2004) Solid water clusters in the size range of tens-thousands of H 2 O: a combined computational/spectroscopic outlook, 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 ¼ 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 quasispherical 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 H 2 O. Smaller clusters are described as compactamorphous.

The optimal structures, interaction energies, and OH stretch frequencies of water cage clusters, (H 2 O) n , n ) 7-10, have been investigated theoretically. The study consisted of MP2 level ab initio calculations of optimal structures and spectra and similar calculations employing an empirical polarizable potential. The study focuses predominantly on structures, that were used for assignment of recently measured size selected OH stretch spectra. The structures are related to octamer cubes. The nonamer and the decamer are derived from the octamer by insertion of one and two two-coordinated molecules, respectively, into the D 2d cube edges. The two heptamer isomers are obtained by removal of a single water molecule from the S 4 octamer cube. The n ) 8-10 clusters correspond to relatively regular structures, characterized by two distinct groups of O‚‚O bond lengths and corresponding stretch frequencies. The more strained and asymmetric heptamers include a broad range of hydrogen bonded configurations, which are reflected by a complex OH stretch spectrum with numerous peaks. Two additional low energy decamer structures were investigated, which correspond to two fused pentamer rings. Following past suggestions, we further explored the possibility of parameterizing OH bond frequency as a function of the electric field at the H atom.

A search for low energy structures of water clusters was performed with a combination of three computational tools: (a) temperature-dependent classical trajectories; (b) hydrogen network improvement; (c) rigid body diffusion Monte Carlo calculation on a smoothed potential energy surface. For the sizes of our main interest, n ) 48, 123, and 293, input configurations included spheroid structures cut from crystalline ice, and amorphous structures. For n ) 48, tube and sandwich minima were explored as well. The lowest energy configurations found were characterized by compact three-dimensional shapes. In the case of n ) 48 and 123, crystallinity was lost in the course of the optimization; for these sizes, one finds four-, five-, and six-membered rings of water molecules, On the other hand, the lowest energy structure found for n ) 293 includes a crystal core, dominated by six-membered rings, and an amorphous surface.

The vibrational OH stretch spectra have been measured for the size-selected pure water clusters (H2O)7. In contrast to (H2O)n, n=8–10 clusters, which exhibit three distinct bands corresponding to three distinct types of OH bonds, the heptamer spectrum displays seven bands spanning the range from 2935 to 3720 cm−1. Calculations suggest that the spectra originate from two isomers, derived from the S4 octamer cube by removal of either one double donor or one double acceptor water molecule.

The vibrational OH-stretch spectra of large water clusters were measured by photofragment spectroscopy after the absorption of pulsed tunable infrared radiation in the frequency range from 3000 to 3800 cm -1 . The mean size of the clusters from 〈n〉 ) 20 to 1960 was measured by threshold photoionization of the water clusters doped with sodium atoms. The largest abundance of the fragments was that of water hexamers. The fragment intensities are measured for different excitation energies and different cluster temperatures as function of the cluster size. For the selected sizes 〈n〉 ) 48, 111, 631, and 1960 complete OH-stretch spectra have been measured. The comparison with calculations revealed that the method is mainly sensitive to the outer cluster surface which has for all sizes an amorphous structure dominated by 3-coordinated and to a lesser extent also by 4-coordinated molecules. The intensity of the hexamer fragments goes through a maximum at n ) 70 and drops to n ) 300 where it levels off with a different slope. This behavior is attributed to the number of available connected 3-coordinated water molecules and the influence of the emerging 4-coordinated molecules in these clusters.