Heteronudear

Spin-Edio Production in a Heteronudear AX Spin System... 

Kessler, H., Griesinger, C., Wagner, K. Peptide conformations. 42. Conformation of side chains in peptides using heteronudear coupling constants obtained by two-dimensional NMR spectroscopy. J. Am. Chem. Soc. 1987, 109, 6927-6933. 

Kessler, H., Griesinger, C., Zarbock, J., Loosli, H. R. Assignment of carbonyl carbons and sequence analysis in peptides by heteronudear shift correlation via small coupling constants with broad-band decoupling in ft (COLOC)./. Magn. Reson. 1984, 57, 331-336. 

Table 2.8 Mean Bond Enthalpies (kJ mol ) Heteronudear Bonds...

FIGURE 3.9 Molecular orbital diagrams for some heteronudear molecules and ions of second-row elements. 

In addition to the homonudear molecules, the elements of the second period form numerous important and interesting heteronudear species, both neutral molecules and diatomic ions. The molecular orbital diagrams for several of these species are shown in Figure 3.9. Keep in mind that the energies of the molecular orbitals having the same designations are not equal for these species. The diagrams are only qualitatively correct. 

It is interesting to note that both CO and CN are isoelectronic with the N2 molecule. That is, they have the same number and arrangement of electrons as the N2 molecule. However, as we will see later, these species are quite different from N2 in their chemical behavior. The properties of many homonudear and heteronudear molecules and ions are presented in Table 3.1. 

Miura, T., Klaus, W., Gsell, B., Miyamoto, C., and Senn, H. Characterization of the binding interface between ubiquitin and class 1 human ubiquitin-conjugating enzyme 2b by multidimensional heteronudear NMR spectroscopy in solution./. Mol. Biol. 1999, 290, 213-28. 

For a homonuclear diatomic molecule, the bond is purely covalent (a, = 0, ac = 1) whereas fora heteronudear diatomic molecule the bond shows mixed covalent-ionic character (a 0, ac 0). In the limit as the separation between the atomic energy levels on the two atoms becomes very large the bond becomes purely ionic (at = 1, ac = 0). 

The experiment is used to separate chemical shifts and J-couplings for homo- and heteronudear systems. In simple cases the chemical shifts and J-couplings may be directly obtained from the 2D spectrum by inspection. For severely overlapped first-order spectra or strongly coupled spin systems the estimated parameters obtained from the spectrum may be used as starting values in a computer assisted spectral analysis as outlined in Modern Spectral Analysis (Volume 3). 

Heteronudear shift correlation spectroscopy is a 2D technique that can be used to determine the connectivity of H and C nuclei (or other heteronuclei), formally bonded together through one or more chemical bonds. The corresponding experiments make use of either the large or the smaller long-range "Jv, couplings for polarization transfer. 

A variety of sequences exist, which differ with respect to the detected interaction ( J, or Jx ) and the mode of detection ( C or H detected, magnitude or phased mode, phase cycling or gradients for coherence selection). In view of the reduced sensitivity of heteronudear experiments with respect to homonuclear COSY experiments and the steadily decreasing sample amounts submitted for NMR experiments, there is no doubt that the inverse ( H) detected, gradient enhanced experiments are currently the best methods to apply. However on older type spectrometers, not equipped for inverse detection the old-fashioned direct C detected experiments are still in use. 

Molecular orbitals in heteronudear diatomic molecules. 167-175 in homonudear diatomic molecules, 160-166 of metallocenes. 670-673 in octahedral complexes, 414-418... 

F g. 5 3 Heteronudear molecule. A B, with large electronegativity difference. Covalency is insignificant (he bond is essentially ionic. 

H and 13C NMR spectroscopy have been used to study successive formation of mononuclear and dinuclear complexes with [SJcryptands.483 A heteronudear (Ag+,Pb2+) complex of (77) was described. In the 13C spectra, significant differences were observed for the carbon atoms adjacent to the bridgehead nitrogen atoms, such that the dinuclear silver complex and dinuclear lead complex could be readily distinguished from the heteronudear spedes. The fact... 

Heteronudear Cluster Chemistry of Copper, Silver, and Gold... 

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