EMPIRICAL GROUP ELECTRONEGATIVITIES FOR VICINAL NMR PROTON-PROTON COUPLINGS ALONG A C-C BOND - SOLVENT EFFECTS AND REPARAMETERIZATION OF THE HAASNOOT EQUATION

Empirical group eletronegativities (substituent parameters lambda(l)), valid for (3)J(HH) in saturated H-C-C-H fragments, were derived from the coupling to methyl in substituted ethanes and isopropyl derivative s according to the equation [(3)J(HH)] = 7.660 - 0.596(lambda(1) + lam bda(2)) - 0.419(lambda(1) lambda(2)) In contrast to earlier work, it w as found advantageous to differentiate between the lambda l, values of hydrogen acting as substituent in CH3 as compared with H in CH2. Spec ial attention was paid to solvent effects, in particular the influence of D2O, on the vicinal couplings and thus on lambda(l). The previousl y derived lambda(l) values remain valid in all common organic solvents but a special effect of D2O on lambda is manifest in cases where the alpha-substituent carries one or two non-conjugated lone pairs of elec trons that readily act as hydrogen bond accepters: Delta lambda = -0.1 1 +/- 0.03 for NH2, NHR, NR(2), OH, OR, R = alkyl. Protonation of NH2 to give NH3+ lowers lambda(l) by 0.28 units. The lambda(1) values for the nucleic acid bases (Ade, Gua, Ura, Thy, Cyt), as determined from t he N-isopropyl derivatives, are 0.56 +/- 0.01 irrespective of the solv ent. Secondary amides display similar values. The parameters of the Ha asnoot equation, originally derived with the aid of a Pauling-type ele ctronegativity scale, were reoptimized on the basis of the present lam bda(l) scale; the previous overall r.m.s. error of 0.48 Hz now drops t o 0.36 Hz and separate parameterization of H-C-C-H fragments with diff erent substitution patterns appears to be no longer necessary.