The effect of bonding electrons on molecular dimensions as determined by X-rays

The central-bond shortening in disubstituted benzenes having an electron-withdrawing group and an electron-releasing group in para positions is well known. However, even in several disubstituted benzenes having electron-withdrawing groups in both para positions, X-ray analysis shows a central-bond shortening. In monosubstituted benzenes, the middle bonds and the bonds farthest from the substituent show bond shortenings which cannot be accounted for by librational motion alone, it is pointed out that these apparent shortenings are caused by the asymmetry of the charge distribution around the C atoms bonded to the H atoms. This charge asymmetry is to a large extent an artifact of the refinement procedure.