• S Natarajan Balasubrahmanyam

      Articles written in Journal of Chemical Sciences

    • Steric effects on the ultraviolet absorption of 1,2-enedioic and related systems

      S Natarajan Balasubrahmanyam H Surya Prakash Rao

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      1,2-Enedioic systems, being sterically perturbed from planarity do not show the effect of the extended conjugation expected of a (formal) trienic entity. In the absence of a model which approximates to a uniplanar situation, the strategy of replacing an ester group in the enedioates by a cyano (for which less stringent steric demand may be presumed) and noting the correction concomitant to this replacement was adopted to arrive at a notional figure for the position of maximal absorption in the planar enedioates. From this the conclusion, subject to substantiation by molecular mechanical or quantum chemical calculations, was drawn that even the E-isomeric and comparatively less substituted enedioates are highly sterically perturbed. An alternative to an earlier explanation of the bathochromic shift of absorption maxima encountered in the 5-cyclic ene-ester and ene-nitrile, relative to the 6-cyclic analogues (observed also with the enedioates and cyanovinyl ester systems), seen later to have been based on unwarranted premises, has been advanced. A comment on the absorption characteristics of enedioic anhydrides has been appended.

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


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      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.

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