Ground state potential energy surface between cyclobutadiene and tetrahedrane looked down from S 1 /S 0 conical intersections Masato Sumita y , Kazuya Saito * Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan article info Article history: Received 25 March 2010 Received in revised form 20 April 2010 Accepted 20 April 2010 Available online 24 April 2010 abstract We have explored the singlet ground state potential energy surface (S 0 PES) between cyclobutadiene (CBD) and tetrahedrane (THD) looked down from S 1 /S 0 conical intersections through multi-configuration self- consistent field theory. On the basis of the obtained S 0 PES, we propose the revised process of the THD to CBD symmetry-forbidden reaction. According to the present result, the THD to CBD rearrangement occurs via plural steps similarly to previous suggestions, but via a tetra-radical species (instead of an endo-species), which is considered for the first time in this paper. Since the endo-species is significantly destabilized when hydrogen atoms are replaced by bulky substituents (such as tert-butyl group), the present one, where endo- species are not involved, would be realized in actual systems having bulky substituents. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Cyclobutadiene (CBD) is a very interesting simple molecule. Many theoretical and experimental chemists often regard it as an anti- aromatic paradigm 1e3 because CBD is a typical antiaromatic molecule in view of Hückel rule. In spite of its simplicity, the structure of CBD has not been established experimentally because of its instability. How- ever, many theoretical and experimental results suggest that its structure should be rectangular D 2h in the singlet ground state (S 0 ). 4e14 Although the property of the unsubstituted CBD is still uncertain, relatively stable derivatives of CBD show very interesting photo- chemical properties. That is, the derivatives having bulky sub- stituents isomerize to corresponding derivatives of tetrahedrane (THD). 15e20 For example, tetra-tert-butyl CBD isomerizes to tetra- tert-butyl THD with light of wavelength >300 nm (<4.12 eV) 15 as shown in Scheme 1 . The CBD to THD rearrangement is a symmetry- forbidden reaction since the occupied and unoccupied orbitals should be crossed during this arrangement as seen in the orbital correlation diagram in Figure 1 . Hence, this rearrangement is Scheme 1. Figure 1. Orbital correlation diagram between CBD and THD. The sign of ‘A’ and ‘S’, which denote ‘Anti-symmetry’ and ‘Symmetry,’ respectively, are assigned to orbitals assuming C 2 symmetry. In CBD (left side), 1S and 1A orbitals are doubly occupied and 2A and 2S are empty (virtual). In THD (right side), 1S and 2S orbitals are doubly oc- cupied and 1A and 2A are empty. When THD4CBD rearrangement occurs, the occu- pied orbital should intersect the virtual orbital. * Corresponding author. Tel.: þ81 29 853 4239; fax: þ81 29 853 6503; e-mail address: kazuya@chem.tsukuba.ac.jp (K. Saito). y Present address: National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2010.04.083 Tetrahedron 66 (2010) 5212e5217