Vicinal protons

We often see splitting patterns in which the intensities of the individual peaks do not match those given in Table 13 2 but are distorted in that the signals for coupled protons lean toward each other This leaning is a general phenomenon but is most easily illus trated for the case of two nonequivalent vicinal protons as shown m Figure 13 18... 

Useful compilations of NMRspectra are , . The H and NMR chemical shifts and the geminal and vicinal proton-proton coupling constants for oxirane and other heterocycles are given in a very readily compared manner,... 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

The couplings of vicinal protons in 1,2-disubstituted alkenes lie in the range 6-12 Hz for cis protons (dihedral angle 0°) and 12-17 Hz for trans protons (dihedral angle 180°), thus also following the Karplus-Conroy equation. Typical examples are the alkene proton AB systems of coumarin (16a, cis) and tra 5-cinnamic acid (16b), and of the cis-trans isomers 17a and b of ethyl isopente-nyl ether, in addition to those in problems 3, 4, 8, 11, 13 and 38. 

The coupling constant of the aldehyde doublet 7.8 Hz) is repeated in the doublet of doublets signal at Sh = 6.3. Its larger splitting of 15.6 Hz is observed also in the doublet at Sh = 7.3 and indicates a CC double bond with a trans configuration of the vicinal protons. 

The 8 Hz coupling indicates a proton in the y-position (B) the 5 Hz coupling locates a vicinal proton in position a (C), the additional 0.9 Hz coupling locates the remaining proton in position a (D) and thereby the p-position of the substituent. 

The relative configurations of vicinal protons follow from the characteristic values of their coupling constants. Thus 16.1 Hz confirms the trans relationship of the protons on C-8 and C-9, 10.8 Hz confirms the cis relationship of the protons on C-6 and C-1. The 2.0 Hz coupling is common to the oxirane protons at = 3.00 and i.27 this value fixes the trans relationship of the protons at C-4 and C-5 following a comparison with the corresponding coupling in the methyloxirane (2.6 Hz). The anti relationship of the protons A-H and h-H can be recognised from their 8.7 Hz coup-... 

The multiplets and coupling constants of the axial) protons at = 3.15, 3.50 and 4.08 moreover confirm the equatorial positions of all three OH groups, as can be seen in formula B. Here the couplings from 10.0 to 11.5 Hz, respectively, identify vicinal protons in diaxial configurations, whilst values of 4.5 and 5.0 Hz, respectively, are typical for axial-equatorial relationships. As the multiplets show, the protons at 5 = 3.50 and 4.08 couple with two axial and one equatorial proton (triplet of doublets) respectively, whereas the proton at = 3.15 couples with one axial and one equatorial proton (doublet of doublets). 

You will find it revealing to construct a splitting diagram similar to that of Figure 13.20 for the case in which the cis and trans H—C=C—H coupling constants are equal. Under those circumstances the four-line pattern simplifies to a triplet, as it should for a proton equally coupled to two vicinal protons. 

The coupling constant, J, between vicinal protons varies with dihedral angle, 0. The relationship between J and 0 is given by the Karplus equation . 

COSY and HETCOR experiments are extremely useful in the structure elucidation of complex organic molecules. The geminal and vicinal protons and their one-bond C-H connectivities are first identified from the HETCOR spectrum, and then the geminal couplings are eliminated from the COSYspectrum, leaving vicinal connectivities. By careful interpretation of the COSY and the one-bond HETCOR spectra, it is then possible to obtain information about the carbon-carbon connectivities of the protonated carbons ( pseudo-INADEQUATE information). In this way the carbon-carbon connectivity information of protonated carbons is obtainable through a combination of COSY and HETCOR experiments. 

The first step of the structure refinement is the appHcation of distance geometry (DG) calculations which do not use an energy function but only experimentally derived distances and restraints which follow directly from the constitution, the so-caUed holonomic constraints. Those constraints are, for example, distances between geminal protons, which normally are in the range between 1.7 and 1.8 A, or the distance between vicinal protons, which can not exceed 3.1 A when protons are in anti-periplanar orientation. 

Reduction is probably stepwise when 0 = 90°. (The plot in fact resembles very closely in form the familiar Karplus plot of the magnitude of vicinal proton-proton coupling constants as a function of 0 48>. The half-wave potentials of 43 and 44 are representative the half-wave potential of a related monohalide (45) is given for reference 46>. These investigators did not examine the products of electrochemical reduction of the vicinal dihalides which they studied. If reduction is concerted, the products should... 

Figure 6.2 The Karplus curve - for relating the observed splitting between vicinal protons to their dihedral...

The related configurations of stereocenters in substituted cyclic nitronates can be determined by analyzing the spin—spin coupling constants between the vicinal protons in the stereoisomer discussed (Chart 3.7) (276). If needed, the results of this analysis are supplemented by special NOE experiments. 

In Figure 1-20, the protected amino acids 56a and 56b are synthetic precursors of the two diastereomers of aminostatine,69 and they can be distinguished by the NOE spectra. The spatial interaction between H-2 and H-3 causes the difference in the NOE in the cyclized derivatives 56a and 56b. For compound 56a, no NOE is observed between these two vicinal protons, while for 56b a significant NOE between H-3 and H-2 can be observed (Fig. 1-20). 

Vicinal protons are those which are separated by three bonds and in such compounds the value of J varies with the dihedral angle. 

For vicinal protons (/) the values of J are always positive and for olefines ( ) J trans > Jcis. 

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