Bonding in negative ions: the role of d orbitals in the heavy analogues of pyridine and furanradical anions

Abstract

We have used a potential wall method to investigate the role of d orbitals in the a₂ singly-occupied molecular orbitals of ²A₂ negative ion states of two molecular series: pyridine, phosphabenzene, arsabenzene, stibabenzene (C₅H₅X, X = {N, P, As, Sb}), and furan, thiophene, selenophene, tellurophene (C₄H₄X, X = {O, S, Se, Te}). Unlike for the lower lying doubly occupied orbitals, heteroatom d–carbon p in-phase (bonding) interactions in these a₂ orbitals are clearly identified and explain the 0.5 eV stabilization of the ²A₂ radical anion state in those compounds where the heteroatoms have d orbitals in the valence shell, compared to compounds where d orbitals are missing in the valence shell of the heteroatoms. The performance of both the potential wall approach and the approximate expression of Tozer and De Proft for calculating negative electron affinities has been also investigated, through a comparison with results obtained using electron-transmission spectroscopy experiments.