Structure, stability, thermodynamic properties and IR spectra of the protonated water cluster H⁺(H₂O)₉

Abstract

It is not clearly known whether H⁺(H₂O)₉ has a 2-dimensional (2D) net-like structure or 3-dimensional (3D) structure. Thus, calculations have been carried out based on density functional theory [DFT], Moller–Plesset second- order perturbation theory (MP2), resolution of identity MP2 theory (RIMP2), and coupled cluster theory with single, double and perturbative triple excitations (CCSD(T)). For the complete basis set (CBS) limit at the CCSD(T) level of theory, two nearly isoenergetic 3D closed structures are the most stable in zero-point energy (ZPE) corrected binding energy at 0 K. However, at 150 K, there are three competing 2D net-like structures. The structure with two acceptor (A) type dangling water molecules is the most stable. This structure is in good agreement with the experimental symmetric and asymmetric OH stretching peaks for the free acceptor (A) type of dangling water molecule(s) at 3650 and 3742 cm⁻¹. On the other hand, another structure shows a better spectral intensity pattern. Thus, these three 2D net-like structures together would be present at ∼150 K.

Keywords:

• molecular cluster
• protonated water
• structure
• IR spectra
• ab initio calculations

Acknowledgements

This work is affectionately dedicated to Professor Henry F. Schaefer III, an outstanding scientist and an exceptional mentor, on the occasion of his 65th birthday. This work was supported by KOSEF (EPB Center: R11-2008-052-01000), BK 21, and GRL (KICOS). Calculations were carried out using supercomputers at KISTI (KSC-2008-K08-0002).