWTC

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