The structure of the methanol radical cation: an artificially short CO bond with MP2 theory

A detailed reinvestigation of the structure of the methanol radical cation has been undertaken using a variety of correlated theoretical procedures. These include the B3-LYP density functional method, second-order Møller-Plesset theory (MP2), quadratic configuration interaction (QCISD, QCISD(T)), Brueckner doubles (BD) and coupled-cluster theory (CCSD(T)). At the highest level of theory employed in this study (CCSD(T)/6–311G(df,p)), the preferred structure of the methanol radical cation has an eclipsed conformation and a CO bond length of 1.370 Å. The structure is found to be strongly influenced by hyperconjugation. MP2, in combination with large basis sets, overestimates the effects of hyperconjugation, predicting CH3OH·+ to have a CO bond length of less than 1.3 Å. The use of a moderately large basis set that includes f-functions on the heavy atoms, and a high-level electron correlation procedure, are important in accurately determining the structure of the methanol radical cation.