NCNCS

NCNCS is the perfect molecule to exhibit quasi-linear behaviour and quantum monodromy.

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Sir Harry Kroto's reflections

"My memory is that we were trying to produce NCBS by pyrolysis of S(CN)2 over crystalline boron at a high temperature. Mike King discovered that S(CN)2 isomerised to NCNCS.

This was a very nice discovery as this molecule is probably the most beautiful example of a quasi-linear system.

Sir Harry Kroto

"Barry Landsberg did a superb job of analysis. He found it was a truly beautiful example in that the rotational spectrum in the lowest vibrational state was that of an asymmetric top. As the bending vibrational quantum increased to about v=3, the molecule was flexing through the linear configuration (it had a sombrero hat potential). For these levels the rotational structure was much more like that of a linear molecule.

"In the case of a linear molecule the l (k) = 1 lines form an l-doublet and lie between the l=0 and l=2 lines whereas for an asymmetric top these latter lines lie between the k (l) = 1 (see my book!). Basically the structure was averaging over a linear configuration for v= 3."

Quantum monodromy in the rotational spectrum of NCNCS

Brenda and Manfred Winnewisser and their colleagues went on to carry out an amazingly elegant and exhaustive study to show that the molecule had even more beautiful tricks up its sleeve. The vibrational quantum number increases even more as the rotational structure reverts back to an asymmetric top pattern at v=6. It is in fact a beautiful example of quantum monodromy.

B P Winnewisser, M Winnewisser, I R Medvedev, M Behnke, F C De Lucia, S C Ross and J Koput, '', Phys Rev Lett, 2005.

Brenda and Manfred Winnewisser said, "The B for K=0 and v = 3 is an absolute minimum because the molecule can just, barely, scrape over the top of the barrier, so it slows down and spends a lot of time there. Therefore, it has its maximum time average extension and moment of inertia for end-over-end rotation, and its minimum B value. The wave function confirming this is shown in a plot in our 2010 paper (PDF, 4.62MB).

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It was the stunning simplicity of the graphical representation of that (quantum-mechanically) obvious fact that blew us way when we plotted our first monodromy plot of B-values.

Brenda and Manfred Winnewisser