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Differentiating Tautomers using 15N chemical shift information

As with many nitrogen-containing compounds, 15N chemical shift information can be critical in elucidating or confirming a candidate structure. Many elucidators extract 15N chemical shifts from such experiments as 15N NMR, 1H-15N HSQC or 1H-15N HMBC.

Below are two tautomers, from a previous blog, that differ in the hybridization state of the nitrogen. The 15N chemical shift for the structure on the left is expected somewhere between 260-300 ppm (referenced to NH3 (liq.)) whereas the 15N chemical shift for the structure on the right is expected around 160-190 ppm.

15n_nhoh_aug52008

3 Replies to “Differentiating Tautomers using 15N chemical shift information”

  1. I assume that carbon shifts also should allow a clear distinction between the two tautomers, since beta-carbons in the enamin part are normally found at higer field (ca. 5ppm ?). Are there carbon shift reported ?

  2. Hello Heinz,
    I agree that these two tautomers can be differentiated by 13C chemical shifts. However, neither reference (Left structure: J.Heterocycl. Chem., 2002, v.39, p.131.) report any 13C NMR data.
    In cases where the unknown contains multiple nitrogen atoms, many elucidators will collect 15N NMR data to assist in the elucidation process.

  3. Hi,
    I just read this contibution (and the older one) and was surprised about the discussion and comments. There are three papers dealing with this topic using 1H, 13C and 15N-NMR:
    Solution Structures of Pyrophthalones, III
    Complementary Application of 14N/15 N-NMR Spectroscopy to Study Solution Structures of Pyrophthalones
    Authors: E.T.K. Haupt, M. Eggers
    Published in: Spectr. Lett., 31(3), 521 (1998)
    Haupt, Erhard T. K.; Tom Dieck, Heindirk; Bonchev, P. Solution structures of pyrophthalones. II. Structure and conformation of iminopyrophthalone – a high-resolution proton, carbon-13, nitrogen-15 and solid state 13C-CP/MAS-NMR study. Zeitschrift fuer Naturforschung, B: Chemical Sciences (1987), 42(11), 1419-23
    Haupt, Erhard T. K.; Tom Dieck, Heindirk; Bonchev, P. Solution structures of pyrophthalones, I. Structure and conformation of 1,3-indandionato-2(2-pyridinium)betaine – a proton/carbon-13 NMR approach. Zeitschrift fuer Naturforschung, B: Chemical Sciences (1987), 42(1), 31-6
    Maybe this is of help….
    Bets regards
    erhard

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