[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.
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.