Frank Lee Emmert III, Stephanie Thompson, and Lyudmila V. Slipchenko Purdue University, West Lafayette, IN 47907

Fullerene formation Bergman Cyclization Ring Closure Retro [2+2] Coalescence and Annealing Hunter, J. M.; Fye, J. L.; Roskamp, E. J.; Jarrold, M. F. J. Phys. Chem. 1994, 98, 1810−1818. Proposed mechanism for the formation of fullerenes.

Fullerene formation We cut the alkyne tails and looked at ethynyl substituted cyclobutadienes.

Fullerene formation We calculated: equilibrium geometries adiabatic S-T gap energies vertical S-T gap energies stabilization energy of the ethynyl substituents spin densities (not discussed) natural charges (not discussed)

Cyclobutadiene orbitals Singlet cyclobutadiene undergoes Jahn-Teller distortion and becomes rectangular. MP2 EOM-SF-CCSD ROMP2 UMP2 EOM-SF-CCSD Triplet Singlet Singlet cyclobutadiene undergoes Jahn-Teller distortions to make a rectangular structure. Spin Flip variant of the Equation of Motion Coupled Cluster with single and double excitations was emplolyed. Accuracy of Møller-Plesset 2 nd order perturbation theory was tested employing both a restricted open shell and unrestricted reference L. V. Slipchenko and A. I. Krylov J. Chem. Phys. 2002, 117, 4694

Optimized singlet geometries EOM-SF-CCSD/cc-pVDZ Singlet Bond Lengths (Å) Molecule c c c short c long c trans c c Ethynyl Substituents (Å) bond56 average Main pattern of alternating bond lengths does not change with substituent addition. The singlet geometries become more square with increased diradical character. Substituent bond lengths remain nearly constant.

Optimized triplet geometries EOM-SF-CCSD/cc-pVDZ Triplet Bond Lengths (Å) Molecule c c c c trans c c1.456 Ethynyl Substituents (Å) bond56 average Square structure is maintained when substituents are added symmetrically. The triplet geometries have increasing bond lengths; decreasing the aromaticity. Substituent bond lengths remain nearly constant.

Adiabatic energies The singlet triplet gap decreases with substituent addition. UMP2 does not follow the trend of decreasing singlet-triplet gap energy. Main source of error is spin contamination of the triplet state.

MP2 vertical energies Vertical S-T gaps should be larger at singlet geometries and smaller at triplet geometries. MP2 cannot properly describe the diradical singlet state at the triplet geometries where the two π-orbitals of the ring are degenerate. Adiabatic S-T Gap Vertical S-T gap at triplet geometry Vertical S-T gap at singlet geometry S T

EOM vertical energies Adiabatic S-T Gap EOM-SF-CCSD shows correct vertical behavior for all substituents. Vertical S-T gap energies decrease with substituent addition at the singlet geometries while remaining almost constant at the triplet geometries. MP2 had a lot of trouble and CCSD(T) would have a lot of trouble because it is mostly a problem of the HF reference. S T

EOM vertical energies Adiabatic S-T Gap The triplet surface is becoming flatter or the singlet geometry is becoming more like the triplet geometry. S T S T

Isodesmic Reactions Isodesmic Homodesmotic Isodesmic reactions preserve the number and type of bonds (single, double, triple). Homodesmotic reactions preserve the hybridization, the number and types of bonds of the carbon atoms, and the number of hydrogen atoms bonded to individual carbon atoms. Wheeler, S.E., et al., J. Am. Chem. Soc., (7): p

Stabilization energies ProductEqn. 1Eqn. 2Eqn. 3.Eqn. 4Eqn. 5 Singlet RMP2 1-c cs cl ct c c Triplet ROMP2 1-c cs cl ct c c Both the triplet and the singlet are stabilized with substituent addition. The triplet is more stabilized then the singlet. Each reaction gives the same pattern and S-T differences.

Conclusions S-T gaps are decreased with ethynyl substituent addition but the singlets are always lower in energy. Results are effected by spin contamination of the triplet states; UMP2 fails to properly describe the system. Based on isodesmic reactions, triplet states becomes more stabilized then the singlet states as substituents are added.

Acknowledgements: Thank you: Professor McMahon – University of Wisconsin Levi Haupert Visualization Software: MacMolPlt ChemBioDraw12 Packages: Q-Chem GAMESS CFOUR Funding provided by: ACS-PRF Purdue University

Thank you

Graphene and fullerene formation Bergman Cyclization Ring Closure Retro [2+2] Coalescence and Annealing Hunter, J. M.; Fye, J. L.; Roskamp, E. J.; Jarrold, M. F. J. Phys. Chem. 1994, 98, 1810−1818.

Cyclobutadiene orbitals Cyclobutadiene undergoes Jahn-Teller distortions. SingletTriplet

Optimized Geometries SingletTriplet

Adiabatic energies The singlet triplet gap decreases with substituent addition. UMP2 does not follow the trend of decreasing singlet-triplet gap energy. Main source of error is spin contamination.

Natural Charges Singlets

Natural Charges Triplets

Spin Densities HF TripletROEOM Triplet

Spin Densities

Optimized singlet geometries EOM-SF-CCSD/cc-pVDZ Singlet Bond Lengths (Å) Molecule c c c short c long c trans c c Ethynyl Substituents (Å) bond56 average Main pattern of alternating bond lengths does not change with substituent addition. The singlet geometries have decreasing single bond lengths and increased double bond lengths; becoming more square. Substituent bond lengths remain nearly constant.

Optimized triplet geometries EOM-SF-CCSD/cc-pVDZ Triplet Bond Lengths (Å) Molecule c c c c trans c c Ethynyl Substituents (Å) bond56 average Square structure is maintained when substituents are added symmetrically. The singlet geometries have decreasing single bond lengths and increased double bond lengths, becoming more square.

Stabilization energies ProductEqn. 1Eqn. 2Eqn. 3.Eqn. 4Eqn. 5 Singlet RMP2 1-c cs cl ct c c Triplet ROMP2 1-c cs cl ct c c ProductEqn. 1Eqn. 2 Singlet RCCSD(T) 1-c cs cl ct c c Triplet ROCCSD(T) 1-c cs cl ct c c