As 14N is a difficult nucleus to work with, 15N is usually used for nitrogen NMR. 15N has very low natural abundance (ca. 0.36%). In addition, 15N has low gyromagnetic ratio, which makes 15N signal sensitivity a great challenge.
Due to the low gyromagnetic ratio of 15N, a 2D 1H-15N spectrum often takes less time than a 1D 15N spectrum. The most useful 2D 1H-15N technique is HMBC. It is fairly easy to run:
- In edc, click Experiment, and select HMBCGP_15N
- be sure to check “getprosol”
- run the experiment like any other NMR experiments: lock, shim, atma, rga, zg.
- When the experiment is running, you can type xfb to check the spectrum.
Following is a 1H-15N HMBC spectrum of 50mM natural abundance cyclosporin, which takes only 20 minutes on the 500. All eleven nitrogen peaks can be resolved, along with their 1H neighbors. Note: 1-bond NH pairs appear at doublets, while 2-bond and 3-bond NH pairs appear as singlets. Weaker peaks are likely 3-bond NH pairs.
