Relationship between proton-proton NMR coupling constants and substituent electronegativities. V—Empirical substituent constants deduced from ethanes and propanes

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Abstract
The electronegativity dependence of the torsion angle‐independent term in the Karplus equation, i.e. of the ‘constant’ A in the Fourier expansion A + B cos ϕ + C cos 2ϕ +…, was investigated. Experimental proton‐proton coupling constants of substituted ethanes and isopropanes appeared to be suitable for this purpose. A data set was constructed which contained 70 couplings, newly measured or remeasured at 300 MHz, and 25 couplings taken from the literature. The accuracy of each data point is estimated as ≤0.02 Hz, with a few exceptions. The actual analysis was carried out on 93 data points, i.e. on J values of 55 mono‐ and of 38 1,1‐di‐substituted ethanes, including 22 isopropyl derivatives. A total of 55 chemical groups is represented in the set; some of these were taken together, leaving 50 distinct groups. Regression analysis of the present data versus standard electronegativities did not yield acceptable results. Instead, substituent parameters λe, valid for 3J(HH) in saturated HCCH fragments, were derived in a least‐squares procedure from the data set. The couplings from mono‐ and 1,1‐di‐substituted ethanes could be accounted for in a simple expression that contains an interaction term C012(λ1λ2). The best equation obtained is The parameters are valid for λe values scaled according to the Huggins electronegativities: λH = 0, λOR = 1.40. The equation fits 84 experimental couplings with a root‐mean‐square deviation of 0.018 Hz and a maximum deviation of 0.06 Hz. Some exceptions occur. (i) CH3CH3, CH3CHCl2 and CH3CHF2 appear to follow a different, but correlated, regression; (ii) C(O)H, C(O)R, SO2Cl and POCl2 groups require different λe values according to the substitution pattern, i.e. mono‐ or 1,1‐di‐substitution. The striking difference between the new λe substituent‐effect scale and other empirical electronegativity scales lies in the inverse correlation of λe with increasing electronegativity of β‐substituents. The inverse relationship is not only found for α‐carbon atoms, but appears to represent a general phenomenon, also seen for substituted α‐hetero atoms (O, N, S).

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