A Density Functional Theory Study For W_nN₂^0,±?n=6-12? Clusters

This article is based on density functional theory?DFT?, using Gaussian09 software package, hybrid density functional B3 LYP method and LANL2 DZ pseudo-potential basis set are selected to optimize W_n N₂0,±?n=6-12? clusters. Then physical and chemical properties are investigated based on the obtained ground state structures. The main results can be summarized as follows:?1?The ground state structures of W_n N₂?n=6-12? clusters are generated when N₂ is inclined to adsorbed on W_n clusters, and there are no significant change in the configurations of the tungsten clusters. We found that among the molecular adsorption states exists mainly in the form of end-on type geometry, and adsorption behavior is nondissociative. Compared with free N₂ molecules, the N-N bond length increased after N₂ adsorption, which demonstrated that the N₂ are activated during adsorption process. The stability analysis shows that W8N₂ and W10N₂ clusters are more stable than other clusters.?2?Based on the ground state structures of W_n N₂?n=6-12? clusters, natural bond orbital?NBO?,the vibrational spectrum, polarizability, the magnetic moment and the rmodynamic properties are analyzed. NBO analysis indicate that mechanism of action between W and N₂ is the interaction between orbitals of W atomic and S-P hybrid orbitals of N₂ molecules. The results show that the strongest intensity of the IR spectrum and the Raman spectrum modes are located at the same frequency, and all of the corresponding vibration modes are assigned to N₂ stretching mode, the wavenumber range is between 1507.76 cm-1 and 1985.93 cm-1. The polarizability values in ?xx??yy??zz direction are major components in each cluster. The total magnetic moment of W_n N₂?n=6-12? clusters varies from 0-6?B.The NICS values of W6N₂?W8N₂?W11N₂ and W12N₂ are negative numbers, so they have aromaticity. From the state density analysis we know that d orbitals of clusters has the greatest effect in the electronic structure.?3?The structures character, stability and chemical activity of W_n N₂±?n=6-12? clusters have been studied. Results show that the ground state structures of ion clusters are similar to the ground state structures or metastable structures of neutral clusters. N-N bond is activated during adsorption process. The N-N bond length in anionic calusters is longer than cationic clusters, which shows that the effect of cationic clusters on the dissociation of N₂ molecules is weaker than that of the corresponding anionic clusters. The average binding energy of cationic cluster is the largest, those of anionic and the neutral clusters are the second and least respectively. The infrared spectrogram of ground state structures of W_n N₂?n=6-12? ion clusters is consistent with those of the ground state structures of neutral clusters for all of the infrared spectrogram only have a strong peak. Average polarizability tensor of anionic clusters is greater than that of corresponding cationic clusters. The magnetic moment of ion clusters is not zero, and the greater the spin multiplicity of clusters, the larger the total magnetic moments. The atomic magnetic moments for symmetric positions are the same.To sum up, in this paper the author have systematically studied the physical and chemical properties of the ground state of clusters, and revealed the internal relation between microstructure and macro-properties on W_n N₂0,±?n=6-12? clusters. This article could provide a reliable theoretical basis for experimental research and an appropriate model for developing new catalytic functional materials.