towards more realistic molecular modeling of homogenous catalysis: combined qm/mm and ab initio...
TRANSCRIPT
Towards More Realistic Molecular Modeling of Homogenous Catalysis:
Combined QM/MM and ab initio Molecular Dynamics Investigations
N
Ni+N
RRiPr
iPriPr
iPr
R
Tom K. Woo, Peter M. Margl, Liqun Deng, Luigi Cavallo, Tom Ziegler
Department of Chemistry, University of Calgary,
Calgary, Alberta, CANADA
Brookhart Polymerization CatalystC&EN Feb. 5, 1996:
“Polymer Catalyst System:Dupont Eyes New Polyolefin Business”
Brookhartcatalyst
highly linear to moderately branched
Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 2343.
• high MWs
• good activities
N
Ni+N
RRiPr
iPriPr
iPr
R
Brookhart Polymerization CatalystC&EN Feb. 5, 1996:
“Polymer Catalyst System:Dupont Eyes New Polyolefin Business”
Brookhartcatalyst
highly linear to moderately branched
Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 2343.
N
Ni+N
RRiPr
iPriPr
iPr
R
temperature: Temp branching
monomer pressure: [Et] branching
bulk of substituents: bulk branching MW
Polymerization Mechanisms
isomerizationisomerization
terminationtermination
insertioninsertion
(branched)(branched)
resting state
Ni
P
HNi
P
H
Ni
P
H
Ni
P
H
Ni
H2CP
H
Ni
CH2
H
P
NiCH3
P
Ni
CH3P
Polymerization Mechanisms
isomerizationisomerization
terminationtermination
insertioninsertion
(branched)(branched)
resting state
barriers: barriers: insertion < isomerization < termination
Ni
P
HNi
P
H
Ni
P
H
Ni
P
H
Ni
H2CP
H
Ni
CH2
H
P
NiCH3
P
Ni
CH3P
“REAL” systemPURE QM
model system
Pure QM Calculations
N
Ni+N
RRiPr
iPriPr
iPr
R
N
NiN
HH
HH
ADF Density Functional Code: Baerends and Ziegler
Becke 88 - Perdew 86 energies and gradients
static, gas phase simulation of model system
Ni
Pure QM Results - Pure QM Results - Calculated BarriersCalculated Barriers
insertion:
isomerization:
termination:
17.5 kcal/mol
12.8 kcal/mol
10.8 kcal/mol
Ni
Ni
Adding the Bulky LigandsAdding the Bulky Ligands
termination
Transition States
insertion
Combined QM/MM - Overview
QMregion
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
NiH H
Ni
N
NiN
RRiPr
iPriPr
iPr
Maseras, F.; Morokuma, K. J. Comp. Chem. 1995, 16, 1170.
Insertion Profiles (kcal/mol)
14.7 13.2
10.0
19.4 16.8
6.7
TS
PURE QM
hybridQM/MM
Ni
P
H
Ni
P
H Ni
P
H
Orientation of the Aryl Rings
N
NiN
RRiPr
iPriPr
iPr
resting state &termination TS
insertion TS
Isomerization
15.3
-0.8
12.8
-1.8
PURE QM
hybridQM/MM
Ni
P
HNi
H2CP
H
NiCH3
P
Termination Profiles (kcal/mol)
TS
19.4
9.7
PURE QM
14.7 18.6hybridQM/MM
Ni
P
H Ni
P
H
Ni
P
H
Comparison with Experiment
Pure QM: 16.8QM/MM: 13.2
EXP: 10 - 11 kcal/mol
QM then MM: 18.5
insertion barrier
QM/MMExp.
insertion < isomerization < termination
insertion < isomerization < termination
Order of Barrier Heights
Comparison with Experiment
13.2 15.3 18.6
Pure QM: 16.8QM/MM: 13.2
EXP: 10 - 11 kcal/mol
QM then MM: 18.5
insertion barrier
Relative to Insertion
Comparison with Experiment
Pure QM (∆∆Hel)
QM/MM (∆∆Hel)
Exp. (∆∆G)
insertion isomerization termination
0.0
0.0
0.0
-4.0 -7.1
2.1
1.3* 5.6
5.4
*monomer capture may be rate limiting for branching process
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
N
NiN
CH3CH3iPr
iPriPr
iPr
Insertion:
• bulkier the groups, the higher the MW
termination:
• monomer capture may dominate
isomerization:
Combined QM/MM ab initio MDCoupled 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
Ni
P
HNi
P
H
N
NiN
CH3CH3iPr
iPriPr
iPr
H
Ni
P
H
40 000 time step simulations
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:
ΔF =∂E
∂λ λ ,Tλ∫ dλ
Termination BarrierExp: 15-16 kcal/mol
Brookhart Termination Movie
QuickTime™ and aAnimation decompressor
are needed to see this picture
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
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
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
• PMMAustrian Fonds zur Förderung deWissenschaftlichen Forschung