``Closs's Diradical: Some Surprises on the Potential Energy Hypersurface,'' C. D. Sherrill, E. T. Seidl, and H. F. Schaefer, J. Phys. Chem. 96, 3712 (1992).

The singlet and triplet potential energy surfaces for 1,3-cyclopentanediyl (Closs's diradical) have been investigated using ab initio electronic structure theory. The triplet C2v structure previously postulated to be an intermediate in the ring inversion of bicyclo[2.1.0]pentane (BCP) is found to correspond to a saddle point, rather than a minimum, on a potential energy surface more complex than that originally proposed by Closs. The singlet and triplet surfaces are similar, but the triplet stationary points lie ~1 kcal/mol below the corresponding singlets. The BCP ground state and the singlet and triplet stationary points for Closs's diradical have been optimized using a DZ+d basis set at the SCF and CISD levels of theory.