Using Computational Chemistry to Study a Reaction Pathway

1. Using Computational Chemistry to Study a Reaction Pathway Jessica L. Case Super Chem II April 30, 2002

2. Goals of the Project • Utilize Gaussian 98 and WebMO for various computational calculations: • Geometry Optimizations • Frequency Calculations • Transition State Determination • IRC Calculations to find Intermediate Structures • Determine the energies of the reactants, intermediates, transition state, and product • Use these methods to determine a reaction pathway • Draw a calculated reaction coordinate diagram

3. Reaction Under Study • Why this reaction? • Studied it last summer as a possible monomer unit to form ladder polymers

4. Geometry Optimization:2-methoxyfuran

5. Geometry Optimization:cyclobutylbenzyne

6. Geometry Optimization:product*

7. Frequency Calculations

8. Locating the Transition State • First, combine the numbering of the atoms in the two reactant structures • Second, combine the Z-matrices of the two geometry optimized reactants • Third, determine the approximate approach of the two molecules will take to react together to form the product • Vary distance between reactants • Vary intermolecular angles • Vary dihedral angles

9. R = 3 Angstroms E = -648.6805142H R = 10 Angstroms E = -648.7616681 H R = 20 Angstroms E = -648.7615803 H R = 80 Angstroms E = -648.7615669 H 2-methoxyfuran: E = -342.5083676 H cyclobutylbenzyne: E = -306.2532483 H sum of reactants: E = -648.7616159 H Lining Up the Reactants*HF / 6-31+G(d)

10. Determining the Transition State Structure • Use the combined Z-matrix of the two reactants and the Z-matrix of the product as input • The STQN method locates a transition structure with the QST2 keyword • Utilizes the input structures to determine a structure of maximum energy in between the reactants’ and product’s structures

11. The Transition State* • Two different inputs yielded very similar structures • E1 = -648.752134712 H = -4.07098452053*105 kcal/mol • E2 = -648.752134717 H = -4.07098452056*105 kcal/mol • The transition state occurred at R = 2.999 Angstroms • Frequency calculations yielded a zero point energy of 0.229644 H and one negative vibrational mode, which is expected for a transition state structure

12. Locating the Intermediates • IRC Calculations • Takes the calculated structure and force field from the optimized transition state and determines intermediate structures along the reaction path • Varied the number of steps away from the transition state: • 2 steps: E = -648.7522613 H • 10 steps: E = -648.7555564 H • 40 steps: E = -648.7571191 H

13. The Intermediate Structures*

14. -648.7521347 D=0.0094812 Energy (H) D=0.037843 -648.7616159 -648.7899777 Reaction Coordinate Reaction Coordinate Diagram

15. And with that, my academic career at Hope College is complete!!! Booyah!