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The uncatalyzed gas phase study of the addition reaction between selenol and alkyne confirms a concerted pathway for the reaction. Reduced natural charge study substantiates that substituents on reactants that stabilize the charges computed on the transition structure facilitate the reaction. Regio- and stereoselectivity in the hydroselenation reactions under radical and base conditions are also investigated in detail and the results are compared with the experimental and the computed uncatalyzed gas phase reactions. While the radical and base catalyzed pathways are pronounced for its high degree of regio- and stereoselectivity leading to E- anti -Markovnikov product, the uncatalyzed reaction prefers Markovnikov product.

The present work provides a deep insight into the mechanistic and energetic aspects of alkyne hydrothiolation reaction. This work can be regarded as the first ever computational attempt to unravel the reaction pathway under transition metal free conditions. The results confirmed a concerted pathway for uncatalyzed gas phase reactions. Reduced natural charge analysis pointed out that the characterized transition structures could be envisaged as a hypothetical ion pair, the stabilization of which led to reduced energy barriers. Hydroxyl radical catalyzed pathway was unraveled for the reaction which was then compared with the uncatalyzed reactions in terms of energy barrier and the feasibility of the mode of addition. The catalyzed reaction was also marked for its high degree of regioselectivity, leading to an unselective mixture of (E/Z) anti-Markovnikov products.

Density functional theory calculations were employed to quantify the effect of hydrogen bonding interactions in the reactions involving cleavage of dichalcogenides in cystines and selenocystines. The computed gas phase reaction has a multistep mechanism involving hydrogen bond transfers from one heteroatom to another across the reaction coordinate along with the anionic charge transfer from the nucleophile to the product. The solution phase mechanism, however, is simple SN2 irrespective of the nature of the reaction centre or hydrogen bonding interaction at the reaction site, or the nucleophile. When compared to similar reactions without any hydrogen bonding interactions at the reaction centre, these were found to be very facile both in gas phase as well as in aqueous solution.

Thione-thiol tautomerization is biologically and synthetically significant proton transfer reaction. Current study analyses the effect of protic solvents on this reaction involving heterocyclic thiocarbonyl compounds having potent bioactivities such as 1,3-dihydroimidazole-2-thione, 2-thiohydantoin and 2-thiouracil. The computed catalytic effect of polar protic solvents water and methanol is explained by the favorable lone pair – antibonding orbital interactions which facilitates the required proton transfer. Microhydration study up to six water molecules has been carried out to get more insight into the explicit water-solute interaction. It has been concluded that four water molecules are enough to model explicit water-solute interaction that favorably affects the tautomerism of heterocyclic thiocarbonyl compounds. Moreover, the thione formation step of the equilibrium gets relatively more facilitated by the polar protic solvents. The results are further validated by considering a reported thiol-thione conversion in enzymatic decarboxylation reaction of 4′-phosphopantothenoylcysteine.

Etude de l'orientation moleculaire du copolymere trisequence du poly(γ-benzyl-L-glutamate)/poly(propylene oxyde) dans les couches monomoleculaires a l'interface air/liquide.