Two-Dimensional CAHA —CXHX Correlation The RELAY Experiment

The COSY sequence for proton-proton shift correlation (HA — Hx relationships, Fig. 2.56) can be combined with a two-dimensional carbon-proton shift correlation (CaHa and CXHX bonds, Fig. 2.53). This experiment, referred to as two-dimensional relayed coherence transfer or RELAY [IQ], permits direct identification of CAHA —HXCX 

The AB and AX systems of all 13C —13C bonds appear in one spectrum when the INADEQUATE pulse sequence (Fig. 2.48) is applied. Complete interpretation usually becomes difficult in practice due to signal overlapping, isotope shifts and AB effects (Section 2.9.4). A separation of the individual 13C— 13C two-spin systems by means of a second dimension would be desirable. It is the frequency of the double quantum transfer (d e) in Fig. 2.48 which introduces a second dimension to the INADEQUATE experiment. This double quantum frequency vDQ characterizes each 13CA — I3CX bond, as it depends on the sum of the individual carbon shieldings vA and vx in addition to the frequency v0 of the transmitter pulse located in the center of the spectrum if quadrature detection is applied [69c, 71]  

To conclude, the second dimension is introduced if the switching time ti (Fig. 2.48) is incremented in a series of single experiments so as to reach all possible double quantum frequencies vDQ within a sample molecule by the reciprocals l/t1. Again, the acquired FID signals will depend on two variable times t1 and t2, respectively. A first Fourier transformation in the t2 domain generates 13C — 13C satellite spectra. The corresponding AB or AX type doublet pairs, however, are modulated by the individual double quantum frequencies which characterize each AB or AX pair. The second Fourier transformation in the tl domain liberates the double quantum frequency as the second dimension Maximum AB or AX 13C—13C subspectra are observed at the corresponding double quantum frequencies, so that each doublet appears with unique coordinates, 

Each pair of connected carbon atoms, CA — CB, is thus segregated according to its sum of chemical shifts, so that the actual magnitude of the carbon-carbon coupling constant is no longer required for identification of the bond. A straightforward method of tracing out the carbon skeleton of a compound is thus established. 

The two-dimensional INADEQUATE experiment also suffers very much from low sensitivity given by the low natural abundance of carbon-13 (about 10 2), so that only 0.01 % or 10-4 of all carbon-carbon bonds contribute to the satellite signals. In fact, the basic experiment can be modified in order to reduce the data matrix and to save measuring time [72], giving COSY-like square correlations as shown in Fig. 2.60. Nevertheless, the two-dimensional INADEQUATE experiment requires several hours of measuring 

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