The electronic structure of Ullman's biradicals: an orthogonal valence bond interpretation

This paper addresses the electronic structure of Ullman's organic biradical, bis(nitronyl) nitroxide, in the frame of ab initio wave function based methods, both using standard (delocalized) molecular orbital methods and a valence bond strategy based on orthogonal localized orbitals. The aim of this study is to clarify the origin of the magnetic coupling of this molecule and to understand the recently observed unexpected behavior of the computed magnetic coupling constant with respect to the use of different theoretical methods. A detailed analysis of the physical effects that govern the correct description of the main characteristics of the electronic structure, in particular the spin density, reveals that the wave functions of the radical fragments are better seen as arising from the interaction of three unpaired electrons and that the intuitive picture of one unpaired electron per fragment is oversimplified and can lead to incorrect results. Furthermore, we establish that both the triplet and the singlet wave functions are well represented by the antisymmetrized product of the two fragment wavefunctions, thus corroborating previous observations concerning the low weight of the ionic structures (hopping of one electron from one fragment to the other) in the biradical singlet wavefunction. The use of the orthogonal valence bond formalism has also allowed us to extract a set of parameters for the fragment (direct exchange, hopping integral, on-site repulsion) which can be used for larger magnetic systems in studies based on model Hamiltonians.