Coupling-constant criterion

It is understandable that in the early days the x/rr distinction was neglected, because no general method of distinguishing between isomeric N - and N -substituted structures in histidine derivatives was available. It is still the case that this distinction is nontrivial, but the problem has been solved in specific cases by crystallography, chemical degradation, or nuclear Overhauser effect measurements, and no exception to the cross-ring coupling constant criterion of Matthews and Rapoport ° is known. 

Magnetically equivalent nuclei must be chemically equivalent and hence must have the same chemical shift that is, they must be isochronous. Nonequivalence resulting from unequal couplings to a single partner nucleus sometimes is referred to as magnetic nonequivalence by the coupling-constant criterion. The term is intended to provide a distinction from chemically nonequivalent nuclei, which have different chemical shifts and illustrate magnetic nonequivalence by the chemical-shift criterion. 

Characterize the indicated protons as (1) homotopic, enantiotopic, or diastereotopic and (2) magnetically equivalent or nonequiva lent (by the coupling-constant criterion). In parts (h) and (i), the Cr(CO)3 ligand remains on one side of the benzene ring. 

Hg and Hb in this system are, therefore, chemically distinct and diastereotopic and would be expected to be anisochronous in either achiral or racemic media. Since they constitute an AB or AX system and are magnetically nonequivalent by the chemical-shift criterion, any considerations of magnetic equivalence by the coupling-constant criterion are not applicable. 

DSO term is very small. However, as we will see latter, the DSO contribution is significant for certain coupling constants and cannot be discarded. Note that our criterion to estimate the order of magnitude of the individual terms is based on an electron in a Bohr orbit of the hydrogen atom. On some occasions this estimate may not give a good indication of the actual magnitude. 

Fattorusso and co-workers identified a component of wormwood called artar-borol. Correlation spectroscopy (COSY) and rotating frame nuclear overhauser effect spectroscopy (ROESY) experiments allowed for deduction of four possible diastereomeric structures of artarborol, 2-5. Low energy conformers of 11-14 were obtained through a molecular mechanics (MM) search. These conformers were screened to identify those having a dihedral angle of around 90 for the C-8 and C-9 protons due to a low coupling constant between these protons. Only conformers of 11 and 13 satisfied this criterion. Next, five low energy conformers, two... 

If R = R2 = H, the appearance of H, Hb, H, and H2 depends upon (i) the relative populations of the three conformers (aia b tmd c), (ii) the four coupling con.stants [ /H(a)H(i) H(a)H(2). H(b)H(i). and 7H(b)H(2)]> and (iii) the rate of rotation about the carbon-carbon bond of HyHbYC-CH H2X. Since the bond rotational speed is almost always sufficiently rapid to average chemical-shift contributions (see earlier), the third criterion is not considered further with respect to the averaging of coupling-constant contributions. 

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