Trapping and Structural Characterization of the XNO2·NO3- (X = Cl, Br, I) Exit Channel Complexes in the Water-Mediated X- + N2O5 Reactions with Cryogenic Vibrational Spectroscopy.

The heterogeneous reaction of N2O5 with sea spray aerosols yields the ClNO2 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)n (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 D2 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.