CASSCF, CASPT2, and MRCI investigations of formyloxyl radical (HCOO.) Original Research Article

The structures of the electronic states of HCOO. were optimized at CASSCF and CASPT2 levels of theory with generally contracted basis sets of atomic natural orbital type: [8s4p]/[3s2p] for hydrogen and [14s9p4d]/[4s3p2d] for carbon and oxygen atoms. Vibrational frequencies were calculated to verify the characteristics for some of the optimized stationary points. MRCI calculations were carried out for the σ radicals at the CASPT2 optimized geometries to provide a test on the higher-order correlation effects. At the CASPT2 level, the lowest electronic state of HCOO. is the 2A1 state. The electronic energy of the 2B2 state is 6 kJ/mol above. The broken symmetry 2A′ state is predicted to lie 25k J/mol above the 2A1 state and to be a transition structure for rearrangement to HOCO.. Single-point MRCI calculations yield the reverse order for the C2v symmetric states, 2A1 state being predicted to be 9.5 kJ/mol above the 2B2 state. The 2A′ state is nearly degenerate with the 2A1 state. Relative energies predicted at the (unoptimized) MRCI level are almost identical to those found by single-reference CI calculations.