ISOMERIZATION OF THE FORMALDEHYDE RADICAL CATION AND THE FAILURE OF MP2

It has been reported that restricted and unrestricted MP2 ab initio calculations of the energy difference between the formaldehyde (H2CO+.) and hydroxymethylene (HCOH+.) radical cations are seriously in error because of poor convergence of the perturbation series. We had reported a study of the formaldehyde-water radical cation and found MP2 to be reliable. A study of the correlation energy contribution to the isomeric structures formaldehyde-water and hydroxymethylene-water radical cations is reported. Both perturbation theory (from MP2 to MP4SDTQ) and configuration interaction (CISD and CISD(Q)) calculations were performed with the 6-311 + G(d, p) basis set at the MP2/6-311 + G(d, p) optimum geometries. The MP2 flaw in formaldehyde/hydroxymethylene is not present in the radical cations derived from the complexes with water. Analysis of our results for formaldehyde/hydroxymethylene shows that the flaw is mainly connected with poor convergence of the perturbation series for the formaldehyde radical cation (and not for the hydroxymethylene). Since proton transfer occurs in the complexes with water, this species is not present in our calculations, so the results are reliable. The hydroxymethylene-water radical cation is predicted to be more stable than the formaldehyde-water radical cation by 29 kJ/mol on the basis of CISD(Q)/6-311 + G(d, p)//MP2/6-311 + G (d, p) calculations.