Computational and experimental study of charge distribution in the α-disulfonyl carbanions

Abstract

The electron densities of the disulfonyl carbanions were determined using experimental 13C chemical shifts. The 13C NMR spectra and electron densities for the disulfonyl, nitro, and cyano carbon acids were calculated at the MP2/cc-pVDZ level of theory. The calculated chemical shifts for disulfonyl carbanions show satisfying correlation with our own experimental data. The calculated π electron densities at the Cα atom correspond roughly to the “experimental” π electron densities estimated from the 13C chemical shifts. The natural charges at Cα in disulfonyl stabilized carbanions are significantly more negative than with other types of carbanions, partly because of the significant negative natural charge of the α carbon in parent carbon acids. The calculated increase of the negative charge caused by ionization is larger for sulfonyl carbon acids than for cyano- and nitroalkanes. The 13C chemical shifts δ of Cα in disulfonyl stabilized carbanions decrease with more negative calculated negative natural charge at Cα, with a slope of 220ppm/electron. The influence of phenyl ring para-substitution on the charge distribution in carbanions and relationship between the 13C chemical shifts and charge density have been discussed. It appears that the π electron density in these planar or nearly planar carbanions has a decisive impact on the chemical shifts.