Nand Kishor Gour

Interstellar dehydrogenated PAH anions: vibrational spectra

Interstellar Polycyclic Aromatic Hydrocarbon (PAH) molecules exist in diverse forms depending on the local physical environment. Formation of ionized PAHs (anions and cations) is favourable in the extreme conditions of the ISM. Besides in their pure form, PAHs are also likely to exist in substituted forms; for example, PAHs with functional groups, dehydrogenated PAHs etc. A dehydrogenated PAH molecule might subsequently form fullerenes in the ISM as a result of ongoing chemical processes. This work presents a Density Functional Theory (DFT) calculation on dehydrogenated PAH anions to explore the infrared emission spectra of these molecules and discuss any possible contribution towards observed IR features in the ISM. The results suggest that dehydrogenated PAH anions might be significantly contributing to the 3.3 μm region. Spectroscopic features unique to dehydrogenated PAH anions are highlighted that may be used for their possible identification in the ISM. A comparison has also been made to see the size effect on spectra of these PAHs.

Theoretical prediction of the mechanistic pathways and kinetics of methylcyclohexane initiated by OH radicals

ABSTRACT In this manuscript, we have systematically depicted the theoretical prediction of H-absorption from methylcyclohexane initiated by OH radical. For this we have performed dual-level of quantum chemical calculations on the gas-phase reactions between methylcyclohexane (MCH) and OH radical. Geometry optimisation and vibrational frequency calculations have been performed at BHandHLYP/6-311G(d,p) level of theory along with energetic calculations at coupled cluster CCSD(T) method using the same basis set. All the stationary points of titled reaction have been located on the potential energy surface. It has also been found that the H-abstraction takes place from –CH site of MCH, which is the minimum energy pathway than others. The rate constant was calculated using canonical transition state theory for MCH with OH radical and is found to be 3.27 × 10−12 cm3 molecule−1 s−1, which is in sound agreement with reported experimental data. The atmospheric lifetime of MCH and branching ratios of the reaction channels are also reported in the manuscript.

Kinetics and mechanism of 3-chloro-2-methyl-1-propene(3-ClMP) initiated by OH radical: an insight from DFT calculations[1]

ABSTRACT In this manuscript, OH-initiated gas phase reactions of 3-chloro-2-methyl-1-propene [CH2=C(CH3)CH2Cl] has been carried out by using density functional theory (DFT) method. We have chosen M06-2X hybrid density functional method along with 6-31 + G(d,p) basis set for optimisation and vibrational frequency calculations of all the relevant molecular species of the titled reaction. Energetic calculations have been further refined at CCSD(T) method along with cc-pVTZ basis set in order to obtain higher degree of precision of thermo-chemical and kinetic results. Energy profile is presented for all stationary points involved in the titled reaction and thermochemistry for all possible reaction channels (H-abstraction and OH-addition reactions) has been analysed. Canonical Transition State Theory (CTST) has been employed to determine the rate constants within the temperature range of 250–450 K and Our reported rate constant at 298 K are found to be close to the reported experimental rate constant. The atmospheric lifetime of the titled molecule has also been reported in this work. We have also reported standard enthalpies of formation of the titled molecule and generated product radicals by using group-balanced isodesmic reaction schemes. GRAPHICAL ABSTRACT

Understanding the Atmospheric Oxidation of HFE-7500 (C3F7CF(OC2H5)CF(CF3)2) Initiated by Cl atom and NO3 Radical From Theory

Kinetics and mechanistic pathways for atmospheric oxidation of HFE-7500 ( n-C3F7CF(OCH2CH3)CF(CF3)2) initiated by Cl atom and NO3 radical have been studied using density functional theory. Oxidative degradation pathways facilitated by H-abstraction from the -OCH2 or -CH3 groups in HFE-7500 have been considered. It has been shown that H-abstraction from the α-site (-OCH2) is favored over other reaction pathways. The rate constants were computed employing transition-state theory and canonical variation transition-state theory incorporating small curvature tunnelling correction, over the temperature range of 250-450 K at atmospheric pressure. Calculated rate constants at 298 K and 1 atm compare well with earlier experiments. Temperature dependence of the rate constants and branching ratios for these pathways contributing to overall reaction are described. It has been shown that the rate constants over the studied temperature range was found to fit well to the modified Arrhenius equation (in cm3 molecule-1 s-1) kCl = 1.10 × 10-14  T0.04 exp(-69.87 ± 1.41/T) and kNO3 = 7.66 × 10-26 T3.30 exp(596.40 ± 1.22/T). Standard enthalpies of formation for the reactant (C3F7CF(OCH2CH3)CF(CF3)2) and the products [C3F7CF(OC•HCH3)CF(CF3)2 and C3F7CF(OCH2C•H2)CF(CF3)2] during H-abstraction are derived using the isodesmic approach. Atmospheric implications of the titled molecule are presented.

Quantum mechanical study on the oxidation of ethyl vinyl ketone initiated by an OH radical

Oxidation of ethyl vinyl ketone (CH2CHCOCH2CH3) by an OH radical was carried out using the M06-2X/6-311++G(d,p) level of theory. For the OH-initiated oxidation of ethyl vinyl ketone (EVK), we have considered six H-atom abstractions and three addition reactions. From the energetic calculation of the species involved therein, the potential energy surface (PES) of all the reaction channels was constructed. From the energy profile, we found that the H-atom abstraction from the methylene group (-CH2-) of CH2CHCOCH2CH3 is energetically more favourable than the other H-abstraction channels. Moreover, we also observed that OH-addition to the α-carbon of the carbon-carbon double bond of the title molecule is energetically and thermodynamically more dominant than β-carbon and carbonyl carbon. The rate coefficients for all the reaction channels were calculated using the canonical transition state theory at the temperature range of 250-450 K and it reveals that among all the reaction channels, OH-addition to α-carbon is kinetically more dominant to the total rate constant. The total rate coefficient for the reaction at 298 K is found to be in good agreement with the reported experimental rate constant. Finally, we have determined the atmospheric lifetime of the title molecule.

Hydrogen atom abstraction from Piperazine by hydroxyl radical: a theoretical investigation

ABSTRACT Dual level of quantum mechanical calculations have been carried out for hydrogen abstraction from Piperazine [HN(CH2CH2)2NH] initiated by OH radical. Geometry optimisation and frequency calculations of all species involved in the titled reaction have been performed at M06-2X/6-31+G(d,p) level of theory. For the accuracy in the thermochemistry and kinetics data, single-point energy calculations have been further carried out at coupled cluster CCSD(T) method along with 6-311G(d,p) basis set. An energy profile diagram for the reaction has been plotted along with pre-reactive and post-reactive complexes at entrance and exit channels. Intrinsic reaction coordinates (IRCs) calculations have been performed for identification of real transition states that connect it via reactant to product. Our result shows that the H-atom abstraction takes place from the C–H position of Piperazine. The rate constant is calculated using canonical transition state theory (CTST) is found to be 2.86 × 10−10 cm3 molecule−1 s−1 which is in good agreement with the reported experimental rate constant (2.38 ± 0.28) × 10−10 cm3 molecule−1 s−1 at 298 K. We have also reported rate constant for the temperature range 300–500 K. Using group-balance isodesmic reaction, the standard enthalpies of formation for Piperazine and product radicals generated by hydrogen abstraction are reported. The branching ratios for both reaction channel (i.e. H-abstraction from –CH2 and –NH position of Piperazine) are found to be 93% and 7%, respectively. The calculated atmospheric life time of Piperazine is found to be 0.97 hour.

Quantum calculation on night-time degradation of 2-chloroethyl ethyl ether (CH3CH2OCH2CH2Cl) initiated by NO3 radical

A dual-level quantum chemical calculations have been carried out on the initiation of night-time degradation of 2-chloroethyl ethyl ether (CH3CH2OCH2CH2Cl) via H-abstraction by NO3 radical. Within the scope of density functional theory, the electronic structure of all the species involved in the titled reaction has been optimized at M06-2X functional along with 6-31+G(d,p) basis set. A higher level of couple cluster CCSD(T) method in conjunction with 6-311++G(d,p) basis set has been used for the refined energy of the species. All minima and saddle states involved in the reaction channel have been characterized on the potential energy surface (PES). From PES, it is confirmed that H-abstraction from methylene (–CH2–) of ethyl (CH3CH2–) part of CH3CH2OCH2CH2Cl follows the minimum energy path. The rate constants (individual and overall) of the titled reaction are obtained using Canonical Transition State Theory (CTST) over the temperature range of 250–350K. The atmospheric lifetime and radiative efficiency of...

Atmospheric chemistry of HFE-7000 (i-C3F7OCH3) and isofluoro-propyl formate (i-C3F7OC(O)H): reactions with OH radicals, atmospheric lifetime and fate of alkoxy radical (i-C3F7OCH2O•) – a DFT study

ABSTRACT The mechanism of hydrogen abstraction reaction between HFE-7000 (i-C3F7OCH3) and OH radicals using M06-2X functional in conjunction with 6-31+G(d,p) basis set is investigated. The pre-reactive and post-reactive complexes from intrinsic reaction coordinate calculations are validated at entrance and exit channels, respectively. The standard enthalpies of formation for the species and bond dissociation energy for C–H bond are also estimated. The rate constants of the titled reactions over the temperature range of 250–450 K are reported. The OH-driven atmospheric life time of i-HFE-7000 is computed to be 3.19 years. The atmospheric fate of the alkoxy radical (i-C3F7OCH2O•) is also explored here for the first time. Three prominent plausible decomposition channels including oxidation are considered in detail. The thermochemical data reveal that reaction with O2 is the dominant path for the decomposition of i-C3F7OCH2O• radical. Moreover, rate constant for the OH-initiated hydrogen abstraction of isofluoro-propyl formate (i-C3F7OC(O)H) is also reported. Graphical Abstract