Electronic and atomic structure of the AlnHn+2 clusters

9/Mayo/2012
Maximiliano De La Higuera Macías

     The electronic and atomic structure of the family of hydrogenated Al clusters Aln Hn+2  with n =4–11 has been studied using the density functional theory with the generalized gradient approximation _ GGA_  for exchange and correlation. All these clusters have substantial gaps between the highest occupied and the lowest unoccupied molecular orbitals _ HOMO-LUMO_ and, consequently, they are chemically very stable. The largest gap of 2.81 eV occurs for Al6 H8 . Five clusters of the family, Al4 H6 , Al5 H7 , Al6 H8 , Al7 H9 , and Al10 H12 , fulfill the Wade–Mingos rule. That is, in Aln Hn+2 , the Al matrix forms a polyhedron of n  vertices and n  H atoms form strong H–Al terminal bonds; one pair of electrons is involved in each of those bonds. The remaining n +1 electron pairs form a delocalized cloud over the surface of the Al cage. The clusters fulfilling the Wade–Mingos rule have wider HOMO-LUMO gaps and are chemically more stable. The trends in the gap have some reflections in the form of the photoabsorption spectra, calculated in the framework of time-dependent density functional theory using the GGA single-particle energies and orbitals and a local density approximation exchange-correlation kernel.

     Motivated by the interest on the Al4 H6  cluster,9  we have studied the family of hydrogenated Al clusters Aln Hn+2  with n =4–11 using the DFT with the GGA approximation for XC. All these clusters have 2n +1 valence electron pairs. The main characteristic is that the clusters have sizable H-L gaps and, consequently, they are chemically very stable. The largest gap of 2.81 eV occurs for Al6 H8 . Five clusters of the family, Al4 H6 , Al5 H7 , Al6 H8 , Al7 H9 , and Al10 H12 , fulfill the WM rule, as earlier discovered for Al4 H6  by Li et al.9  That is, in the Aln Hn+2 , the Al matrix forms a polyhedron of n  vertices and n  hydrogen atoms of the cluster form strong H–Al terminal bonds. One pair of electrons is involved in the formation of each of those bonds. The remaining n +1 electron pairs form a cloud delocalized over the surface of the Al cage.

     The other three clusters, Al8 H10 , Al9  H11 , and Al11 H13 do not conform to this rule. The clusters fulfilling the WM rule have wider H-L gaps and are then chemically more stable _ although the gap of Al10 H12  is smaller than the gap of Al8 H10 , it is a local maximum with respect to neighbor clusters. The trends in the H-L gap have some reflection in the behavior of the photoabsorption spectrum. This has been calculated by the Casida method in the frame of TDDFT, using PBE single-particle energies and orbitals and an LDA XC kernel.

 Martínez, J. I., and J. A. Alonso. "Electronic And Atomic Structure Of The Alnhn+2 Clusters." Journal Of Chemical Physics 129.7 (2008): 074306. Academic Search Complete. Web. 10 May 2012.