Distribution of Topologically Distinct Isomers of Water Clusters and Dipole Moments of Constituent Water Molecules at Finite Atmospheric Temperatures

Hydrogen-bonding
(HB) patterns correspond to topologically distinct
isomers of a water cluster and can be expressed by digraphs. We make
use of the HB pattern to divide the configuration space of a water
cluster ((H2O)n, n = 3–8) at a finite temperature. Each configuration of a water
cluster generated in Monte Carlo (MC) simulation is classified into
an HB pattern. The number of observed HB patterns increases exponentially
with the cluster size, whereas the population of the most abundant
HB pattern decreases. The populations of the HB patterns are transformed
into the relative Helmholtz energies. At a finite temperature, it
can be observed that isomers other than local minimum structures on
the potential energy surface are highly populated. The dipole moment
of a constituent molecule in a water cluster is enhanced, depending
on the molecular circumstance. The change is rationalized by the difference
in the local HB type of the water molecule in the HB network.