Methoxymethane C-O Bond Strengths: Do Their Changes Result from Hyperconjugation or Polar Effects?

The methoxymethanes have been studied and compared with the fluoromethanes. The energies and atomic charges were calculated using MP2/aug-cc-pVTZ, and the group separation energies and bond dissociation enthalpies were calculated using CBS-QB3. The group separation energies are endothermic and the BDE increases with additional substitution as a result of the increase in charge at the central carbon. The greater charge leads to a stronger bond to the new substituent as well as to the original substituents. With the methoxymethanes, there is a linear relationship between the BDE and the atomic charge at C. The energies of the several methoxymethane conformers were calculated, and their energies usually increase with increasing values of the electronic spatial extent in accord with a proposal by Gillespie. The role of hyperconjugation in these cases is not settled.