On the suggestion of a heteronuclear C∴O through-space three-electron bond in tetrahydropyran radical

Following the suggestion, based on experimental data, that the tetrahydropyran radical might exhibit the first known example of a C therefore O through-space three-electron bond rather than being a classical radical with a well localized unpaired electron, this system is compared to other radicals for which the delocalized nature of the unpaired electron is well established. Qualitative considerations and ab initio calculations on model systems are used in a first step, to confirm that through-space three-electron homonuclear interactions are likely to take place in 1,4-dioxane and 1,4-dithiane cations, and to rule out the possibility of a heteronuclear C therefore O interaction in a neutral system like tetrahydropyran in vacuum. In a second step, ab initio Hartree-Fock and Moller-Plesset calculations are performed on tetrahydropyran in real size, This radical is found to be more stable in its chair form than in the twist-boat form that brings the oxygen and para carbon atoms in front of each other, in agreement with expectations based on pure steric effects and excluding ally stabilization of the boat form by through-space interaction. The absence of any such interaction in tetrahydropyran radical is further confirmed by a spin density analysis that shows that the unpaired electron is not delocalized on both the oxygen and the para carbon, but remains fully localized on the latter atom. It is concluded that the tetrahydropyran exhibits a classical structure and does not show any sign of a through-space C therefore O three-electron bond in the gas phase. (C) 1998 Elsevier Science B.V.