|The first direct measurements of the strength of hydrogen bonds in
individual molecules have been claimed by an international team of
physicists. Unlike chemical bonds, which involve the sharing or transfer
of electrons, hydrogen bonds are dipole-dipole interactions between
certain molecules containing hydrogen. As well as playing key roles in
defining the properties of proteins and nucleic acids, hydrogen bonds
are also responsible for the relatively high boiling point of water.
Researchers from the University of Basel in Switzerland and Aalto
University in Finland used an atomic force microscope (AFM) to study
hydrogen bonds in molecules called propellanes - which arrange
themselves on surfaces such that two hydrogen atoms are pointing
upwards. Their AFM tip comprised a single oxygen atom, which was
positioned so close to a propellane molecule that a hydrogen bond formed
between the oxygen atom and the two hydrogen atoms.
Then, the AFM was used to measure the strength of the bond as a function
of the separation between the oxygen and hydrogen atoms. The
measurements confirmed that the hydrogen bond is much weaker than
chemical bonds, but much stronger than van der Waals forces - which is a
dipolar interaction that is weaker than hydrogen bonding. The
measurements were also in agreement with calculations of bond strength
done by members of the team. The technique could be used to identify 3D
molecules such as DNA and polymers.