A high-level ab initio investigation of identity and nonidentity gas-phase SN2 reactions of halide ions with halophosphines

Theis I. Solling, Addy Pross, Leo Radoma

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The high-level ab initio procedures G2(+) and G2(+)[ECP(S)]have been employed in an investigation of SN2 reactions at neutral tri-coordinated phosphorus. The process has been modeled by identity and nonidentity substitution reactions involving a series of halophosphines PH2-X and halide ions. We find that the reaction proceeds without an intervening barrier by way of a tetra-coordinated phosphorus anion intermediate (X-PH2-Y-). This contrasts with the corresponding process for the carbon and nitrogen analogues, where the tetra-coordinated species is a transition structure. The threshold for inversion of the phosphorus intermediate is found to lie below the reaction energy in the cases where F- is the leaving group, but above it in all the other cases. The SN2 reaction will therefore lead to racemization when F- is expelled. In the other cases, it is possible in principle that the reaction can be controlled to proceed with inversion, but because of the relatively low barriers for inversion, this is not a very likely outcome. We predict that the tetra-coordinated intermediate should be detectable, if not isolable, and that the SN2 reaction at neutral tri-coordinated phosphorus is exothermic when the reactant halide ion is more electronegative than the product halide ion, and endothermic when the reverse applies.

Original languageEnglish
Pages (from-to)1-11
Number of pages11
JournalInternational Journal of Mass Spectrometry
StatePublished - 14 Sep 2001


  • ECP
  • G2
  • Halophosphines
  • Nucleophilic substitution
  • Pseudorotation


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