Photoelectron Spectroscopy of Heterocycles. Indene Analogs

The He (I) photoelectron (PE) spectra of indene, indole, benzofuran, benzothiophene, 1-, 2-, 3-, 4and 6-methylindole and 4,5,6,7-tetrafluoroindole were measured and analysed. A close correlation between the jr-ionisation energies of these compounds and the eigenvalues of the corresponding HMO-orbitals is found. A qualitative scheme for the assignment of the ^-electron ionisations by orbital interaction of the constituents of the indene analogs i. e. of styrene and the p-orbital of the corresponding heteroatom, indicates that the energy levels in all the compounds investigated can be described by the same type of interaction and yields a value for the inductive effect of the hetero group (-CHS and >CH2: 0.3 eV, >NH: 0.4, > S : 0.1, > 0 : negligible).