Carbenes remain an active area of interest for computational chemists, as seen in Chapter 5 of my book. For many carbenes, the triplet is the ground state, and that is true of diphenylcarbene 1. Can solvent play a role in the stability of carbene spin states? Surprisingly, the answer, provided in a recent paper by Sander,1 is yes!
In the gas phase, the singlet-triplet gap of 1 is computed to be 5.62 kcal mol-1 at (U)B3LYP/6-311++G(d,p) (and this reduces to 5.06 at (U)B3LYP+D3/6-311++G(d,p)) with the ground state as a triplet. If a single methanol molecules is allowed to approach 1, then the complex involving the singlet has a short hydrogen bond distance of 1.97 Å but the triplet has a much longer distance of 2.33 Å. These structures are shown in Figure 1. This manifests in a dramatic change in the relative stability, with the singlet complex now 0.26 kcal mol-1 (0.44 with the D3 correction) lower in energy than the triplet.
Figure 1. (U)B3LYP/6-311++G(d,p) optimized geometries of the compelxes of methanol with singlet or triplet 1.
IR spectroscopy of 1 in an argon matrix doped with a small amount of methanol confirms the presence of the singlet carbene, and detailed description of the difference in the reactivities of the singlet and triplet are provided.
(1) Costa, P.; Sander, W. "Hydrogen Bonding Switches the Spin State of Diphenylcarbene from Triplet to Singlet," Angew. Chem. Int. Ed. 2014, 53, 5122-5125, DOI: 10.1002/anie.201400176.