Rapid Acquisition of ¹⁰³Rh Solid-State NMR Spectra by ³¹P Detection and Sideband Selective Methods

¹⁰³Rh solid-state nuclear magnetic resonance (SSNMR) spectroscopy is potentially a powerful method for investigating the molecular and electronic structure of rhodium compounds. However, ¹⁰³Rh is a difficult nucleus to study by NMR spectroscopy because of its small gyromagnetic ratio, broad chemical shift range, and long spin-lattice relaxation times (T₁). While there are many prior reports demonstrating acquisition of ¹⁰³Rh solution NMR spectra, there are few examples of ¹⁰³Rh SSNMR spectra in the literature. Here, we utilize the large ³¹P-¹⁰³Rh scalar couplings (J-couplings) to efficiently acquire ³¹P-detected high-resolution ¹⁰³Rh SSNMR spectra. ³¹P{¹⁰³Rh} J-resolved NMR experiments were used to measure ³¹P-¹⁰³Rh J-couplings. Sideband selective SSNMR techniques originally developed for wide-line ¹⁹⁵Pt SSNMR experiments were then used to rapidly acquire ¹⁰³Rh SSNMR spectra. Notably, we were able to acquire MAS ¹⁰³Rh SSNMR spectra in experiment times on the order of 30 min to a few hours and from only a few mg of materials. The sideband selective experiments offer significant time savings as compared to existing direct detection methods, which require days of acquisition to obtain a directly detected MAS spectrum, or yield low-resolution static powder patterns. Numerical fits of the SSNMR spectra provide ¹⁰³Rh chemical shift tensor parameters, with the experimental spectra agreeing well with the DFT-calculated spectra.