Accurate Computed Rate Coefficients for the Hydrogen Atom ...

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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

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.

)()()( CVT/MTT-MSCVT/MT-MS TkTFTk

))(exp(

)()()(Φ

),()(1),(min)( CVT

*MEP2

1

RRHOSS

R,

el

R,rel

CVT

*

RRHO-SS

GTS

el

GTSGTSCVT sV

TQTQT

sTQTQ

hsTkTk

i

ii

s

)()()( CVTMTCVT/MT TkTTk

t

jjj

J

j

jj fZQUQQ1

,

HO

1

,rot

T-MS

rovib-con exp

2

1

TMS

R,

TMS

TST-MS

)(

)()(

i

i TF

TFTF

)(

)()(

RRHO-SS

T-MS

rovib,-conTMS

TQ

TQTF

m

m

m

)(

)(

)(

)()()()(

HO-MS

T-MS

rovib,-con

RRHO-SS

HO-MS

rovib,-conTMSTMS

TQ

TQ

TQ

TQTFTFTF

m

m

m

m

mmm

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)

Rate Coefficients (R1a and R1b)

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n-Butanol + Hydroperoxyl Radical

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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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