Preparation and Characterization of the Halogen-Bonding Motif inthe Isolated Cl-center dot IOH Complex with Cryogenic Ion Vibrational Spectroscopy br

In the presence of a halide ion, hypohalous acids can adopt two binding motifsupon formation of the ion-molecule complexes [XHOY]-(X, Y = Cl, Br, I): a hydrogen (HB)bond to the acid OH group and a halogen (XB) bond between the anion and the acid halogen.Here we isolate the X-bonded Cl-middotIOH ion-molecule complex by collisions of I-middot(H2O)nclusters with HOCl vapor and measure its vibrational spectrum by IR photodissociation of theH2-tagged complex. Anharmonic analysis of its vibrational band pattern reveals that formationof the XB complex results in dramatic lowering of the HOI bending fundamental frequencyand elongation of the O-I bond (by 168 cm-1and 0.13 A, respectively, relative to isolatedHOI). The frequency of the O-I stretch (estimated 436 cm-1) is also encoded in the spectrumby the weakv=0 -> 2 overtone transition at 872 cm(-1)