Heteronuclear interactions

The HMBC spectrum of podophyllotoxin is shown. The cross-peaks in the HMBC spectrum represent long-range heteronuclear H/ C interactions within the same substructure or between different substructures. Interpretation should start with a readily assignable carbon (or proton), and then you identify the proton/s (or carbon/s) with which it has coupling interactions. Then proceed from these protons, and look for the carbon two, three, or, occasionally, four bonds away. One-bond heteronuclear interactions may also appear in HMBC spectrum. 

The one-bond hetero-COSYspectrum of 7-hydroxyfrullanoIide exhibits interactions for all nine protonated carbons. The most downfield crosspeaks, K and L, represent one-bond heteronuclear correlations of the two vinylic exomethylenic protons resonating at 8 5.71 and 6.06 with the C-13 carbon (8 120.5). The C-6a proton, which resonates downtield at 8 4.97 due to the directly bonded oxygen atom, displays correlation with the carbon resonating at 8 80.9 (cross-peak D). Cross-peaks G and M represent h interactions of the C-1 methylene protons (8 1.33 and 1.31, respectively) with C-1 (8 38.1). Similarly, cross-peaks E and F display heteronuclear interactions of the C-8 methylenic protons (8 1.48 and 1.72) with C-8 (8 30.7), while cross-peak C couplings of C-3 methylene protons at 8 1.97 and 1.99 with C-3 (8 32.5). Couplings between the C-1 methylene protons and C-1 (8 38.1) can be inferred from cross-peak A, though in this case both the C-1 a and protons resonate very close to each other (i.e., 8 1.31 and 1.33). Cross-peak C is due to C-9 methylene, while cross-peak I represents the C-15 methyl. The heteronuclear interactions between the most upheld C-2 methy-... 

The most downfield cross-peaks, V-Y, are due to heteronuclear couplings of the aromadc or vinylic protons and carbons. For instance, cross-peak Y represents heteronuclear interaction between the C-1 vinylic proton (8 5.56) and a carbon resonating at 8 134.0 (C-1). The downfield cross-peaks, V and W, are due to the heteronuclear correlations of the ortho and meta protons (8 7.34 and 7.71) in the aromatic moiety with the carbons resonating at 8 128.3 and 126.9, respectively. The remaining cross-peak X is due to the one-bond correlation of the C-4 aromatic proton (8 7.42) with the C-4 carbon appearing at 8 131.4. The cross-peak U displays direct H/ C connectivity between the carbon at 8 77.9 (C-6) and C-6 methine proton (8 4.70). The crosspeak T is due to the one-bond heteronuclear correlation of carbon... 

In the case of pharmaceutical solids that are dominated by carbon and proton nuclei, the dipole-dipole interactions may be simplified. The carbon and proton nuclei may be perceived as dilute and abundant based upon then-isotopic natural abundance, respectively (Table 1). Homonuclear 13C—13C dipolar interactions essentially do not exist because of the low concentration of 13C nuclei (natural abundance of 1.1%). On the other hand, H—13C dipolar interactions contribute significantly to the broad resonances, but this heteronuclear interaction may be removed through simple high-power proton decoupling fields, similar to solution-phase techniques. 

Since dipolar interaction is a distance-dependent interaction, the heteronuclear interaction between spins 1 and S can be exploited to get information about the distance between these nuclei. Heteronuclear dipolar interactions that are averaged by magic angle spiiming can be reintroduced by suitable dephasing pulses that are synchronized with the sample spinning. It is a difference spectroscopy... 

Hence, originally designed to eliminate dipolar interactions, MAS has the main effect on the removal of the chemical shift anisotropy, which is rather pronounced in 13C NMR spectroscopy. Since heteronuclear interactions could be viewed as a perturbation of abundant spins (I) on the energy states of rare spin system (S), a more convenient way of reducing HD to zero is high power decoupling of the I nuclei < u = 0). 

In point-charge simulation this electronic rearrangement is of no immediate consequence except for the assumption of a reduced interatomic distance, which is the parameter needed to calculate increased dissociation energies. However, in Heitler-London calculation it is necessary to compensate for the modified valence density, as was done for heteronuclear interactions. The closer approach between the nuclei, and the consequent increase in calculated dissociation energy, is assumed to result from screening of the nuclear repulsion by the excess valence density. Computationally this assumption is convenient and effective. 

This is only rigorous for an ideal covalent bond. For heteronuclear interactions, one intuitively recognises that the more electronegative element will attract a greater share of the overlap density. The Mulliken procedure ignores... 

Unusually large spin-spin couplings between F and Pt over seven bonds, /FPt = 2.9Hz (3.1Hz), and between F and P over eight bonds, /fp= 11-8 Hz (13.2Hz), have been observed by Zenkina et al. for two analogous platinum stilbene- and stilbazole-based complexes, whose structures are shown in Fig. 8 below. These heteronuclear interactions are independent of temperature, solvent and concentration, which is indicative of through-bond spin-spin coupling. 

Let us consider two /-coupled nuclear spins denoted as I and S. The simplest case to examine is that of heteronuclear interaction ( C and nuclei in the chloroform molecule, for example), so that the magnitude of the /-coupling is much smaller than the difference between the resonant frequencies of the two nuclei. This is named the AX system in the... 

By taking electronegativity differences into account, dissociation energies for heteronuclear interactions are calculated as... 

Exhaustive testing has shown the formula to hold for all heteronuclear interactions... 

Fig. 1 Covalence curves in dimensionless units. Homonuclear interactions are described by the curve BFC and heteronuclear interactions map into the crescent CFA...

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