Assignments of NMR signals

C-NMR, dipole moments, Kerr constants, and IR and UV spectroscopy have been employed. The assignment of NMR signals to the separate con-formers seems (78JCS(P2)99) to cause the misleading results obtained when C chemical shifts alone are used in conformational analysis of -acylazoles and N-acylindole. 

Decarboxylation is sometimes useful in structure investigations and in assignment of NMR signals. Chelidonic acid (488 R = H), which is widely distributed in plants, is doubly decarboxylated to pyran-4-one (487) on heating with copper powder. The monoester is decarboxylated to comanic acid ester (489). 

There are some situations where the assignment of NMR signals is a very difficult task in the absence of calculated chemical shifts. For instance, in the Mo complex... 

This paper reports the synthesis of 5-alkylsilylborazines by rhodium catalyzed hydroboration with A -trialkylborazines. The characterization of these molecules by NMR spectroscopy gives valuable facts about the regioselectivity and completeness of the reactions and should enable a more facile assignment of NMR signals from precursors of Si-B-N-C ceramics based on borazines and vinylsilanes as educts. 

In the case of copolymerization using catalysts that do not rearrange the siloxane chain and under non-equilibrium conditions a theoretical model can be developed to describe the microstructure. This same microstructure can be analysed by NMR of proton or silicon-29 nuclei. The concordance between theory and experiment enables confirmation of the correctness of either the theoretical mathematical model or the assignment of NMR signals. Quantitative information on the microstructure of copolymers as a function of time or at any conversion or for any comonomer composition can be obtained. For copolymerization under less precise conditions, the microstructure can be derived from the siloxane copolymer itself by direct observation of the magnetic resonance signals of the silicon-29 nuclei. 

Another serious problem in applying these methods is that unambiguous assignments of NMR signals to monomer sequences are, as yet, only available for a few systems. Moreover, assignments are complicated by the fact that the sensitivity of chemical shifts to tacticity may be equal or greater than their... 

Synthesis of Reference Compounds. Synthesis of THpCs were according to the methods described before (27, 22), and assignment of NMR signals was confirmed by GROESY, H-H COSY, HMQC, and HMBC experiments. 

The assignments of NMR signals for cytochalasin D (6) were made by single-frequency decoupling and PRFT, as for cytochalasin B (26) (Graf et al, 1974). 

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