The ${}^{103}$Rh NMR Spectroscopy and Relaxometry of the Rhodium Formate Paddlewheel Complex

Abstract

The NMR spectroscopy of spin-1/2 nuclei with low gyromagnetic ratio is challenging, due to the low NMR signal strength. Methodology for the rapid acquisition of ${}^{103}$Rh NMR parameters is demonstrated for the case of the rhodium formate "paddlewheel" complex $\mathrm{R}{\mathrm{h}}_2(\mathrm{HC}{\mathrm{O}}_2{)}_4$. A scheme is described for enhancing the ${}^{103}$Rh signal strength by polarization transfer from ${}^1$H nuclei and which also greatly reduces the interference from ringing artifacts, a common hurdle for the direct observation of low-$\gamma$ nuclei. The ${}^{103}$Rh relaxation time constants $T_1$ and $T_2$ are measured within 20 minutes using ${}^1$H-detected experiments. The field-dependence of the ${}^{103}$Rh $T_1$ is measured. The high-field relaxation is dominated by the chemical shift anisotropy (CSA) mechanism. The ${}^{103}$Rh shielding anisotropy is found to be very large: $|\Delta \sigma |=9900\pm 540\mathrm{ppm}$. This estimate is compared with density functional theory calculations.

submitted to JCP