A Multireference Ab Initio Study of the Diradical Isomers of Pyrazine

Abstract

Three diradical pyrazine isomers were characterized using highly correlated, multireference methods. The lowest lying singlet and triplet state geometries of 2,3didehydropyrazine (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 2,6didehydropyrazine (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 2,3didehydropyrazine (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.

Half-Projection of the Strongly Orthogonal Unrestricted Geminals’ Product Wave Function

“…Singlet−triplet gaps of diradical isomers of pyrazine follow a trend different from benzynes, the para isomer exhibiting a significant gap (on the order of 1 eV), while splitting of the ortho and meta isomers is rather small. 79 Calculations based on USLG involve four correlated geminals, in a vein similar to para-benzyne, nitrogen π orbitals replacing carbon π orbitals at the substituted positions. Results of the computations are collected in Table 6.…”

Twenty years of exceptional success: The molecular education and research consortium in undergraduate computational chemistry (MERCURY)

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Simplified Treatment of Electronic Structures of the Lowest Singlet and Triplet States of Didehydropyrazines

“…At this point, it is worth mentioning that the diradical isomers of benzene need MR-based calculations to accurately describe the optimized geometrical parameters as well as energy separations between multiplets. 5−9 Very recently, the structures and other properties of spectroscopic interest of pyrazine isomers have been clarified by Scott et al 10 second-order complete active space second-order perturbation theory [CASPT2], 12 and multiconfigurational pair density functional theory [MC-PDFT] 13,14 ) with larger complete active spaces, CAS (such as CAS (8,8) and CAS (12,10)).…”

“…The calculated structures and properties of these diradical isomers show a strong dependence on the level of theory and dimension of CAS used. Scott et al 10 showed that the extended CAS is needed for the proper physical description of these diradical isomers. They used a CAS (12,10) to explore the nature of interaction of the unpaired electrons on the radical center with the nitrogen lone pair orbitals.…”