Abstract
The substituent effects on π-orbital energies in three benzene derivatives have been examined on the basis of data provided by HeI ultraviolet photoelectron spectroscopy (UPS). We focus on cooperative action of substituents when acting on the π-electrons of benzene ring. We present UPS of substituted benzenes: 4-methoxybenzoic acid (MBA), 2,3,5,6-tetramethyl-nitroaniline (TMNA) and pentafluorobenzaldehyde (PFBA) which serve as examples of interplay of complex interactions between substituents where aromatic ring acts as electron density relay. We have shown that a simple method: linear combination of bond orbitals (LCBO) can be used to deduce the magnitude of resonance substituents effects on π energy levels in MBA. The splitting of two lowest π-energy levels was used to detect steric inhibition of resonance in TMNA. The resonance effects of CHO and COOH groups on π-orbitals are very small. On the other hand, the resonance effects of OCH3, NH2 and NO2 groups are significant.
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