Ab initio conformational and stereopermutational analyses of phosphoranyl radicals HP(OR)₃ and P(OR)₄ [R = H or CH₃]

The potential energy hypersurfaces for H_nP(OH)_4-n and H_nP(OCH₃)_4-n, n = 0 or 1, have been explored at correlated levels using polarized valence double-ζ and triple-ζ basis sets. Numerous local minima and several distinct types of stereopermutational transition states interconnecting them have been identified. These transition states correspond to (i) bond rotation, (ii) pseudorotation, (iii) double pseudorotation, and (iv) pseudoinversion. The stereochemical consequences of these processes are analyzed in detail as is the nature of the singly occupied molecular orbital throughout. Geometric and natural bond orbital analyses indicate the importance of stabilizing hyperconjugative interactions in these systems. The latter effect is maximized for interactions between equatorial substituents in trigonal bipyramidal (TBP) phosphoranyl structures. Unlike the TBP local minima of the mono- and dihydroxyphosphoranyls, where the unpaired electron always localizes in an equatorial site, tri- and tetrahydroxyphosphoranyl minima localize their unpaired electrons in both equatorial and apical positions.