An equation utilizing empirically derived substituent constants for the prediction of vicinal coupling constants in substituted ethanes

Abstract

The equation is a cosine series in 8, the torsion angle between vicinal hydrogens. The feature which distinguishes this equation from similar equations is the inclusion of ASi terms, which describe the magnitude of each substituent's effect. These substituent constants have been defined from experimental data. An orientation effect which is dependent on the torsion angle(s) between substituent(s) and a vicinal hydrogen is included. Substituent constants have been defined for 39 groups, of which 15 have been experimentally determined herein. The parameters for the equation have been defined from 49 torsion angles in 19 conformationally rigid compounds. The torsion angles have been determined from x-ray crystal structure data and molecular mechanics calculations. The multiplicity of structures used to determine the substituent constants should allow for the application of this equation to a wide variety of structures.

Structure of 5-hydroxy-2,2-dimethyl-5-phenyl-2,5-dihydrofuran-3-carbonitrile

“…This conformation leads to some particularities in the JH NMR spectra. Despite its limitations (Karplus, 1963), the Karplus equation, which relates 3Jn_r~ coupling constants and dihedral angles, is a very valuable model for probing molecular geometries (Colucci, Jungk & Gandour, 1985). The structure assignment of (1), made at the same time by two-dimensional NMR techniques, has revealed some surprising vicinal coupling constants especially between HI6 and H10 (Jcalc = 3"8, Jobs = 4"7 Hz), H12 and H10 (Jcalc = 2"3, Jobs = 3 Hz) and H15 and HI0 (Jca~ --1.2, Jobs = 0Hz), which could be explained by the corresponding dihedral angles [H16--CI6--C13/H10--C13--C16 = 50 (2); H12~ C14--C13/HI0--C13--C14 = 61 (2); H15--C16--C13/H10---C13--C16 = 108 (2) The analysis establishes the structure of this reaction product.…”

The Dependence of Vicinal Proton-Proton Coupling Constants on Molecular Structure

“…The contri bution of the distance between coupling protons is of lesser importance, whereas the influence of the elec tronegativities of the substituents, bonded to a-C atom, is only of marginal statistical importance. Figure 1 illustrates how the coupling constants cal culated according to (7) correlate with the observed ones. It is obvious that the points are uniformly dis tributed over the entire range from 1 Hz to 13 Hz.…”

“…It is of interest to examine the importance of each term in (7). The order of contributions of the terms was determined by excluding each term, one by one, Fig.…”

“…In addition to this, all necessary data for its application are easy to determine. Although (7) does not include the depen dence of the vicinal proton-proton coupling constants on all existing conformational factors, it appears to be a successful and useful approximation for serving all stereochemical needs.…”

“…Unfortunately, these theoretical calculations were not straightforwardly applicable to large molecules, which are of importance in conformational analysis. This was the main reason that (1) was revised by various investigators [5][6][7][8]. Among these proposed modifica tions, there is Pachler's equation [5] (2)…”