Arup Kumar Chakrabartty

Mechanistic and kinetics study of the gas phase reactions of methyltrifluoroacetate with OH radical and Cl atom

Theoretical investigations are carried out on the title reactions by means of ab initio and DFT methods. The optimized geometries, frequencies and minimum energy path are obtained at MPWB1K/6-31+G(d,p) level. Single point energy calculations are performed at MP2 and QCISD(T) levels of theory. Energetics were further refined by calculating the energy of the species with a high level G2(MP2) method. The rate constant of the two reactions are calculated at 298 K and 1 atm using Canonical Transition State Theory (CTST) utilizing the ab initio data obtained during the present study. The rate constant values are found to be 5.5 × 10−14 and 5.9 × 10−14 cm3 molecule−1 s−1, respectively which are in good agreement with the experimental data.

Theoretical investigation of atmospheric chemistry of volatile anaesthetic sevoflurane: reactions with the OH radicals and atmospheric fate of the alkoxy radical (CF3)2CHOCHFO: thermal decomposition vs. oxidation†

A theoretical study on the mechanism and kinetics of the gas phase reactions of a volatile anaesthetic compound (CF3)2CHOCH2F (Sevoflurane) with the OH radicals has been carried out using the hybrid HF–density functional M06-2X/6-31+G(d,p) method. Three conformations are predicted for the Sevoflurane molecule. Among the three conformers, the most stable one is considered for a detailed study. Reaction profiles are modeled including the formation of pre-reactive and post-reactive complexes at entrance and exit channels. Single point energy calculations have been performed by using the 6-311++G(d,p) basis set. The hydrogen abstraction from the –CH2F group is found to be the dominant reaction channel for hydrogen abstraction by OH radicals. Theoretically the calculated rate constant is found to be in good agreement with the experimentally measured ones. Using group-balanced isodesmic reactions, the standard enthalpies of formation for (CF3)2CHOCH2F, (CF3)2COCH2F and (CF3)2CHOCHF radicals are also reported for the first time. The atmospheric fate of the alkoxy radical, (CF3)2CHOCHFO, is also investigated for the first time using the same level of theory. Out of four prominent plausible decomposition channels including oxidation, our results clearly point out that reaction with O2 is the dominant path for the decomposition of (CF3)2CHOCHFO in the atmosphere involving the lowest energy barrier which is in accord with recent experimental findings.

Theoretical study on rate constants for the reactions of CF3CH2NH2 (TFEA) with the hydroxyl radical at 298 K and atmospheric pressure

Theoretical investigations are carried out on reaction mechanism of the reactions of CF3CH2NH2 (TFEA) with the OH radical by means of ab initio and DFT methods. The electronic structure information on the potential energy surface for each reaction is obtained at MPWB1K/6-31+G(d,p) level and energetic information is further refined by calculating the energy of the species with a Gaussian-2 method, G2(MP2). The existence of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation. Our calculation indicates that the H abstraction from –NH2 group is the dominant reaction channel because of lower energy barrier. The rate constants of the reaction calculated using canonical transition state theory (CTST) utilizing the ab initio data. The agreement between the theoretical and experimental rate constants is good at the measured temperature. From the comparison with CH3CH2NH2, it is shown that the fluorine substution decreases the reactivity of the C-H bond.

Theoretical investigation of the gas-phase reactions of CF2ClC(O)OCH3 with the hydroxyl radical and the chlorine atom at 298 K

A Theoretical study on the mechanism of the reactions of CF2ClC(O)OCH3 with the OH radical and Cl atom is presented. Geometry optimization and frequency calculations have been performed at the MPWB1K/6-31+G(d,p) level of theory and energetic information is further refined by calculating the energy of the species using G2(MP2) theory. Transition states are searched on the potential energy surface involved during the reaction channels and each of the transition states are characterized by presence of only one imaginary frequency. The existence of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation. Theoretically calculated rate constants at 298 K and atmospheric pressure using the canonical transition state theory (CTST) are found to be in good agreement with the experimentally measured ones. Using group-balanced isodesmic reactions as working chemical reactions, the standard enthalpies of formation for CF2ClC(O)OCH3, CF2ClC(O)OCH2 and CF3C(O)OCH3 are also reported for the first time.

Catalytic oxidation of NO by Au2− dimers: a DFT study

Density functional theory (DFT) calculations are performed to study the mechanistic details of NO oxidation promoted by gold dimer anions. Furthermore, we studied a full catalytic cycle producing two NO2 molecules. The reaction is explored along three possible pathways. Our theoretical results show that anionic gold dimers present catalytic activity towards NO oxidation, as indicated by the calculated low energy barriers and high exothermicities. The present results enrich our understanding of the catalytic oxidation of NO by Au-cluster based catalysts. For the first time, we have presented a systematic study on the structure and energetics of various reaction intermediates involved in NO oxidation by Au2− clusters using density functional theory (DFT).

A computational perspective on the kinetics and thermochemistry of the gas phase reactions of 1, 1-dichlorodimethylether (DCDME) with OH radical at 298 K

Kinetics and thermochemistry of the gas phase reactions between CH3OCHCl2 (DCDME) and OH radical are investigated theoretically. The geometries and all the stationary points on the potential energy surface are calculated at BHandHLYP/6-311G(d,p) method. The energy information is further refined at CCSD(T)/6-311G(d,p) level of theory. Reaction profiles are modelled including the formation of two pre-reactive and post-complexes. The rate constants, which are evaluated by Canonical Transition State Theory (CTST) including tunnelling correction at 298 K, are in very good agreement with the available experimental data. The percentage contributions of both reaction channels are also reported at 298 K. The hydrogen abstraction reaction from the –CHCl2 group is found to be dominant leading to the formation of CH3OCCl2 + H2O. Using group-balanced isodesmic reactions, the standard enthalpies of formation for CH3OCHCl2, CH3OCCl2 and CH2OCHCl2 are also reported.

Theoretical investigation on unimolecular decomposition of malonic acid: a potential sink for ketene†

DFT and ab initio calculations are performed to study the unimolecular decmposition pathways of malonic acid. The reaction is explored along three plausible decomposition pathways. The calculated results shows that decarboxylation is a two step process with an energy barrier of 32.16 kcal mol−1 at the CCSD(T)/6-311++G(d,p) level which is in reasonable agreement with available data. In contrast to the usual interpretations, novel features of this study are dehydration and in situ decarboxylation–dehydration pathways that involve energy barriers of 67.13 and 44.75 kcal mol−1, respectively at the CCSD(T)/6-311++G(d,p) level leading to the formation of carbon-suboxide and ketene along with conventional H2O and CO2. The thermal rate constants for the above decomposition pathways are also evaluated using Canonical Transition State Theory at 298 K.

Catalytic activity of anionic Au–Ag dimer for nitric oxide oxidation: a DFT study

Bimetallic nanoparticles composed of two different metals such as Au–Ag show novel catalytic behavior based on the effect of the second metal element added. Considering this fact, we have performed density functional theory (DFT) calculations to study the oxidation pathway of NO promoted by anionic Au–Ag− dimer. During our investigations, we have considered two most plausible pathways of NO oxidation. We found that the anionic Au–Ag− dimer can effectively catalyze NO oxidation reaction. In Au–Ag−, the Au site is more active than the Ag site, and the calculated energy barrier values for the rate determining step of the Au-site catalytic reaction are remarkably lower than those for both the Ag-site catalytic reactions. The present results enrich our understanding of the catalytic oxidation of NO by Au–Ag cluster based catalyst. For the first time, we have presented a systematic study on the structure and energetics of various reaction intermediates involved in NO oxidation by Au–Ag− dimer using DFT. The T1 diagnostic calculation suggests that the multi-reference character is not an issue for the present study.