Resolved Two-Dimensional 13C NMR Spectra

Since JCH coupling is only in operation during one t,/2 period in the J-modulated spin-echo sequence with gated proton decoupling, as drawn in Fig. 2.51, the second Fourier transformation provides only one half of the actual Jal magnitudes. Therefore, resolution of smaller couplings will be poor. 

J-resolved two-dimensional carbon-13 NMR spectra [60, 61] separate chemical shifts and coupling constants in the two-dimensional JCH, 5C plane. Their practical value can be estimated on looking at Fig. 2.52. In the one-dimensional 13C NMR spectrum of 

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Fig. 2.50. Generation of /-resolved two-dimensional 13C NMR spectra (a) flux diagram (b) /-modulation of Cl l doublets, CH2 triplets and CH3 quartets during evolution, vector diagrams in the x y plane and cosine curves described by the signal maxima (c) series of 13C NMR spectra of CHn groups with t1 dependent /-modulation of the signal amplitudes (d) series of /-resolved two-dimensional 13C NMR spectra formethine, methylene and methyl carbon atoms, stacked plots and contour plots (e).

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

Fig. 2.52.. /-Resolved two-dimensional 13C NMR spectra of biotin (100.6 MHz for 13C, 400.1 MHz for H 40 mg in 0.1 mol/ L aqueous sodium hydroxide measuring time 25 min transform time 12.5 min) stacked plot before (a) and after rotation (b) of the matrix by 90 " [62] (c) contour plot with one-dimensional 13C NMR spectra recorded with and without proton decoupling.

Figure 2.3. J-resolved two-dimensional 13C NMR spectra series of a-pinene (1) [in (CDafeCO, 25 °C, 50 MHz],...

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