PENNING IONIZATION ELECTRON-SPECTROSCOPY OF HALOGENOTOLUENES - ORTHO-, META-, AND PARA-CH3C6H4X (X = CL, BR, I)

He*(2(3)S) Penning ionization electron spectra (PIES) and He I ultraviolet photoelectron spectra (UPS) were measured to study the electronic structures and the orbital reactivities of halogenotoluenes; o-, m-, and p-CH3C6H4X (X = Cl, Br, I). On the basis of the feature of PIES, which provides direct information on the electron distribution of individual molecular orbitals, all the bands in UPS were assigned. The relative reactivity of the orbital upon electrophilic attack can be related to the relative intensity of the band in PIES. It was found that the reactivities of the n and pi-orbitals depend on the electronic effect due to the size of the halogen p orbitals and the degree of the n-pi-interaction and also on the steric effect due to the methyl and halogeno substituents shielding some orbitals from the impact of metastable atoms. The reactivities of the sigma-orbitals were also discussed in terms of the steric shielding effect.