Effects of Substituents on the Stability of Phosphoranyl Radicals

Jennifer L. Hodgson, Michelle L. Coote

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14 Citations (Scopus)


The effect of substituents on the geometries, apicophilicities, radical stabilization energies, and bond dissociation energies of •P(CH 3)3X (X = CH3, SCH3, OCH 3, OH, CN, CF3, Ph) were studied via high-level ab initio molecular orbital calculations. Two alternative definitions for the radical stabilization energy (RSE) were considered: the standard RSE, in which radical stability is measured relative to H-P(CH3)3X, and a new definition, the α-RSE, which measures stability relative to P(CH 3)2X. We show that these alternative definitions yield almost diametrically opposed trends; we argue that α-RSE provides a reasonable qualitative measure of relative radical stability, while the standard RSE qualitatively reflects the relative strength of the P - H bonds in the corresponding H - P(CH3)3X phosphines. The •P(CH3)3X radicals assume a trigonal-bipyramidal structure, with the X-group occupying an axial position, and the unpaired electron distributed between a 3pα-type orbital (that occupies the position of the "fifth ligand"), and the σ* orbitals of the axial bonds. Consistent with this picture, the radical is stabilized by resonance (along the axial bonds) with configurations such as X- P•+(CH3)3 and X• P(CH3)3. As a result, substituents that are strong σ-acceptors (such as F, OH, or OCH3) or have weak P - X bonds (such as SCH3) stabilize these configurations, resulting in the largest apicophilicities and α-RSEs. Unsaturated π-acceptor substituents (such as phenyl or CN) are weakly stabilizing and interact with the 3pσ-type orbital via a through-space effect. As part of this work, we challenge the notion that phosphorus-centered radicals are more stable than carbon-centered radicals.

Original languageEnglish
Pages (from-to)10013-10021
Number of pages9
JournalJournal of Physical Chemistry A
Issue number44
Publication statusPublished - 1 Nov 2005
Externally publishedYes


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