Noncovalent Halogen Bonding as a Mechanism for Gas-Phase Clustering

Gas-phase clustering of nonionizable iodylbenzene (PhIO2) is attributed to supramolecular halogen bonding. Electrospray ionization results in the formation of ions of proton-charged and preferably sodium-charged clusters assignable to [H(PhIO2)n]+, n = 1–7; [Na(PhIO2)n]+, n = 1–6; [Na2(PhIO2)n]2+, n = 7–20; [HNa(PhIO2)n]2+, n = 6–19; [HNa2(PhIO2)n]3+, n = 15–30; and [Na3(PhIO2)n]3+, n = 14–30. The largest cluster detected has a supramolecular mass of 7147 Da. Electronic structure calculations using the M06-2X functional with the 6-311++G(d,p) basis set for C, H, and O, and LANL2DZ basis set for I and Na predict 298 K binding enthalpies for the protonated and sodiated iodylbenzene dimers and trimers are greater than 180 kJ/mol. This is exceptionally high in comparison with other protonated and sodiated clusters with well-established binding enthalpies. Strongly halogen-bonded motifs found in the crystalline phases of PhIO2 and its derivatives serve as models for the structures of larger gas-phase clusters, and calculations on simple model gas-phase dimer and trimer clusters result in similar motifs. This is the first account of halogen bonding playing an extensive role in gas-phase associations.