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

The correlation of various properties of water clusters (H[sub 2]O)[sub 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: ν[sub 3][sup da] (single donor-single acceptor) ≅ν[sub 3][sup daa] (single donor-double acceptor) >ν[sub 3][sup dda] (double donor-single acceptor) >ν[sub 1][sup dda]>ν[sub 1][sup da]> (or ≅) ν[sub 1][sup daa]. 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 (ν[sub 3][sup da] and ν[sub 3][sup daa]). The intensities of ν[sub 1][sup daa] and ν[sub 1][sup da] 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 ν[... [ABSTRACT FROM AUTHOR]