A significant portion of Chapter 4 of my book is devoted to phenylnitrene 2 and phenylcarbene. Phenyloxenium cation 1 is isoelectronic with phenylnitrene and so one might expect similar behavior of the two. Winter has reported a nice computational study of the singlet and triplet phenyloxenium cation and finds some very striking differences between phenyloxenium cation and phenylnitrene.¹

Phenylnitrene has a triplet ground state, with the ¹A₁ state about 18 kcal mol^-1 higher in energy, and the ¹A₂ state higher still. CASPT2/pVTZ//CASSCF(8,8)/pVTZ computations of 1 find the singlet ¹A₁ to be the ground state. The lowest triplet is 22.1 kcal mol^-1 higher in energy, and the lowest ¹A₁ state lies 30.8 kcal mol^-1 above the ground state singlet. (The structures of the lowest singlet and triplet of 1 are shown in Figure 1.) Reanalysis of the ultraviolet photoelectron spectrum of the phenoxy radical² switches the assignments of the observed transitions and is in excellent agreement with these computed values. G3 and CCSD(T)/cc-pVTZ predicts a similar value for the singlet-triplet gap. B3LYP, MPW1PW91, and some other DFT methods predict the singlet to be lower in energy than the triplet, but with a gap half of the correct value of 22 kcal mol^-1.

1 singlet (1A1)

1 triplet (3A2)

Figure 1. CASSCF(8,8) optimized geometries of the lowest singlet and triplet states of 1.

The origin of the difference between 1 and 2 lies in the description of the singlet state. The singlet state of 1 places the two lone pairs on oxygen into the sp-like orbital and into the in plane p orbital. However, in 2, the singlet is described by two determinants, one with the nitrogen lone pairs in the sp and in plane p orbital and the second determinant has them in the sp orbital and in the perpendicular p orbital. For 1, this single determinant allows for the positive charge to delocalize into the phenyl ring and off the very electronegative oxygen; this is manifest in a short C-O bond (1.211 Å). The greater electronegativity of oxygen then nitrogen brings the perpendicular p orbital lower in energy and better able to mix with the phenyl π-orbitals. In other words, the greater electronegativity of O over N results in a large symmetry break of the degenerate p orbitals.

References

(1) Hanway, P. J.; Winter, A. H., "Phenyloxenium Ions: More Like Phenylnitrenium Ions than Isoelectronic Phenylnitrenes?," J. Am. Chem. Soc., 2011, 133, 5086-5093, DOI: 10.1021/ja1114612

(2) Dewar, M. J. S.; David, D. E., "Ultraviolet photoelectron spectrum of the phenoxy radical," J. Am. Chem. Soc., 1980, 102, 7387-7389, DOI: 10.1021/ja00544a050