Stabilities, Vibrational States and Hydrogen Bond Characteristics of Water Clusters
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36 low-energy isomers of (H2O) n (n = 6–21) are optimized using ab initio methods. Their vibrational frequencies are calculated and the properties of hydrogen bond (H-bond) are analyzed. The structure of a water cluster is decided by the number and the strength of the H-bonds formed in the cluster. The H-bonds in the hexagonal rings are strongest but the clusters building up by the cubes and pentamers can form more H-bonds. When the energies are corrected by the zero point energy and the basis set superposition error, the H-bond strength is about 0.182 eV. The thermodynamic properties of water clusters are decided by the intermolecular vibrational states. Similar to the density of states of bulk ice, the intermolecular vibrational frequencies of the clusters are divided into two subbands, the lower frequencies correspond to the translational and the higher frequencies to librational modes. The vibrational frequencies of the clusters are more extended and shift to higher frequencies. The O–H stretching band of water clusters is the fingerprint of the structures and it also reflects the strength of the H-bonds that the donor O–H forms. Analysis on the stretching frequencies shows that the strength of the H-bonds depends mainly on the characteristic of the donor molecule. The H-bonds formed by DAA as H-donor are strongest and the bonds formed by DDA are weakest.
Keywords(H2O)n (n = 6–21) Stabilities Vibrational states H-bond characteristics
This work is financially supported by the National Science Foundation of China (Grant No. 11164024, Grant No. 11547253) and the Natural Science Foundation of Gansu Province (Grant No. 11547253). We also thank Gansu and Shenzhen Computing Center for computation resources.
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