Isotope effect produces new type of chemical bond

Posted by: Tarun Kumar
Researchers believe they have confirmed the
existence of a new type of chemical bond , first
proposed some 30 years ago but never
convincingly demonstrated because of the lack of
experimental evidence and the relatively poor
accuracy of the quantum chemistry methods that
prevailed at the time. The new work also shows
how substituting isotopes can result in
fundamental changes in the nature of chemical
bonding.
In the early 1980s it was proposed that in certain
transition states consisting of a very light atom
sandwiched between two heavy ones, the system
would be stabilised not by conventional van der
Waal’s forces, but by vibrational bonding, with
the light atom shuttling between its two
neighbours. However, despite several groups
searching for such a system none was
demonstrated and the hunt fizzled out.
Now, Jörn Manz , of the Free University of Berlin
and Shanxi University in China, and colleagues
believe they have the theoretical and experimental
evidence to demonstrate a stable vibrational
bond.
The researchers carried out a series of theoretical
experiments looking at the reaction of BrH with Br
to create the radical BrHBr, but using different
isotopes of hydrogen. By using muons –
elementary particles that are similar to an
electron but have greater mass – the team added
a range of hydrogen isotopes to BrHBr from the
relatively hefty muonic helium , H, to the
extremely light muonium , Mu, with a mass nearly
40 times smaller than H.
The team mapped two key parameters: the
potential energy surface of the system – the
three-dimensional potential energy ‘landscape’
relating the energy of the surface, with hills and
valleys – to the geometry; and a quantum
mechanical parameter, the vibrational zero point
energy or ZPE.
Classically, a bond will form if there is a net
reduction in the potential energy of the system.
However, in certain circumstances, if there is a
sufficiently large decrease in the vibrational ZPE,
this can overcome the need for a decrease in
potential energy and the system can be stabilised
by a vibrational bond.
The team used state of the art quantum
chemistry to calculate these values and the
behaviour of the hydrogen isotopes and showed
that for the three heavier isotopes, bonding in the
triatomic system was through classical van der
Waal’s interactions. However, for the ultralight
muonium, a large decrease in vibrational ZPE
resulted in stabilisation of the system through
vibrational bonding, despite an increase in
potential energy.
Furthermore, earlier laboratory experiments by
Manz’s co-worker Donald Fleming suggested that
the BrMuBr radical can exist, and is not merely a
theoretical construct .
‘In BrMuBr we have shown the first system where
there is vibrational bonding,’ says Manz. ‘And
also we have an isotope effect on the nature of
the chemical bonding in a system.’
Commenting on the work, David Clary at the
University of Oxford in the UK says: ‘The study
provides rigorous theoretical evidence for
vibrational bonding and suggests that isotopic
substitution can have a dramatic effect on the
ability of molecules to form bound states.’
REFERENCES
1 D G Fleming et al ., Angew. Chem. Int. Ed.,
2014, DOI: 10.1002/anie.201408211
2 D G Fleming et al , Phys. Chem. Chem.
Phys. , 2013, 14, 10953 (DOI: 10.1039/
C2CP41366C)