Vicinal proton-proton coupling constants

Abstract

An equation for the prediction of the vicinal coupling constants aJHn in substituted HCCH fragments is formulated as a truncated Fourier series in the torsion angle q9 between the coupling hydrogens with coefficients expanded as a Taylor series in a substituent parameter 2. The different terms in this series have definite meanings: (1) the independent terms k. give the angular dependence of 3JHH in ethane, (2) the linear terms kni2i represent the effects upon 3Jan of the individual substituents, and (3) the cross terms k.ij 2i 2~ account for the effects of interactions between pairs of substituents. This interpretation is based on a model for the effect of the substituents that explains the Fourier coefficients as a sum of contributions from ethane, from individual substituents and from interactions between pairs of substituents. The model reduces to the equation when several simplifying assumptions are made. Otherwise, some refinements can be introduced in the equation. The invariances of the aJnn couplings under reflections and rotations of the nuclear coordinates enabled us to derive some relations between the 3Jnn couplings with different orientations of the substituents with respect to the coupled protons. Equivalent relations may also be established between the coefficients kni or k,i j with the consequent reduction in the number of terms to be included in the equations.

Vicinal proton-proton coupling constants

“…The remaining K x coefficients, however, cannot be measured. Therefore, the corresponding AK x effects should be assessed theoretically or by 0026 The empirical parametrization of an equation for predicting 3JHH couplings is complicated in practice [1]. The accurate determination of the individual effects of substituents AK x by this way seems to be impracticable.…”

“…A substituent effect model for predicting the vicinal proton proton coupling constants 3JHH (-J), which accounts for the dependence upon the dihedral angle ~b between the coupled protons and for the effect of the substituents attached to the HCCH fragment, was proposed in part I of this series [1]. The angular dependence of 3J~H is represented in this model as a truncated Fourier series in ~b (see equations (3) and (4) below) with coefficients K, (C, and S,) expressed as a sum of contributions from ethane, from individual effects of substituents and from interactions between substituents [1].…”

“…The angular dependence of 3J~H is represented in this model as a truncated Fourier series in ~b (see equations (3) and (4) below) with coefficients K, (C, and S,) expressed as a sum of contributions from ethane, from individual effects of substituents and from interactions between substituents [1]. After the angular dependence in ethane, the next factor that determines the magnitude of the 3JHH couplings is the individual effect of substituents which may be described by the terms AK x computed as the difference between the K x coefficients for monosubstituted ethanes CH3CH2X and the K ~ coefficients for ethane (see equation (5) below).…”

Determination of the Relative Configuration of a Five-Membered Lactone from Residual Dipolar Couplings

“…and/or angular restraints from 3 J coupling constants. [2][3][4] Information concerning interatomic angles can also be extracted on basis of the recently introduced projection angle restraints from cross-correlated relaxation.…”

“…[2][3][4] Information concerning interatomic angles can also be extracted on basis of the recently introduced projection angle restraints from cross-correlated relaxation. [5][6][7] Reasons why these conventional approaches may fail are manifold.…”