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Intuitive Vs Predicted N15 Spectral Windows
figureposted on 30.12.2011, 13:45 by Antony Williams
Intuitive vs. calculated 15N F1 spectral windows. The F1 spectral window from 50-200 ppm was selected by an investigator with some knowledge of 15N chemical shifts based on the premise that the N1 nitrogen resonance of harmaline, would be similar to that of indole, in the range of ~130 ppm. The experimental result is shown in panel A; the N1 resonance was observed near 150 ppm via a long-range correlation from the vinyl methyl group. The observed 15N chemical shift is however folded and since it was reasonable relative to the spectroscopist’s intuitive thinking, that incorrect 15N chemical shift could be propagated in the literature. However, when the 15N shifts of harmaline were calculated, the calculated 15N shifts were found to be 131.6 for the indole N6 and 298.1 for the N1 resonance. Opening the F1 spectral window to cover 100-320 ppm gave the experimental result shown in B. The N1 resonance is “unfolded” and now appears quite near the predicted 15N chemical shift value.