SUBSTITUENT EFFECTS ON IMIDAZOLE BASICITY AND PHOTOELECTRON-SPECTROSCOPY DETERMINED IONIZATION ENERGIES

The nitrogen lone pair orbital (n) and the two highest occupied tt orbital vertical ionization energies(7V) from the UV photoelectron spectra of 1-methyl-, 2-methyl-, 1, 2-dimethyl-, 1-acetyl-, and 1 -(trifluoroacetyl)imidazole and imidazole and the complete PE spectrum of benzimidazole are compared with the results of modified CNDO/2 calculations. Only in the PE spectra of 1-acetyl-and l-(trifluoroacetyl)imidazole are the n and x2 orbital Iv resolvable. A linear relationship between substituted imidazole pKa values and CNDO lone pair orbital energies is found to be limited to substituents which leave the n orbital electron density at nitrogen unchanged. Perturbation molecular orbital arguments are developed which suggest that this result is the equivalent of a restriction on the observation of linear 7v-pKs regressions that substituent interaction mechanisms be limited to inductive and field effects, with the exclusion of through bond resonance Interactions between the substituent and nitrogen lone pair. © 1979, American Chemical Society. All rights reserved.