Combustion science also has a complexity gap of sorts. Tremendous effort using a wide range of methods has been directed at understanding the small-molecule end of the combustion process. Prototypical fuels such as methane have been thoroughly studied under the well-controlled environment of laminar flames, and detailed models of the chemical pathways have evolved from years of careful studies of the individual reaction steps. On the other hand, in fuel-rich flames, soot is a major combustion product, with its large, graphite-like aggregates which are macroscopic in size. The question is, how one gets from small to large. Not only is the number of pathways extremely large, but each intermediate in the scheme will need, in addition to its chemical formula, a growing number of additional markers that identify its various conformational and structural isomers. Some of these isomers will react quickly, while others will not. Some will prefer one type of product, others another.

We operate, as before, at the entrance to this complexity gap, pushing into chemistry and spectroscopy that lead from the first aromatic ring to larger, more complex fused ring and highly substituted molecules. These molecules are a good place to focus attention, because they constitute a reasonable fraction (~30%) of the gasoline we put into our gas tanks. Not surprisingly, we have been particularly interested in substituted aromatics with flexible side chains, where we can do single-conformation spectroscopy and study the conformational isomerization dynamics.

Our current work includes extension of population transfer methods to conformationally-flexible free radicals, in order to measure the barriers to isomerization, especially immediately adjacent to the free radical site.