Trapping and Structural Characterization of the XNO 2 ·NO 3 - (X = Cl, Br, I) Exit Channel Complexes in the Water-Mediated X - + N 2 O 5 Reactions with Cryogenic Vibrational Spectroscopy.

The heterogeneous reaction of N2O5 with sea spray aerosols yields the CINO2 molecule, which is postulated to occur through water-mediated charge separation into NO3- and NO2+ ions followed by association with Cl-. Here we address an alternative mechanism where the attack by a halide ion can yield XNO2 by direct insertion in the presence of water. This was accomplished by reacting X-(D2O) (X = Cl, Br, I) cluster ions with N2O5 to produce ions with stoichiometry [XN2O5](-). These species were cooled in a 20 K ion trap and structurally characterized by vibrational spectroscopy using the D-2 messenger tagging technique. Analysis of the resulting band patterns with DFT calculations indicates that they all correspond to exit channel ion molecule complexes based on the association of NO3- with XNO2, with the NO3- constituent increasingly perturbed in the order I > Br > Cl. These results establish that XNO2 can be generated even when more exoergic reaction pathways involving hydrolysis are available and demonstrate the role of the intermediate [XN2O5](-) in the formation of XNO2.