Methylene "anti" Jahn-Teller effect Yesterday a Reddit user posted a page from Morrison's Organic Chemistry in which it is said that singlet methylene is less stable than triplet methylene. Another user asked basically the same I'm asking here, as he got no answer and probably won't get any.
I'm used to see radicals (and therefore diradicals) as extremely reactive and lone pairs as stable, when not reacting in some Lewis base fashion.
Also, Jahn-Teller effect explanations usually say that unpaired electrons configurations with each electron on a degenerate orbital are less stable than the paired configuration with some deformed structure, and therefore the structure deforms to &quot;de-degenerate&quot; the orbitals and achieve a lower energy configuration, hence the effect.
What we see here is exactly the opposite, isn't it?
What is it then that makes the triplet state more stable?
Wikipedia confirms the observation and adds a value to the energy difference: 38 kJ.
Wikipedia also says it is normal for carbenes to be more stable in the triplet state.
The Jahn-Teller effect is not relevant here. The triplet state is observed in many molecules to be of lower energy than the singlet. The Pauli exclusion principle, makes the states symmetric or anti-symmetric to the exchanging electrons.
A triplet state has unpaired spins while in a singlet they are paired. The triplet is lower in energy than the corresponding singlet. One reason for this is that the electrons try to ‘avoid’ one another in the triplet; (see Salem 'Electrons in Chemical Reactions', publ Wiley 1982 for a detailed discussion). This means that the spatial part of the triplet wavefunction is asymmetric to exchange of the electrons;
$$\varphi_T = \psi_1(1)\psi_2(2) − \psi_1(2)\psi_2(1) $$
and the singlet spatial part is symmetric which means that the electrons tend to crowd together raising the energy
$$\varphi_S = \psi_1(1)\psi_2(2) + \psi_1(2)\psi_2(1) $$
The numbers in brackets refer to the electrons’ coordinate. As the total wavefunction is anti-symmetric to exchange of electrons the corresponding spin parts have the opposite symmetry to the spatial parts.
If the spins are on two separate atoms then the singlet and triplet energies can be very close to one another and the order can be changes by external magnetic fields. There is more detail on this effect in this answer How does spin flipping of triplet carbenes occur?