• Jayaraman Chandrasekhar

      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.

    • Factors contributing to phantom bonds in inorganic molecules: Interpretations and predictions

      A Rathna Jayaraman Chandrasekhar

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      The validity of various qualitative proposals for interpreting and predicting the existence of short contacts between formally non-bonded atoms, as in cyclodisiloxane and related inorganic ring systems, is critically evaluated. The models range from simple considerations of geometric constraints, lone pair repulsions and π-complex formation to proposals such as the unsupported π-bond model and the σ-bridged-π bond concept. It is pointed out that a unified description based on a combination of closed and open 3-centre 2-electron bonds is possible. The role of hybridisation is emphasized in the short phantom bond computed in an earlier model system. These insights are used to predict structures with exceptionally short Si..Si and B..B phantom bonds. The proposals are confirmed byab initio calculations.

    • Analysis of geometric and strain effects in homo-Diels-Alder reactions

      Animesh Pramanik Jayaraman Chandrasekhar

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      Molecular mechanics calculations have been carried out to quantify the key geometric and strain effects which are likely to control the homo-Diels-Alder reactivity of 1,4-dienes. The criteria considered include C1..C5 and C2..C4 distances in the diene, twist angle of the twoπ units, and the magnitude of strain increase as a result of cycloaddition. By first considering these factors in a number of nonconjugated dienes with known reactivity, the ranges of values within which the reaction is favoured are proposed. Calculations are also reported on several substrates which have not been investigated so far. Promising systems for experimental study are suggested which, in addition to being intrinsically interesting, would place the present proposals on a firm basis.

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