The proton transfer reaction H3 + + CO → HCO+/HOC+ + H2 has gained considerable attention in the literature due to its importance in interstellar chemistry. The reaction productsformyl cation (HCO+) and isoformyl cation (HOC+)are known to initiate multiple chemical reaction networks, resulting in complex molecules found in space. Several experimental and theoretical studies probing the structure and energetics of the [H3CO]+ system, HCO+/HOC+ product branching ratios, reaction mechanisms, etc., have been reported in the literature. In the present work, we investigated the H3 + + CO bimolecular reaction in the gas phase using direct dynamics methodology. The simulation conditions were chosen to mimic recently reported velocity map imaging experiments on the same reaction. The calculations were performed using the density functional PBE0/aug-cc-pVDZ level of electronic structure theory. Internal energy and scattering angle distributions of reaction products found from the simulations are in qualitative agreement with the experiment. However, the product branching ratios at low collision energies were in contrast with the experimental predictions. Interesting dynamical features were observed in the simulations, and detailed atomic level mechanisms are presented.

A great deal of attention has been given to the decomposition chemistry of halons (halomethanes) due to their role in stratospheric ozone depletion. Knowledge of certain aspects of dissociation of halons such as the competition between radical and molecular pathways and their mechanistic details is limited. Halon molecules can isomerize to an iso form containing a halogen–halogen bond and such iso-halon forms have been identified as intermediates in condensed phase chemistry. Recently, a quantum chemistry study of role of iso-halons in the gas phase decomposition of halomethanes has been reported. In the present work, we have investigated the ground state dissociation chemistry of select halon molecules - CF2Cl2, CF2Br2, CHBr3, and CH2BrCl using electronic structure theory calculations and direct chemical dynamics simulations. Classical trajectories were generated on-the-fly using density functional PBE0/6-31G* level of theory at a fixed total energy. Simulation results showed that molecular products, in general, were dominant for all the four molecules at the chosen energy. A variety of mechanisms such as direct dissociation via multicenter transition states, decomposition via isomerization, radical recombinations, and roaming pathways contributed to the formation of molecular products. Atomic level mechanisms are presented, and the role of iso-halons in the gas phase chemistry of halomethanes is clearly established.

Curtius rearrangement is the elimination of N 2 from carbonyl azides RC(O)N 3 to form isocyanates RNCO. Two mechanisms, viz., stepwise and concerted have been proposed in the literature for this reaction. The stepwise mechanism involves the formation of a nitrene RC(O)N by elimination of N 2 followed by an intramolecular rearrangement of the nitrene to form the isocyanate. The concerted mechanism is a single-step pathway forming the N 2 + RNCO products directly. Previous experimental and theoretical studies have indicated that the mechanism is usually concerted for thermal reactions and both stepwise and concerted are preferred under photochemical conditions. In the present work, we investigated the mechanism of Curtius rearrangement of two carbonyl azides with different substituents (R = CH 3 and F). Atomic level reaction mechanisms were studied using chemical dynamics simulations under thermal reaction conditions. Classical trajectories were generated on-the-fly at the density functional B3LYP/6-31+G* level of electronic structure theory with similar initial conditions for both the molecules. Simulation results showed a dominant concerted mechanism for CH 3 C(O)N 3 and the operation of both the mechanisms for FC(O)N 3 . The fluorocarbonyl nitrene FC(O)N had an appreciable lifetime before undergoing intramolecular rearrangement to form the isocyanate. In a small number of trajectories, the product isocyanate produced via the concerted dissociation of FC(O)N 3 , isomerized back to the nitrene form.

3-Oxetanone is a strained cyclic molecule which plays an important role in synthetic chemistry. A few studies exist in the literature about the equilibrium properties of this molecule and the dissociation patterns of substituted 3-oxetanones. For the unsubstituted 3-oxetanone, formation of ketene (CHCO) and formaldehyde (HCHO) was considered to be the major dissociation pathway. In a recent work, pyrolysis products of 3-oxetanone molecule in the gas phase were investigated by Fourier transform infrared spectroscopy and photoionization mass spectrometry. In this study, an additional dissociation channel forming ethylene oxide (c-CHO) and carbon monoxide CO was reported. In the present work, gas phase dissociation chemistry of 3-oxetanone was investigated by electronic structure theory, ab initio classical chemical dynamics simulations, and Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant calculations. The barrier height for the ethylene oxide channel was found to be much higher than the ketene pathway. The dynamics simulations were performed at three different total energies, viz., 150, 200, and 300 kcal/mol, and multiple reaction pathways and varying branching ratios observed. A new dissociation channel involving a ring-opened isomer of ethylene oxide was identified in the simulations. This pathway has a lower energy barrier and was dominant in our dynamics simulations.