A theoretical approach to substituent effects. Structural consequences of electrostatic and orbital interactions in model mono‐ and disubstituted methanes

Ab initio molecular orbital theory is used to examine the effect of substituents on bond lengths in mono‐ and disubstituted methanes. The relative importance of electrostatic and orbital interaction terms are assessed. The results suggest that for substituents (X) which show powerful σ effects and weak π interactions (e.g., F), the changes in bond length are due primarily to the electrostatic component except in some disubstituted methanes in which case the change in the hyperconjugative ability of the C—X bond is also important. On the other hand, substituents X which show weak σ effects but powerful π interactions (e.g., NH₂) affect bond lengths primarily through hyperconjugative interaction of a filled or vacant π‐type orbital on X with the adjacent bonds.