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Gas‐phase acidity of some α‐keto aldoximes: Experiment and theory
Author(s) -
Bouchoux G.,
Jaudon P.,
Decouzon M.,
Gal J.F.,
Maria P.C.
Publication year - 1991
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610040505
Subject(s) - chemistry , deprotonation , delocalized electron , ion cyclotron resonance , fourier transform ion cyclotron resonance , ion , substituent , oxime , resonance (particle physics) , analytical chemistry (journal) , computational chemistry , conjugate , conjugate acid , stereochemistry , medicinal chemistry , atomic physics , organic chemistry , cyclotron , physics , mathematical analysis , mathematics
Experimental gas‐phase acidities of 2‐oxopropanaloximes, XCH 2 COCHNOH (X H, CH 3 S, CH 3 SO, CH 3 SO 2 ; compounds 1–4), were determined by Fourier transform ion cyclotron resonance (FT‐ICR) spectrometry. The values are δ G acid ° = 1401, 1381, 1360 and 1351 kJ mol −1 for 1, 2, 3, and 4, respectively. Molecular orbital calculations using the semi‐empirical AM1 method provided information on the geometry and relative energy of neutrals species 1–4 and their conjugate bases, together with charge distributions and entropies of deprotonation. It is demonstrated that the proton abstraction occurs preferentially at the oxime function; the formation of an enolate as a conjugate base is unfavourable by 70–140 kJ mol −1 . The large variation of the gas‐phase acidities for 1–4 is explained in terms of the field/inductive empirical substituent constant σ F . The variation of solution acidities appears to be comparatively strongly attenuated. This attenuation is attributed mainly to charge delocalization in the anion, which was confirmed by charge density calculations.