Sourab Sinha

Reaction Mechanism for m-Xylene Oxidation in the Claus Process by Sulfur Dioxide

In the Claus process, the presence of aromatic contaminants such benzene, toluene, and xylenes (BTX), in the H2S feed stream has a detrimental effect on catalytic reactors, where BTX form soot particles and clog and deactivate the catalysts. Among BTX, xylenes are proven to be most damaging contaminant for catalysts. BTX oxidation in the Claus furnace, before they enter catalyst beds, provides a solution to this problem. A reaction kinetics study on m-xylene oxidation by SO2, an oxidant present in Claus furnace, is presented. The density functional theory is used to study the formation of m-xylene radicals (3-methylbenzyl, 2,6-dimethylphenyl, 2,4-dimethylphenyl, and 3,5-dimethylphenyl) through H-abstraction and their oxidation by SO2. The mechanism begins with SO2 addition on the radicals through an O-atom rather than the S-atom with the release of 180.0-183.1 kJ/mol of reaction energies. This exothermic reaction involves energy barriers in the range 3.9-5.2 kJ/mol for several m-xylene radicals. Thereafter, O-S bond scission takes place to release SO, and the O-atom remaining on aromatics leads to CO formation. Among four m-xylene radicals, the resonantly stabilized 3-methylbenzyl exhibited the lowest SO2 addition and SO elimination rates. The reaction rate constants are provided to facilitate Claus process simulations to find conditions suitable for BTX oxidation.

Variation of reactivity of aziridinium ion during alkylation

Reactivity/stability of the tricyclic aziridinium ion intermediate of the mustine drug molecule varies with the ∠NCC bond angle of the tricyclic ring during alkylation of guanine. A sharp variation in reactivity of the aziridinium ion intermediate is observed along the intrinsic reaction coordinate. Further, shifting of the lowest unoccupied molecular orbital towards the carbon centre that interacts with guanine is also observed. Thermochemical analysis shows that the alkylation reaction is exothermic and presence of polar solvent effect activation energy.

QM/MM Studies on Cyclodextrin-Alcohol Interaction

The main objective of this study is to provide an insight into the interactions involved during adsorption of the alcohols on β-CD composite nanostructured membrane. Interactions between β-cyclodextrin (β-CD) and alcohols (methanol, ethanol and butanol) are studied using the QM/MM method. Magnitude of interaction energies show that the alcohols are adsorbed on the membrane. In addition, the thermochemical analysis suggests that the formation of these host-guest complexes is enthalpy driven.

Understanding the influence of external perturbation on aziridinium ion formation

ABSTRACT A density functional theory study is performed to understand the effect of discrete water molecules during Az+ ion formation in nitrogen mustards. A comparative study in gas phase, and implicit and explicit solvation models of three drug molecules (mustine, chlorambucil and melphalan) is reported. Noteworthy changes in the structure and C–N stretching frequencies of the transition states have been observed in the presence of explicit water molecules. Presence of explicit water molecules reduces the positive charge around the tricyclic Az+ ring, and hence stabilising it. Both activation energy and rate constants are seen to be significantly affected in the presence of discrete water molecules.

Formation of Polymeric Housene Molecules of Group 15 Elements (N, P, As, and Sb): A DFT Study

Recently, the formation of the dimeric stibahousene molecule, bis(stibahousene), has been reported. In line with the report, the formation of dimeric housene molecules with N, P, and As is examined in light of density functional theory. Moreover, the extension of the study from dimeric to tetrameric and hexameric molecules (N, P, As, and Sb) is also performed. The study supports the formation of such polymeric housene analogues.

Density Functional Studies of Bis-alkylating Nitrogen Mustards

Abstract: Nitrogen mustards are the most extensively used chemotherapeutic agent since their evolution in the mid-1940s. The high degree of cytotoxicity of these drugs is attributed to their ability to form DNA interstrand cross-linked adducts, thereby inhibiting DNA replication. Interstrand cross-linking occurs via formation of an unstable intermediate, the aziridinium ion and formation of mono-adducts. Mustine, the first member of this family, suffers from some serious drawbacks such as high rate of hydrolysis. Therefore its stable analogs have been sought; and since its discovery hundreds of analogs have been synthesized. This article presents a brief introduction to nitrogen mustards and deliberates on the works already devoted to establishing the mechanism of action of this class of drug. A brief discussion on DFT and DFRT is also furnished in section 1.2. Further, computational studies performed on nitrogen mustards are discussed in section 1.3 and 1.4. Section 1.4 of the article consists of research works from our group and has special reference to DFT and DFRT.