Application of ¹⁴N decoupled HETCOR for ¹⁹F-mediated amide hydrogen bonds with ROYALPROBE™ HFX

Measurements of ¹H NMR with ¹⁴N decoupling

NMR spectroscopy is a powerful tool for analysis of hydrogen bonds, and its J coupling value provides the information of hydrogen bond. Fig.1(A) shows ¹H NMR spectrum of 1. Amide ¹H signal is observed in the lowest magnetic field region, ca 8.77 ppm. In this case, severe line broadening is arising from quadrupolar ¹⁴N. In contrast, measurement of ¹H NMR under the ¹⁴N decoupling condition, resulted in the sharp amide ¹H signal observation as double of a doublet (Fig.1(B)). This splitting signal is derived from bifurcated hydrogen bond bridges between N-H (11) and two F (F5, F6) atoms.

Fig.1 (A) ¹H NMR spectrum of 1 (B) ¹H{¹⁴N} NMR spectrum of 1

Sample: 36 mg fluorinated benzanilide in chloroform-d
Equipment: JNM-ECZ400S with ROYALPROBE™ HFX^※
※Special modification for ¹⁴N nuclei required.

Measurements of ¹H–¹⁹F HETCOR with ¹⁴N decoupling

In order to confirm two hydrogen bonds between the amide ¹H and ¹⁹F atoms, we collected ¹H–¹⁹F HETCOR under the ¹⁴N decoupling condition. 1D-HETCOR spectra of 1 are shown in Fig.2. As expected, we could observe cross peaks to amide ¹H with both F atoms (F5, F6). Furthermore, by using anti phase signals in 1D-HETCOR spectra, J constants were clearly observed as ca. 16 Hz (amide-F5) and 3 Hz (amide-F6), respectively in the case of ¹⁴N decoupling conditions. 2D-HETCOR spectrum with ¹⁴N decoupling is also available, which is shown in Fig.3.

Fig.2 1D-HETCOR spectra of 1 with or without ¹⁴N decoupling (A: selective excitation at F5; B: selective excitation at F6).

Fig.3 ¹⁴N decoupling 2D-HETCOR spectrum of 1

Reference

G. N. Manjunatha Reddy, M. V. Vasantha Kumar, T. N. Guru Row, N. Suryaprakash, Phys. Chem. Chem. Phys., 12, 13232–13237 (2010).

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