In13Chyphen;enriched molecules,13Cndash;13C dipolar interactions can contribute significantly to the relaxation of13C nuclei which are not directly bonded to protons. Comparison of the relaxation behavior of the13Cndash;13C multiplet lines with that of the singlet for which there are no13Cndash;13C dipolar interactions allows a quantitative analysis of this interaction. However, since13Cndash;13C dipolar interactions are necessarily weak, substantial contributions to the relaxation of nonprotonated carbon atoms are made by protons attached to adjacent atoms. The general features of the spin lattice relaxation behavior of an AMlcub;Xrcub; system in which lcub;Xrcub; represents the decoupled protons, A, the nonprotonated carbon, and M, the adjacent protonated carbon, have been derived. In general, the A relaxation will be nonexponential and dependent on the initial state of the Mzmagnetization. A particularly useful application of these measurements on13Chyphen;enriched systems is found in their sensitivity to internal and/or anisotropic motions. Such rotations affect the13Cndash;13C and13Cminus;1H dipolar interactions differently, depending on the particular geometry involved and the axis of rotation. These techniques have been applied to the observed relaxation behavior of 6hyphen;phosphogluconate enriched to the 79percnt; level. In this case the contributions to the intramolecular dipolar relaxation of the carboxyl carbon consist primarily of the13Cndash;13C interaction with its adjacent carbon, the13Cminus;1H interaction with the proton on the adjacent carbon, and a significant contribution from the proton attached to the third carbon in the chain. For this case, it is found that the carboxyl relaxation will be, to a very good approximation, exponential and independent of theinitialvalue of Mz. Furthermore, a significantly improved fit of the data results if anisotropic rotation effects are considered.
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