Orbitals for the Water Pentamer, (H2O)5
A water pentamer (H2O)5 structure.
This is molecular orbital eleven out of twenty-five showing
the electron overlap possible for extended hydrogen bonding;
an interactive structure with orbitals is available (COW [Plug-in, ActiveX] only), also without
orbitals (Jmol). The (H2O)5 structure is a local minimum-energy structure but not a global
minimum for a pentamera or even for a cyclic pentamer as all the non-hydrogen-bonded
hydrogen atoms are cis; the global minimum for a
water pentamer has a chiral mostly trans cyclic structure
(Jmol) but also possesses a similar
molecular orbital eleven (COW [Plug-in, ActiveX] only). The occupied molecular orbitals were calculated
using the Restricted Hartree-Fock wave function (RHF) using
the 6-31G** basis set. The presence of extensive molecular orbitals allows the possibility that this planar pentameric structure may be responsible for the absorption at about 270 nm noticed in water under certain circumstances where it is thought not due to impurities .
For both of these cyclic pentamers,
the bond energies indicate that cooperation of two hydrogen
bonds increase the average bond strength by 22%, cooperation
of three hydrogen bonds increase the average bond strength
by 40-41% and cooperation of all five hydrogen bonds increase
the average bond strength by 52-53%. Water pentameric clusters
can be produced by bubbling inert gas through the liquid water
followed by adiabatic expansion into a vacuum .
The vibrationally- averaged structure involving pseudorotation
of the ring pucker is a flattened planar structure with C5h symmetry.
The large orbitals indicated by semiempirical and density-functional
computations  are supported
by this ab initio method. Such orbitals, indicated in isolated
clusters (that is, in vacuo), should be taken as indicative
only of the type of orbitals that exist in liquid water clusters.
a There are 161 topologically
distinct hydrogen-bonded pentamer (that is, consisting
of five molecules) arrangements of water possible .
A recent computational study gives this ring (as the global minimum) a puckered conformation with one free hydrogen bond approximately at right-angles to the plane of the ring . [Back]