Substitution effects in isomeric para and meta derivatives of benzonitrile revealed by UV photoelectron spectroscopy

Abstract

The UV photoelectron spectra of isomeric para and meta derivatives of benzonitrile with substituents NO2, COCH3, Cl, Br, CH2I and NH2 were measured. Their assignment was carried out with the aid of MNDO calculations. The splitting of the highest π orbitals e1gS, e1gA is in all para derivatives greater than in their meta isomers, the greatest difference of splittings 0.25 eV being caused by the amino group. The level ordering e1gS, A exists in all molecules except meta NO2, and COCH3 derivatives where it is believed to be reverse. The π'CN - πCN splitting keeps nearly constant and it is close to that in benzonitrile. The ionization potentials of σ orbitals e1uS, e1uA, b2u and b1u in meta NO2 derivative are, in agreement with our expectation, higher than in benzonitrile. It is not true for para isomer where on the contrary, they are lower and they differ from meta isomer by 0.71-1.8 eV.