Comparison of the capacity of nitrogen, oxygen, sulfur, and selenium atoms to exist in onium states. quantum-chemical investigation of isomeric model systems with two heteroatoms
HF, B3LYP, and MP2 calculations with the 6-31+G(d) basis set with correction to the energy of zero-point vibrations were carried out to determine the energy characteristics of model molecules containing two heteroatoms in the sp3- and sp2-hybrid states; different combinations of N, O, S, and Se atoms were studied. The stability of the onium states of the nitrogen atom was found to be greater than for its chalcogen analogs and the relative stability of onium states of the chalcogen analogs was found to depend on the hybridization of these atoms. Analysis of these results permitted us to construct a stability series of onium derivatives and to interpret the positional selectivity in electrophilic substitution reactions of five-membered heterocyclic compounds with one heteroatom.