PNNL

Abstract

A series of silicon clusters containing four atoms but with different charge states (Si42+, Si4, Si42-, and NaSi4-) were studied by photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series were interpreted in terms of aromaticity and antiaromaticity, which allowed the prediction of how structures of the four-atom silicon clusters change upon addition or removal of two electrons. It is shown that Si42+ is square-planar, analogous to the recently discovered aromatic Al42- cluster. Upon addition of two electrons, neutral Si4 becomes s-antiaromatic and exhibits a rhombus distortion. Adding two more electrons to Si4 leads to two energetically close structures of Si42-: either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of doubly charged Si42-, a stabilizing cation (Na+) was used to produce Si42- in the gas phase in the form of Na+[Si42-], which was characterized experimentally by photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+[Si42-] species is highly unusual.