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

• Published in: Magnetic resonance in chemistry 2007-03, Vol.45 (3), p.220-230
• Main Authors: Afonin, Andrei V., Ushakov, Igor A., Mikhaleva, Al'bina I., Trofimov, Boris A.
• Format: Article
• Language: English
• Subjects: 13C; 15N; bifurcated hydrogen bonds; Carbon Isotopes; DFT calculations; Hydrogen Bonding; Magnetic Resonance Spectroscopy - methods; Magnetic Resonance Spectroscopy - standards; Models, Chemical; Molecular Structure; Nitrogen Isotopes; NMR; Protons; pyrroles; Pyrroles - chemistry; Quantum Theory; Reference Standards; Sensitivity and Specificity; Stereoisomerism
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.1949

According to the 1H, 13C and 15N 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 1J(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 1J(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 19F and 15N nuclei, as well as the proton. Copyright © 2007 John Wiley & Sons, Ltd.