We investigate reaction paths for the formation of ground-state 2-cyclopentenone and 3-cyclopentenone arising from the electrophilic addition of O(P-3) to a double bond of cyclopentadiene. Our calculations show that one possible mechanism for these reactions is the formation initially of a triplet diradical that undergoes intersystem crossing to a diradical singlet followed by passage of the system through the transition state to ground-state products. Equilibrium geometries and total energies are computed for all species using the Hartree-Fock method, the local spin density approximation, four generalized gradient approximations, the G2 model chemistry method, and the quantum Monte Carlo (QMC) approach in both the variational (VMC) and diffusion (DMC) variants. Using DMC as a gauge of accuracy, a comparison of these approaches reveals that the density functional methods show mixed performance, with the gradient-corrected B3PW91 functional giving the best reaction pathway energetics overall for the group. The G2 energetics are in good agreement with DMC for stable species but differ significantly for transition states. A resonance found in precursors to formation of 3-cyclopentenone (3CP) provides a possible explanation for the appearance of 3CP in trace amounts in cyclopentadiene combustion reactors.