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.

Asphaltene nanoaggregates from three diverse source materialscoal-derived asphaltenes dominated by aromatic carbon, petroleum asphaltenes with comparable abundances of aromatic and aliphatic carbon, and immature source-rock asphaltenes dominated by aliphatic carbonare examined by means of surface-assisted laser desorption ionization mass spectrometry (SALDI-MS) coupled with laser desorption laser ionization mass spectrometry (L2MS). All three types of asphaltenes form nanoaggregates with aggregation numbers close to 7. Molecular dynamics calculations for proposed island molecular structures show the important roles that π-stacking and alkane steric hindrance play in nanoaggregate formation and structure. These results are discussed in terms of entropy and enthalpy changes. All results are consistent with the Yen-Mullins model, which bodes well for its expanded use in oilfield reservoir evaluations.

The developments of the open-source chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes can address, while showing that is an attractive platform for state-of-the-art atomistic computer simulations.