Three diradical pyrazine isomers were characterized using highly correlated, multireference methods. The lowest lying singlet and triplet state geometries of 2,3-didehydropyrazine (ortho), 2,5-didehydropyrazine (para), and 2,6-didehydropyrazine (meta) were determined. Two active reference spaces were utilized. The complete active space (CAS) (8,8) includes the σ and σ∗ orbitals on the dehydrocarbon atoms as well as the valence π and π∗ orbitals. The CAS (12,10) reference space includes two additional orbitals corresponding to the in-phase and out-of-phase nitrogen lone pair orbitals. Adiabatic and vertical gaps between the lowest lying singlet and triplet states, optimized geometries, canonicalized orbital energies, unpaired electron densities, and spin polarization effects were compared. We find that the singlet states of each diradical isomer contain two significantly weighted configurations, and the larger active space is necessary for the proper physical characterization of both the singlet and triplet states. The singlet-triplet splitting is very small for the 2,3-didehydropyrazine (ortho) and 2,6-didehydropyrazine (meta) isomers (+1.8 and -1.4 kcal/mol, respectively) and significant for the 2,5-didehydropyrazine (para) isomer (+28.2 kcal/mol). Singlet geometries show through-space interactions between the dehydocarbon atoms in the 2,3-didehydropyrazine (ortho) and 2,6-didehydropyrazine (meta) isomers. An analysis of the effectively unpaired electrons suggests that the 2,5-didehydropyrazine (para) isomer also displays through-bond coupling between the diradical electrons.
Bibliographical noteFunding Information:
This work was supported in part by the Department of Energy (Grant DE-SC0001093 (C.P.)) and the National Science Foundation (Grant CHE-1213271 and CHE-18800014 (C.P.) and Grant CHE-1213263 (H.L.)). C.P. acknowledges the Donors of the American Chemical Society Petroleum Research Fund. Computational resources were provided, in part, by the MERCURY supercomputer consortium under NSF grant CHE-1626238. This work was also supported in part (L.G.) by the National Science Foundation by grant no. CHE-1464536. T.S. acknowledges that this material is also based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. CON-75851, project 00074041. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. T.S. and B.Z. acknowledge support from the University of Richmond Arts and Sciences Undergraduate Research Committee. A.L. acknowledges support from the Arnold and Mabel Beckman Foundation through receipt of a Beckman Scholars award.
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