Author

Thais Scott

Date of Award

2017

Document Type

Thesis

Degree Name

Bachelor of Science

Department

Chemistry

First Advisor

Dr. Carol Parish

Abstract

Three diradical pyrazine isomers, 2,3-didehydropyrazine (2,3), 2,6-didehydropyrazine (2,6), and 2,5-didehydropyrazine (2,5), were characterized using MCSCF and MR-CISD, with and without the a posteriori Davidson correction, for both the lowest lying singlet and triplet states. In addition, the method MR-AQCC was used for each triplet isomer with both cc-pVDZ and cc- pVTZ basis sets. Initial molecular orbitals were generated using the ROHF/cc-pVDZ method and assigned irreducible representation labels under C2v symmetry for 2,6 and 2,3 and under C2h for the 2,5 isomer. CAS(8,8) active space orbitals included each molecule’s specific σ and σ* orbitals and the π and π* orbitals. Orbitals were further optimized using MCSCF to generate the reference wavefuntion. Geometry optimizations and single point calculations were performed on the lowest lying singlet and triplet states to generate adiabatic and vertical energy gaps using the Gaussian and Columbus software packages. The smallest adiabatic gap was observed in the 2,6 isomer and the largest gap occurred in the 2,5 isomer. Analysis of the final geometries showed the radical carbons moving closer to each other in the singlet state of the 2,6 isomer. The 2,3 isomer showed a strong bonding interaction forming in the singlet state. Finally for the 2,5 isomer there was stabilizing interactions between the lone pair on the nitrogen atom and the radical carbon in both the singlet and triplet states. The 2,5 isomer is the most stable while the 2,6 is the least stable.

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