Carbon coupling longer-range

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

Using HMBC, the longer range connectivities are detected from a particular H to the carbon atom(s) either two bonds away (H-C-C coupling) or three bonds away (H-C-C-C coupling), so that,... 

Key experiments useful for substructure determination by NMR include the DEPT sequence (c.. Figs. 2.44-2.46) for analysis of CH multiplicities, as well as the two-dimensional CH correlation for identification of all CH bonds (e.g. Fig. 2.55 and Table 2.2) and localization of individual proton shifts. If, in addition, vicinal and longer-range proton-proton coupling relationships are known, all CH substructures of the sample molecule can be derived. Classical identification of homonuclear proton coupling relationships involves homonuclear proton decoupling. A two-dimensional proton-proton shift correlation would be an alternative and the complementary experiment to carbon-proton shift correlation. Several methods exist [68], Of those, the COSTsequence abbreviated from Correlation spectroscopy [69] is illustrated in Fig. 2.56. 

Longer-Range Carbon-Proton Couplings The Jcul-hth Ratio... 

Table 3.8. Longer-Range Carbon-Proton Coupling Constants in Relation to Comparable Proton-Proton Couplings.

Compound Formula 2 Jcc ( z) Table 3.11 (b). Longer-Range Carbon-Carbon Coupling Constants 2JCC and 3Jqc, Data from Ref. [133 a],... 

Longer-Range Carbon-Carbon Couplings (2JCC, 3/cc)... 

Table 4.51. One-Bond (J and Longer-range (2J, 3J, 4J) Carbon-Phosphorus Coupling Constants of Representative Phosphines [372], Phosphonium Salts [365], Phosphine oxides [366, 372, 373], Phosphonium Ylides [365, 373], Phosphonates [368, 372], Phosphites [374], and Phosphates [375] (in Hz). 

Several techniques have been developed that enable the number of hydrogens attached to the carbon to be determined. An older technique, called off-resonance decoupling, allows hydrogens and carbons that are directly bonded to couple but removes any longer-range coupling. In an off-resonance decoupled spectrum, a CH3 appears as... 

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