For Chemistry, Seeing is Believing

As a scientist, you learn to accept one thing very early on in the laboratory: theory and experiment do not always match up.

Chemistry is no exception. (In fact, the author finds it hard to believe that any empirical science would be an exception to this rule.) Time and time again, reactions will yield products that are to some degree different to those predicted by the underlying theory. This is fairly normal, if inconvenient. What is more unusual, however, is when a reaction which gives the desired product occurs via a different route than the one proposed.

This is such a strange phenomenon because it is difficult to determine the exact mechanism of the transformation of starting materials to products. Proposed mechanisms are typically supported by evidence such as isolated intermediates (‘stepping stone’ compounds which form en route to the final product, which are stable enough to be removed from the reaction). However, not all intermediates are long-lived enough to be isolated and detected. Very often only theoretical approaches are possible.

Recently, however, microscopy has been used to garner highly detailed information on chemical systems, with atomic force microscopy (AFM) gaining a large amount of interest¹. AFM comprises of an atomically-sharp tip at the end of a controlled cantilever, which scans the surface of a sample in order to build up a topographical image. Images of structures, covalent bonds and even individual atoms can be built up in this way.

A similar approach was taken by a group of researchers studying a surface-catalysed reaction, a class of reaction which is of high importance to the chemical industry². In this instance, the researchers used non-contact AFM to study the coupling of two organic molecules deposited onto a solid silver surface. Theoretically, it was predicted that the reaction should proceed via an eight-step mechanism. However, only two of these eight possible intermediates were identified from the AFM images obtained.

Fischer et al noted that their results could only be explained if other factors, such as entropic changes throughout the course of the reaction, were taken into account. This means that the theory behind industrially important surface-catalysed reactions may be due for a re-evaluation, with the possibility of improving catalysts and finally gaining a full understanding of how these processes work.

Edited by Sarah Spence

1. For examples, see http://www.rsc.org/chemistryworld/2015/08/afm-microscopy-charge-distribution-chemical-bonds and http://www.rsc.org/chemistryworld/2015/08/afm-takes-first-step-unravel-asphaltene-make, and http://www.rsc.org/chemistryworld/2015/07/first-snapshot-elusive-aryne-intermediate-supplies-surprise
2. F. R. Fischer et al, Nature Chemistry [Advanced Publication], 2016, DOI: 10.1038/NCHEM.2506