Halogen bonding (XB) non-covalent interactions can be observed in compounds containing chlorine, bromine, or iodine which can form directed close contacts of the type R1-XY-R2, where the halogen X acts as a Lewis acid and Y can be any electron donor moiety including electron lone pairs on hetero atoms such as O and N, or π electrons in olefin double bonds and aromatic conjugated systems. In this work, we present the first evidence for the formation of ionic halogen bonds (IXBs) in the hydration of bromobenzene and iodobenzene radical cations in the gas phase. We present a combined thermochemical investigation using the mass-selected ion mobility (MSIM) technique and density functional theory (DFT) calculations of the stepwise hydration of the fluoro, chloro, bromo, and iodobenzene radical cations. The binding energy associated with the formation of an IXB in the hydration of the iodobenzene cation (11.2 kcal mol) is about 20% higher than the typical unconventional ionic hydrogen bond (IHB) of the CHOH interaction. The formation of an IXB in the hydration of the iodobenzene cation involves a significant entropy loss (29 cal mol K) resulting from the formation of a more ordered structure and a highly directional interaction between the oxygen lone pair of electrons of water and the electropositive region around the iodine atom of the iodobenzene cation. In comparison, the hydration of the fluorobenzene and chlorobenzene cations where IHBs are formed, -ΔS° = 18-21 cal mol K consistent with the formation of less ordered structures and loose interactions. The electrostatic potentials on the lowest energy structures of the hydrated halogenated benzene radical cations show clearly that the formation of an IXB is driven by a positively charged σ-hole on the external side of the halogen atom X along the C-X bond axis. The size of the σ-hole increases significantly in bromobenzene and iodobenzene radical cations which results in strong interaction potentials with the electron lone pairs of the oxygen atom of the water molecules and thus IXBs provide the most stable hydrated structures of the bromobenzene and iodobenzene radical cations. The results clearly distinguish the hydration behaviors resulting from the ionic hydrogen and halogen bonding interactions of fluorobenzene and iodobenzene cations, respectively, and establish the different bonding and structural features of the two interactions.

We report on the gas phase association of the small polar and aprotic solvent molecules acetonitrile (CH 3 CN) and acetone (CH 3 COCH 3 ) with the halogenated benzene radical cations (C 6 H 5 X •+ , X = F, Cl, Br, and I) using the mass-selected ion mobility technique and density functional theory calculations. The association energies (−ΔH°) of CH 3 CN (CH 3 COCH 3 ) with C 6 H 5 F •+ and C 6 H 5 I •+ are similar [13.0 (13.3) and 13.2 (14.1) kcal/mol, respectively] but higher than those of CH 3 CN (CH 3 COCH 3 ) with

The recent discovery of benzonitrile (C6H5CN), one of the simplest nitrogen-bearing polar aromatic molecules, in the interstellar medium motivates structural characterization of the benzonitrile-containing molecular ions as potential precursors for nitrogen-containing complex organics in space. Herein, we present mass-selected ion mobility measurements combined with density functional theory (DFT) calculations to reveal, for the first time, the structures of the benzonitrile dimer radical cation, the protonated dimer, and the protonated hydrated small clusters in the gas phase. The measured collision cross sections of the investigated ions in helium are in excellent agreement with the calculated values of the lowest energy DFT structures. Unlike the dimer radical cations of nonpolar aromatic molecules which adopt parallel sandwich configurations, the (C6H5CN)2·+ displays a symmetrically planar geometry with a double hydrogen bond formed between the nitrogen and hydrogen atoms. The protonated dimer has the structure of a proton-bound dimer (C6H5CNH+NCC6H5) where the bridging proton connects the nitrogen atoms in the two benzonitrile molecules resulting in a calculated collision cross section of 101.1 Å2 in excellent agreement with the measured value of 103.3 Å2. The structure of the hydrated protonated trimer consists of a hydronium ion core solvated by three benzonitrile molecules. By locating the proton on the lower proton affinity water molecule, the resulting hydronium ion can be fully solvated by forming three ionic hydrogen bonds with the benzonitrile molecules. These unique structural motifs could be useful for the molecular design and recognition involving charged aromatic systems and also for the search of nitrogen-containing complex organics in space.