Two-Dimensional Carbon-Proton Shift Correlation

A constant mixing period A follows evolution (Fig. 2.53). Its purpose is to transfer proton chemical shift information, as given by the phase angle q , to carbon-13 magnetization. A polarization transfer takes place. Therefore, the mixing time A is adjusted to an average value of all possible JCH coupling constants (usually 145 Hz), so that the half-periods will be 

It is an essential feature of the sequence (Fig. 2.53) that only the x components of the doublet vectors 1 and 2 will be rotated by the 90° pulse in (f). The magnitudes of these components depend on the phase angle cp, which is related to the proton shift 5H. To conclude, the extent of polarization transfer (Fig. 2.53 (f-g)) is a function of proton chemical shift. After the first Fourier transformation in the t2 domain, carbon-13 signals with modulated amplitudes will be obtained when ty is varied. Chemical shifts of the attached protons are the modulation frequencies. Therefore, a second Fourier transformation in the lL domain provides maximum signals located at the chemical shifts b H and 5C of the coupling proton and carbon-13 nuclei. 

The data flux of a two-dimensional carbon-proton shift correlation is similiar to that described in Fig. 2.50(a) for a. /-resolved 2D CMR experiment, with one difference Instead of carbon-proton couplings JCH, proton chemical shifts (iH are stored in the evolution time tl. Fourier transformation in the (2 domain thus yields a series of NMR spectra with carbon-13 signals modulated by the attached proton Larmor frequencies. A second Fourier transformation in the domain generates the dH, Sc matrix of a two-dimensional carbon-proton correlation. 

Useful modifications of the basic experiment include CH correlations with proton-proton couplings removed in the t ( 5H) domain [66] and long-range carbon-proton shift correlations with the mixing period in Fig. 2.53 adjusted to an average value of 2/CH and 

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. 

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Two-dimensional carbon-proton shift correlation m one-bond CH coupling... 

Fig. 2.53. Pulse sequences in the proton and carbon-13 channels and 13C — 11 doublet vectors controlled by pulses and /-modulation for acquisition of FID signals in a two-dimensional carbon-proton shift correlation [63],...

Fig. 2.54 presents a two-dimensional carbon-proton shift correlation of D-lactose after mutarotational equilibration (40% a-, 60% / -D-lactose in deuterium oxide), demonstrating the good resolution of overlapping proton resonances between 3.6 and 4 ppm by means of the larger frequency dispersion of carbon-13 shifts in the second dimension. The assignment known for one nucleus - carbon-13 in this case - can be used to analyze the crowded resonances of the other nucleus. This is the significance of the two-dimensional CH shift correlation, in addition to the identification of CH bonds. For practical evaluation, the contour plot shown in Fig. 2.54(b) proves to be more useful than the stacked representation (Fig. 2.54(a)). In the case of D-lactose, selective proton decoupling between 3.6 and 4 ppm would not afford results of similiar quality. 

Fig. 2.54. Two-dimensional carbon-proton shift correlation of mutarotated D-lactose (1 mol/L in deuterium oxide , 3C 100.6 MHz H 400.1 MHz measuring time 90min transform lime 2.5 min) (a) stacked plot [64] (b) contour plot. 

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... 

Fig. 5.17. Carbon-13 signal assignment of aflatoxin B, [603] by two-dimensional carbon-proton shift correlation (30 mg in 0.4 mL of hexadeuteriodimethyl sulfoxide, 30 C, 100.576 MHz for 13C, 400.133 MHz for H full and strong contours correlations via one-bond couplings empty and weaker contours correlations via two- and Lhree-bond couplings). Boldface printed substructures in Lhe formula can be directly derived from this figure the carbon nucleus at 91.4 ppm, for example, is correlated with the proton at 6.72 ppm via one-bond coupling this proton is additionally correlated with the adjacent carbon nuclei at 165.1, 161.4, 107.2, and 103.5 ppm as indicated by correlation signals via two- and three-bond couplings.

The cover displays a stacked plot of a two-dimensional carbon-proton shift correlation. The plot is reprinted here with its numerical data. Isopinocampheoxyisoprene in deute-riochloroform is the sample. The aliphatic carbon and proton shift ranges are shown. All carbon-proton connectivities of the bicyclic substructure can be clearly derived, For example, the carbon atom whose signal occurs at 35.6 ppm is attached to the protons with signals at 1.76 and 2.37 ppm. These signals make up an AB system in the proton NMR spectrum and overlap with other proton signals in the one-dimensional spectrum. 

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