Probing the O⋯Br–Br halogen bonding in X-ray crystal structures with ab initio calculations

Abstract

Halogen bonding is a class of non-covalent interaction in which a halogen atom interacts with an electronegative atom such as oxygen or nitrogen in an attractive fashion. In this work, we investigate the X-ray crystallographically observed non-bonded C–O⋯Br–Br interactions with methanol, 1,4-dioxane and acetone by quantum chemical calculations. The C–O⋯Br–Br interaction was further extended with dimethyl ether, 1,3-dioxane and formaldehyde. The CBS-QB3 calculated results show that the oxygen and bromine non-bonded interactions are in the order of 3–5 kcal mol⁻¹, which are comparable to the typical O–H⋯O and N–H⋯O type hydrogen bond strengths [S. J. Grabowski, Chem. Rev., 2011, 111, 2597]. The AIM calculations show good correlation between the density at the intermolecular critical point and the interaction energy. This study has also explored the directionality of bromine molecule addition to the lone pairs at the sp² and sp³ oxygen atoms of methanol, 1,4-dioxane, acetone, dimethyl ether, 1,3-dioxane and formaldehyde. The calculated results show that the directionality of bromine molecules towards interacting with the oxygen atoms of 1,4-dioxane and acetone is in accord with the observed X-ray crystal structure analysis. However, in the case of methanol, the approach of the bromine molecule seems to be influenced by the crystal forces. The influence of stereoelectronic effects towards the approach of the bromine molecule to 1,3-dioxane is more prominent than that of the 1,4-dioxane system. The existence and magnitude of the positive regions (σ-hole) on the other side of the bromine molecule complexed with the donor molecule seem to facilitate the bridge formation as observed in the X-ray crystal structures.