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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 C&EN Feb. 5, 1996: “Polymer Catalyst System: Dupont Eyes New Polyolefin Business” Brookhart catalyst highly linear to moderately branched Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 2343. high MWs good activities

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Brookhart Polymerization Catalyst C&EN Feb. 5, 1996: “Polymer Catalyst System: Dupont Eyes New Polyolefin Business” Brookhart catalyst highly linear to moderately branched Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 2343. temperature: Temp branching monomer pressure: [Et] branching bulk of substituents: bulk branching MW

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Polymerization Mechanisms isomerization termination insertion (branched) resting state

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Polymerization Mechanisms isomerization termination insertion (branched) resting state barriers: barriers: insertion < isomerization < termination

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“REAL” system PURE QM model system Pure QM Calculations 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: isomerization: termination: 17.5 kcal/mol 12.8 kcal/mol 10.8 kcal/mol

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Adding the Bulky Ligands termination Transition States insertion

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Combined QM/MM - Overview QM region MM region QM region (active site) MO calculation MM region MM force field regions are treated simultaneously QM - MM regions interact via vdW & Coulomb QM electronic system capped “real” system QM system Maseras, F.; Morokuma, K. J. Comp. Chem. 1995, 16, 1170.

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Insertion Profiles (kcal/mol) 14.7 13.2 10.0 19.4 16.8 6.7 TS PURE QM hybrid QM/MM

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Orientation of the Aryl Rings resting state & termination TS insertion TS

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Isomerization 15.3 -0.8 12.8 -1.8 PURE QM hybrid QM/MM

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Termination Profiles (kcal/mol) TS 19.4 9.7 PURE QM 14.7 18.6 hybrid QM/MM

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Comparison with Experiment Pure QM: 16.8 QM/MM: 13.2 EXP: 10 - 11 kcal/mol QM then MM: 18.5 insertion barrier

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QM/MM Exp. insertion < isomerization < termination Order of Barrier Heights Comparison with Experiment 13.215.318.6 Pure QM: 16.8 QM/MM: 13.2 EXP: 10 - 11 kcal/mol QM then MM: 18.5 insertion barrier

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Relative to Insertion Comparison with Experiment Pure QM (∆∆H el ) QM/MM (∆∆H el ) Exp. (∆∆G) insertionisomerizationtermination 0.0 -4.0-7.1 2.1 1.3*5.6 5.4 *monomer capture may be rate limiting for branching process

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Primary Effects of Bulky Ligands destabilizes resting state N-C(aryl) torsion important bulkier the groups, the higher the activity diimine methyl important in chain termination Insertion: bulkier the groups, the higher the MW termination: monomer capture may dominate isomerization:

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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 slow growth simulation 2 weeks on an IBM 3CT workstation with 64 MB ram 40 000 time step simulations

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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 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 successfully applied QM/MM method to transition metal based homogenous catalysis

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Future Work multiple time step AIMD electrostatic coupling in AIMD for solvent simulations validation and calibration of QM/MM method simulation of monomer trapping in Brookhart catalyst

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Acknowledgments NSERC of Canada Novacor Chemicals of Calgary Alberta Heritage Scholarship Fund Killam Memorial Foundation NSERC TKW Visit our Home Page at: http://www.chem.ucalgary.ca/groups/ziegler Maurice Brookhart Peter Blöchl at IBM Zürich PMM Austrian Fonds zur Förderung de Wissenschaftlichen Forschung

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