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

The correlation of various properties of water clusters (H2O)(n=1-10) to th e cluster size has been investigated using extensive ab initio calculations . Since the transition from two dimensional (2-D) (from the dimer to pentam er) to 3-D structures (for clusters larger than the hexamer) is reflected i n the hexamer region, the hexamer can exist in a number of isoenergetic con formers. The wide-ranging zero-point vibrational effects of the water clust ers having dangling H atoms on the conformational stability by the O-H flap ping 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 (s imilar to 0.1) kcal/mol). Large dipole moments (> 2.5 D) are found in the d imer and decamer, and significant dipole moments (similar to 2 D) are obser ved in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this in crease tapers down beyond the tetramer. The O-H vibrational frequencies ser ve as sensitive indicators of the status of proton donation ("d") and accep tance ("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) (sin gle donor-single acceptor) congruent to nu(3)(daa) (single donor-double acc eptor) >nu(3)(dda) (double donor-single acceptor) >nu(1)(dda)>nu(1)(da)> (o r congruent to) nu(1)(daa). The increase in the cluster size has a pronounc ed effect on the decrease of the lower frequencies. However, there are smal l changes in the higher frequencies (nu(3)(da) and nu(3)(daa)). The intensi ties 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 experiment al 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 wo uld be useful in the assignments of the experimental O-H stretching and HOH bending modes. (C) 2000 American Institute of Physics. [S0021-9606(00)3032 2-1].