Higher sensitivity through selective (13)C excitation in solid-state NMR spectroscopy.

A notable drawback of NMR spectroscopy is its inherently low sensitivity: 95% of the measuring time consists solely of idle delays during which nuclei regain their Boltzmann equilibrium. Here, a strategy for solid-state (13)C NMR experiments is presented that allows the user to acquire spectra in time periods that are notably shorter than previously necessary. Experiments that are band-selective in nature may utilize the cooling potential of unperturbed nuclei to lower the spin temperature of their excited neighbors. As we demonstrate, it becomes possible to replace the recycle delay in a series of scans by a time period during which proton-driven spin diffusion causes relaxation enhancement by a lower spin temperature of adjacent spins (RELOAD). Typically, a duration of approximately 200 ms suffices for this step, and for 1D (13)C NMR experiments, it is shown that the omission of recycle delays (typically of 2 s length) reduces the measuring time substantially. RELOAD is applied to 2D homonuclear (13)C NMR experiments, and it is demonstrated that for experiments in which correlations between (13)C backbone atoms are detected, the measurement time is reduced by a factor of 10 through a time-saving combination of a smaller number of increments in the indirect dimension and RELOAD.