*4.3.4.3 Carbon nuclear magnetic resonance spectroscopy (13C-NMR)*

13C-NMR spectra can provide structural information of organic compounds, including the number, types, and chemical environment of carbon atoms [27]. It is one of the important means for the structural identification of organic compounds. Especially, where there are serious signal peak overlaps in the 1 H-NMR spectrum, or the molecules contain several quaternary carbon atoms, 13C-NMR spectra will provide crucial information for the structure identification. The chemical shifts of common carbon signals are shown in **Figure 4** [4].

Common 13C-NMR techniques include proton broadband decoupling, off resonance decoupling (OFR), insensitive nuclei enhanced by polarization transfer (INEPT), and distortionless enhancement by polarization transfer (DEPT). Proton broadband decoupling and DEPT spectra are most commonly used at present.

**63**

**Figure 5.**

*The DEPT spectrum of Arctiin (CD3OD).*

*Analytical Methods of Isolation and Identification DOI: http://dx.doi.org/10.5772/intechopen.88122*

Proton broadband decoupling spectrum is measured after 1

*4.3.4.3.2 Distortionless enhancement by polarization transfer (DEPT)*

, 90o

, only CH groups show positive signals; when *θ* = 135o

It is a kind of chemical shift correlation spectrum between <sup>1</sup>

with broadband electromagnetic radiation. At this point, the couplings between 1

and 13C are completely eliminated, and all 13C signals are shown as singlets, so it is very convenient to determine the chemical shift of 13C signals. In addition, because of

be increased, while the quarterly carbon signal will show weak absorption peaks.

, and 135o

*4.3.4.4 Two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR)*

carbons could show different strengths and signs. The results are similar with INEPT

show positive signals, while CH2 groups show negative signals. Quarterly carbons show no signal peaks in DEPT spectra. An example of DEPT spectra is shown in **Figure 5**.

Two-dimensional correlation spectroscopy (2D-COSY) is the most important and widely used in 2D-NMR spectroscopy. 2D-COSY spectra can be divided into homonuclear and heteronuclear correlation spectra. Both abscissa and ordinate represent chemical shifts in 2D-COSY. Common correlation spectrum types are show as follows.

coupling correlation spectrum between protons in the same coupling system. The adjacent hydrogen groups could be determined by their coupling relationships (3

It is an improved method of INEPT, in which a *J*-modulation is accompanied by a polarization transfer from the protons to coupled carbons, leading to significant improvement in sensitivity. In DEPT spectrum, by changing the pulse width (*θ*),

H after irradiation, the signal of 13C signal connected with 1

, during irradiation of 1

, all CH, CH2, and CH3 groups display positive signals; when

H nuclei are saturated

H, different

, both CH and CH3 groups

H and 1

H. It is the

*J*)

H

H will

*4.3.4.3.1 Proton broadband decoupling*

the NOE effect of 1

which could be designed as 45o

spectrum. When *θ* = 45o

*θ* = 45o

*4.3.4.4.1 1*

shown in 1

*H-<sup>1</sup>*

H-1

*H COSY spectrum*

H COSY spectra.

*Analytical Methods of Isolation and Identification DOI: http://dx.doi.org/10.5772/intechopen.88122*
