Accurate Computed Rate Coefficients for the Hydrogen Atom ...
Transcript of Accurate Computed Rate Coefficients for the Hydrogen Atom ...
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
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
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 laser-photolysis experimental technique coupled with laser-absorption 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”
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Alcohols + Hydroperoxyl Radical: Motivation
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RMG: n-BuOH combustion mechanism highly sensitive to rate of reaction with HO2 at low and intermediate combustion temperatures
HO2 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
Theoretical Approach
The Reactions: CH3OH + HO2 → CH2OH + HOOH (R1a)
CH3OH + HO2 → CH3O + HOOH (R1b)
CH3(CH2)3OH + HO2 → CH3(CH2)2CHOH + 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 kCVT/MT (direct dynamics/dynamics on MCSI PES)
kCVT/MT are combined with MS-T partition functions to calculate accurate final result: kMS-CVT/MT 5
Potential Energy (Best Estimates)
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Rel
ativ
e E
ner
gy
(kca
l m
ol-1
)
9.89
16.92
0.00
23.65
18.99
*Best estimates: experiment for reaction energies, CCSDT(2)Q/CBS + CV + R for barrier heights.
MeOH + Hydroperoxyl Radical:
Validations
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Alecu, I. M; Truhlar, D. G. J. Phys. Chem. A 2011, 115, 2811.
MS-CVT/MT: An Overview
8 Zheng, Yu, Papajak, Alecu, Mielke, Truhlar, Phys. Chem. Chem. Phys., 2011, 13, 10885.
Yu, Zheng, Truhlar, Chem. Science, 2011, DOI: 10.1039/C1SC00225B.
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Single-Structure Canonical VTST with Multidimensional Tunneling
Multi-Structural Partition Functions
Multi-Structural Canonical VTST with Multidimensional Tunneling
F-Factors (R1a and R1b)
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FMS(TS)
FMS-T(TS)
FT(ROH)
FT(TS)
FMS-T(R1a)
FMS(TS)
FMS-T(TS)
FT(ROH)
FT(TS)
FMS-T(R1b)
F-Factors and Rate Coefficients (R2a)
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FMS(ROH)
FMS(TS)
FMS-T(TS)
FMS-T(ROH)
FT(ROH)
FT(TS)
FMS-T(R2a)
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-of-magnitude errors in the rate coefficient at temperatures of interest to combustion
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