¹H-detected measurement of spin relaxation constants in ultrawide solid-state NMR line shapes via progressive saturation of the proton reservoir

Nuclear magnetic resonance relaxation time constants provide valuable insights into the dynamic processes and structure of a system. However, determination of these relaxation parameters is often challenging for low-γ nuclides, which exhibit low sensitivity and are typically subject to large anisotropic interactions. Here, we introduce two new pulse sequences, PROSPR-T₁ and PROSPR-T₂, for measuring longitudinal and transverse relaxation time constants in solids. These sequences are modifications of the PROgressive Saturation of the Proton Reservoir (PROSPR) experiment, developed to facilitate the detection of insensitive nuclides. Following an initial analysis of the adiabatic requirements of the PROSPR experiment, the performance of PROSPR to measure T₁/ T 2 ′ is thoroughly evaluated on a range of spins, including ¹¹⁹Sn (I = 1/2), ³⁵Cl (I = 3/2), and ¹⁴N (I = 1). Experimental results demonstrate that optimized versions of PROSPR can be up to an order of magnitude more sensitive than their original implementation. These new methods offer accurate relaxation time constants, even for spectra spanning over 1 MHz, without demanding frequency stepping throughout the powder pattern.