Theoretical study of substituted PBPB dimers: structural analysis, tetraradical character, and excited states.

The radicaloid nature of para and meta 1,3-diborata-2,4-diphosphoniocyclobutane-1,3-diyl doubly substituted benzene is assessed from several electronic structure perspectives. Orbital occupation numbers computed by perfect pairing (PP), complete active space SCF (CASSCF), and restricted active space double spin-flip (RAS-2SF) reveal the presence of less than one unpaired electron in the planar molecules. Thus, the surprising stability of the "para tetraradical" can be rationalized by its moderate extent of radical character. Estimation of the delocalization energy, low-lying excited states, and short and long-range magnetic coupling constants all indicate a rather weak interaction to occur between two singlet PBPB units. Communication between two triplet units was found to be negligible. Comparison between para and meta isomers confirms a distinctly larger communication via the pi framework for the former. However, this communication, which was recently proposed to be the main factor for the different behavior of meta and para isomers regarding their preferred geometries, was found to account for only one-third of their energy difference. The study shows the important contribution of steric and/or electronic effects of the bulky (i)Pr and (t)Bu substituents on P and B.