Effect of halogen substituents on carbon is ionization energies in haloethenes and haloethanes

Abstract

Carbon 1 s ionization energies have been measured for ethene and ethane and for their monosubstituted halogen derivatives, CH 2CHX and CH 3CH 2X (X = F, Cl, Br, I). The shift in ionization energies relative to the unsubstituted compounds is large for the carbon atoms directly attached to the halogen atom, but much less for the terminal carbon atoms. The effect of substitution is, however, remarkably similar for the two series of molecules. Ab initio calculations give results which are in good agreement with experiments and permit an analysis of the results in terms of initial-state potential energies and final-state relaxation energies. It is shown that the relative ability to accept charge at a carbon position is mainly determined by the initial-state potential and to a lesser extent by the relaxation energy. The effect of fluorine substitution on the initialstate potentials is almost exclusively confined to the directly bonded carbon, C1. The effect becomes less for C1 and is transmitted more to the terminal carbon C2 as the substituent is changed from fluorine to iodine, the combined effect being almost constant for both series of compounds. Molecular charge distributions have been calculated both for the initial state and for the coreionized state to aid interpretation. Results from integrated Bader populations on haloethenes show that the traditional point-charge model is inadequate to explain the shifts in initial-state potentials.