Preparation and Characterization of the Halogen-Bonding Motif in the Isolated Cl - ·IOH Complex with Cryogenic Ion Vibrational Spectroscopy.

In the presence of a halide ion, hypohalous acids can adopt two binding motifs upon 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 - ·IOH ion-molecule complex by collisions of I - ·(H 2 O) n clusters with HOCl vapor and measure its vibrational spectrum by IR photodissociation of the H 2 -tagged complex. Anharmonic analysis of its vibrational band pattern reveals that formation of the XB complex results in dramatic lowering of the HOI bending fundamental frequency and elongation of the O-I bond (by 168 cm -1 and 0.13 Å, respectively, relative to isolated HOI). The frequency of the O-I stretch (estimated 436 cm -1 ) is also encoded in the spectrum by the weak v = 0 → 2 overtone transition at 872 cm -1 .