文摘
Establishing the carbon skeleton of a molecule greatly facilitates the process of structure elucidation, both manual and computer-assisted. Recent advances in the family of ADEQUATE experiments demonstrated their potential in this regard. 1,1-ADEQUATE, which provides direct 13C-13C correlation via 1JCC, and 1,n-ADEQUATE, which typically yields 3JCC and 1JCC correlations, are more sensitive and more widely applicable experiments than INADEQUATE and PANACEA. A recently reported modified pulse sequence that semi-selectively inverts 1JCC correlations in 1,n-ADEQUATE spectra provided a significant improvement, allowing 1JCC and nJCC correlations to be discerned in the same spectrum. However, the reported experiment requires a careful matching of the amplitude transfer function with 1JCC coupling constants in order to achieve the inversion, and even then some 1JCC correlations could still have positive intensity due to the oscillatory nature of the transfer function. Both shortcomings limit the practicality of the method. We now report a new, dual-optimized inverted 1JCC 1,n-ADEQUATE experiment, which provides more uniform inversion of 1JCC correlations across the range of 29-82 Hz. Unlike the original method, the dual optimization experiment does not require fine-tuning for the molecule鈥檚 1JCC coupling constant values. Even more usefully, the dual-optimized version provides up to two-fold improvement in signal-to-noise for some long-range correlations. Using modern, cryogenically-cooled probes, the experiment can be successfully applied to samples of 鈭? mg under favorable circumstances. The improvements afforded by dual optimization inverted 1JCC 1,n-ADEQUATE experiment make it a useful and practical tool for NMR structure elucidation and should facilitate the implementation and utilization of the experiment.