Oxygen-17 NMR Linewidths as Influenced by Proton Exchange in Water

Abstract

Linewidths for the ¹⁷O nuclear magnetic resonance have been obtained at 29.0°±0.2°C in 10% H₂¹⁷O as a function of acidity with the sideband technique at 8 MHz. Computer methods have been used to solve the set of coupled linear equations from which line shapes as functions of the residence time and 2δ, the proton—¹⁷O spin—spin coupling constant, have been calculated. It has been possible to obtain a linear relationship between k₁, the specific rate constant for the proton catalyzed exchange process, and 2δ. The spin—spin coupling constant has been redetermined by the acetone‐dilution procedure, and a value of 82±1 Hz has been obtained. Because of quadrupole relaxation of the ¹⁷O, the true coupling constant will be somewhat larger than the value obtained under these conditions of slow exchange. With the assumption of a value of 2δ of 580 rad sec⁻¹, the acid and base catalyzed specific rate constants at 29.0°C are k₁=8.2×10⁹ mole⁻¹ liter sec⁻¹ and k₂=4.6×10⁹ mole⁻¹ liter sec⁻¹, respectively. However, it appears that the spin—spin coupling constant cannot be obtained in an unambiguous manner and accordingly, the values of the specific rate constants for the exchange processes are uncertain.