A G2(+) level investigation of the gas-phase identity nucleophilic substitution at neutral oxygen

Yi Ren, Joel L. Wolk, Shmaryahu Hoz

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Abstract

G2(+) level molecular orbital calculations have been carried out for the identity nucleophilic substitution at saturated oxygen, X- + HOX → HOX + X- (X = F, Cl, Br, I). A comparison with data for the analogous reactions at saturated nitrogen, X- + NH2X → NH2X + X-, and at saturated carbon, X- + CH3X → CH3X + X-, indicate that the substitution reaction at saturated oxygen proceeds via a classic SN2 pathway. The calculated intrinsic barriers ΔHcent for substitution at oxygen are found to be much higher than the corresponding barriers for substitution at carbon and nitrogen, decreasing in the order F(106.3 kJ/mol) > Cl(92.5 kJ/mol) > Br(70.3 kJ/mol) > I(58.6 kJ/mol). Stabilization energies of the ion-molecule complexes decrease in the order F(187.9 kJ/mol) > Cl(97.5 kJ/mol) > Br(81.2 kJ/mol) > I(66.5 kJ/mol), that are also significantly higher than the corresponding values at carbon and nitrogen, and correlate well with the halogen electronegativities. The overall barriers relative to the reactants (ΔHovr) are negative for all halogens F(-81.7 kJ/mol), Cl(-5.1 kJ/mol), Br(-10.7 kJ/mol), I(-8.1 kJ/mol). These trend is similar to that for the analogous reaction at nitrogen, but contrasts to that for the reactions at carbon where the ΔHovr are negative only for X = F.

Original languageEnglish
Pages (from-to)1-10
Number of pages10
JournalInternational Journal of Mass Spectrometry
Volume220
Issue number1
DOIs
StatePublished - 15 Sep 2002

Bibliographical note

Funding Information:
This work was supported by the Barbara and Kort Sino-Israel Post-Doctoral Fellowship Program.

Funding

This work was supported by the Barbara and Kort Sino-Israel Post-Doctoral Fellowship Program.

FundersFunder number
Barbara and Kort Sino-Israel

    Keywords

    • Ab initio calculation
    • Reaction mechanism
    • S2 at neutral oxygen

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