Solution to doubling spectroscopy challenge.

Abstract

The reason for the signal doubling of the H-6 hydrogen signal in the H-NMR spectrum of dexelvucitabine (DFC) is the presence of the fluorine at position 5 (Fig. 1). Fluorine has only one isotope, F, with a spin 1⁄2 nucleus. Since both H-6 and F-5 are connected in DFC by a vicinal coupling JH6,F5, the NMR signal of each H-6 nucleus is split by the interaction with the F-5 nucleus and vice versa. Therefore, no cross-peak will be observed for the H-6 doublet in the H,H-COSY spectrum. As expected, noticeable crosspeaks were observed in the NOESY spectrum for interactions between H-6, H-2′, H-5′, and –OH protons, indicating correct signal assignment (Fig. 2). Considerable cross-peaks were also observed between the pair of twin amino proton signals in the NOESY spectrum. However, the phase of these cross-peaks (red color) is opposite to that of the other NOESY cross-peaks (blue color). This indicates that there is a slow exchange process between both amino protons during the 700-ms continuous mixing time at room temperature. The energy barrier for this exchange process can be calculated using the experimentally determined coalescence temperature Tc and the frequency difference Δν between the separate signals of the amino-group protons. Both of these parameters were determined twice at different magnetic field strengths: