• S Lalitha

      Articles written in Journal of Chemical Sciences

    • Is tetramethyleneethane a ground state triplet?

      Aparna Chakrabarti I D L Albert S Ramasesha S Lalitha Jayaraman Chandrasekhar

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      We have examined a number of possible ways by which tetramethyleneethane (TME) can be a ground state triplet, as claimed by experimental studies, in violation of Ovchinnikov’s theorem for alternant hydrocarbons of equal bond lengths. Model exact π calculations of the low-lying states of TME, 3,4-dimethylenefuran and 3,4-dimethylenepyrrole were carried out using a diagrammatic valence bond approach. The calculations failed to yield a triplet ground state even after (a) tuning of electron correlation, (b) breaking alternancy symmetry, and (c) allowing for geometric distortions. In contrast to earlier studies of fine structure constants in other conjugated systems, the computedD andE values of all the low-lying triplet states of TME for various geometries are at least an order of magnitude different from the experimentally reported values. Incorporation of σ mixing by means of UHF MNDO calculations is found to favour a singlet ground state even further. A reinterpretation of the experimental results of TME is therefore suggested to resolve the conflict.

    • Ab initio study of α- and β- silyl substituent effects on vinyl radicals

      S Lalitha Jayaraman Chandrasekhar

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      The geometries of α- and β-silyl substituted vinyl radicals and of α, β-disilylvinyl radical have been optimised with the STO-3G and the STO-3G * basis sets. The relative stabilities of various conformera have been determined at the UMP2/6-31G* level. The stabilisation of vinyl radicals through α-silyl substitution is larger than that due to corresponding alkyl groups. The presence of an α-silyl group also leads to a tendency towards linearisation of the vinyl radical centre and a corresponding reduction in the inversion barrier. In marked contrast, the β-silyl effect is negligible. The geometric, conformational and energetic consequences are insignificant. Overall, the silyl substituent effect at vinyl radicals is very different from that computed earlier for the vinyl cations, but qualitatively similar to that found in carbanions.

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