Computational evidence for back donation in an N → O group based on modes of transmission of substituent effects in 3-(4′-substituted) phenylfuroxans

• Published in: Journal of chemical sciences (Bangalore, India) India), 2021, Vol.133 (1), Article 31
• Main Authors: BALASUBRAHMANYAM, S NATARAJAN, GANGULY, BISHWAJIT, LO, RABINDRANATH, RAJAN, MUSIRI M BALAKRISHNA, SREERAG, MOORKKANNUR N, SHARAFUDEEN, P C, OSHIYA, R A, RAJENDRAN, NACHIAPPAN
• Format: Article
• Language: English
• Subjects: Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Electrons; Mathematical analysis; Nitrogen; Perturbation; Regular Article; Styrenes; Substitutes
• ISSN: 0974-3626; 0973-7103
• DOI: 10.1007/s12039-021-01885-7

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 the C-4 substituent to C-β. From geometry optimizations through high-level MO theory calculations, it was first ensured that the N-2–C-3 liaison in 3-(4′-substituted)phenyl furoxans retains as much double-bond character as it does in the case of furoxan bearing no substituents and that the para- substituted phenyl and furoxan rings maintain near uniplanarity. The calculations, carried out for such furoxans, chosen to represent a spectrum of effects from electron-donor to electron-acceptor, showed how the change in the 4′-substituent affects electron redistribution within N-oxide group in the way expected: while the residual positive charge at N increases the residual negative charge at O decreases. An increase in the N-oxide bond order (as measured by the Wiberg bond index), together with a small reduction in the N-2–O-6 bond length, was also found. That these effects were not artefacts of the calculation procedure was ensured when the calculations, repeated using a different functional, showed not only inverse dependence of positive N-2 and negative O-6 net charges on N-2–O-6 bond lengths but also confirmatory evidence from N-oxide bond dissociation and second-order perturbation energies. 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 π-polarisation involving the substituent at the para -position of the phenyl group offering a spectrum of effects from electron-releasing to electron-withdrawing. Graphical abstract 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.