Pseudorotational interconversion of the (2)A(1) and B-2(2) states of HCOO1

The lowest potential energy surface for the sigma formyloxyl radical (sigma-HCOO) is investigated with an equation-of-motion coupled cluster technique and basis sets that contain up to 195 contracted Gaussian functions. The B-2(2) and (2)A(1) states of symmetric (C-2v) sigma-HCOO both lie on this surface and can be interconverted adiabatically by transporting the nuclei through nonsymmetric, but planar, intermediate geometries. In accord with previous high-level calculations, the potential energy surface that governs this pseudorotation process is predicted to be extremely flat. While the (2)A(1) stale is found to be the highest point on the interconversion pathway when basis sets of modest size are used, this symmetric form is preferentially stabilized by basis set augmentation. Calculations with very large basis sets suggest that both the (2)A(1) and B-2(2) C-2v structures are local minima on the surface, separated by a barrier of about 500 cm(-1). These results are in qualitative agreement with conclusions reached in a recent electron photodetachment experiment.