Structure and Spectra of Three-Dimensional(H2O)nClusters,

Abstract

The vibrational OH stretch spectra have been measured for size-selected pure water clusters ͑H 2 O͒ n , in the size range n 8 10. Comparison between experiment and calculations suggests that the spectra originate from a small number of "microcrystalline" structures, based on the cubic octamer. The n 8 spectra are caused by two isomers of D 2d and S 4 symmetry. The proposed lowest energy nonamer and decamer structures are derived from the octamer by insertion of one and two two-coordinated molecules, respectively, into the cube edges. [S0031-9007 (98)05604-X] PACS numbers: 36.40.MrWater clusters have been the focus of intense interest during the last several years. One of the objectives is to understand how the properties of ͑H 2 O͒ n evolve towards condensed phase behavior. One may ask, for example, at which size a cold cluster starts displaying attributes of a small crystal. The initial stages of the evolution as a function of n have been largely understood. Recently, an elegant series of far-infrared vibration-rotation-tunneling [1,2] and infrared [3] laser spectroscopic studies demonstrated conclusively a cyclic structure for n 3 5, and a transition towards a three-dimensional cage structure at n 6 [4]. Similar conclusions were drawn from the double resonance ion-dip infrared experiments on water clusters connected to a benzene molecule [5]. At present, the challenge is to understand the evolution in the "cage" regime n $ 7. A number of theoretical studies addressed this problem [6-9]. The only pertinent experimental results focus on the OH stretch spectroscopy of larger clusters without size selection [10,11] and n 7, 8 clusters attached to benzene [5,9].The present Letter reports the first measurement of the infrared spectroscopy of the OH stretch mode of pure water clusters in the size range n 8 10. The OH stretch spectra of hydrogen bonded H 2 O, which are redshifted by hundreds of cm 21 with respect to gaseous H 2 O, are known to be strikingly sensitive to hydrogen bond coordination and to bonding geometry [7,12]. Thus they can be used to probe the cluster structure. Moreover, the spectroscopy of clusters can serve as a benchmark for the calibration of flexible force fields for studies of condensed H 2 O and H 2 O surfaces.The experimental method which we apply is a combination of size selection by momentum transfer in a scattering experiment with atoms, with the infrared depletion technique [13,14]. This technique has been developed in our laboratory in Göttingen and mainly applied to cluster sizes n # 6. In the first step the different clusters are dispersed into different angles according to their masses and detected by a mass spectrometer. Then the OH stretch vibrational mode of the water molecules is excited by infrared laser radiation. The detector records the depletion in the cluster signal caused by the clusters which are dissociated by the absorbed radiation.The experimental setup consists of a crossed molecular beam apparatus with an angular dependent detection of the scattered beam with a resol...

The thermodynamic and ground state properties of the TIP4P water octamer

“…The characteristic vibrational frequencies have been computed with two different potentials. 6,13 Thermodynamic properties have been computed in the NVT ensemble, 23,28 in the NPT ensemble, 29 and in the presence of external electric fields. 41 Isomerizations and the melting transition have been studied using molecular dynamics.…”

“…Yet, large quantum effects can be expected given the light mass of the nuclei involved. The intermolecular contributions of the zero point energy for the D 2d and the S 4 configurations have been computed with the rigid body diffusion Monte Carlo ͑DMC͒, 13 whereas the equilibrium properties of the quantum water octamer have been simulated with the variational Gaussian wave packet method. 43 One of our groups has been involved in the development of quantum methods for the efficient simulation of molecular aggregates.…”

Water pair potential of near spectroscopic accuracy. II. Vibration–rotation–tunneling levels of the water dimer

“…Many-body interactions in water have also been probed by molecular-beam spectroscopy: high-resolution far-infrared spectra were recorded for the water trimer, tetramer, pentamer, and hexamer, [18][19][20][21][22][23][24][25] mid-and near-infrared spectra for clusters up to the decamer. [26][27][28][29] The temperature of these clusters is usually around 5 K, which is so cold that only a few levels are thermally occupied. The observed lines in the high-resolution spectra correspond to transitions between the individual quantum levels of the clusters, with well-defined rotational quantum numbers J and K and with even minute tunneling splittings ͑less than 1 MHz͒ resolved.…”

Identification of CH3OH2+ and H3O+-centered cluster isomers from fragment-dependent vibrational predissociation spectra of H+(CH3OH)4H2O

“…This is particularly the case for neutral clusters whose sizes are difficult to determine unambiguously. Although there have been several successful examples in this direction, [3][4][5] the range of exploration is expected to be severely limited by the lack of proper mass-selection means. For charged clusters, this limitation is apparently removed since they can be easily sizeselected by mass spectrometry.…”