Computational studies of the properties of phenyloxenium ions: A comparison with phenylnitrenium and phenylcarbenium ions

Properties of phenyloxenium ion 13a, phenylnitrenium ion 14a, and their 4-methyl and 4-phenyl analogues have been studied at the HF/6-31G* and pBP/DN*//HF/6-31g* levels to explain differences in their relative ease of formation and their stabilities. The phenyloxenium ions 13 are ground-state singlets but S₀-T₁ gaps are smaller than those of the corresponding nitrenium ions. The S₀ states are stabilized by donor methyl and phenyl substituents in both classes of ions, but phenyloxenium ion has much greater charge localization on the ring, primarily at the 4 position. Evidence for this difference stems from ground-state HF/6-31G*geometries, dipole moments, and vibrational frequencies. Nitrenium ions exhibit some quinoidal character, but the calculated C―N bond lengths are longer than those of their 4-hydroxy-2,5-cyclohexadienone imine hydration products 17 and the symmetric C-N stretching frequencies are ca. 60-100 cm⁻¹ less than those of 17. However, the C―O bond legths and stretching frequencies of the phenyloxenium ions are slightly shorter and greater, respectively than those of their 4-hydroxy-2,5-cyclohexadienone hydration products (16). The oxenium ions are best described by their 4-oxo-2,5-cyclohexadienyl carbenium resonance structures. Accordingly, a 4-phenyl group stabilizes the phenyloxenium ion more than the phenylnitrenium ion leading to a planar geometry and considerably more charge in the distal ring, thus accounting for regioselectivities of azide reactions. Isodesmic comparison of the energy difference between phenyloxenium and phenylnitrenium ion and their neutral hydration products explains their relative stabilities under aqueous conditions; whereas 4-biphenylyloxenium ion 13c has a lifetime in water of 12 ns as opposed to the corresponding nitrenium ion 14c (300 ns), the 4-methylphenyloxenium ion 13b is less stable to hydration by 18.7 kcal mol⁻¹ (1 cal = 4.184 J) and cannot be observed under the conditions used to generate 13c.