JACS Abstract

Abstract

Despite a long history of experimental and theoretical investigation, the mechanism of the Diels-Alder (DA) reaction has been controversial since its discovery 80 years ago. From these investigations, two schools of thought have emerged, namely that the reaction can proceed via a concerted, symmetric or asymmetric mechanism or via a non-concerted mechanism involving a zwitterions or diradical as an intermediate. Here, we employ finite temperature ab initio molecular dynamics simulations, employing forces computed "on the fly" from electronic structure calculations, to investigate the microscopic mechanism of DA adduct formation between 1,3-butadiene and the Si(100)2x1 surface. Free energy profiles and nonequilibrium trajectories strongly suggest a non-concerted mechanism that forms a zwitterionic intermediate state. This mechanism, which begins with a nucleophilic attack of the C=C double bond on the positive member of an charge-asymmetric buckled Si-Si dimer, was previously shown to be common to the formation of a wide range of adducts that can form on the surface.