The FIDS Principle

In the absence of a large associated coupling J(A,C), which is a requirement for the successful application of E. COSY or DQ/ZQ methodology, the FIDS (fitting of doublets from singlets) procedure can be applied. The basic principle of this experiment is outlined in Fig. 7.8. 

In a B,C spin system, the multiplet of spin B contains the /(B, C) coupling constant among other couplings. This coupling can be determined by comparison of two spectra a coupled spectrum in which the J(B, C) coupling evolves and a decoupled reference experiment in which the J(B,C) is removed. 

The procedure works when each detected spin B can be selectively decoupled from a spin C, although other couplings may be present. If this is the case, the spectra obtained in the coupled experiment and the reference experiment are identical except for the additional splitting due to the coupling of interest J(l , C). This splitting can be determined by convolution ( in Fig. 7.8) of the decoupled spectrum with a stick doublet to yield the coupled spectrum. 

A sequence suitable for measurement of J(H, P) and J(C, P) couplings is shown in Fig. 7.9a. The pulse sequence is a constant-time [13C, H]-HSQC (heteronuclear single-quantum correlation), in which 31P decoupling is applied in ot, in the first experiment and in co2 in the second. 

Traces through the spectrum along co2 are shown in Fig. 7.10 together with the fitting of the coupled to the decoupled spectrum after convolution by an in-phase stick doublet. The fit delivers the coupling constant with high precision. The sensitivity of this experiment is practically identical to that of the HSQC experiment since the splitting is normally smaller or in the order of the line widths. 

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