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

The heterogeneous reaction of NO with sea spray aerosols yields the ClNO molecule, which is postulated to occur through water-mediated charge separation into NO and NO ions followed by association with Cl. Here we address an alternative mechanism where the attack by a halide ion can yield XNO by direct insertion in the presence of water. This was accomplished by reacting X(DO) (X = Cl, Br, I) cluster ions with NO to produce ions with stoichiometry [XNO]. These species were cooled in a 20 K ion trap and structurally characterized by vibrational spectroscopy using the D 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 NO with XNO, with the NO constituent increasingly perturbed in the order I > Br > Cl. These results establish that XNO can be generated even when more exoergic reaction pathways involving hydrolysis are available and demonstrate the role of the intermediate [XNO] in the formation of XNO.