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Measurement and Computation of Molecular Potential Energy Surfaces Polik Research Group Hope College Department of Chemistry Holland, MI 49423
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Measurement and Computation of Molecular Potential Energy Surfaces Jennica Skoug, David Gorno, & Eli Scheele Polik Research Group Hope College Department of Chemistry Holland, MI 49423
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Outline Potential Energy Surfaces Dispersed Fluorescence Spectroscopy –Molecular Beam –Lasers –Monochromator Resonant Polyad Model –Harmonic and Anharmonic Terms –Vibrational State Mixing Computation of PES’s and Vibrational Levels
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Potential Energy Surfaces A Potential Energy Surface (PES) describes how a molecule’s energy depends on geometry Chemical structure, properties, and reactivity can be calculated from the PES
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Measuring PES’s & Vibrational States Measuring highly excited vibrational states allows characterization of the PES away from the equilibrium structure of the molecule
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Molecular Beam for Sample Preparation A molecular beam cools the sample to 5K Molecules occupy the lowest quantum state and simplify the resulting spectrum
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Lasers for Electronic Excitation Laser provide an intense monochromatic light source Lasers motes molecules to an excited electronic state
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Monochromator for Detection A monchromator disperses molecular fluorescence E vibrational level = E laser – E fluoresence
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Dispersed Fluorescence Spectrum 3 1 HFCO
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Summary of Assignments MoleculePrevious #Current # Energy Range (cm -1 ) Year H 2 CO812790 - 12,5001996 D 2 CO72610 - 12,0001998 HFCO443820 - 22,5002002 H 2 CO H 2 +COdissociation barrier 28,000 cm -1 HFCO HF+COdissociation barrier 17,000 cm -1
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Harmonic and Anharmonic Models A harmonic oscillator predicts equally spaced energy levels Anharmonic corrections shift vibrational energy levels as the PES widens Harmonic Energy Anharmonic Correction
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Polyad Model Groups of vibrational states interacting through resonances are called polyads Resonances mix vibrational energy levels Energy levels are calculated from the Schrodinger Eqn 22652265 k 26,5 215164215164 k 26,5 52635263 k 44,66 224263224263 k 26,5 2142516221425162 k 26,5 425261425261 k 44,66 224461224461 k 26,5 214451214451
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Diagonal Elements: Off-Diagonal Elements: Harmonic Energy Anharmonic Correction Resonant Interactions Matrix Form of Schrödinger Equation
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H 2 CO Anharmonic Polyad Model Fits ParameterFit 1Fit 2Fit3Fit 4 ω1°ω1°2818.92812.32813.72817.4 ω6°ω6°1260.61254.81251.51251.9 x 11 -40.1-29.8-30.7-34.4 x 66 -5.2-2.8-2.1-2.2 k 26,5 148.6146.7138.6 k 36,5 129.3129.6135.1 k 11,55 140.5137.4129.3 k 44,66 21.623.3 k 25,35 18.5 Std Dev23.44.343.342.80
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Model Fits to Experimental Data
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Polyad Quantum Numbers H 2 CO D 2 CO HFCO k 1,44 N vib = v 2 +v 3 +v 5 ( ultimately destroyed ) k 44,66 N CO = v 2 ( remains good! ) k 36,5 N res = 2v 1 +2v 2 +v 3 +v 4 +2v 5 +v 6 ( remains good! ) k 2,66 N polyad = 2v 2 +v 6 others? v 1, v 3, v 4, v 5 may remain good k 36,5 N oop = v 4 ( destroyed by k 44,66 ) k 26,5 N vib = v 1 +v 4 +v 5 +v 6 ( destroyed by k 1,44 and k 1,66 ) k 11,55 N res = 2v 1 +v 2 +v 3 +v 4 +2v 5 +v 6 ( remains good! )
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Polyad Quantum Numbers H 2 CO D 2 CO HCCH k 1,44 N vib = v 2 +v 3 +v 5 ( ultimately destroyed ) k 44,66 N CO = v 2 ( remains good! ) k 36,5 N res = 2v 1 +2v 2 +v 3 +v 4 +2v 5 +v 6 ( remains good! ) many N str = v 1 +v 2 +v 3 ( ultimately destroyed ) reson- N l = l 4 +l 5 ( ultimately destroyed ) ances N res = 5v 1 +3v 2 +5v 3 +v 4 +v 5 ( remains good! ) k 36,5 N oop = v 4 ( destroyed by k 44,66 ) k 26,5 N vib = v 1 +v 4 +v 5 +v 6 ( destroyed by k 1,44 and k 1,66 ) k 11,55 N res = 2v 1 +v 2 +v 3 +v 4 +2v 5 +v 6 ( remains good! )
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Computation of PES’s The Potential Energy E can be represented by a Taylor series expansion of the geometry coordinates q i A quartic PES requires computation of many high- order force constants (partial derivatives) Force constants predict vibrational energy level shifts and mixing
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Parallel Computing Force constants are computed as numerical derivatives, i.e., by calculating energies of displaced geometries PES calculation takes hours instead of weeks with parallel computing
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Computation of PES and Vibrations
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Conclusions DF spectroscopy is a powerful technique for measuring excited states (general, selective, sensitive) Resonances shift and mix vibrational states The anharmonic polyad model accounts for resonances and assigns highly mixed spectra ( , x, k) Polyad quantum numbers remain at high energy (N res always conserved) High level quartic PES calculations and polyad model accurately predict excited vibrational states
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Acknowledgements H 2 CO Rychard Bouwens (UC Berkeley - Physics), Jon Hammerschmidt (U Minn - Chemistry), Martha Grzeskowiak (Mich St - Med School), Tineke Stegink (Netherlands - Industry), Patrick Yorba (Med School) D 2 CO Gregory Martin (Dow Chemical), Todd Chassee (U Mich - Med School), Tyson Friday (Industry) HFCO Katie Horsman (U Va - Chemistry), Karen Hahn (Med School), Ron Heemstra (Pfizer - Industry), Ben Ellingson (U Minn – Chemistry) Funding NSF, Beckman Foundation, ACS-PRF, Research Corporation, Wyckoff Chemical, Exxon, Warner-Lambert
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