Results of preliminary calculations at the semi-empirical level using the AM1 method and at the ab initio STO-3G level for different molecular ions C3H5X+. (X = F, Cl, Br and I) are presented. The structure of the molecular ion of allyl bromide is discussed in more detail. This ion exists in lower energy structures obtained by elongation of the carbon-bromide bond: one corresponds to a symmetric bromonium-type ion (VI)+., and the other to an asymmetric structure with the Br atom out of the carbon plane (VII)+