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-Brδ+- -NCCH3 and C-Brδ+- -OC(CH3)2 or C-Iδ+- -NCCH3 and C-Iδ+- -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.