Accurate Computed Rate Coefficients for the Hydrogen Atom Abstraction Reactions from Methanol and n-butanol by the Hydroperoxyl Radical.

Transcription

1 Accurate Computed Rate Coefficients for the Hydrogen Atom Abstraction Reactions from Methanol and n-butanol by the Hydroperoxyl Radical John Alecu Second Annual CEFRC Conference August 17, 2011

2 Acknowledgments Prof. Donald Truhlar Dr. Jingjing Zheng Dr. Steven Mielke Dr. Xuefei Xu Dr. Prasenjit Seal Tao Yu Ewa Papajak Prof. William Green Dr. Michael Harper

3 Research Plan Improve on the existing n-buoh combustion mechanism by accurately computing or measuring the rate coefficients of several critical elementary reactions Year 1: high-level QM calculations of rate coefficients, including multidimensional tunneling as well as torsional and multiple-structure anharmonicity (Minnesota) Year 2: measurement of rate coefficients using the laserphotolysis experimental technique coupled with laserabsorption and/or time-of-flight mass spectrometry (MIT) Use these new accurate rate coefficients to refine the kinetic model for butanol combustion and simulate important combustion properties using RMG Help fulfill CEFRC s mission: The development of a validated, predictive, multi-scale combustion modeling capability to optimize the design and operation of evolving fuels in advanced engines for transportation applications 3

4 Alcohols + Hydroperoxyl Radical: Motivation RMG: n-buoh combustion mechanism highly sensitive to rate of reaction with HO 2 at low and intermediate combustion temperatures HO 2 challenging to study experimentally Difficult to generate/detect directly Reaction with alcohols too slow Thermal degradation at elevated temperatures Excellent opportunity for theory to contribute Size of system allows high-level QM treatment Complex (many torsions for reactants/products/ts) Analogous methanol reactions as prototypes Understand important features at reduced cost Find suitable methods for treating class of reactions These reactions important in methanol combustion 4

5 Theoretical Approach The Reactions: CH 3 OH + HO 2 CH 2 OH + HOOH (R1a) CH 3 OH + HO 2 CH 3 O + HOOH (R1b) CH 3 (CH 2 ) 3 OH + HO 2 CH 3 (CH 2 ) 2 CHOH + HOOH (R2a) Stage I: Validations CCSD(T)/CBS used for accurate reaction energetics DFT validations against CCSD(T)/CBS results M08-HX55/MG3S for R1a/b (MUE = 0.23 kcal/mol) M08-SO/MG3S for R2a (MUE = 0.10 kcal/mol) Stage II: Anharmonic Partition Functions Multi-Structural method that accounts for torsions (MS-T) Includes contribution from all structures Physical: no assigned torsions, accounts for coupling Practical: No barrier information, cheaper than Feynman path integrals or configuration integrals Stage III: Rate Coefficients k CVT/MT (direct dynamics/dynamics on MCSI PES) k CVT/MT are combined with MS-T partition functions to calculate accurate final result: k MS-CVT/MT 5

6 Relative Energy (kcal mol -1 ) Potential Energy (Best Estimates) *Best estimates: experiment for reaction energies, CCSDT(2) Q /CBS + CV + R for barrier heights. 6

7 MeOH + Hydroperoxyl Radical: Validations Alecu, I. M; Truhlar, D. G. J. Phys. Chem. A 2011, 115,

8 MS-CVT/MT: An Overview Single-Structure Canonical VTST with Multidimensional Tunneling k CVT min k s GTS ( T, s) 1 h k Φ CVT/MT Q rel el GTS Q 2 i1 Q MT SS-RRHO GTS el R, i Q k CVT ( T, s CVT * SSRRHO R, i ) exp( V MEP ( s CVT * )) Q MS-T con -rovib Multi-Structural Partition Functions J j1 Q rot, j exp t HO U j Qj Z j Multi-Structural Canonical VTST with Multidimensional Tunneling 1 f j, F F MST m MS-T 2 F i1 m MST TS F MST R, i F MST m MS-T Qcon-rovib, m( T) SS-RRHO Q MS-HO MS-T MS T Qcon-rovib, m( T) Qcon-rovib, m( T) F ( ) ( ) m T Fm T SS-RRHO MS-HO Qm Qm k MS-CVT/MT F MS-T k CVT/MT Zheng, Yu, Papajak, Alecu, Mielke, Truhlar, Phys. Chem. Chem. Phys., 2011, 13, Yu, Zheng, Truhlar, Chem. Science, 2011, DOI: /C1SC00225B. 8

9 F-Factors (R1a and R1b) F MS-T (R1a) F MS (TS) F MS-T (TS) F MS-T (R1b) F MS (TS) F MS-T (TS) F T (TS) F T (ROH) F T (TS) F T (ROH) 9

10 Rate Coefficients (R1a and R1b) 10

11 n-butanol + Hydroperoxyl Radical 11

12 F-Factors and Rate Coefficients (R2a) F MS-T (TS) F MS (ROH) F MS-T (ROH) F T (TS) F MS (TS) F MS-T (R2a) F T (ROH) 12

13 Conclusions Rate coefficients that cannot be measured can be calculated accurately using modern computational chemistry methods this is crucial to CEFRC s mission of attaining combustion modeling capability MS-CVT/MT can provide highly-accurate results for reaction systems comprised of complex species with multiple torsions Neglecting to account for multi-structure and torsional anharmonicity can lead to order-ofmagnitude errors in the rate coefficient at temperatures of interest to combustion 13