Effect of paramagnetic relaxation reagent concentration on the nuclear overhauser enhancement and on the use of gated proton decoupling in quantitative carbon-13 nuclear magnetic resonance spectroscopy

Abstract

To determine the effect of the concentration of the paramagnetic relaxation reagent chromium (tris)acetylacetonate [Cr(acac)3] when used in quantitative carbon-13 nuclear magnetic resonance spectroscopy (13C NMR), the spectrum of pyrene, a compound with extreme values of spin lattice relaxation times (T1) and nuclear Overhauser enhancements (NOE), was studied. The NOE, line broadening and shifting of the five pyrene resonances were determined for up to 0.3 M Cr(acac)3. The quaternary and protonated carbons displayed different NOE's until all were completely quenched with the Cr(acac)3 greater than 0.25 M. Employing this Cr(acac)3 concentration in quantitative NMR analyses eliminated errors due to differing NOE's, but led to considerable line broadening and shifting. The use of lower Cr(acac)3 concentrations in these analyses, such as the commonly reported 0.05 - 0.15 M, necessitates the joint use of gated proton decoupling to eliminate the NOE differences which still exist at these concentrations.