SPIN-FORBIDDEN AND SPIN-ALLOWED CYCLOPROPENONE (c-H₂C₃O) FORMATION IN INTERSTELLAR MEDIUM

Abstract

Three proposed mechanisms of cyclopropenone (c-H 2 C 3 O) formation from neutral species are studied using highlevel electronic structure methods in combination with nonadiabatic transition state and collision theories to deduce the likelihood of each reaction mechanism under interstellar conditions. The spin-forbidden reaction involving the singlet electronic state of cyclopenylidene (c-C 3 H 2 ) and the triplet state of atomic oxygen is studied using nonadiabatic transition state theory to predict the rate constant for c-H 2 C 3 O formation. The spin-allowed reactions of c-C 3 H 2 with molecular oxygen and acetylene with carbon monoxide were also investigated. The reaction involving the ground electronic states of acetylene and carbon monoxide has a very large reaction barrier and is unlikely to contribute to c-H 2 C 3 O formation in interstellar medium. The spin-forbidden reaction of c-C 3 H 2 with atomic oxygen, despite the high probability of nonadiabatic transition between the triplet and singlet states, was found to have a very small rate constant due to the presence of a small (3.8 kcal mol −1 ) reaction barrier. In contrast, the spin-allowed reaction between c-C 3 H 2 and molecular oxygen is found to be barrierless, and therefore can be an important path to the formation of c-H 2 C 3 O molecule in interstellar environment.

The interstellar chemistry of H2C3O isomers

“…The efficiency of OH + C 3 H 2 reactions to produce H 2 C 3 O isomers is linked to the fact that this channel is a direct OH addition followed by H elimination associated to the fact that CO production in its ground state is spin forbidden and also to the fact that HOC radical production, which involve also few steps, has a very low stability and then the C 2 H 2 + HOC channel is kinetically unfavored versus H 2 C 3 O + H production. Ahmadvand et al (2014) proposed three reactions to produce cyclopropenone, O + c-C 3 H 2 , O 2 + c-C 3 H 2 and CO + C 2 H 2 reactions. Among them, the O + c-C 3 H 2 reaction has an open bimolecular exit channel so that the association reaction will be negligible and the CO + C 2 H 2 reaction shows a very high barrier so it will also be negligible.…”

Nonadiabatic transition state theory: Application to intersystem crossings in the active sites of metal‐sulfur proteins

“…The magnetic properties of transition metal complexes and materials can be controlled by light, temperature, and pressure by initiating spin‐crossover transitions between the low‐spin and high‐spin states . Among many examples of processes where intersystem crossings play a central role are combustion, reactions in the atmosphere and in interstellar space, transition metal‐based catalysis, and binding of small molecules to the active sites of metalloproteins . Intersystem crossing has applications in photodynamic therapy, free‐radical polymerization reactions, organic‐light emitting diodes, and metal–organic frameworks .…”

Spin‐Forbidden Branching in the Mechanism of the Intrinsic Haber–Weiss Reaction

“…In this case, the probability of the transition from the doublet to the quartet surface becomes important.A lthough ar elatively small spin-orbitc oupling matrix element can be expected for light elements, high probabilities of spin conversion can be achieved, depending on the characteristics of the two PES at the MECP [15,16] and on the velocity of the systema tt he MECP. [17] According to the transition probability based on the Landau-Zener model appliedt os pinforbidden reactions, if, for instance, relativelyl ow velocities (in comparison with the spin-orbit coupling between the two spin states) are achieved,t he probability of spin conversion can be increased, as can be seen from Equation (1) of Ref. [17].Apreference for as pin-forbidden pathwayh as been observed previously for ar eaction between species containing light elements, [18] and many cases have been illustrated in gas-phase ion chemistry.…”

“…[17] According to the transition probability based on the Landau-Zener model appliedt os pinforbidden reactions, if, for instance, relativelyl ow velocities (in comparison with the spin-orbit coupling between the two spin states) are achieved,t he probability of spin conversion can be increased, as can be seen from Equation (1) of Ref. [17].Apreference for as pin-forbidden pathwayh as been observed previously for ar eaction between species containing light elements, [18] and many cases have been illustrated in gas-phase ion chemistry. [19] Ap otential energy calculation along the O···O bond distance of hydrogen peroxidea nd one of the O···OH bond angles showsaflat region involving 2 Int-1, 2 TS-2, and 2/4 MECP (see Figure 3).…”