Empirical group electronegativities for vicinal NMR proton–proton couplings along a CC bond: Solvent effects and reparameterization of the Haasnoot equation
✍ Scribed by Cornelis Altona; Robert Francke; Rudy de Haan; Johannes H. Ippel; Godefridus J. Daalmans; Aldert J. A. Westra Hoekzema; John van Wijk
- Publisher
- John Wiley and Sons
- Year
- 1994
- Tongue
- English
- Weight
- 891 KB
- Volume
- 32
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
Abstract
Empirical group electronegativities (substituent parameters λ~i~), valid for ^3^J(HH) in saturated HCCH fragments, were derived from the coupling to methyl in substituted ethanes and isopropyl derivatives according to the equation
In contrast to earlier work, it was found advantageous to differentiate between the λ~i~ values of hydrogen acting as substituent in CH~3~ as compared with H in CH~2~. Special attention was paid to solvent effects, in particular the influence of D~2~O, on the vicinal couplings and thus on λ~i~. The previously derived λ~i~ values remain valid in all common organic solvents but a special effect of D~2~O on λ is manifest in cases where the α‐substituent carries one or two non‐conjugated lone pairs of electrons that readily act as hydrogen bond acceptors: Δλ= −0.11 ± 0.03 for NH~2~, NHR, NR~2~, OH, OR, R = alkyl. Protonation of NH~2~ to give NH~3~^+^ lowers λ~i~ by 0.28 units. The λ~i~ values for the nucleic acid bases (Ade, Gua, Ura, Thy, Cyt), as determined from the N‐isopropyl derivatives, are 0.56 ± 0.01 irrespective of the solvent. Secondary amides display similar values. The parameters of the Haasnoot equation, originally derived with the aid of a Pauling‐type electronegativity scale, were reoptimized on the basis of the present λ~i~ scale; the previous overall r.m.s. error of 0.48 Hz now drops to 0.36 Hz and separate parameterization of HCCH fragments with different substitution patterns appears to be no longer necessary.