Grain-Surface Hydrogen-Addition Reactions as a Chemical Link Between Cold Cores and Hot Corinos: The Case of H2CCS and CH3CH2SH.

Recently, searches were made for H2CCS and HCCSH in a variety of interstellar environments─all of them resulted in nondetections of these two species. Recent findings have indicated the importance of destruction pathways, e.g., with atomic hydrogen, in explaining the consistent nondetection of other species, such as the H2C3O family of isomers. We have thus performed ab initio calculations looking at reactions of H2CCS, HCCSH, and related species with atomic hydrogen. Our results show that H2CCS and HCCSH are both destroyed barrierlessly by atomic hydrogen, thus providing a plausible explanation for the nondetections. We further find that subsequent reactions with atomic hydrogen can barrierlessly lead to CH3CH2SH, which has been detected. Astrochemical simulations including these reactions result not only in reproducing the observed abundance of H2CCS in TMC-1 but also show that CH3CH2SH, produced via our H-addition pathways and subsequently trapped on grains, can desorb in warmer sources up to abundances that match previous observations of CH3CH2SH in Orion KL. These results, taken together, point to the importance of grain-surface H-atom addition reactions and highlight the chemical links between cold prestellar cores and their subsequent, warmer evolutionary stages.

Atmospheric Fate of Criegee Intermediate Formed During Ozonolysis of Styrene in the Presence of H2O and NH3: The Crucial Role of Stereochemistry.

A gas-phase mechanistic investigation of the unimolecular, water/ammonia-assisted decomposition reactions of the α-hydroxy hydroperoxides (HPs) and hydroperoxide arylamines (a-HPs) produced during the styrene ozonolysis has been carried out theoretically in the present article. The instrumental role of stereochemistry in controlling the outcome of individual reactions has been discussed. Thermodynamic parameters (Δ G298K, Δ H298K, Δ E0K) associated with individual reactions have also been computed. The rate constants estimated for individual reactions using conventional transition state theory (TST) combined with statistical mechanics provide a comprehensive understanding of the reaction mechanism and also elucidate the atmospheric fate of Criegee intermediates. Considering the feasibility of reactions from thermodynamic and kinetic points of view, while aldehyde (PhCHO) formation pathway originating from bimolecular decomposition of HP is found to be kinetically favored, benzoic acid formation pathway remains favored thermodynamically. A similar consideration for the bimolecular reactions of a-HP reveals the phenylmethanimine formation pathway to be kinetically favored, while the benzamide formation pathway is favored thermodynamically. Our findings appear to be in excellent agreement with the experimental observations.

Exploration of Unimolecular Gas-Phase Detoxication Pathways of Sarin and Soman: A Computational Study from the Perspective of Reaction Energetics and Kinetics.

A mechanistic investigation has been carried out to explore all possible gas phase unimolecular isomerization as well as decomposition pathways of toxic organophosphorus compounds (OPCs), namely, sarin (GB) and soman (GD), which are better known as nerve agents. We have identified a total of 13 detoxication pathways for sarin, where the α-H, ß-H, and γ-H take part in the H-transfer process. However, for soman, due to the presence of ω-H, three additional detoxication pathways are obtained, where the ω-H is involved in the H-transfer process. Among all the pathways, the D3 decomposition pathway, where the phosphorus oxoacid derivative and alkene are generated via the formation of a six-membered ring in the transition state, is identified as the most feasible pathway from the perspective of both activation barrier and reaction enthalpy values. Moreover, we have studied the feasibility of the isomerization and decomposition pathways by performing the reaction kinetics in the temperature range of 300 K-1000 K using the one-dimensional Rice-Ramsperger-Kassel-Marcus (RRKM) master equation. From the RRKM calculation also, D3 pathway is confirmed as the most feasible pathway for both OPCs. The rate constant values associated with the D3 pathway within the temperature range of 600 K-700 K imply that the degradation of the OPCs is possible within this temperature range via the D3 pathway, which is in good agreement with the earlier reported experimental result. It is also observed that at higher temperature range (∼900 K), the increased rate constant values of other detoxication pathways indicate that along with D3, all other pathways become more or less equally feasible. Therefore, the entire work provides a widespread idea about the kinetic as well as thermodynamic feasibility of the explored detoxication pathways of the titled OPCs.

Exploration of Binding Interactions of Cu(2+) with d-Penicillamine and its O- and Se- Analogues in Both Gas and Aqueous Phases: A Theoretical Approach.

We have theoretically explored the entire binding phenomena of d-penicillamine and its O- and Se-analogues with Cu(2+) in both gas and aqueous phases. At first, a brief conformational analysis has been performed via -XH and -COOH rotations to investigate such conformers that are suitable for binding in both bidentate as well as tridentate fashions. The stability of each bidentate and tridentate complex is determined on the basis of relative energy (ΔE) and gas phase metal ion affinity (MIA) along with the bonding analysis by using atoms in molecule theory. The effect of conformational change on the stability of the complexes is also examined thoroughly. By analyzing the MIA values, we have shown that the side chain substitution makes an impact on the binding process. To delve into the binding phenomena in aqueous phase, we have introduced both the first and second hydration sphere models. In first hydration sphere model, to realize the precise effect of water molecules we have considered stable octahedral hexa-aqua copper complex, [Cu(H2O)6](+2) and accordingly substituted water molecules depending on the bidentate or tridentate nature of the chelating agents. The influence of bulk water molecules on the energetics and geometries of the first hydrated sphere complexes have also been investigated by employing second hydration sphere model assuming physiological pH through the implementation of implicit COSMO and polarizable continuum models, respectively. In the second hydration sphere model, the zwitterionic structures of the amino acids and their side chain deprotonated forms are also included to study the binding phenomena with Cu(2+). The complete work furnishes both the binding properties and the energetics of the copper-artificial amino acid complexes in both gas and aqueous phases that will reflect a realistic overview of the entire binding phenomena.

Hydrolysis of ammonia borane and metal amidoboranes: A comparative study.

A gas phase mechanistic investigation has been carried out theoretically to explore the hydrolysis pathway of ammonia borane (NH3BH3) and metal amidoboranes (MNH2BH3, M = Li,Na). The Solvation Model based on Density (SMD) has been employed to show the effect of bulk water on the reaction mechanism. Gibbs free energy of solvation has also been computed to evaluate the stabilization of the participating systems in water medium which directly affects the barrier heights in the potential energy surface of hydrolysis reaction. To validate the experimentally observed kinetics studies, we have carried out transition state theory calculations on these hydrolysis reactions. Our result shows that the hydrolysis of both the metal amidoboranes exhibits greatly improved kinetics over the neat NH3BH3 hydrolysis which corroborates well with the experimental observation. Between the two amidoboranes, hydrolysis of LiNH2BH3 is found to be kinetically favored over that of NaNH2BH3, making it a better candidate for releasing molecular hydrogen.

Comprehensive study of methylation on the silicon (100)-2 × 1 surface: a density functional approach.

A detailed mechanistic investigation of Si-Me formation over the silicon (100)-2 × 1 surface using the Si9H12 cluster model has been performed using various reagents, based on two basic mechanisms: dissociation and substitution. The reagents CH4, CH3Cl for dissociation and CH3Li, CH3MgBr for substitution mechanism are used to explore the methylation process on the silicon surface at the M062X/6-311+G(2d, p) level of theory. The associated potential energy surfaces explored here are aimed to unveil the most favored pathway of methylation with appropriate reagents. Dissociation of methane forms a monomethylated product (D1) through an energetically unfavorable pathway. All the adsorption modes of CH3Cl over the silicon surface are also detected and analyzed. Methyl chloride dissociates to form another monomethylated product D2 and its derivative D3 in the entrance channel, while, in the next step, bridged compounds I1 (Cl-bridged) and I2 (H-bridged) are produced from them, respectively. The C-Cl dissociation leads to the formation of D2 having a lower activation barrier. With a comparably high activation barrier in the C-H dissociation, producing D3, very interestingly carbene intermediate has been detected in the reaction pathway. Detection of energetically unfavored conversions from D2 to I1 and D3 to I2 ensured that the methylation process will not be hampered through these interconversions. For substitution, HCl- and Cl2-passivated Si surfaces are taken, where chlorine is to be substituted by the methyl group of both of the methylating agents. With both substituents, HCl-passivated Si9H12 gives D1. The substitution process on Cl2-passivated Si9H12 leads to the formation of D2 in the first step and dimethylated product (S1) in the final step. In all the above substitution processes, methyl lithium proved to be the better substituent for the formations of D1, D2, and S1 on HCl- or Cl2-passivated surfaces. The present work not only demonstrated methyl lithium as one of the best methylating agents but also revealed the interrelation among the dissociative adsorption modes of CH3Cl, reported earlier, in a single potential energy surface with a remarkable detection of carbene intermediate formed in the pathway of C-H dissociation.

Towards a comprehensive understanding of the chemical vapor deposition of titanium nitride using Ti(NMe2)4: a density functional theory approach.

A gas phase mechanistic investigation of the chemical vapor deposition (CVD) of titanium nitride (TiN) from the decomposition of Ti(NMe2)4, tetrakis(dimethylamido)titanium (TDMAT) as a single source precursor as well as from the reaction of Ti(NMe2)4 with NH3, i.e., the ammonia assisted mechanism is carried out and reported herein within the framework of density functional theory. Contrary to the theoretical result reported previously for a model TDMAT, metallacycle formation and ß-H elimination pathways are found to be the major decomposition pathways responsible for the decomposition of TDMAT, and this finding is in accord with the experimental observation. Interestingly, agostic interaction is found to play a key role in promoting ß-H elimination in the decomposition of TDMAT. A new additional pathway of decomposition of TDMAT has been identified theoretically in this present study. Exploration of the complex gas phase mechanism and thereby a detailed identification of the reaction intermediates enable us in realizing the origin of incorporation of carbon contamination in TiN films produced from TDMAT alone and then how the contamination is removed in the presence of ammonia. The ammonia assisted mechanism is found to proceed through the formation of a pre-equilibrium complex. The computed barrier height of 7.3 kcal mol(-1) for the initial transamination process associated with the Ti(NMe2)4 + NH3 reaction is found to be in very good agreement with the experimental activation energy. The total rate constant ktot for the ammonia assisted mechanism is calculated to be 1.28 × 10(-51) cm(3) molecule(-1) s(-1) at 298.15 K.

Structure, stability, and dissociation of small ionic silicon oxide clusters [SiO(n)+ (n = 3, 4)]: insight from density functional and topological exploration.

The structures, energies, isomerization, and decomposition pathways of small ionic silicon oxide clusters, SiO(n)(+) (n = 3, 4), on doublet and quartet energy surfaces are investigated by density functional theory. New structural isomers of these ionic clusters have been obtained with this systematic study. The energy ordering of the isomeric cluster ions on doublet spin surface is found to follow the same general trend as that of the neutral ones, while it differs on the quartet surface. Our computational results reveal the energetically most preferred decomposition pathways of the ionic clusters on both spin surfaces. To comprehend the reaction mechanism, bonding evolution theory has also been employed using atoms in molecules formalism. The possible reasons behind the structural deformation of some isomers on quartet surface have also been addressed. Our results are expected to provide important insight into the decomposition mechanism and relative stability of the SiO(n)(+) clusters on both the energy surfaces.