Complex homonuclear

Heteronuclear 3D experiments, using the chemical shift dispersion of or 15n, can also be used to edit a complex homonuclear 2D spectrum. In contrast to heteronuclear 3D experiments, the homonuclear 3D spectrum contains a larger number of cross peaks, which is a disadvantage, but provides multiple confirmations of the origin of the NOE interaction found in the homonuclear 2D spectrum. 

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). 

In sharp contrast to the stable [H2S. .SH2] radical cation, the isoelectron-ic neutral radicals [H2S.. SH] and [H2S. .C1] are very weakly-bound van der Waals complexes [125]. Furthermore, the unsymmetrical [H2S.. C1H] radical cation is less strongly bound than the symmetrical [H2S.. SH2] ion. The strength of these three-electron bonds was explained in terms of the overlap between the donor HOMO and radical SOMO. In a systematic study of a series of three-electron bonded radical cations [126], Clark has shown that the three-electron bond energy of [X.. Y] decreases exponentially with AIP, the difference between the ionisation potentials (IP) of X and Y. As a consequence, many of the known three-electron bonds are homonuclear, or at least involve two atoms of similar IP. 

Few examples of complex structures containing homonuclear Ge-Ge or Si-Si bonds have been described in the Hterature (see Fig. 14.1b). Isostructural ternary materials with the formula (A = Na, K M" =Si, Ge Q=S, Se, Te)... 

A few additional structural comparisons between the homonuclear gold and silver complexes and the mixed gold and silver complexes are of interest. In the dimer of the... 

Figure 18 Hypothetical CnHn—C2H2—C3H3 fragment within a more complex molecule, with heteronuclear coupling constants Vhici, 2Jmc2> Vh2ct> 21h2ci> respectively, and homonuclear coupling constants Vhih2, Vmm, respectively.

Heteronuclear species may also be prepared - the nmr studies indicate that a heteronuclear Ag(i)/Pb(n) species is produced in solution when a 1 1 mixture of the respective dinuclear Ag(i) and Pb(n) complexes is dissolved. However, the reaction is not quantitative and the heteronuclear complex exists in equilibrium with both of its homonuclear precursors. 

Fig. 10.19. IDR-HSQC-TOCSY spectrum of the complex marine polyether toxin brevetoxin-2 (7). The data were recorded overnight using a 500 pg sample of the toxin (MW = 895) dissolved in 30 pi of d6-benzene. The data were recorded at 600 MHz using an instrument equipped with a Nalorac 1.7 mm SMIDG probe. Direct responses are inverted and identified by red contours relayed responses are plotted in black. The IDR-HSQC-TOCSY data shown allows large contiguous protonated segments of the brevetoxin-2 structure to be assembled, with ether linkages established from either long-range connectivities in the HMBC spectrum and/or a homonuclear ROESY spectrum.

Fig. 11.15 Diagram showing the relative proximity of the two ligands Glp and S3P, which form a stable ternary complex with the enzyme EPSP synthase. The distance constraints were obtained from both homonuclear and heteronuclear dipolar couplings obtained using the REDOR and DRAMA pulse sequence, together with a model showing a...

Proceeding in the spirit above it seems reasonable to inquire why s is equal to the number of equivalent rotations, rather than to the total number of symmetry operations for the molecule of interest. Rotational partition functions of the diatomic molecule were discussed immediately above. It was pointed out that symmetry requirements mandate that homonuclear diatomics occupy rotational states with either even or odd values of the rotational quantum number J depending on the nuclear spin quantum number I. Heteronuclear diatomics populate both even and odd J states. Similar behaviors are expected for polyatomic molecules but the analysis of polyatomic rotational wave functions is far more complex than it is for diatomics. Moreover the spacing between polyatomic rotational energy levels is small compared to kT and classical analysis is appropriate. These factors appreciated there is little motivation to study the quantum rules applying to individual rotational states of polyatomic molecules. 

Bielawski et al. have developed Janus-head dicarbene ligands which are able to act as a bridge between two metal centers, thereby leading to dinuclear complexes of type 96 [58-60] (Fig. 32). More recently homonuclear bimetallic ruthenium(II) and iron(II) complexes 97 have been synthesized. It was hoped that the dicarbene ligand would interconnect the redox-active metal centers, but the... 

Many complexes with stable homonuclear diatomic ligands like Oj and N2 are well known and have been extensively studied. Only in recent years has the chemistry of metal complexes containing one or more coordinated homonuclear sulfur ligands, (with ra = 2 or n > 2), been developed systematically. Complexes with units can be obtained for many metals under a variety of conditions, and coordinated ligands exhibit an especially rich structural chemistry. 

Table 8.1 compiles the most common stable homonuclear neutral binary carbonyls. They can be prepared by direct carbonylation of pure metals or made from available metallic precursors by reductive carbonylation. However, most of them are commercially available. Moreover, heteronuclear carbonyl compounds are known, and they are usually prepared by reaction between homonuclear binary carbonyls or carbonyl-derivative complexes. 

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