Published in Henry Rzepa's Blog

Steve Bachrach on his own blog has commented on a recent article[cite]10.1002/anie.201505934[/cite] discussing the structure of the trimer of fluoroethanol. Rather than the expected triangular form with three OH—O hydrogen bonds, the lowest energy form only had two such bonds, but it matched the microwave data much better. Here I explore this a bit more.

References

General ChemistryCatalysis

Search for a Strong, Virtually “No‐Shift” Hydrogen Bond: A Cage Molecule with an Exceptional OH⋅⋅⋅F Interaction

Published in Angewandte Chemie International Edition
Authors Mark D. Struble, Courtney Kelly, Maxime A. Siegler, Thomas Lectka

AbstractReported herein is the synthesis of a molecule containing an unusually strong hydrogen bond between an OH donor and a covalent F acceptor, a heretofore somewhat ill‐defined if not controversial interaction. This unique hydrogen bond is to a large extent a product of the tight framework of the rigid caged system. Remarkably, the interaction shows little to no perceptible shift in the OH stretch of the IR spectrum relative to appropriate nonhydrogen‐bound standards in fairly non‐interactive solvents. This fascinating example of what has been termed a virtual “no‐shift” hydrogen bond is investigated through NMR (coupling constants, isotopic chemical shift perturbations, proton exchange rates) and IR studies which all tell a consistent story.

General ChemistryCatalysis

Unusual H‐Bond Topology and Bifurcated H‐bonds in the 2‐Fluoroethanol Trimer

Published in Angewandte Chemie International Edition
Authors Javix Thomas, Xunchen Liu, Wolfgang Jäger, Yunjie Xu

AbstractBy using a combination of rotational spectroscopy and ab initio calculations, an unusual H‐bond topology was revealed for the 2‐fluoroethanol trimer. The trimer exhibits a strong heterochiral preference and adopts an open OH⋅⋅⋅OH H‐bond topology while utilizing two types of bifurcated H‐bonds involving organic fluorine. This is in stark contrast to the cyclic OH⋅⋅⋅OH H‐bond topology adopted by trimers of water and other simple alcohols. The strengths of different H‐bonds in the trimer were analyzed by using the quantum theory of atoms in molecules. The study showcases a remarkable example of a chirality‐induced switch in H‐bond topology in a simple transient chiral fluoroalcohol. It provides important insight into the H‐bond topologies of small fluoroalcohol aggregates, which are proposed to play a key role in protein folding and in enantioselective reactions and separations where fluoroalcohols serve as a (co)solvent.