Relative stability of planar versus double-ring tubular isomers of neutral and anionic boron cluster B_(20)and B_(20)~-

摘要

High-level ab initio molecular-orbital methods have been employed to determine the relative stability among four neutral and anionic B_(20)isomers,particularly the double-ring tubular isomer versus three low-lying planar isomers.Calculations with the fourth-order Moller-Plessset perturbation theory [MP4(SDQ)] and Dunning's correlation consistent polarized valence triple zeta basis set as well as with the coupled-cluster method including single,double,and noniteratively perturbative triple excitations and the 6-311G(d)basis set show that the double-ring tubular isomer is appreciably lower in energy than the three planar isomers and is thus likely the global minimum of neutral B_(20)cluster.In contrast,calculations with the MP4(SDQ)level of theory and 6-311 +G(d)basis set show that the double-ring anion isomer is appreciably higher in energy than two of the three planar isomers.In addition,the temperature effects on the relative stability of both ~(10)B_(20)~-and ~(11)B_(20)~-anion isomers are examined using the density-functional theory.It is found that the three planar anion isomers become increasingly more stable than the double-ring isomer with increasing the temperature.These results are consistent with the previous conclusion based on a joint experimental/simulated anion photoelectron spectroscopy study [B.Kiran et al,Proc.Natl.Acad.Sci.U.S.A.102,961 (2005)],that is,the double-ring anion isomer is notably absent from the experimental spectra.The high stability of the double-ring neutral isomer of B_(20)can be attributed in part to the strong aromaticity as charaterized by its large negative nucleus-independent chemical shift.The high-level ab initio calculations suggest that the planar-to-tubular structural transition starts at B_(20)for neutral clusters but should occur beyond the size of B_(20)~-for the anion clusters.