The interstellar gas-phase formation of CO 2 – Assisted or not by water molecules?

Abstract

Using state of the art methods of quantum chemistry, potential energy surfaces for the formation of CO 2 ( 1 Σ g + ) and CO 2 ( 3B 2) from CO + O ( 1D) and CO + O ( 3P), respectively, have been studied. At the MRSDCI level, we show that the formation of CO 2 ( 1 Σ g + ) from O ( 3P) is strongly connected with the height of the barrier localized on the CO + O ( 3P) entrance channel. At the CCSD(T) level with a large basis set we calculate this barrier to be 5.9 kcal/mol. Consequently, we confirm that the gas-phase formation of CO 2 in interstellar molecular clouds is inefficient. To mimic the formation of CO 2, through the Eley–Rideal mechanism, on the water ice surfaces of interstellar grains, we have extended our study to consider the formation of CO 2 in the presence of water molecules. We show, using density functional and CCSD(T) methods, that the barrier located on the CO + O ( 3P) reaction entrance channel is hardly affected by the presence of water molecules. We therefore suggest that CO 2 formation, through the Eley–Rideal mechanism, on the water ice surfaces of interstellar grains, should be inefficient too.