Sanyasi Sitha

Formation of a pre-reaction hydrogen-bonded complex and its significance in the potential energy surface of the OH + SO2→ HOSO2 reaction: A computational study

Using computational calculations, we have revisited the potential energy surface (PES) of the reaction between OH and SO2, which is believed as the rate-limiting step in the atmospheric formation of H2SO4. In this work, we report for the first time the presence of a pre-reaction hydrogen-bonded complex between OH and SO2 in the reaction PES. Based on this finding, it has been shown that the reaction can be considered as a two-step process in which the first step is the formation of the pre-reaction complex and the second step is the transformation of this complex to the product. It was observed that due to the presence of this pre-reaction complex as a potential well in the reaction PES, the barrier height got increased by around two-fold for the second step. Based on this observation, it has been proposed that the kinetics of the reaction is going to be affected. Also based on the analysis of the geometries of this pre-reaction complex and the transition state, it has been argued that the step involving the transformation of this pre-reaction complex to the product via the transition state is going to be the slowest step as this transformation involves large structural changes of the stationary points involved.

Reactivity of phosphorus mononitride and interstellar formation of molecules containing phospazo linkage: A computational study on the reaction between HSi (X2Π) and PN (X1Σ+)

Phosphorus mononitride (PN) shows some interesting chemistry due to its low dissociation energy (compared to N2) and small dipole moment (zero dipole moment for N2). In this work, a reaction between HSi (X2Π) and PN (X1Σ+) has been studied using various computational methods. Analysis of the doublet surface of the HSi+PN reaction indicates that the reaction is exothermic in nature leading to the formation of various products. In view of the barrierless association of the reactants and exothermic nature for the product formation, it is suggested that species like HPNSi, cyclic-SiN(H)P (these two most stable isomers have phosphazo linkage) and HSiNP (third most stable isomer has phosphdiazo linkage) can possibly be detected in the interstellar medium. In view of the potential applications of phosphazo compounds in amide synthesis and pervasive nature of amide linkages in the nature, possible interstellar prebiotic applications can be advocated for these compounds.

Potential application of compliance constants in predicting the mass spectral fragmentation of metabolites.

RATIONALE Metabolomics is a qualitative and quantitative measurement of the metabolite content of any biological system under a given physiological status. Due to the chemically diverse nature of these samples, metabolite identification is a difficult task, and development of alternative approaches, such as those based on mass spectrometry (MS), aimed at proper metabolite identification is required. METHODS Compliance constants, a direct measure of mechanical bond strength, were used for the first time to predict the MS fragmentation patterns of different regional isomers of a ubiquitous plant metabolite, caffeoylquinic acid (CQA). The compliance constant of an ester bond linking caffeic acid and a quinic acid molecule in CQA was calculated using density functional theory and Wilsons G-matrix formalism to distinguish the isomers. The predicted fragmentation patterns were compared with mass spectra obtained using negative ion electrospray ionization ultra-high-performance liquid chromatography quadrupole time-of-flight mass spectrometry (UHPLC/QTOFMS). RESULTS Our in silico results were found to be in correlation with our UHPLC/QTOFMS results, suggesting a potential application of compliance constant algorithms for the rationalization of complex mass spectrometric data. The results also show that the different configurations in stereochemistry that exist between different regional isomers contribute to the underlying energy of the surrounding bonds and the fragmentation thereof. CONCLUSIONS The results of our pilot study suggest that computational modelling can be applied for metabolite identification during metabolomic data mining and Natural Product research in general.