Multiple homonuclear

While TipH is about the same for all protons throughout the sample as a result of spin diffusion from multiple homonuclear dipole-dipole couplings, 7 ipc and Tch are different for different parts of the molecule. They need to be known for a quantitative analysis of CP signals [Voel]. 

Figure 4 Signal loss in respect of multiple homonuclear couplings in the HSQC whenis presumed to be 5% of the Jch (see Equation (7)).

Because of the complexity of the polyether antibiotics tittle progress has been made in stmcture determination by the chemical degradation route. X-ray methods were the techniques most successfully applied for the early stmcture elucidations. Monensin, X206, lasalocid, lysocellin, and salinomycin were included in nineteen distinct polyether x-ray analyses reported in 1983 (190). Use of mass spectrometry (191), and H (192) and nmr (141) are also reviewed. More recently, innovative developments in these latter techniques have resulted in increased applications for stmcture determinations. Eor example, heteronuclear multiple bond connectivity (hmbc) and homonuclear Hartmann-Hahn spectroscopy were used to solve the stmcture of portimicin (14) (193). East atom bombardment mass spectrometry was used in solving the stmctures of maduramicin alpha and co-factors (58). 

Despite its very simple electronic configuration (Is ) hydrogen can, paradoxically, exist in over 50 different forms most of which have been well characterized. This multiplicity of forms arises firstly from the existence of atomic, molecular and ionized species in the gas phase H, H2, H+, H , H2" ", H3+. .., H11 + secondly, from the existence of three isotopes, jH, jH(D) and jH(T), and correspondingly of D, D2, HD, DT, etc. and, finally, from the existence of nuclear spin isomers for the homonuclear diatomic species. 

II. Homonuclear Multiple Bonding Involving the Heavier Main Group 3... 

HOMONUCLEAR MULTIPLE BONDING INVOLVING THE HEAVIER MAIN GROUP 3 ELEMENTS... 

The new developments in homonuclear bonding described in this section for A1 -> T1 have precedent in the synthesis of molecular species with boron-boron multiple bonds. In essence, it has been shown that tetraor-ganodiboron compounds38-40 can undergo two successive one-electron reductions9,41 to give multiple B—B bonds as shown by Eq. (1). 

The first-row homonuclear diatomic molecules A2 of main-group elements (A = B, C, N, O, F) exhibit a well-known diversity of ground-state multiplicities, bond lengths, and bond energies. Calculated potential-energy curves for low-lying singlet and triplet states of these species are pictured in Fig. 3.27 and summarized in Table 3.13 (with comparison experimental values). Because these homonuclear... 

Table 3.13. Calculated spin multiplicity, bond length Re, and dissociation energy De of first-row homonuclear diatomic molecules, with comparison experimental valued1 in parentheses...

Let us also briefly examine the corresponding behavior in second- and third-row homonuclear diatomics. Figures 3.31(a) and (b) display the calculated potential-energy curves for these species (ground-state multiplicities only) and Table 3.16 summarizes the equilibrium bond lengths and bond energies. 

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