Transient NOE driven signal enhancement of INADEQUATE solid-state NMR spectroscopy for the structural analysis of rubbers

INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) is one of the most important techniques in revealing the carbon skeleton of organic solids in solid-state NMR spectroscopy. Nevertheless, its use for structural analysis is quite limited due to the low natural abundance of ¹³C–¹³C connectivity (∼0.01%) and thus low sensitivity. Particularly, in semi-solids like rubbers, the sensitivity will be further significantly reduced by the inefficient cross polarization from ¹H to ¹³C due to molecular motions induced averaging of ¹H–¹³C dipolar couplings. Herein, in this study, we demonstrate that transient nuclear Overhauser effect (NOE) can be used to efficiently enhance ¹³C signals, and thus enable rapid acquisition of two-dimensional (2D) ¹³C INADEQUATE spectra of rubbers. Using chlorobutyl rubber as the model system, it is found that an overall signal-to-noise ratio (SNR) enhancement about 22% can be achieved, leading to significant time-saving by about 33% as compared to the direct polarization-based INADEQUATE experiment. Further experimental results on natural rubber and ethylene propylene diene monomer (EPDM) rubber are also shown to demonstrate the robust performance of transient NOE enhanced INADEQUATE experiment.