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”

3

Alcohols + Hydroperoxyl Radical: Motivation

4

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)

6

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

7

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

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j

jj fZQUQQ1

,

HO

1

,rot

T-MS

rovib-con exp

2

1

TMS

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TST-MS

)(

)()(

i

i TF

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)(

)()(

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)

10

n-Butanol + Hydroperoxyl Radical

11

F-Factors and Rate Coefficients (R2a)

12

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

13