• G Narahari Sastry

      Articles written in Journal of Chemical Sciences

    • Measures to evaluate heteroaromaticity and their limitations: Story of skeletally substituted benzenes

      U Deva Priyakumar G Narahari Sastry

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      Ab initio HF, MP2, CCSD(T) and hybrid density functional B3LYP calculations were performed on a series of skeletally mono- and di-substituted benzenes, (CH)5Z and (CH)4Z2, Z = C-, N, O+, Si-, P, S+, Ge-, As, Se+, BH-, NH+, AlH-, SiH, PH+, GaH-, GeH and AsH+. Various measures of aromaticity such as the bond length equalization, homodesmic equations, singlet-triplet energy difference (AEs-t), chemical hardness (η) and out-of-plane distortive tendency are critically analysed. The relative energy ordering in skeletally disubstituted benzenes displays trends that are inexplicable based on conventional wisdom. In general, the orthoisomer is found to be the least stable when the substituent is from the second row, whereas if the substituent is from the fourth row, the ortho-isomer is the most stable. Various qualitative arguments, including (a) lone pair-lone pair repulsion, (b) the sum of bond strengths in the twin Kekule forms, and (c) the rule of topological charge stabilization (TCS), are used to explain the observed relative energy trends. The rule of TCS in conjunction with the sum of bond strengths is found to predict the relative energy ordering reasonably well. The reactivity of this class of compounds is assessed based on their singlet-triplet energy differences, chemical hardness and the frequencies corresponding to out-of-plane skeletal distortions. These reactivity indices show less kinetic stability for the compounds with substituents from the fourth row and point to the fact that the thermodynamically most stable compounds need not be the least reactive ones. The ‡Es-t values indicate that the π-framework of benzene weakens upon skeletal substitutions.

    • A theoretical study of porphyrin isomers and their core-modified analogues:cis-trans isomerism, tautomerism and relative stabilities

      M Punnagai Saju Joseph G Narahari Sastry

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      Semiempirical (AM1 and PM3) and density functional theory (DFT) calculations were performed on about 50 porphyrin isomers with 25 each of 1,2 (syn) and 1,3 (anti) tautomeric forms. The corresponding oxa-and thia-core-modified analogues were also computed. The variations of relative energies and stabilities of the core-modified analogues were compared with parent porphyrin1 and the corresponding oxa-and thia-analogues. The trends in relative energies are not significantly changed while going from parent system to oxa-and thia-core-modified porphyrins in case of bothsyn andanti tautomers. Isomers of types [2·2·0·0], [3·0·1·0], [3·1·0·0], and [4·0·0·0] are destabilized due to the absence of methine bridge, which results in angle strain for tetrapyrroles. Isomers having [2·1·1·0], [2·1·0·1], [2·0·2·0] and [2·2·0·0] connectivity, the Z isomers, are more stable compared to the correspondingE isomers in bothsyn andanti forms of parent and core-modified analogues.

    • Bottlenecks in the prediction of regioselectivity of [4 + 2] cycloaddition reactions: An assessment of reactivity descriptors

      G Gayatri G Narahari Sastry

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      B3LYP/6-31G(d) calculations were performed to obtain all the transition states and products for the 128 distinct reaction channels of Diels-Alder reactions by taking all possible combinations from a series of dienes (1N-a, 1N-b, 2N, 1P-a, 1P-b, 2P, 1O, 1S) and dienophiles (NE, PE, OE, SE, AE, OHE, MeE, CNE). The predictive ability of the values to gauge the regioselectivity of the putative [4 + 2] cycloaddition reactions is analysed. No correlation is obtained between the reaction energies and activation energies. The extent of asynchronicity is measured based on the bond order analysis. DFT-based descriptors such as the local softness (sk+ andsk), Fukui function indices (fk+ andfk), global electrophilicity index (Ω) and local electrophilicity index (Ωk) were found to be better than the conventional FMO predictions.

    • Metal ion binding with dehydroannulenes - Plausible two-dimensional molecular sieves

      B Sateesh Y Soujanya G Narahari Sastry

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      Theoretical investigations have been carried out at B3LYP/6-311++G∗∗ level of theory to study the binding interaction of various metal ions, Li+, Na+ and K+ with dehydroannulene systems. The present study reveals that alkali metal ions bind strongly to dehydroannulenes and the passage through the central cavity is controlled by the size of metal ion and dimension of dehydroannulene cavity.

    • Hydrogen bonded networks in formamide [HCONH2]$_n$ ($n = 1 − 10)$ clusters: A computational exploration of preferred aggregation patterns

      A Subha Mahadevi Y Indra Neela G Narahari Sastry

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      Application of quantum chemical calculations is vital in understanding hydrogen bonding observed in formamide clusters, a prototype model for motifs found in protein secondary structure. DFT calculations have been performed on four arrangements of formamide clusters [HCONH2]$_n$, ($n = 1 − 10$) linear, circular, helical and stacked forms. These studies reveal the maximum cooperativity in the stacked arrangement followed by the circular, helical and linear arrangements and is based on interaction energy per monomer. In all these arrangements as we increase cluster size, an increasing trend in cooperativity of hydrogen bonding is observed. Atoms-in-molecule analysis establishes the nature of bonding between the formamide monomers on the basis of electron density values obtained at the bond critical point (BCP).

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