• R H DUNCAN LYNGDOH

      Articles written in Journal of Chemical Sciences

    • Comparison of alkyl group labilities in O-and N-alkylated DNA bases: A semiempirical molecular orbital study

      R H Duncan Lyngdoh

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      All O-alkylated DNA bases and nucleosides possess alkyl groups considerably more labile than those in N-alkylated bases and nucleosides, being prone to degradation through loss of the alkyl group at strongly acidicpH. The strength of the bond between the alkyl group and the atom on the base to which it is bound is calculated here using the semiempirical INDO-SCF-MO method, comparison being made between O6-alkylguanines, O4-alkylthymines and N7-alkylguanines. The results, calculated for many different alkyl groups, predict that the strength of this bond at acidicpH would be appreciably lower for the O-alkylated bases than for the N7-alkylguanines, but that increase ofpH would serve to stabilise this bond for the O-alkylated bases. These predictions are in good accord with experimental findings.

    • Proton transfer reactions of nucleic acid bases: A semiempirical molecular orbital study

      Divi Venkateswarlu R H Duncan Lyngdoh

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      The possibilities open for tautomerism, for protonation and for deprotonation in the five nucleic acid bases are examined theoretically using the semiempirical AM1 SCF-MO methodology. The tautomers predicted to be the most stable, other than the usual forms, all involve proton shifts to adjacent sites. The sites predicted to be the most favourable for protonation are the N7-G, N3-A, N3-C, O4-T and O4-U positions of guanine, adenine, cytosine, thymine and uracil respectively. The protons predicted to be the most acidic for each base are the N1-G, N9-A, N1-C, N3-T and N3-U protons. These predictions accord well with the conclusions drawn from experimental work so far as assignments of acidic protons and basic sites for the particular bases are concerned. However, the relative feasibility of these reactions for the different bases is not well predicted by these gas-phase calculations.

    • Uncatalyzed thermal gas phase aziridination of alkenes by organic azides. Part I: Mechanisms with discrete nitrene species

      S PREMILA DEVI TEJESHWORI SALAM R H DUNCAN LYNGDOH

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      Alkene aziridination by azides through uncatalyzed thermal gas phase routes has been studiedusing the DFT B3LYP/6-31G(d,p) method, where the possible role of discrete nitrene intermediates is emphasized.The thermal decomposition of azides is studied using the MP2/aug-cc-pVDZ strategy as well. The MP2(but not the B3LYP) results discount the existence of singlet alkylnitrenes where the alkyl group has an α-hydrogen. Addition of the lowest lying singlet and triplet nitrenes R-N (R = H, Me, Ac) to four different alkenesubstrates leading to aziridine formation was studied by the B3LYP method. Singlet nitrenes with alkenes canyield aziridines via a concerted mechanism, where H-N insertion takes place without a barrier, whereas Me-Nshows larger barriers than Ac-N. Methyl substitution in the alkene favors this reaction. Triplet nitrene additionto alkenes is studied as a two-step process, where the initially formed diradical intermediates cyclize to formaziridines by ISC (intersystem crossing) and collapse. Scope for C-C bond rotation in the diradical leads to lossof stereochemical integrity for triplet nitrene addition to cis- and trans-2-butenes. Geometries of the transitionstates in the various reaction steps studied here are described as “early” or “late” in good accordance with theHammond postulate.

    • Uncatalyzed gas phase aziridination of alkenes by organic azides. Part 2. Whole azide reaction with alkene

      S PREMILA DEVI R H DUNCAN LYNGDOH

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      The B3LYP/6-31G(d,p) DFT method was used to study alkene aziridination by azides through uncatalyzed thermal gas phase routes which involve the whole azide reactant molecule without dissociation.Two mechanisms were studied – Route I involving concerted azide addition to alkene with the elimination of N2, and the multi-step Route II involving 1,3-dipolar cycloaddition between azide and alkene. Three azides RN3 (R = H, Me, Ac) are reacted with alkene substrates forming aziridine products. The concerted addition–elimination step of Route I is exothermic with an appreciable barrier, where the facility order Ac > Me > Hpoints to electrophilicity of the azide reactant. The initial 1,3-dipolar cycloaddition step of Route II involves smaller barriers than Route I, while thermal decomposition of the triazoline intermediate to aziridine and N2 involves two more steps with an N-alkylimine intermediate. The very high barrier for N-alkylimine cyclization to aziridine could be offset by the high exothermicity of the previous step. Geometries of the transition states for various reaction steps studied here are described as ‘early’ or ‘late’ in good accordance with the Hammondpostulate. Two other mechanisms (Routes A and B) studied earlier (involving discrete nitrene intermediates) are compared with Routes I and II, where Route II involving 1,3-dipolar cycloaddition is predicted to be energeticallythe most favored of all the four mechanisms for thermal gas-phase aziridination of alkenes by azides.

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