Proton-decoupled carbon-13 relaxation in 13CH2 and 13CH3 spin systems

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Abstract
The transient behavior of the carbon magnetization in proton decoupled spectra is discussed in detail for 13CH2 and 13CH3 spin systems. The influence of multispin dipolar cross correlation effects is shown to influence the decoupled inversion‐recovery experiment and lead to a predicted biexponential recovery of the carbon magnetization. It is rationalized that for methyl relaxation, the interference effects will play an inconsequential role. However, in the methylene case, it is demonstrated that there may arise many possible instances when it will become necessary to consider these correlation effects in much greater detail. It is also shown that if (1) extreme narrowing arguments are invalid or (2) other relaxation mechanisms compete with dipolar interactions, then the NOE enhancement factors deviate from those predicted from conventional treatments. For many conceivable situations, the inferred relaxation rate will underestimate the sum of the dipolar and nondipolar contributions whereas the usage of Overhauser enhancements will lead to an overestimation of the relative importance of nondipolar contributions. Again, this facet of cross correlation effects appears to be of more general concern in CH2 than CH3 studies.