Reinvestigation of the ordering of the low-lying electronic states of cyclobutanetetraone with CASPT2, CCSD(T), G3B3, ccCA, and CBS-QB3 calculations

Abstract

Previous calculations on cyclobutanetetraone by Gleiter et al. and by Jiao et al. have found that this seemingly simple, closed-shell, organic molecule actually has four low-lying electronic states–a ¹A_1g state with eight π electrons, a ¹B_2u and a ³B_2u state with nine π electrons, and a second ¹A_1g state with ten π electrons. However, the previous calculations have left some doubt as to whether, in violation of Hund's rule, ¹B_2u lies below ³B_2u, and which of the closed-shell singlets is lower in energy. These questions have been addressed by performing multi-reference (16/16)CASPT2/6-31G(d) calculations, CCSD(T)/aug-cc-PVDZ calculations, and calculations with three composite methods–G3B3, ccCA, and CBS-QB3. The calculations find the amounts of correlation energy that are recovered variationally, even with a (16/16)CASSCF wave function, are very different for eight, nine, and ten π electrons; so (16/16)CASPT2 is really useful only for unequivocally showing that ³B_2u is lower in energy than ¹B_2u. The CCSD(T), G3B3, ccCA, and CBS-QB3 calculations all find that the triplet is the ground state and that the 8π singlet is lower in energy than the 10π singlet.

Keywords:

• Hund's rule
• electronic state
• singlet
• triplet
• ground state

Acknowledgements

We thank the National Science Foundation and the Robert A. Welch Foundation for support of this research. Some of the calculations reported here were performed on computers that were purchased with funds provided by the National Science Foundation under grant CHE-0741936. We dedicate this paper to Henry F. Schaefer III on the occasion of his 65th birthday, and we wish Fritz a very happy birthday and many more years of making important contributions to chemistry.