The number of water molecules needed to form the smallest ice crystals has proven challenging to pinpoint experimentally. This information would help to better understand the hydrogen-bonding interactions that account for the macroscopic properties of water. Here, we report infrared (IR) spectra of precisely size-selected (H(2)O)(n) clusters, with n ranging from 85 to 475; sodium doping and associated IR excitation-modulated photoionization spectroscopy allowed the study of this previously intractable size domain. Spectral features indicating the onset of crystallization are first observed for n = 275 ± 25; for n = 475 ± 25, the well-known band of crystalline ice around 3200 cm(-1) dominates the OH-stretching region. The applied method has the potential to push size-resolved IR spectroscopy of neutral clusters more broadly to the 100- to 1000-molecule range, in which many solvents start to manifest condensed phase properties.

Using cluster studies to approach the electronic structure of bulk water: Reassessing the vacuum level, conduction band edge, and band gap of water

Water clusters with internally solvated water molecules are widespread models that mimic the local environment of the condensed phase. The appearance of stable (H2O)n cluster isomers having a fully coordinated interior molecule has been theoretically predicted to occur around the n = 20 size range. However, our current knowledge about the size regime in which those structures become energetically more stable has remained hypothetical from simulations in lieu of the absence of precisely size-resolved experimental measurements. Here we report size and isomer selective infrared (IR) spectra of (H2O)20 clusters tagged with a sodium atom by employing IR excitation modulated photoionization spectroscopy. The observed absorption patterns in the OH stretching region are consistent with the theoretically predicted spectra of two structurally distinct isomers of exceptional stability: a drop-like cluster with a fully coordinated (interior) water molecule and an edge-sharing pentagonal prism cluster in which all atoms are on the surface. The drop-like structure is the first experimentally detected water cluster exhibiting the local connectivity found in liquid water.

The structures of sodium doped water trimers are characterized on the basis of their infrared action spectra in the OH-stretching region and a global optimization approach to identify the lowest energy minima. The most stable structure is an open ring with two contacts of terminal water molecules to the Na atom. This structure explains the dominating feature in the IR depletion spectrum around 3410 cm(-1). Three additional isomer classes were found in an energy window of 12 kJ mol(-1) with vertical ionization energies ranging from ∼3.83 eV to ∼4.36 eV. These structures show different hydrogen bonding and sodium coordination patterns and are identified by specific spectral features in the IR spectra. The significant abundance of closed rings with an external Na atom, resembling the undoped water trimer, suggests that for larger clusters the picture of the sodium atom being situated on the cluster surface seems adequate.

Size resolved IR action spectra of neutral sodium doped methanol clusters have been measured using IR excitation modulated photoionisation mass spectroscopy. The Na(CH(3)OH)(n) clusters were generated in a supersonic He seeded expansion of methanol by subsequent Na doping in a pick-up cell. A combined analysis of IR action spectra, IP evolutions and harmonic predictions of IR spectra (using density functional theory) of the most stable structures revealed that for n = 4, 5 structures with an exterior Na atom showing high ionisation potentials (IPs) of ~4 eV dominate, while for n = 6, 7 clusters with lower IPs (~3.2 eV) featuring fully solvated Na atoms and solvated electrons emerge and dominate the IR action spectra. For n = 4 simulations of photoionisation spectra using an ab initio MD approach confirm the dominance of exterior structures and explain the previously reported appearance IP of 3.48 eV by small fractions of clusters with partly solvated Na atoms. Only for this cluster size a shift in the isomer composition with cluster temperature has been observed, which may be related to kinetic stabilisation of less Na solvated clusters at low temperatures. Features of slow fragmentation dynamics of cationic Na(+)(CH(3)OH)(6) clusters have been observed for the photoionisation near the adiabatic limit. This finding points to the relevance of previously proposed non-vertical photoionisation dynamics of this system.