Planarization of B_7~- and B_(12)~- Clusters by Isoelectronic Substitution: AIB_6~-and AIBn_(11)~-

摘要

Small boron clusters have been shown to be planar from a series of combined photoelectron spectroscopy and theoretical studies. However, a number of boron clusters are quasiplanar, such as B_7~- and B_(12)~-. To elucidate the nature of the nonplanarity in these dusters, we have investigated the electronic structure and chemical bonding of two isoelectronic Al-doped boron clusters, ALB_6~- and A1B_(11)~-. Vibrationally resolved photo-electron spectra were obtained for A1B_6~-, resulting in an accurate electron affinity (EA) for AlB_6 of 2.49 ± 0.03 eV. The photoelectron spectra of AlB_(11)~- revealed the presence of two isomers with EAs of 2.16 ± 0.03 and 2.33 ± 0.03 eV, respectively. Global minimum structures of both AlB_6~- and AlB_(11)~- were established from unbiased searches and comparison with the experimental data. The global minimum of AlB_6~- is nearly planar with a central B atom and an A1B_5 six membered ring, in contrast to that of B_7~-, which possesses a C_(2v) structure with a large distortion from planarity. Two nearly degenerate structures were found for ALB_(11)~- competing for the global minimum, in agreement with the experimental observation. One of these isomers with the lower EA can be viewed as substituting a peripheral B atom by Al in B_(12)~-, which has a bowl shape with a B_9 outer ring and an out-of-plane inner B_3 triangle. The second isomer of AlB_(11)~- can be viewed as an Al atom interacting with a B_(11)~- cluster. Both isomers of A1B_(11)~- are perfectly planar. It is shown that Al substitution of a peripheral B atom in B_7~- and B_(12) induces planarization by slightly expanding the outer ring due to the larger size of Al.