Towards More Realistic Molecular Modeling of Homogenous Catalysis: Combined QM/MM and ab initio Molecular Dynamics Investigations

1
Towards More Realistic Molecular Modeling of
Homogenous Catalysis Combined QM/MM and ab
initio Molecular Dynamics Investigations

• Tom K. Woo, Peter M. Margl, Liqun Deng, Luigi
  Cavallo, Tom Ziegler
• Department of Chemistry, University of Calgary,
• Calgary, Alberta, CANADA

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Brookhart Polymerization Catalyst
CEN Feb. 5, 1996
Polymer Catalyst System Dupont Eyes New
Polyolefin Business
Brookhart catalyst
highly linear to moderately branched

• high MWs
• good activities

Johnson, L. K. Killian, C. M. Brookhart, M. J.
Am. Chem. Soc. 1995, 117, 2343.
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Brookhart Polymerization Catalyst
CEN Feb. 5, 1996
Polymer Catalyst System Dupont Eyes New
Polyolefin Business
Brookhart catalyst
highly linear to moderately branched

• temperature Temp ? branching ?
• monomer pressure Et ? branching ?
• bulk of substituents bulk ? branching ? MW ?

Johnson, L. K. Killian, C. M. Brookhart, M. J.
Am. Chem. Soc. 1995, 117, 2343.
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Polymerization Mechanisms
insertion
resting state
termination
(branched)
isomerization
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Polymerization Mechanisms
insertion
resting state
termination
(branched)
isomerization
barriers insertion lt isomerization lt termination
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Pure QM Calculations
PURE QM model system
REAL system
ADF Density Functional Code Baerends and Ziegler
Becke 88 - Perdew 86 energies and gradients
static, gas phase simulation of model system
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Pure QM Results - Calculated Barriers
insertion
17.5 kcal/mol
isomerization
12.8 kcal/mol
termination
10.8 kcal/mol
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Adding the Bulky Ligands
insertion
termination
Transition States
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Combined QM/MM - Overview
Maseras, F. Morokuma, K. J. Comp. Chem. 1995,
16, 1170.
QM region (active site)
MO calculation
MM region
QM region
MM region
MM force field

• regions are treated simultaneously

• QM - MM regions interact via vdW Coulomb

• QM electronic system capped

real system
QM system
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Insertion Profiles (kcal/mol)
PURE QM
19.4
16.8
6.7
hybrid QM/MM
13.2
14.7
10.0
TS
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Orientation of the Aryl Rings
resting state termination TS
insertion TS
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Isomerization
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Termination Profiles (kcal/mol)
TS
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Comparison with Experiment
insertion barrier
EXP 10 - 11 kcal/mol
QM/MM 13.2
Pure QM 16.8
QM then MM 18.5
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Comparison with Experiment
insertion barrier
EXP 10 - 11 kcal/mol
QM/MM 13.2
Pure QM 16.8
QM then MM 18.5
Order of Barrier Heights
13.2
15.3
18.6
insertion lt isomerization lt termination
QM/MM
Exp.
insertion lt isomerization lt termination
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Comparison with Experiment
Relative to Insertion
insertion
isomerization
termination
Pure QM (??Hel)
0.0
-4.0
-7.1
QM/MM (??Hel)
0.0
2.1
5.4
Exp. (??G)
0.0
1.3
5.6
monomer capture may be rate limiting for
branching process
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Primary Effects of Bulky Ligands
Insertion

• destabilizes resting state

• bulkier the groups, the higher the activity

• N-C(aryl) torsion important

termination

• bulkier the groups, the higher the MW

• diimine methyl important in chain termination

isomerization

• monomer capture may dominate

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Combined QM/MM ab initio MD
Coupled AMBER with Car-Parrinello AIMD code
PAW code - Peter Blöchl of IBM Zürich
Becke 88 - Perdew 86 QM potential
300 K simulation temperature
40 000 time step simulations
slow growth simulation
2 weeks on an IBM 3CT workstation with 64 MB ram
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Reaction Barriers with AIMD

• thermodynamic integration ?? free energy barriers

• reaction coordinate constrained during dynamics

• RC slowly varied, leading system over barrier

?

• relative free energies along RC calculated by

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Termination Barrier
Exp 15-16 kcal/mol
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Brookhart Termination Movie
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Conclusions
successfully applied QM/MM method to transition
metal based homogenous catalysis
implemented QM/MM ab initio molecular dynamics
method
determined free energy barrier with QM/MM AIMD
methods show good promise
QM/MM boundary is dubious
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Future Work

• validation and calibration of QM/MM method

• multiple time step AIMD

• electrostatic coupling in AIMD for solvent
  simulations

• simulation of monomer trapping in Brookhart
  catalyst

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Acknowledgments

• NSERC of Canada

• Novacor Chemicals of Calgary

• Peter Blöchl at IBM Zürich

• Maurice Brookhart

• PMM

Austrian Fonds zur Förderung de Wissenschaftlichen
Forschung

• TKW

NSERC
Alberta Heritage Scholarship Fund
Killam Memorial Foundation
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