Solid water clusters in the size range of tens–thousands of H2O: a combined computational/spectroscopic outlook

Abstract A joint computational and experimental effort was directed towards the understanding of large solid water clusters. The computations included structure optimizations and calculations of OH stretch spectra for select sizes in the range n = 20–931. The measurements focused predominantly on OH stretch spectroscopy as a function of mean cluster size. FTIR spectra are discussed for the size range of tens to hundreds-of-thousands of molecules. Photofragment spectroscopy in molecular beams is shown to be a sensitive probe of the outer cluster surfaces. The crucial element of the different experimental approaches is the control and the estimation of the mean cluster sizes. The combined experimental and computational results are consistent with the physical picture of quasi-spherical nanocrystals with disordered reconstructed surface layers. The surface reconstruction can be viewed as the outcome of recombination of surface dangling atoms, to increase the number of hydrogen bonds. The hydrogen bonds within the mostly crystalline subsurface layer are stretched by the interaction with the disordered component. The emergence of the (strained) crystal core occurs at a size of a few hundred H2O. Smaller clusters are described as compact-amorphous. Acknowledgements Israel Science Foundation and the National Science Foundation of the USA are acknowledged for funding. The work in Göttingen was supported by the Deutsche Forschungsgemeinschaft in SFB 357 and in Graduiertenkolleg 782. Notes Best agreement with experiment was obtained by placing the centre of interaction on the OH bond axis, at a distance 0.62 Å from O Citation[27]. Later, extension was made to include the effect of intermolecular vibrational averaging on the electric field. The potential minimum is a ‘special’ point at which the hydrogen bonds are particularly strong, and the electric fields are particularly large. The averaging was included, to account for delocalization due to zero-point and thermal intermolecular motions, and to represent temperature dependence of the cluster spectra Citation[105–Citation107]. However these averaging effects are not included in the present study, which focuses on fairly large and computationally expensive clusters. In the squared expression for intensity, there are cross-terms including contributions of molecules of different coordinations. In figure 7, each coordination was assigned half of such a contribution. Similar division was used to calculate surface contribution to the spectra in figure 6; that is, half of each surface-interior cross-term was assigned to the surface. In the unrelaxed (H2O)931 sphere cut from ice Ic crystal there are 168/194 d-O/d-H atoms. Recombination during direct minimization reduces the dangling atoms’ numbers to 128/126. Further reduction to 96/86 is obtained in the lowest energy optimized cluster structure.