title: 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.
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.
authors.affiliation: Institute of Chemistry, Favorsky St. 1, 664033 Irkutsk, Russia. nmr@istu.irk.ru
authors.forename: Andrei V
authors.initials: AV
authors.lastname: Afonin
authors.forename: Igor A
authors.initials: IA
authors.lastname: Ushakov
authors.forename: Al'bina I
authors.initials: AI
authors.lastname: Mikhaleva
authors.forename: Boris A
authors.initials: BA
authors.lastname: Trofimov
mesh_terms: 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
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D011522:Protons; D011758:Pyrroles