GAS-PHASE ACIDITY OF SOME ALPHA-KETO ALDOXIMES - EXPERIMENT AND THEORY

Experimental gas-phase acidities of 2-oxopropanaloximes, XCH2COCH = NOH (X = H, CH3S, CH3SO, CH3SO2; compounds 1-4), were determined by Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. The values are DELTA G(acid)-degrees = 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 sigma-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.