On the formation of propylene oxide from propylene in space: gas-phase reactions

Abstract

In the present article, we have investigated the possibility of forming propylene oxide (PO) from propylene (PE) by bi-molecular reactions. Propylene oxide is the first chiral molecule observed in the interstellar medium, and studying the thermodynamics and kinetics of formation can suggest possible synthetic routes. We have focused our attention on gas-phase reactions, and the presence of an environment is discussed in particular for the possibility of forming it by association reactions. In particular, we have considered radical and ion–molecule reactions. Results show that the main gas-phase route to PO formation is represented by ion–molecule reactions which turn out to be compatible with astrophysical conditions, notably: $$\hbox {PE} + \hbox {O}^{+}$$ and $$\hbox {PE} + \hbox {HO}_2^{+}$$ . Their final product is not PO, but its ionized variant $$\hbox {PO}^{+}$$ that can be neutralized by electron capture. The only thermodynamically and kinetically allowed reaction which can directly lead to neutral PO is a collision of PE with a singlet-excited $$\hbox {OH}^{+}$$ but two competing reactions (leading to $$\hbox {PE}^{+}$$ and $$\hbox {PO}^{+}$$ ) are thermodynamically favored and thus more plausible in space.