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Bifurcated hydrogen-bonding effect on the shielding and coupling constants in trifluoroacetyl pyrroles as studied by 1H, 13C and 15N NMR spectroscopy and DFT calculations

✍ Scribed by Andrei V. Afonin; Igor A. Ushakov; Al'bina I. Mikhaleva; Boris A. Trofimov

Publisher
John Wiley and Sons
Year
2007
Tongue
English
Weight
231 KB
Volume
45
Category
Article
ISSN
0749-1581

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✦ Synopsis

Abstract

According to the ^1^H, ^13^C and ^15^N NMR spectroscopic data and DFT calculations, bifurcated NH···N and NH···O intramolecular hydrogen bond is shown to be present in 2‐trifluoroacetyl‐5‐(2′‐pyridyl)‐pyrrole. This bifurcated hydrogen bond causes an increase in the absolute size of the ^1^J(N,H) coupling constant by about 6 Hz, and the deshielding of the bridge proton by 2 ppm. DFT calculations show that the influence of the NH···N and NH···O intramolecular hydrogen bonds on the ^1^J(N,H) coupling and proton shielding is almost additive, although the components of the bifurcated hydrogen bond slightly weaken each other. In 2‐trifluoroacetyl‐5‐(2′‐pyridyl)‐pyrrole, the coupling constants involving the fluorine and the NH covalent bond nuclei depend dramatically on the spatial position of the pyridine ring. The pyridine ring rotation operates as a quantum switch controlling the spin information transfer between the ^19^F and ^15^N nuclei, as well as the proton. Copyright © 2007 John Wiley & Sons, Ltd.

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