Protonated Water Clusters (H2O)nH+(n=2-6):
The complicated nature of deceptively simple systems
Massimo Mella
School of Chemistry, Cardiff University,
Main Building,
Park Place, Cardiff CF10 3AT, UK
MellaM@cardiff.ac.uk
A variety of computational methods has been used to investigate the
properties
of protonated water clusters in the gas phase.
Their energy landscape has been explored using both model potentials
and
electronic structure techniques providing a set of local minima for
clusters up to the hexamer.
The energy ordering of the isomers is found to be sensitive to the
level of
description, especially when quantum effects are taken into account.
The most sophisticated level of description employed in this work
indicates that many isomers are extremely close in energy and subjected
to fast interconversion. Isomerization rates have been computed using
Molecular Dynamics simulations, suggesting a likely mechanism for D/H
scrambling following a collision with heavy water. The presence of
important non-statistical effects has also been highlighted.
In this respect, the Zundel cation (H5O2+)
represents a striking example, its conversion dynamics showing clusters
of reactive events
separated by less than a single vibration of the oxygen skeleton.
This behaviour appears to be responsible for a far from statistical
branching
ratio between HD2O+ and H2DO+,
which are produced during the collision between
H3O+ and D2O.