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Despite the importance of rhodium complexes in catalysis, and the favorable 100% natural abundance of the spin-1/2 103Rh nucleus, there are few reports of 103Rh nuclear magnetic resonance (NMR) parameters in the literature. In part, this is the consequence of the very low gyromagnetic ratio of 103Rh and its dismal NMR sensitivity. In a previous paper [Harbor-Collins et al., J. Chem. Phys. 159, 104 307 (2023)], we demonstrated an NMR methodology for 1H-enhanced 103Rh NMR and demonstrated an application to the 103Rh NMR of the dirhodium formate paddlewheel complex. In this paper, we employ selective 18O labeling to break the magnetic equivalence of the 103Rh spin pair of dirhodium formate. This allows the estimation of the 103Rh–103Rh spin–spin coupling and provides access to the 103Rh singlet state. We present the first measurement of a 18O-induced 103Rh secondary isotope shift as well as the first instance of singlet order generated in a 103Rh spin pair. The field-dependence of 103Rh singlet relaxation is measured by field-cycling NMR experiments. [ABSTRACT FROM AUTHOR]

The nuclear magnetic resonance (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 103Rh NMR parameters is demonstrated for the case of the rhodium formate "paddlewheel" complex R h 2 (H C O 2 ) 4 . A scheme is described for enhancing the 103Rh signal strength by polarization transfer from 1H nuclei, which also greatly reduces the interference from ringing artifacts, a common hurdle for the direct observation of low-γ nuclei. The 103Rh relaxation time constants T1 and T2 are measured within 20 min by using 1H-detected experiments. The field dependence of the 103Rh T1 is measured. The high-field relaxation is dominated by the chemical shift anisotropy mechanism. The 103Rh shielding anisotropy is found to be very large: |Δσ| = 9900 ± 540 ppm. This estimate is compared with density functional theory calculations. [ABSTRACT FROM AUTHOR]

Recently developed polarisation transfer techniques are applied to the 103Rh nuclear magnetic resonance (NMR) of the 103Rh(acac)3 coordination complex in solution. Four-bond 1H – 103Rh J couplings of around 0.39 Hz are exploited to enhance the 103Rh NMR signal and to estimate the 103Rh T1 and T2 relaxation times as a function of field and temperature. The 103Rh longitudinal T1 relaxation in 103Rh(acac)3 is shown to be dominated by the spin–rotation mechanism, with an additional field-dependent contribution from the 103Rh chemical shift anisotropy. [ABSTRACT FROM AUTHOR]

Recently-developed polarisation-transfer techniques are applied to the 103Rh NMR of the Rh(acac)3 coordination complex in solution. Four-bond 1H-103Rh J-couplings of around 0.39 Hz are exploited to enhance the 103Rh NMR signal and to estimate the 103Rh T 1 and T 2 relaxation times as a function of field and temperature. The 103Rh longitudinal T 1 relaxation in Rh(acac)3 is shown to be dominated by the spin-rotation mechanism, with an additional field-dependent contribution from the 103Rh chemical shift anisotropy. [ABSTRACT FROM AUTHOR]