Posts Tagged ‘Hofmann’

The colour of purple

Thursday, February 24th, 2011

One of my chemical heroes is William Perkin, who in 1856 famously (and accidentally) made the dye mauveine as an 18 year old whilst a student of August von Hofmann, the founder of the Royal College of Chemistry (at what is now  Imperial College London). Perkin went on to found the British synthetic dyestuffs and perfumeries industries. The photo below shows Charles Rees, who was for many years the Hofmann professor of organic chemistry at the very same institute as Perkin and Hofmann himself, wearing his mauveine tie. A colleague, who is about to give a talk on mauveine, asked if I knew why it was, well so very mauve. It is a tad bright for today’s tastes!

Charles Rees, wearing a bow tie dyed with (Perkin original) mauveine and holding a journal named after Perkin.

The first thing to note about mauveine is that it is not a single compound; actual samples can contain up to 13 different forms! These all vary in the number of methyl groups present which range from none up to four, in various positions. These compounds all have absorption maxima λmax in the range 540-550nm, the colour of purple. The structure of one of these, known as mauveine A, is shown below.

Mauveine A. Click to load 3D

You can see from this that something is missing. The so-called chromophore is a cation, and an anion needs to be provided to balance the charge. We will now attempt to predict the color of purple using purely the power of quantum mechanics (for many years, accurate prediction of colour was a holy grail amongst dye chemists for obvious reasons). The anion can be chloride, and the colour is often measured in methanol as solvent. So the first task is to calculate this ion-pair. This used to be easier said than done (and in the past, the anion was often simply neglected). But using the ωB97XD density functional procedure (to get the van der Waals interactions modelled correctly) and a 6-311++G(d,p) basis set, coupled with a smoothed-cavity continuum solvation procedure, and two molecules of water (standing in for methanol, which is a bit bigger) as explicit solvent molecules, we get the structure apparent when you click on the diagram above (DOI: 10042/to-7320). Application of time-dependent density function theory (TD-DFT) gives a measure of the UV-optical spectrum (below, loaded as a scaleable SVG image. If you are using a modern browser, it should display. If not, try the latest FireFox, Chrome, Safari etc).

 

 

This has several noteworthy aspects.

  1. The visible (right hand side) part of the spectrum is very monochromatic, with λmax ~440nm. In other words, mauveine has a pure and intense colour.
  2. This λmax is hardly affected by the presence of the counterion.
  3. The electronic transition responsible for this band is a simple HOMO (highest-occupied-molecular-orbital) to LUMO (lowest-unoccupied-molecular-orbital) excitation of an electron.
  4. These orbitals are shown below.
    LUMO HOMO

    Mauveine A. LUMO. Click for 3D

    Mauveine A. HOMO. Click for 3D
  5. Note how the excitation involves the central region of the molecule, and one of the pendant aryl groups, but not the other. One might presume that tuning the colour would only work if changes are made to the first of these aryl groups.
  6. There is a real mystery about the calculated value of λmax, which differs from the observed value by about 100nm (the wrong colour, making mauveine orange rather than purple). Normally, this sort of time dependent density functional theory has errors no greater than 15-20nm. The calculated value of λmax is not sensitive to the basis set, or the presence or not of the counter ion and solvent. Clearly, a discrepancy of this magnitude must have some other explanation. Watch this space!

So this post ends with a bit of a mystery. The fanciest most modern computational theory gets the colour of mauveine wrong by ~100nm. Why?

Tags:, , , , , , , , , , , , , , , , , ,
Posted in General, Interesting chemistry | 16 Comments »

A short history of molecular modelling: 1860-1890.

Saturday, February 5th, 2011

In 1953, the model of the DNA molecule led to what has become regarded as the most famous scientific diagram of the 20th century. It had all started 93 years earlier in 1860, at a time when the tetravalency of carbon was only just established (by William Odling) and the concept of atoms as real entities was to remain controversial for another 45 years (for example Faraday, perhaps the most famous scientist alive in 1860 did not believe atoms were real). So the idea of constructing a molecular model from atoms as the basis for understanding chemical behaviour was perhaps bolder than we might think. It is shown below, part of a set built for August Wilhelm von Hofmann as part of the lectures he delivered at the Royal College of Chemistry in London (now Imperial College).

The original August Wilhelm von Hofmann molecular model

This grand-daddy of all molecular models does have some interesting features. The most obvious is that the carbon atom at the centre is square planar (tetrahedral carbon was still 14 years in the future). What HAS survived to the present day is the colour scheme used (black=carbon, white=hydrogen, and not shown here, red=oxygen, blue=nitrogen, green=chlorine).  But another noteworthy aspect is the relative size of the white hydrogen, which is larger than the black carbon. This deficiency was however very soon rectified in 1861 by Josef Loschmidt, who published  a famous pamphlet in which he set out his ideas for the structures of more than  270 molecules (many of which by the way were cyclic, and this some four years before Kekule’s dream!). An example (#239) is shown below, which gets the relative sizes of the atoms more or less correct (OK, chlorine is shown with rather an odd shape). To get an idea of how good Loschmidt’s model actually was, click on the diagram to load a modern model, and compare the two! Even more impressive, these diagrams pre-date van der Waals work on the finite sizes of atoms, first presented in 1873.

Loschmidt’s molecular models. Click for 3D

To conclude, I cannot resist showing one more model. Hermann Sachse believed cyclohexane could not be planar. To try to convince people, in 1890 he included a  “flat-packed” model in the pages of a journal article,  evidently believing that people would cut it out, and assemble it into a 3D shape.

Flat-packed molecular model of cyclohexane

You might have noticed a theme in the present blog of presenting 3D models for many of the molecules I discuss (include the Loschmidt one above). For the historians amongst you, I note our 1995 article in which we updated[1] Sachse’s origami with an article featuring how to incorporate interactive models into journals (still sadly only too rare). Perhaps a history of the molecular model, and how it has been presented over 150 years might be an interesting one to trace!

References

  1. O. Casher, G.K. Chandramohan, M.J. Hargreaves, C. Leach, P. Murray-Rust, H.S. Rzepa, R. Sayle, and B.J. Whitaker, "Hyperactive molecules and the World-Wide-Web information system", Journal of the Chemical Society, Perkin Transactions 2, pp. 7, 1995. https://doi.org/10.1039/p29950000007

Tags:, , , , , , , , , , , , , , , , , , ,
Posted in General | 3 Comments »