Gas phase hydration of halogenated benzene cations. Is it hydrogen or halogen bonding?

Abstract

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.

Energy Decomposition Analysis for Interactions of Radicals: Theory and Implementation at the MP2 Level with Application to Hydration of Halogenated Benzene Cations and Complexes between CO2–· and Pyridine and Imidazole

“…Ionic hydrogen bonds (IHBs) constitute a subclass of hydrogen bonding between radical ions and polar molecules with a binding energy ranging from −20 to −140 kJ/ mol. 62,105 Halogen bonds are defined according to IUPAC as a linear binding motif R−X•••Y, where X denotes a covalently bonded halogen atom acting as a Lewis acid (electron-poor), and Y is an electron-rich Lewis base (e.g., halide anion, water, etc.). 106 Halogen bonding is a type of intermolecular interaction that, in part, arises from the favorable electrostatic interaction between the halogen atom and the acceptor (Lewis base), which is often described in terms of the so-called σ-hole.…”

“…In this work the ALMO-MP2-EDA scheme is generalized to unrestricted and restricted open-shell MP2 to permit the study of intermolecular interactions of radicals with a correlated wavefunction method, for which DFT methods are shown to be error prone. 39,61 We verify that the correct asymptotic behaviour of frozen, polarization, dispersion and charge transfer is operative, and the method is applied to understand the hydration of halogenated benzene radical cations., 62 as well as the different interaction motifs of the CO 2 −• radical anion with N-heterocycles in the gas phase. 63,64 2 Theory…”

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

“…The experimentally measured −ΔH°values are consistently greater than the −ΔH°values corresponding to the first hydration step of the halogenated benzene cations previously determined. 18 For instance, −ΔH°values for the hydration of fluoro-, chloro-, bromo-, and iodobenzene radical cations (9.0, 7.9, 8.4, and 11.2 kcal/mol, respectively) are less than those for the association of acetonitrile with the same ions (13.0, 10.5, 10.9, and 13.2 kcal/mol, respectively). This can be explained by the larger dipole moment of acetonitrile (4.1 D) as compared to that of water (1.9 D) which leads to stronger iondipole interactions in the acetonitrile-associated than in the hydrated ions.…”

“…The electrostatic potentials of the lowest energy structures of the hydrated bromobenzene and iodobenzene radical cations show clearly the formation of a positively charged σ-hole on the external side of the halogen atom along the C−X bond axis which interacts favorably with water, thus providing the most stable hydrated structures of these radical cations. 18 In the present work, we extend the hydration study of the halogenated benzene cations to include other important polar solvents such as acetonitrile and acetone. In these systems, both uIHBs and IXBs can be formed through the interactions of the hydrogen or the halogen atoms, respectively, of the halogenated benzene cations with the N or the O atoms of the solvent molecules.…”

Open-Shell Variant of the London Dispersion-Corrected Hartree–Fock Method (HFLD) for the Quantification and Analysis of Noncovalent Interaction Energies

“…This set is labeled in this study as IB8. A combined mass-selected ion spectroscopy and DFT study 92 demonstrated that the binding motif (ionic H-bond vs ionic X-bond) of 2 H 5 C 6 X + is sensitive to the halogen atom X (F, Cl, Br, and I). The TA13 set 94 of radical–solvent binary complexes in the MGCDB84 database.…”