The N-oxide nitrogen in C-4^′ substituted 3-phenyl furoxans occupies a position analogous to C-β in 4-substituted styrenes that have been examined for modes of transmission of substituent effects from theC-4 substituent to C-β. From geometry optimizations through high-level MO theory calculations, it was firstensured that the N-2–C-3 liaison in 3-(4^′-substituted)phenyl furoxans retains as much double-bond characteras it does in the case of furoxan bearing no substituents and that the para-substituted phenyl and furoxan ringsmaintain near uniplanarity. The calculations, carried out for such furoxans, chosen to represent a spectrum ofeffects from electron-donor to electron-acceptor, showed how the change in the 4^′-substituent affects electronredistribution within N-oxide group in the way expected: while the residual positive charge at N increases theresidual negative charge at O decreases. An increase in the N-oxide bond order (as measured by the Wibergbond index), together with a small reduction in the N-2–O-6 bond length, was also found. That these effectswere not artefacts of the calculation procedure was ensured when the calculations, repeated using a differentfunctional, showed not only inverse dependence of positive N-2 and negative O-6 net charges on N-2–O-6bond lengths but also confirmatory evidence from N-oxide bond dissociation and second-order perturbationenergies. These results are interpreted as demonstrating graded back donation from O to N within the N → O group caused by a combined action of mesomerism and ᴨ-polarisation involving the substituent at the parapositionof the phenyl group offering a spectrum of effects from electron-releasing to electron-withdrawing.

Calculations at the B₃LYP/6-31++G^** level have shown that back donation from O to N in N → O in 3-(4^′-X-phenyl) furoxans increase with a change of X from electron-donor to electron-acceptor.