Quenching the Nuclear Overhauser Effect in NMR Spectra of Carbon-13

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Abstract
Fourier transform techniques have been employed to determine the spin‐lattice relaxation time and the enhancement of the intensity of the carbon‐13 signal of formic acid upon strong proton irradiation—the nuclear Overhauser effect. Addition of paramagnetic hexaquochromic perchlorate is found to decrease the relaxation time and the enhancement factor in the same proportion. Beyond a concentration of about 0.1M, the Overhauser enhancement is effectively quenched, line broadening is only just becoming apparent, and the loss of intensity can be fully recovered by readjustment of the pulse conditions to take advantage of the faster relaxation. The carbon‐13 spin‐lattice relaxation in neat formic acid is shown to proceed predominantly through dipolar interaction with the protons, and when paramagnetic ions are added they are found to act directly on the carbon nuclei rather than via the proton spins, a negligible ``three‐spin effect.'' The quenching technique has been employed to establish uniform intensities for the four carbon‐13 resonances of vinyl acetate, where normally there is a wide disparity in signal strength due to unequal Overhauser effects.