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The ionization spectra of furan, pyrrole, thiophene, and selenophene have been calculated within the framework of the nonempirical quantum-chemical method with the Green's one-particle function in the approximation of the third order algebraic diagram construction [ADC(3)]. The calculated energies and the intensity of vertical transitions pertaining to the ionization of outer and inner shells are compared with the newest experimental data. The good agreement of theoretical and experimental results enabled a detailed assignment and interpretation of the observed photoelectron spectra to be carried out. Problems of disturbing the picture of orbital ionization are considered; the mechanism of formation of low-lying photoelectron satellites is explained. Certain general rules and trends of the behavior of the spectra of the systems studied are considered.

By means of 1H NMR spectroscopy it was demonstrated that 2-(2-thienyl)pyrrole and its 1-vinyl-substituted derivatives are protonated by adds (HSO3F, CF3CO2H, HCl, and HBr) at the C5 atom of the pyrrole ring. The reaction with the superacid system HSO3F/SbF5/SO2 leads to an equilibrium mixture of pyrrolium and thiophenium ions. The MNDO method was used to calculate the heats of formation (δH), charges, and partial electron densities of the highest occupied molecular orbitals (HOMO) of 2-(2-thienyl)pyrrole and its protonated forms. The calculated δH values of the thienylpyrrolium and pyrrolylthiophenium ions are in agreement with the experimentally established relationship.

The electron and conformational structures, as well as the internal rotation, of 2-(2-furyl)pyrrole and its α-protonated forms were studied by the MNDO method with complete optimization of the geometry. In conformity with the experiments (PMR), the two delocalized cations with a cis orientation of the heteroatoms that are formed as a result of protonation of the pyrrole or furan ring have the greatest and virtually equal stabilities (δH=738.7 and 740.6 kJ/mole).

It was shown by 1H NMR spectroscopy that 2-(5-R1-2-furyl)-3-R2-1-R1-pyrroles (R1= H, Me; R2=H, Me; R3=H, Et, CH=CH2) are protonated by acids (HSO3F, HCO2CF3, HCl, HBr) either at the C(5) atom of the pyrrole ring or at the C(5′) atom of the furan ring, depending on the conditions. The energies of formation (δH), the charges, and the partial electron densities of the boundary orbitals of 2-(2-furyl)pyrrole (I) and its protonated forms (IA and IB) were calculated by the MNDO method. The calculated δH values for the IA and IB forms are in agreement with their experimental ratio. According to the calculated reactivity indexes, the protonation of pyrrole is subject to orbital control and may include the prior formation of less stable protonated forms.