PNNL

Abstract

The concept of aromaticity was first invented to account for the unusual stability of planar organic molecules with 4n + 2 delocalized p electrons. Recent photoelectron spectroscopy experiments on all-metal MAl4- systems with an approximate square planar Al42- unit and an alkali led to the suggestion that Al42- is aromatic. The square Al42- structure was recognized as the prototype of a new family of aromatic molecules. High level ab initio calculations based on extrapolating CCSD(T)/aug-cc-pVnZ (n = D,T,Q) to the complete basis set limit were used to calculate the electron affinities of Aln, n = 0 ? 4. The electron affinities are: Based on the high level ab initio quantum chemical calculations, we can estimate the resonance energy and show that it is quite large, large enough to stabilize Al42- with respect to Al4. Analysis of the calculated results shows that the aromaticity of Al42- is unusual and different from that of C6H6.