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.
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.
Afonin
Andrei V
AV
Institute of Chemistry, Favorsky St. 1, 664033 Irkutsk, Russia. nmr@istu.irk.ru
Ushakov
Igor A
IA
Mikhaleva
Al'bina I
AI
Trofimov
Boris A
BA
D002247:Carbon Isotopes; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy / Q000379:methods* / Q000592:standards*; D008956:Models, Chemical; D015394:Molecular Structure; D009587:Nitrogen Isotopes; D011522:Protons; D011758:Pyrroles / Q000737:chemistry*; D011789:Quantum Theory; D012015:Reference Standards; D012680:Sensitivity and Specificity; D013237:Stereoisomerism
D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D011522:Protons; D011758:Pyrroles