Coupled protons constants

Vicinal coupling constants Jhh indicate very clearly the relative configuration of the coupling protons. Their contribution depends, according to the Karplus-Conroy equation... 

For the stable conformers 13a-c of a substituted ethane the vicinal HH coupling constants 3Hz for syn-protons and 15 Hz for anti-protons can be derived from Fig. 2.18. If there is rotation around the C-C single bond, the coupling protons pass through the syn configuration twice and the anti configuration once. 

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

The connectivities found in the HH COSY diagram reveal two partial stuctures A and B according to Table 44.1. These are supported by the identical proton-proton coupling constants of the coupling protons. 

FIGURE 13.18 The appearance of the splitting pattern of two coupled protons depends on their coupling constant J and the chemical shift difference Av between them. As the ratio Av/J decreases, the doublets become increasingly distorted. When the two protons have the same chemical shift, no splitting is observed. 

The first step was found to be a fast pre-equilibrium (Scheme 12-8). The dependence of the measured azo coupling rate constants on the acidity function and the effect of electron-withdrawing substituents in the benzenediazo methyl ether resulting in reduced rate constants are consistent with a mechanism in which the slow step is a first-order dissociation of the protonated diazo ether to give the diazonium ion (Scheme 12-9). The azo coupling proper (Scheme 12-10) is faster than the dissociation, since the overall rate constant is found to be independent of the naphthol con-... 

First order multiplet First order multiplets are those in which the difference in chemical shifts of the coupled protons, in hertz (Hz), divided by the coupling constant between them is about 8 or more (Av/y>8). These multiplets have a symmetrical disposition of the lines about their midpoints. The distance between the two outermost peaks of a first order multiplet is the sum of each of the coupling constants. 

These results suggest that the signals arise from dipole-dipole coupled protons. Kreis et al. confirmed this finding by measurements using one-dimensional zero- and double-quantum filtering, two-dimensional J-resolved spectroscopy, two-dimensional constant time COSY and longitudinal order separation... 

In aromatic systems, the coupling constant between protons attached to an aromatic ring is characteristic of the relative position of the coupled protons i.e. whether they are ortho, meta or para. 

Similarly in condensed polynuclear aromatic compounds and heterocyclic compounds, the magnitude of the coupling constants between protons in the aromatic rings reflects the relative position of the coupled protons. 

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