• R H Duncan Lyngdoh

      Articles written in Journal of Chemical Sciences

    • Generation, structure and reactivity of arynes: A theoretical study

      Peter G S Dkhar R H Duncan Lyngdoh

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      The semiempirical AM1 SCF-MO method is used to study the benzyne mechanism for aromatic nucleophilic substitution of various m-substituted chlorobenzenes and 3-chloropyridine. The calculations predict that most of the fixed substituents studied here would induce the formation of 2,3-arynes through their electron-withdrawing resonance or inductive effects. The geometry and electronic structure of the 2,3- and 3,4-arynes investigated here, confirm the generally acceptedo-benzyne structure postulated for arynes. The sites of nucleophilic addition to arynes as predicted here are in fair agreement with expectation and experimental findings.

    • Molecular orbital studies on the Wagner-Meerwein migration in some acyclic pinacol—pinacolone rearrangements

      Zodinpuia Pachuau R H Duncan Lyngdoh

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      The semi-empirical PM3 SCF-MO method is used to investigate the Wagner-Meerwein migration of various groups during the pinacol-pinacolone rearrangement of some acyclic systems. Pinacol first protonates and dehydrates to form a carbocation that undergoes a 1,2-migration to form a protonated ketone, which then deprotonates to yield the pinacolone product. We study the Wagner-Meerwein migration of hydride, methyl, ethyl, isopropyl,t- butyl, phenyl and heterocylic 2-, 3-and 4-pyridyl groups in various acyclic 1,2-diol (pinacol) systems as they rearrange to pinacolones. This 1,2-migration involves a three-centred moiety in the cationic transition state. The migratory aptitude predicted here follows the order: hydride >t-butyl > isopropyl > ethyl > methyl > phenyl, which accords well with available experimental data and/or chemical intuition, reflecting also on the ability of the group involved to carry positive charge in the transition state. The structure of the migrating group (whether aliphatic or aromatic) within the transition state also supports the stabilising role of delocalisation of positive charge for reaction feasibility. Geometrical and thermodynamic considerations coincide in assigning the following order to relative “earliness” of the transition state along the reaction pathway:t-butyl > isopropyl > phenyl > methyl > 2-pyridyl > 4-pyridyl

    • Isomerization of propargyl cation to cyclopropenyl cation: Mechanistic elucidations and effects of lone pair donors

      Zodinpuia Pachuau Kiew S Kharnaior R H Duncan Lyngdoh

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      This ab initio study examines two pathways (one concerted and the other two-step) for isomerization of the linear propargyl cation to the aromatic cyclopropenyl cation, also probing the phenomenon of solvation of this reaction by simple lone pair donors (NH3, H2O, H2S and HF) which bind to the substrate at two sites. Fully optimized geometries at the B3LYP/6-31G(d) level were used, along with single point QCISD(T)/6-311+G(d,p) and accurate G3 level calculations upon the DFT optimized geometries. For the unsolvated reaction, the two-step second pathway is energetically favoured over the one-step first pathway. Lone pair donor affinity for the various C3H$^+_3$ species follows the uniform order NH3 > H2S>H2O>HF. The activation barriers for the solvated isomerizations decrease in the order HF>H2O>H2S>NH3 for both pathways. The number of lone pairs on the donor heteroatom as well as the heteroatom electronegativity are factors related to both these trends. Compared to the unsolvated cases, the solvated reactions have transition states which are usually ‘later’ in position along the reaction coordinate, validating the Hammond postulate.

    • Exploration of minima on the C7H$^+_7$ surface: Structural, stabilityand charge-related considerations

      Kiew S Kharnaior R H Duncan Lyngdoh

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      Four ab initio quantum chemical approaches were used to study the relative stability, structure and charge distribution of 33 putative C7H$^+_7$ isomeric structures. All of them proved to be true minima on the C7H$^+_7$ potential energy surface. Stability orders for all 33 isomers as derived from the four computational strategies used here showed a fair degree of correlation with one another, especially between the density functional theory (DFT) and MP2 data sets. These computational results reproduced the stability order tropyl > benzyl > mtolyl > o-tolyl > p-tolyl derived from previous results of high-level theory and experiment. Structural features such as aromaticity, cyclic or acyclic structure, allylic and other conjugation effects, carbon atom hybridization state and small ring strain were invoked as factors which augment or diminish cation stability. The NICS index for aromaticity could distinguish well between aromatic, non-aromatic and anti-aromatic ring moieties in the cations. Application of these factors proved to be quite consistent in rationalizing in a broad manner the calculated stability orders for the entire set. Joint consideration of the optimized geometry and charge distribution led to the choice of a single canonical form or hybrid best representing each C7H$^+_7$ isomer.

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