Intramolecular hydrogen bonding in 8-quinolinol N-oxides, quinaldinic acid N-oxides and quinoline-2-carboxyamide N-oxide. Deuterium isotope effects on 13C chemical shifts

Abstract

Secondary isotope effects on 13C chemical shifts have been measured in a series quinolinols, quinaldinic aci N-oxides and quinoline-2-carboxyamide N-oxide. For 8-quinolinol N-oxides a good correlation was found between δOH and nΔC(OD) isotope effects. The OH and 13C chemical shifts and n ΔC(OD) show very small temperature dependences. The primary isotope effects are small, positive and temperature insensitive. Furthermore, they increase with increasing n ΔC(OD). All features point towards a localised hydrogen bond in an asymmetric double well potential. The quinaldinic acid N-oxides show long-range isotope effects on 13C chemical shifts of both signs with 2 ΔCO(OD) rather small. The primary isotope effects of the quinaldinic acid N-oxide is of order of 0.5 ppm, whereas for its 4-ethoxy-derivative is smaller, ∼0.3 ppm. The OH chemical shifts resonate at the low field ∼18–20 ppm and the OH resonance is fairly broad at room temperature, especially for the 4-ethoxy-derivative. The temperature effects on the chemical shifts, primary and secondary isotope effects are small. For quinaldinic acid N-oxides the asymmetric broad quasi-single potential is suggested. For quinoline-2-carboxyamide N-oxide the isotope effects are small, indicating rather weak hydrogen bond