Errors in the measurement of cross-correlated relaxation rates and how to avoid them

Cross-correlated relaxation rates Gamma are commonly obtained from constant time experiments by measuring the effect of the desired cross-correlated r elaxation on an appropriate coherence during the constant time T. These mea surements are affected by systematic errors, which derive from undesired cr oss-correlated relaxation effects taking place before and after the constan t time period T. In this paper we discuss the sources and the size of these errors in an example of two pulse sequences. Higher accuracy of the measur ed data can be obtained by recording a set of experiments with different T values. Cross-correlated relaxation rates me measured in constant time expe riments either from the differential relaxation of multiplet components (J- resolved Gamma experiments) or from the efficiency of magnetization transfe r between two coherences (quantitative Gamma experiments). In this paper we calculate analytically the statistical errors in both J-resolved and quant itative Gamma experiments. These formulae provide the basis for the choice of the most efficient experimental approach and parameters for a given meas urement time and size of the rate. The optimal constant time T for each met hod can be calculated and depends on the relaxation properties of the molec ule under investigation. Moreover, we will show how to optimize the relativ e duration of cross and reference experiments in a quantitative Gamma appro ach. (C) 2000 Academic Press.