Group ElectronegativitiesIt has long been recognized that there is a relationship between the electron withdrawing power of substituents and proton chemical shifts. A good correlation was found between the difference in α and β proton chemical shifts in substituted ethanes and electronegativity. Calculated from 1H chemical shift differences between CH2 and CH3 in ethyl derivatives.
Values in column two are calculated using equ. 1 (based on 30 MHz NMR, 1955) and values in column three by equ. 2 (based on 60 MHz NMR, 1961). The electronegativities are taken from Huggins.
Whilst a cursory inspection of the derived electronegativities suggest that there is a correlation with the chemical shifts there are a number of inconsistencies. For example the value for the cyano group is lowest of all groups dispite the fact that all chemical data point to a much higher electronegativity.
Equation 1χ=0.6945*(δCH2 -δCH3 )+1.71
Equation 2χ=0.684*(δCH2 -δCH3 )+1.78