My distinguished colleague, Professor Mitch Winnik, used to give a really interesting talk entitled “Watching paint dry”. This slightly facetious title is meant to represent a popular belief that watching paint dry is one of the most boring things known to man. If you know something about colloidal chemistry, you would quickly realize that this complex and multifaceted process is anything but boring.
Today I want to talk about infrared (IR) spectroscopy. Does this sounds boring? I think the majority of organic chemists look at IR spectroscopy as the chore of compound characterization. I think you will all agree that synthetic organic students rarely use IR in order to answer questions pertaining to mechanism and/or compound characterization (unless they are putting together their theses or supplementary materials for papers). There is indeed something to be said about other methods that provide way more information and are almost as fast (eg NMR). Accordingly, there are more streamlined means to ascertain product purity and identity, unless you are an inorganic chemist studying the structure of antimony pentachloride. In this case you have no choice but to use IR…
This week I started teaching my second year organic chemistry class (CHM 249), which is one of my favourite courses. I always start with spectroscopy and my first week is all about IR. I asked myself if I can think of an example where IR has enabled structural assignment and led to a valuable insight. I did not have to go far in order to dig out an example… When compared to cyanides, isocyanides are distinguished by a shift in the characteristic CN absorbance of about 100cm-1 in their IR spectrum. Here is a classic paper from the 1960’s that takes advantage of this difference: http://pubs.rsc.org/en/content/articlelanding/1968/c1/c19680001347#!divAbstract. In it, Booth and Frankiss showed a very peculiar property of trimethylsilyl cyanide. This molecule is in equilibrium with its isocyanide form, albeit the latter accounts for a very small percentage of the mixture. Curiously, the composition is pressure-dependent and the amount of the isocyanide component increases with pressure (someone should use this property, by the way).
The relatively small amount of the isocyanide form in TMSCN did not prevent Hulme and co-workers from using this reagent in a multicomponent reaction: http://www.sciencedirect.com/science/article/pii/S0040403906003583.