MO-SCF calculations on protonated butatriene and 1,2,3-pentatriene†

Energies are calculated for the cations obtained by protonation of butatriene and 1,2,3-pentatriene at various positions. The relative stabilities of the various ions are discussed. The ion of lowest energy is an allylic cation obtained by rotation around a CC bond after or concomitant with protonation. Experimentally, instead of this ion the next higher in energy is formed. The latter is a propargylic ion, formed by proton attack at the terminal carbon atom. Apparently, rotation is not appreciable in the transition state of the experimental protonation.