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

Three proposed mechanisms of cyclopropenone (c-H2C3O) formation from neutral species are studied using high-level 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-C3H2) and the triplet state of atomic oxygen is studied using nonadiabatic transition state theory to predict the rate constant for c-H2C3O formation. The spin-allowed reactions of c-C3H2 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-H2C3O formation in interstellar medium. The spin-forbidden reaction of c-C3H2 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-C3H2 and molecular oxygen is found to be barrierless, and therefore can be an important path to the formation of c-H2C3O molecule in interstellar environment.