Monday, July 4, 2011

Seeing how degenerate radicals can be

I have been slowly digesting a really nice combined theoretical and experimental paper The Lowest Singlet and Triplet States of the Oxyallyl Diradical which was featured on the cover of Angewandte Chemie in 2009 [For physicists this is the European counterpart to the prestigious JACS = Journal of the American Chemical Society].
[See also the commentary by Bettinger].

Here are a few interesting things I learnt:
  • The ground state is a singlet but only 55 meV in energy below the lowest lying triplet. [In most organic molecules the energy difference is ~ 1-2 eV].
  • C-C-C angle bending has a frequency of about 400 cm-1 and couples to the electronic transitions. 
  • Another vibrational mode which couples strongly to electronic transitions is the C-O stretch [with a frequency of order 1700 cm-1].
  • The singlet state is unstable to "disrotatory ring closure" to form cyclopropanone
The relevant valence bond structures are
and provide a natural framework to understand the above observations.

This paper is of particular interest to me because the oxyallyl diradical is a simple example of a methine dye [cf. green fluorescent protein, Malachite green] and a ketocyanine dye whose minimal quantum chemical description requires 4 electrons in 3 orbitals. [To physicists 4 electrons in a 3 site Hubbard model]. The paper though uses the framework of 4 electrons in 4 orbitals due to the analogue with trimethylenemethane (TMM) which actually has a triplet ground state. TMM is one of the simplest non-Kekule molecules.

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