CH3CH=S and [CH3CHS]3 cyclic trimer

Sir Harry Kroto speaking about the cyclic trimer

"At the time, SRC (in its infinite wisdom) had decided that the microwave spectrometer (which I, an experimentalist, had applied for together with a theoretician in Reading) should be sited with the theoretician at Reading! 

"It is interesting to conjecture how such a decision could have been made when I and the theoretician had agreed it should be at Sussex. This meant that for the next five years, my experimental group had to travel to reading to do our experiments. 

"I had done all the work in working out what to buy and wrote essentially all the proposal, especially the technical part. There was a silver lining however in that my senior colleague, Mike Lappert, let me use his Photoelectron spectrometer. 

"Fortuitously the sampling conditions in PES and MW are closely similar and thus one technique can piggy back on the other. Sometimes it is much easier to detect a species by one technique and knowing it is present work hard to detect it by the other technique. This became a key part of my research strategy to detect new molecules.

"One of my favourite examples is the thermolysis of the trimer of thioacetaldehyde [CH3CHS]3. The trimer has a beautifully simple first two PE bands in which the sulphur orbitals form A and E MOs, in which the A overlap is more bonding and about half the intensity of the less bonding doubly degenerate E MOs.

"On thermolysis the spectrum was replaced by that of the monomer CH3CH=S. Thus we knew the condition for producing the monomer and we got a beautiful microwave spectrum.

"Take note of the nice doubling of the outer lines due to internal rotation. Barry Landsberg did a great job of analysing this MW spectrum."

For more information, refer to page 448 in the Tilden lecture (PDF, 6.4MB).