期刊
JOURNAL OF PHYSICAL CHEMISTRY A
卷 123, 期 10, 页码 2049-2057出版社
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpca.8b12440
关键词
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资金
- Department of Energy [DE-SC0001093]
- National Science Foundation [CHE-1213271, CHE-18800014, CHE-1213263, CHE-1464536]
- NSF [CHE-1626238]
- National Science Foundation Graduate Research Fellowship Program [CON-75851, 00074041]
- University of Richmond Arts and Sciences Undergraduate Research Committee
- Arnold and Mabel Beckman Foundation
- U.S. Department of Energy (DOE) [DE-SC0001093] Funding Source: U.S. Department of Energy (DOE)
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 sigma and sigma* orbitals on the dehydrocarbon atoms as well as the valence pi and pi* 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.
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