# Low-Energy Isomers of the Magic Number H+(H2O)21 Cluster

## TL;DR

This paper explores different low-energy structures of a water cluster with a proton, focusing on how the proton and an extra water molecule are arranged.

## Contribution

The study identifies new isomer classes with distinct ring arrangements in the H+(H2O)21 cluster.

## Key findings

- Ten isomer classes have an internal water molecule in six 5-membered rings.
- One isomer class has an internal water in a 4-membered ring and other ring combinations.
- Isomers with two 4-membered and two 6-membered rings are found near the lowest energy.

## Abstract

Electronic structure
calculations are used to characterize low-energy
isomers of H+(H2O)21. Eleven different
classes of isomers, based on the (H2O)20 pentagonal
dodecahedron with the excess proton localized on the surface (as a
hydrated hydronium ion) and the “extra” water molecule
located in the interior of the cluster, are characterized. In 10 of
these classes, the internal water molecule is engaged in six 5-membered
rings, but in the remaining class, which is predicted to start at
only 0.6 kcal/mol above the global minimum, the internal water is
engaged in a 4-membered ring, an additional 6-membered ring, and four
5-membered rings. In addition, isomers with two 4-membered rings and
two 6-membered rings on the cluster surface are predicted to start
at only ∼1.3 kcal/mol above the lowest-energy dodecahedral-based
structure.