Ab Initio Conformational and Stereopermutational Analyses of Phosphoranyl Radicals HP(OR)3 and P(OR)4 [R = H or CH3]

Abstract

The potential energy hypersurfaces for HnP(OH)4-, and HnP(OCH3)4-n3 n = 0 or 1, have been explored at correlated levels using polarized valence double-5 and triple-5 basis sets. Numerous local minima and several distinct types of stereopemutational 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.

Experimental and Computational Investigations into Trivalent Phosphorous-Mediated Radical Thiol Desulfurization

“…Minimized structures with overlaid spin density volumes and labelled atomic spin populations for select intermediates are shown in Figure 8 (See SI for additional information). Structurally, [P(OEt)4]• and [P(SMe)(OEt)3]• are best described as distorted trigonal bipyramidal geometry with the thiyl ligand preferentially apical in the latter, in agreement with past ab initio 43 and more recent DFT calculations. 44 The conformer with an equatorial thiyl group was also a stationary point, but the calculated hyperfine splitting value overestimated the experimentally observed value by ~100 Gauss.…”

Ab initio calculations on P?C bond cleavage in phosphoranyl radicals: implications for the biodegradation of organophosphonate derivatives

“…trihydroxyphosphoranyl). 51,55 Four TBP minima all exhibiting equatorial localization of the unpaired electron are also found, and these minima differ either by the axial/equatorial substitution pattern of the ligands-1a has the methyl group equatorial whereas 1c, 1d and 1e place it axial-or as hydroxyl group rotamers amongst the latter three. The hydrogen bonding and hyperconjugative interactions that control the hydroxyl group orientations have been extensively discussed for other hydroxy-substituted phosphoranyl radicals, 48,51,52,55 and we find similar results here; in particular, the greater apicophilicity of hydroxyl compared with methyl 66 is offset by favorable hyperconjugative interactions between two equatorial hydroxyl groups, making the energies of the lowest energy conformers, 1a and 1c, very similar at all levels of theory.…”

“…There is generally good agreement between UHF, MP2 and DFT for the relative energies of all species except 1b. We have noted elsewhere the tendency for correlation effects to preferentially stabilize TBPs with axially localized unpaired electrons, 52,55 but have noted that this effect is overestimated at the MP2 level, 52 suggesting that the DFT and UHF results are probably reasonably accurate.…”

Molecular orbital calculations on the P—S bond cleavage step in the hydroperoxidolysis of nerve agent VX

“…33,34 The stability of a particular pseudorotamer depends upon many factors, including the apicophilicity of the various ligands (correlated with electronegativity) and a ligand's ability to hyperconjugate with the central phosphorus atom. 25,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] Elimination from trigonal bipyramidal species is typically most facile from apical positions, and hence apicophilic ligands tend to be good leaving groups. 35 Since fluorine is the most electronegative element, it is strongly apicophilic, and it is the preferred leaving group in the reaction of G-series nerve agents with hydroxide.…”