Trans coupling constant

Likewise, the vicinal fluorines in l-chloro-l,2-difluoroethenylben-zene (Scheme 2.3) would appear as doublets due to the three-bond F—F coupling between the two fluorine nuclei. Note that the trans coupling constant is much larger (127 Hz) than the respective cis coupling constant (12 Hz). (Note also that the cis-vicinal fluorines deshield each other significantly, relative to the tram-vicinal pair.)... 

The chemical shifts for hydrogens and methyl groups at C-4 of 5-hydroxy-and 5-amino-A2-1,2,3-triazolines depend on the orientation relative to the hetero substituent at C-5. This has been extensively used for assignment of relative configurations at C-4 and C-5 of variously substituted A2-tria-zolines.216,259 lH-NMR spectra show that 5-alkoxy- and 5-hydroxy-A2-1,2,3-triazolines prefer an envelope conformation218 (63) with the hetero substituent at C-5 pseudoaxial at the flap and the N-l substituent pseudoequatorial, probably because of the anomeric effect. The cis and trans coupling constants in the 5-amino-, 5-hydroxy-, and 5-aIkoxy-A2-l,2,3-triazolines are very constant, being 7.0-9.8 and 2.0-3.4 Hz, respectively.218... 

Compounds 1-4 were characterized by conventional spectroscopic techniques. For the stilbene compounds 1-3, the trans conformation of the double bond was confirmed by XH NMR analysis. For example, in the XH NMR spectrum of 1, the olefinic protons were detected as a doublet of doublets with resonances at 7.23 and 7.01 ppm, with a trans coupling constant of 16.3 Hz. In addition, the 13C NMR spectra of the stilbene compounds indicated the presence of a single isomer that was consistent with the desired structures. The UV/visible spectra in tetrahydrofuran solution showed a X value for 1-3 at 378 nm (e 2.6 x 10 ) and for 4 at 490 nm (e 3. ax 101 ). 

Proton Hb is farther from the deshielding influence of the phenyl group, giving rise to the multiplet centered at 8 5.65 in the styrene NMR spectrum. Hb is also split by two nonequivalent protons It is split by Ha with a trans coupling constant... 

Subsequently, the coupling constants of twenty-two monosubstituted cyclopropanes were determined with great precision though some of the substituents are rather similar. A small sample of the data is given in Table 4. From the data a plot of cis and trans coupling constants for the J -protons shows quite a good correlation. 

The same order holds when the vicinal coupling constants are between a proton and fluorine. This is shown in the cis coupling constant 3/HF between -4 and 20 Hz in molecules such as 46, and the corresponding trans coupling constants VHF between 20 and 100 Hz in, for example, 47 and related molecules. 

Hz is a typical value for an olefinic trans coupling constant and identifies the formation of the corresponding (i )-alkene by trans-hydrogenation of the substrate. 

The relationship between the magnitudes of Jpj (gem) and Jpj (cis and trans) coupling constants is similar to that observed in coupling constants. Jp (trans) is frequently larger than Jp (cis), but Jp (gem) can be larger than Jp (trans). The electron density on phosphorus has a pronounced effect on Jp (gem) and Jp (cis). Coordination to a transition metal markedly increases both, whereas electronegative substituents on phosphorus effect only Jp (cis) in a variable manner. The effect on Jpjj (trans) is small, but nonetheless significant. 

A simpler case than benzene, which has chemical equivalence (due to symmetry) but not magnetic equivalence, is 1,1-difluoroethene. Both hydrogens couple to the fluorines ( F, spin = ) however, the two hydrogens are not magnetically equivalent, because Ha and Hb do not couple to Fa with the same coupling constant (Vhf)- One of these couplings is cis CJds) and the other is trans CJtrans)- In Table 5.4 it was shown that cis and trans coupling constants in alkenes were different in... 

The chemical shift of H is 56.6 because it is deshielded by both the vinyl group and the aromatic ring. is coupled to H with a typical trans coupling constant. 

Differences in the cis and trans coupling constants can also be taken as a manifestation of the general Karplus relationships. These differences are commonly used as indicators of stereochemistry because, for the same combination of substituents, is always greater than (Figure 12.72). 

Figure 12.72 Illustrative differences in the cis and trans coupling constants.

Table 7.4 lists some representative three-bond conpling constants. Notice that in the alkenes the trans coupling constant is always larger than the cis coupling constant. Spin-spin coupUng in... 

---