Ab initio coupled-cluster and multi-reference configuration interaction studies of the low-lying electronic states of 1,2,3,4-cyclobutanetetraone

ABSTRACT

The four, closely spaced, lowest energy electronic states of the challenging, D_4h-symmetric, 1,2,3,4-cyclobutanetetraone (C₄O₄) molecule have been investigated using high-level ab initio methods. The calculated states include the closed-shell singlet 8π(¹A_1g) state, the singlet 10π(¹A_1g) state, in which the π-type lowest unoccupied molecular orbital (LUMO) of the 8π(¹A_1g) reference is doubly occupied and the σ-type highest occupied molecular orbital (HOMO) is empty, and the open-shell singlet and triplet states, designated as 9π(¹B_2u) and 9π(³B_2u), respectively, originating from single occupancy of the HOMO and LUMO. Our focus is on single-reference coupled-cluster (CC) approaches capable of handling electronic near-degeneracies in diradicals, especially the completely renormalised CR-CC(2,3) and active-space CCSDt methods, along with their CCSD and EOMCCSD counterparts. The internally contracted multi-reference configuration interaction calculations with a quasi-degenerate Davidson correction are performed as well. Our computations demonstrate that the state ordering is 9π(³B_2u) < 8π(¹A_1g) < 9π(¹B_2u) < 10π(¹A_1g) and that the 8π(¹A_1g) − 9π(³B_2u) gap is in the 7–11 kJ/mol range, in reasonable agreement with the negative ion photoelectron spectroscopy measurements, which give 6.27 ± 0.5 kJ/mol. In addition to the theory level used, geometry relaxation and basis set play a significant role in determining the state ordering and energy spacings. In particular, it is unsafe to use lower level, non-CC geometries and smaller basis sets.

GRAPHICAL ABSTRACT

Keywords:

• 1,2,3,4-cyclobutanetetraone
• coupled-cluster theory
• multi-reference configuration interaction
• singlet–triplet gap
• electronic excitations

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work has been supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy [grant number DE-FG02-01ER15228; P.P]. The research at UNT was supported by grants from the National Science Foundation [grant number CHE-0910527] and the Robert A. Welch Foundation [grant number B0027].