A linear relation between the ionization potentials of HeI photoelectron spectroscopy and NMR chemical shifts. A PES study of the electronic structure of alkyl isocyanides

HeI photoelectron spectra (PES) of alkyl isocyanides RNC, where R is -CH3, -CH2CH3, -CH2CH2CH3, -CH(CH3)2, -C(CH3)3 and -cyclo-C6H11, are reported. The assignment of the bands is made with the aid of bandshapes, fine vibrational structure and ab initio SCF MO calculations with STO-6G basis sets for each of the molecules studied. The first sharp peak with the lowest ionization potential and with different vibrational spacings is assigned to electron ionization of the HOMO which is composed of a dominant contribution of the carbon atom, similar to the ‘lone-pair’ orbital of the carbon atom in the isocyano group. A linear relationship between the lowest PES experimental ionization potentials and the 13C NMR chemical shifts of the carbon in the isocyano group is found.