Cooperative O-H$\cdots\pi$ and C-H$\cdots$O Hydrogen Bonding in Benzene-Methanol Solution: Strong Structures from Weak Interactions

TL;DR

This study uses neutron diffraction and isotopic substitution to reveal that weak hydrogen bonds like O-H$ o\pi$ and C-H$ o ext{O}$ in benzene-methanol solution form highly ordered, cooperative structures, challenging the notion that such interactions are insignificant.

Contribution

It provides the first detailed spatial and orientational characterization of O-H$ o\pi$ and C-H$ o ext{O}$ hydrogen bonds in solution, demonstrating their role in forming ordered molecular motifs.

Findings

O-H$ o\pi$ interaction is highly localized and directional.

Methanol forms symmetrical, templated solvation structures around benzene.

Weak hydrogen bonds can produce strongly ordered, cooperative structures in liquids.

Abstract

Weak hydrogen bonds, such as O-H$\cdots\pi$ and C-H$\cdots$O, are pivotal in a wide range of important natural and industrial processes including biochemical assembly, molecular recognition, and chemical selectivity. In this study we use neutron diffraction in conjunction with comprehensive H/D isotopic substitution to obtain a detailed spatial and orientational picture of the structure in benzene-methanol solution. This system provides us with a prototypical situation where the aromatic ring can act as an hydrogen bond acceptor (via the $\pi$ electron density) and/or a hydrogen bond donor (via the CH groups), with the potential for cooperative effects. Our analysis places benzene at the centre of our frame-of-reference, and reveals for the first time that in solution the O-H$\cdots\pi$ interaction is highly localised and directional, the hydrogen atom being located directly above/below the ring centroid at a distance of 2.30 {\AA} and with the hydroxyl bond axis normal to the aromatic plane. The tendency of methanol to form chain and cyclic motifs in the bulk liquid is manifest in a highly templated, symmetrical equatorial solvation structure; the methanol molecules surround the benzene so that the O-H bonds are coplanar with the aromatic ring while the oxygens interact with C-H groups through simultaneous bifurcated hydrogen bonds. By contrast, C-H$\cdots\pi$ interactions are relegated to the role of more distant spectators. The experimentally observed solvation therefore demonstrates that weak hydrogen bonding can give rise to strongly-ordered cooperative structural motifs also in the liquid phase.