Abstract
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.
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Acknowledgement
The authors thank the reviewer for constructively suggesting how to confirm the small change observed of the calculated N-oxide bond lengths with a change in C-4′ substituent in the furoxan series is not merely an artefact of the calculation procedure. We have included confirmatory calculations and discussed the results.
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BALASUBRAHMANYAM, S.N., GANGULY, B., LO, R. et al. Computational evidence for back donation in an N → O group based on modes of transmission of substituent effects in 3-(4′-substituted) phenylfuroxans. J Chem Sci 133, 31 (2021). https://doi.org/10.1007/s12039-021-01885-7
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DOI: https://doi.org/10.1007/s12039-021-01885-7
Keywords
- 3(4′-substituted)phenylfuroxans
- ranges of electron-releasing and electron-attracting substituents
- substituent-dependent N-oxide bond lengths from B3LYP/6-311++G
- inverse residual charge changes at N-2 and O-6
- N-oxide bond dissociation and second-order perturbation energies from B3LYP/6-31++G and BLYP/6-31++G
- back donation from O to N in N–O dative bond