ISOMERIZATION OF THE MOLECULAR ION OF ALLYL BROMIDE

The molecular ion of allyl bromide has been characterized by nb initio molecular orbital calculations at the MP4(SDTQ) level with optimized geometries at the MP2 level in order to account for experimental data suggesting the presence of two isomers. The calculations predict the e xistence of an allyl bromide molecular ion with structural parameters resembling the neutral species except for a lengthening of the double bond. This structure is calculated to be more stable than a cyclic bro monium radical cation structure, Rearrangement of the molecular ion of allyl bromide to that of 1-bromopropene is shown to be possible throu gh a transition state represented by the distonic ion, (BrHCCH2CH2.)-B r-+, lying just below the dissociation limit of the allyl bromide mole cular ion. Studies based on ion/molecule reactivity of C3H5Br.+ ions g enerated from allyl bromide and 1-bromopropene with ammonia, methanol, allyl bromide, and charge transfer reactions strongly suggest that a small fraction of the molecular ions of allyl bromide isomerize to the 1-bromopropene molecular ion as predicted by the calculation. These e xperiments cannot establish unequivocally whether the allyl bromide mo lecular ions retain the structure of the parent molecules as predicted by the calculations or undergo ion/molecule reactions mediated by a b romonium type complex. Charge transfer experiments also suggest the ad iabatic ionization energy of allyl bromide to be 9.83 +/- 0.07 eV.