Ionic Hydrogen and Halogen Bonding in the Gas Phase Association of Acetonitrile and Acetone with Halogenated Benzene Cations

We report on the gas phase association of the small polar and aprotic solvent molecules acetonitrile (CH₃CN) and acetone (CH₃COCH₃) with the halogenated benzene radical cations (C₆H₅X^•+, X = F, Cl, Br, and I) using the mass-selected ion mobility technique and density functional theory calculations. The association energies (-Δ H°) of CH₃CN (CH₃COCH₃) with C₆H₅F^•+ and C₆H₅I^•+ are similar [13.0 (13.3) and 13.2 (14.1) kcal/mol, respectively] but higher than those of CH₃CN (CH₃COCH₃) with C₆H₅Cl^•+ and C₆H₅Br^•+ [10.5 (11.5) and 10.9 (10.6) kcal/mol, respectively]. However, the electrostatic potentials of the lowest energy structures of C₆H₅Br^•+(CH₃CN) and C₆H₅Br^•+(CH₃COCH₃) or C₆H₅I^•+(CH₃CN) and C₆H₅I^•+(CH₃COCH₃) complexes clearly show the formation of the ionic halogen bonds (IXBs) C-Br^δ+- -NCCH₃ and C-Br^δ+- -OC(CH₃)₂ or C-I^δ+- -NCCH₃ and C-I^δ+- -OC(CH₃)₂ driven by positively charged σ-holes on the external sides of the C-Br and C-I bond axes of the bromobenzene and iodobenzene radical cations, respectively. For the C₆H₅F^•+(CH₃CN) complex, the dominant interaction involves a T-shaped structure between the N atom of CH₃CN and the C atom of the C-F bond of C₆H₅F^•+. The structure of the C₆H₅Cl^•+(CH₃CN) complex shows the formation of unconventional ionic hydrogen bonds (uIHBs) between the N atom of CH₃CN and the C-H bonds of the C₆H₅Cl^•+ cation. Similar results are obtained for the association of acetone with the halogenated benzene radical cations. The formation of IXBs of the iodobenzene cation with acetonitrile or acetone involves a significant entropy loss (-Δ S° = 25-27 cal /(mol K)) resulting from the formation of more ordered and highly directional structures between the nitrogen or oxygen lone pair of electrons of acetonitrile or acetone, respectively, and the electropositive region around the iodine atom of the iodobenzene cation. In comparison, for the association of acetonitrile or acetone with the fluorobenzene, chlorobenzene, and bromobenzene cations, -Δ S° = 16-23 cal/(mol K), consistent with the formation of less ordered structures and loose interactions. The lowest energy structures of the C₆H₅Br^•+(CH₃COCH₃)₂ and C₆H₅I^•+(CH₃COCH₃)₂ clusters show a novel combination of ionic halogen bonding and hydrogen bonding where the oxygen atom of one acetone molecule forms the halogen bond while the oxygen atom of the second acetone molecule becomes the hydrogen acceptor from the methyl group of the first acetone molecule.