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 (similar to 0.5 kcal/mol) between two different orientations of each dangling H atom are not large (similar to 0.1) kcal/mol). Large dipole moments (> 2.5 D) are found in the dimer and decamer, and significant dipole moments (similar to 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 (nu) for each cluster can be arranged in the following order: nu(3)(da) (single donor-single acceptor) congruent to nu(3)(daa) (single donor-double acceptor) >nu(3)(dda) (double donor-single acceptor) >nu(1)(dda)>nu(1)(da)> (or congruent to) nu(1)(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 (nu(3)(da) and nu(3)(daa)). The intensities of nu(1)(daa) and nu(1)(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 nu(1)(dda) 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: nu(2)(dda)>nu(2)(daa)congruent to nu(2)(da). The present study therefore would be useful in the assignments of the experimental O-H stretching and HOH bending modes. (C) 2000 American Institute of Physics. [S0021-9606(00)30322-1].