Vicinal and Geminal Coupling

The absorptions between about 1585 Hz and 1525 Hz contain the peaks from both the H-8 and H-10 protons. At the left side, there is an 18 Hz doublet representing one of the H-8 protons coupled tram across the double bond to H-7. These doublet peaks are at 1585 Hz and 1567 Hz. 

In the (1R, 3R) compound, Ha and Hb are not magnetic equivalent since they do not identically couple to Ht or to Hd Hc and Hd also are not magnetic equivalent since they do not identically couple to Ha or Hb. But since the J values approximately average out by free rotation, the spin system is treated as A2X2 and the spectrum would show two triplets. In the (IS, 3R) 

In the above compound, both chiral centers have the same substituents. In a compound with two chiral centers in which such is not the case, there is no symmetry element none of the protons would be interchangeable. 

Coupling between protons on vicinal carbon atoms depends primarily on the dihedral angle between the H—C—C and the C—C —H planes. This angle can be visualized by an end-on view (Newman projection) of the bond between the vicinal carbon atoms 

FIGURE 3.57 The vicinal Karplus correlation. Relationship between dihedral angle (cj ) and coupling constant for vicinal protons. 

Most of the protons can be assigned on the basis of chemical shifts, integration ratios, and coupling patterns as follows. 

The entry points for analysis of the spectrum are protons that have distinctive chemical shifts and/or couplings, such as the methyl groups just discussed. The one-proton multiplet at 8 3.82 must be the deshielded Z/COH proton that is coupled to two sets of diastereotopic protons. If all couplings were equal, the multiplicity would be 5 obviously they are not equal. 

The conjugated olefinic protons are distinctively at the deshielded end of the spectrum. The isolated =CH2 group is at 8 5.08. The CH = CH2 moiety accounts for the remainder of the patterns with the CH= proton centered at about 8 6.40 and the =CH2 protons centered at about 8 5.25 and 8 5.14. This is a slightly distorted AMX system in which the AM coupling is barely detectable at the expansion shown (see Section 4,14). 

At the shielded end of the spectrum, from right to left, we see the identified diastereotopic methyl groups, each of the H-3 diastereotopic protons, a mysterious two-proton multiplet at 8 1.8, and the individual H-5 protons, each of which consists of a doublet of doublets with different coupling constants. The mysterious absorption at 51.8 consists of the highly coupled H-2 proton superimposed on the broad OH absorption. 

The CH, CH2, and CH3 peaks thus identified are confirmed by the 13C/DEPT spectrum (Section 5.5). Finally, the H and 13C peaks can be correlated (Chapter 6). 

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Scalar coupling Coupling interaction between nuclei transmitted through chemical bonds (vicinal and geminal couplings). 

Chemical shifts and coupling constants of substituted 1,2,3,4-tetra-hydroquinoxalines indicate that the heterocyclic ring in these derivatives is in the half-chair form.263 The variation of the cis vicinal and geminal couplings resulting from acylation on nitrogen indicates that the acylated derivatives have a slightly flattened half-chair conformation.263... 

COSY-p Simple and robust. Magnitude processing well suited to automated operation. Simplification of crosspeak structures reduces peak overlap. Vicinal and geminal couplings can be distinguished in some cases from tilt of peaks. Usually requires magnitude-mode presentation as phase-sensitive variant has mixed-phase lineshapes. 

The vicinal and geminal coupling constants J H, Jh s, and have been determined for many undistorted mono- and disubstituted olefins (184). Whereas Jh.h>s n many cases close to 10 Hz, J u is larger and reaches 19.1 Hz in ethylene (185). In terminal olefins 0-4 Hz and is thus much smaller than the vicinal coupling constants. Theory predicts that JH H (vicinal) in olefins should decrease with an increase of the H-C-C bond angle (186,187) and hence with an increase of s-character in the carbon hybrid orbitals. 

In view of the possible ambiguities which can attend conformational assignments based on vicinal coupling constants, it is fortunate that both long-range and geminal couplings each exhibit stereospecific dependences. 

Chemical shifts of magnetically active nuclei, such as [79MI171], Vicinal and geminal scalar H,H and H,C coupling constants Line widths of proton resonances of tertiary protons... 

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