Carbon-13 T1 Measurements under Proton Coupled and Decoupled Conditions

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Abstract
A theoretical treatment is presented of carbon‐13 spin‐lattice relaxation observed with and without proton decoupling, with particular emphasis on the influence of dissolved oxygen. It is shown that the recovery of carbon‐13 magnetization after population inversion does not follow a pure exponential in the absence of proton irradiation, and contains terms involving proton transition probabilities. This apparent carbon‐13 relaxation time can differ significantly from the relaxation time measured under proton decoupled conditions, tending to be longer when measured by pulse methods (which excite all the spin multiplet components) and shorter when measured by adiabatic rapid passage (where only one multiplet component is normally affected). Experimental results for benzene are used to illustrate the importance of these effects including the presence of dissolved oxygen. The significantly larger effect of oxygen on the apparent relaxation times, observed without proton decoupling, can be explained on the basis of the oxygen's greater direct influence on the proton transition probabilities.