THE ENERGY DIFFERENCE BETWEEN FORMALDEHYDE AND HYDROXYMETHYLENE RADICAL CATIONS - FAILURE OF UNRESTRICTED (UMP2) AND RESTRICTED (RMP2) MOLLER-PLESSET PROCEDURES

Ab initio calculations at a variety of levels of theory, including full configuration interaction, have been performed to determine the energy difference between the formaldehyde (H2CO+.) and hydroxymethylene (HCOH+.) radical cations, UMP2 and RMP2 calculations fail even to produce the correct sign of the energy difference. Our best calculations, at the QCISD(T) / 6-311 + G(3df, 3pd) // QCISD(T) / 6-31 G (d, p) level, predict an energy difference of 23 kJ mol-1 (at 0 or 298 K after correction for zero-point vibrational energy), in good agreement with the experimental 27 kJ mol-1.