A study of the multiradical nature of graphene as affected by the total spin state, modeled by acenes and periacenes

This study examines the radical nature and spin symmetry of the ground state and low-lying ionic states of the quasi-linear acene and two-dimensional periacene series. For this purpose, high-level ab initio calculations have been performed using the multireference averaged quadratic coupled cluster theory and the COLUMBUS program package. A complete active space (CAS) approach is taken for the π-conjugated space, correlating 16 electrons in 16 orbitals with the most open-shell character for the acenes and a similar approach with 8 electrons in 8 orbitals for the periacenes. For the ground state it is shown that a focus on the π space with a 6-31G basis set is sufficient to describe the major features of the electronic character of these compounds. The findings of this treatment suggest that the ground state is a singlet for the smaller members of these series, but that for the larger ones singlet and triplet states are quasi-degenerate. Both the acenes and periacenes exhibit significant polyradical character beyond the traditional diradical. However, for the low-lying ionic states, the π space with a 6-31G basis is found to be insufficient in describing the major features of the electronic character of these compounds. Accordingly, an all valence calculation is made with extended basis sets to include polarized and diffuse functions. The findings of this treatment suggest that all members of the acene and periacene series are bound except for naphthalene, which agrees with experiment. All ions for both the acenes and periacenes exhibit a significant decrease in radical character as compared to the neutral state.