Spectroscopic separation of mixed organic fluorine compounds by 2D ¹⁹F–¹⁹F TOCSY
—To extract 1D ¹⁹F NMR spectrum of each compound—

Perfluoroalkyl compounds are often found in research fields such as pharmaceuticals and agrochemicals. There are increasing demands for 1D ¹⁹F NMR, which is a powerful tool to characterize these compounds. Sensitivity of ¹⁹F is the second highest after ¹H, which facilitates to obtain a ¹⁹F NMR spectrum. Furthermore, the observation range of ¹⁹F chemical shifts in fluorocarbons is very large and signals show good dispersion. However, ¹⁹F NMR analysis of a mixture of these compounds are generally very complicated. This is because similarity of the chemical properties of each compound often occurs overlaps of the signals. For the same reason, purification by using chromatographic techniques is difficult. Therefore, it’s convenient if a 1D ¹⁹F NMR spectrum of each compound is directly obtained from a mixture.

2D TOCSY is known as a method which enables to obtain a 1D NMR spectrum from a mixture. To measure a ¹⁹F–¹⁹F TOCSY [1] with high field NMR equipment, a radio frequency (RF) covering wide spectrum width of ¹⁹F is necessary. However, it’s difficult to use such a RF, so ¹⁹F TOCSY has not been generally used. Recently, to address the wide ¹⁹F chemical shift range, 2D ¹⁹F TOCSY with BURBOP spin lock (Fig. 1) was reported [2]. Here, this application note explains a method to extract a 1D ¹⁹F spectrum of each compound from a mixture by using 2D ¹⁹F TOCSY with BURBOP spin lock.

Fig. 1. ¹⁹F TOCSY pulse sequence with BURBOP spin lock.

Spectroscopic separation of a mixture by 2D ¹⁹F TOCSY

In 2D TOCSY, when measured with a sufficiently enough long mixing time, magnetization transfer occurs in all coupled spins. Therefore, a 1D ¹⁹F NMR spectrum of each compound can be extracted. As an example, Figure. 2 shows a 2D ¹⁹F TOCSY of two organic fluorine compounds. Two sliced data were made at -121.8 ppm (red dotted line) and -126.8 ppm (blue dotted line), respectively. Figure. 3 shows (a) the 1D ¹⁹F spectrum of the mixture, (b) the sliced data at -121.8 ppm, and (c) at -126.8 ppm. Each sliced data is equivalent to the 1D ¹⁹F spectrum of each compound. As you can see, each 1D ¹⁹F spectrum is successfully extracted from 2D ¹⁹F TOCSY.

Equipment: JNM-ECZL400S (Delta NMR software V6.3), ROYALPROBE™ HFX, Pulse sequence: 19f_tocsy_burbop_abs_pfg.jxp,
Sample: 2,2,3,3-tetrafluoro-1-propanol and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol in CDCl₃

[1] Anal. Chem., 65, (1993) 752-758, [2] J. Magn. Reson., 285, (2018) 143-147.

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