PNNL

Abstract

A series of silicon clusters four atoms, but with different charge states (Si42+, Si4, Si42-, and NaSi4-), are studied using photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series are interpreted in terms of aromaticity and antiaromaticity, allowing prediction of how structures of how structures of the four-atom silicon cluster change upon addition or reduction of two electrons. It is shown that Si42+ is square planar, analogous to the recently discovered aromatic A142- cluster. Upon addition of two electrons, the neutral Si4 becomes s-antiaromatic, resulting in 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 the 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 using photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+ [Si42] species is highly unusual in chemistry.