Radical-molecule reaction C3H+H2O on amorphous water ice:: A promising route for interstellar propynal

Intriguing propynal (HCCCHO) has been attracting chemist's attention since 1955. However, to date, no satisfying conclusion concerning its formation mechanism in the interstellar medium ( ISM) has been reached, although a variety of gas-reaction models, including ion-molecule, radical-molecule, and molecule-molecule, have been postulated. In this paper, we consider for the first time the gas-grain interaction model that involves heterogeneous reaction at the surface of dust grain or in the icy mantles to account for the propynal's formation. Based on the detailed density functional theory (DFT) and Gaussian-3 potential energy surface studies, we found that although the gaseous process C3H + H2O -> propynal + H must surmount a considerable entrance barrier (around 10 kcal mol(-1)), amorphous water ice can significantly catalyze the propynal's formation to be barrierless. So, this model should be a more reasonable one for propynal's formation in the low-temperature interstellar space. This result may also represent one rare case of the water-catalyzed reaction associated with a molecular radical in space. Future experimental studies are greatly desired to probe such interesting processes.