A computational study of some nitrofluoromethanes

A group of eight nitrofluoromethanes has been studied by the ab initio SCF GAUSSIAN 82 computational procedure. All structures were optimized at the 3-21G level and then used to compute bond orders, as measures of relative bond strengths, and electrostatic potentials, as guides to the molecular charge distributions. The C-F bonds were found to be strengthened by the presence of other fluorines or nitro groups; this is attributed to inductive electron withdrawal by these other substituents, which leads to significant double-bond character in the C-F bond, through resonance back-donation by the fluorine. This simultaneously weakens or even eliminates the negative electrostatic potential normally associated with the fluorine. The interesting observation that there are marked differences between the strengths of the two N-O bonds in some nitro groups can be quantitatively related to the environment of each oxygen. The C-NO₂ bond strengths are relatively insensitive to the presence of other-NO₂or-F substituents. However, CHF(NO₂)₂ is found to be significantly more stable than CH(NO₂)₃, presumably because the crowding of three -NO₂ groups together on the same carbon is partially relieved.