The electronic structure of conjugated molecules. Non-empirical calculations for the benzenium, pyridinium, pyrylium, and thiopyrylium cations and a comparison of the last with phosphorin

Abstract

Non-empirical calculations on the title compounds and of the allyl carbonium ion are reported. For the second row compounds(1a and g), the role of the 3d-orbitals is that of polarisation functions rather than strongly bonding orbitals. The orbital energies of all the cyclic cations can be correlated directly with benzene and phosphorin, such minor rearrangements of orbital ordering that occur can be readily interpreted in terms of the heteroatom orbital energies. Support for the highest occupied orbital sequence of binding energies in phosphorin, 3b₁ < 1a₂ < 13a₁ < 8b₂, as suggested by Schweig is given. The population analyses for the compounds are discussed. The hydrogen atoms absorb much of the positive charge in the cations (1a—e). The internal electric fields arising from the populations are evaluated, and compared with the observed chemical shifts; the anomalous shifts in the thiopyrylium cation (1a) are attributed to ring current effects, which are substantial in (1a) but probably much smaller or absent in the other cations, as evidenced by the dissociations of the π-electrons into well separated pairs. Some reactions of the second row compounds arise directly from the molecular orbital ordering and are discussed.