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

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 4 + C cos 24 + . . . , 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 GO.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 l,l-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 A ' , valid for 'J(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,l-di-

Relationship between proton—proton NMR coupling constants and substituent electronegativities. IV—An extended karplus equation accounting for interactions between substituents and its application to coupling constant data calculated by the Extended Hückel method

“…It should also be noted that the aforementioned invariance property does not apply to Eqns (4) and (5), which means that the choice of reference compound (ethane) is by no means immaterial. The use of the absolute value I Axi I in Eqns (4) and (5) is one possible way to ensure that substituents which differ in the sign of their electronegativity difference, Axi, will display the opposite asymmetric behaviour [in Eqn (5) represented by the sin (24) term] with respect to the reference compound ethane. In Eqn (2) the assumed strict linear Ax dependence of the sine terms takes care of this aspect.…”

An NMR investigation of the conformational equilibria of 2‐(2′‐pyridyl)ethylphosphonic acid in several solvents

“…Altona electronegativites k for the pyridine and phosphonic acid substituents were derived from the coupling constants of 2-ethylpyridine (Aldrich) and ethylphosphonic acid (Aldrich) using the Altona procedure. 5,6 These Altona electronegativities were found to vary somewhat with pH in water; consequently, different values were used for cationic, neutral, and anionic species of 1, even though small variations in the k values did not cause signi®cant changes in calculated coupling constants.…”

“…The proportions of gauche and trans conformers were estimated with the aid of expected couplings calculated by the Altona-Haasnoot equation, 5,6 which relates predicted vicinal coupling constants to dihedral angles and empirical substituent electronegativities. Perfectly staggered rotational angles h of 60°and 180°were assumed for the gauche and trans conformers throughout, although that assumption is arguable, because it is clear with monoanionic 1,4-butanedioic acid that intramolecular hydrogen bonding results in increasing the rotational angle h 2,3 (also unpublished research by D.R.…”

“…1). The percent gauche was calculated for each ionic species from the solution with the highest concentration of that ionic species with the aid of the measured vicinal protonÀproton coupling constants of the methylene hydrogens and the Altona-Haasnoot procedure 5,6 for estimating the coupling constants of the gauche and trans conformations.…”

Determination of rotamer populations and related parameters from NMR coupling constants: a critical review

“…To consider the substituent electronegativity more precisely, they defined a group electronegativity: (8) where ∆ξ group is not a real group electronegativity because it takes into account the electronegativity effect of β substituents but it ignores the hybridization of the α atom and the effects from beyond the β position. To further improve the precision of Haasnoot's equation, Altona et al derived empirical group electronegativities (substituent parameters) from the coupling constants of monosubstituted and 1,1-disubstituted ethanes [49] and reparametrized Eq. (8) [50].…”