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

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.

#### *4.3.4.4.1 1 H-<sup>1</sup> H COSY spectrum*

It is a kind of chemical shift correlation spectrum between 1 H and 1 H. It is the coupling correlation spectrum between protons in the same coupling system. The adjacent hydrogen groups could be determined by their coupling relationships (3 *J*) shown in 1 H-1 H COSY spectra.

**Figure 5.** *The DEPT spectrum of Arctiin (CD3OD).*

In addition, for compounds of aromatic systems, double bond systems, and some particular configuration systems, 1 H-1 H COSY spectra can show 4 *J* coupling or longer coupling relationships of hydrogen groups. It is very important for the elucidation of an unknown structure.

#### *4.3.4.4.2 HSQC (HMQC) spectrum*

1 H detected heteronuclear single quantum coherence (HSQC) and <sup>1</sup> H detected heteronuclear multiple quantum coherence (HMQC) can display the correlations between 1 H and 13C. HSQC possesses higher sensitivity and wider application than HMQC. In the HMQC or HSQC spectrum, the signals occurred at the crosses of chemical shifts generated by corresponding carbons and protons (**Figure 6**).
