Structures, energies, vibrational spectra, and electronic properties of water monomer to decamer

Abstract

The correlation of various properties of water clusters (H2O)n=1–10 to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O–H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O–H vibrational frequencies serve as sensitive indicators of the status of proton donation (“d”) and acceptance (“a”) (i.e., the structural signature of H-bond type) for each water monomer in the cluster. In general, the magnitudes of the O–H frequencies (ν) for each cluster can be arranged in the following order: ν3da (single donor–single acceptor) ≅ν3daa (single donor–double acceptor) >ν3dda (double donor–single acceptor) >ν1dda>ν1da> (or ≅) ν1daa. The increase in the cluster size has a pronounced effect on the decrease of the lower frequencies. However, there are small changes in the higher frequencies (ν3da and ν3daa). The intensities of ν1daa and ν1da are very high, since the increased atomic charges can be correlated to the enhanced H-bond relay effect. On the other hand, the intensities of the ν1dda modes are diminished by more than half. Most of the above data have been compared to the available experimental data. Keeping in view the recent experimental reports of the HOH bending modes, we have also analyzed these modes, which show the following trend: ν2dda>ν2daa≅ν2da. The present study therefore would be useful in the assignments of the experimental O–H stretching and HOH bending modes.

Polarizable Atomic Multipole Water Model for Molecular Mechanics Simulation

“…For the same set of cyclic clusters, both the AMOEBA model and MP2/TZ2P++ optimization 98 yield average O‚‚‚O nearest neighbor distances that decrease with increasing cluster size. However, our O‚‚‚O distances are systematically longer than ab initio estimates.…”

Molecular Dynamics Simulations of Water with Novel Shell-Model Potentials

“…The SW trimers are very good; the r OO matches the experimental distance, 59 while the energy and dipole are almost identical to ab initio results. 38,60 However since the ab initio r OO does not agree with the experimental value, it is not possible to determine how accurate the energy and dipole are. In case of the tetramer the SW models all have too large r OO as compared to experiment 61 and, somewhat surprising, even larger r OO than in the trimer.…”

Fingerprints in IR OH vibrational spectra of H2O clusters from different H-bond conformations by means of quantum-chemical computations

“…[10,11,13,22,23] At the same time, theoretical IR spectra of water clusters are frequently simulated based on quantum-chemical computations to help determine the molecular structure of water clusters through comparing with experimental results. [5,6,[24][25][26] Regarding quantum-chemical studies of vibrations of larger clusters, the IR spectra of different (H 2 O) 20 isomers were calculated by Fanourgakis et al at the level of MP2 theory, where they demonstrated the different spectral features for the isomers and that these features could be related to the spectra of their constituent fragments. [25] Lenz and Ojamäe studied the evolution of IR spectra with increasing cluster size for various clusters containing up to 30 molecules by B3LYP hybrid density functional theory, and the contributions to the spectrum from different types of H-bond donor and acceptor configurations were analyzed.…”