Probing isotope shifts in 103Rh and 195Pt NMR spectra with density functional theory

Structure and bonding in rhodium coordination compounds: a 103Rh solid-state NMR and relativistic DFT study

This study demonstrates the application of 103Rh solid-state NMR (SSNMR) spectroscopy to inorganic and organometallic coordination compounds, in combination with relativistic density functional theory (DFT) calculations of 103Rh chemical shift tensors and their analysis with natural bond orbital (NBO) and natural localized molecular orbital (NLMO) protocols, to develop correlations between 103Rh chemical shift tensors, molecular structure, and Rh-ligand bonding. 103Rh is one of the least receptive NMR nuclides, and consequently, there are very few reports in the literature. We introduce robust 103Rh SSNMR protocols for stationary samples, which use the broadband adiabatic inversion-cross polarization (BRAIN-CP) pulse sequence and wideband uniform-rate smooth-truncation (WURST) pulses for excitation, refocusing, and polarization transfer, and demonstrate the acquisition of 103Rh SSNMR spectra of unprecedented signal-to-noise and uniformity. The 103Rh chemical shift tensors determined from these spectra are complemented by NBO/NLMO analyses of contributions of individual orbitals to the 103Rh magnetic shielding tensors to understand their relationship to structure and bonding. Finally, we discuss the potential for these experimental and theoretical protocols for investigating a wide range of materials containing the platinum group elements.

NMR spectroscopy of a 18O-labeled rhodium paddlewheel complex: Isotope shifts, 103Rh-103Rh spin-spin coupling, and 103Rh singlet NMR

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.

Isotopomeric Elucidation of the Mechanism of Temperature Sensitivity in 59Co NMR Molecular Thermometers

Understanding the mechanisms governing temperature-dependent magnetic resonance properties is essential for enabling thermometry via magnetic resonance imaging. Herein we harness a new molecular design strategy for thermometry─that of effective mass engineering via deuteration in the first coordination shell─to reveal the mechanistic origin of ⁵⁹Co chemical shift thermometry. Exposure of [Co(en)₃]³⁺ (1; en = ethylenediamine) and [Co(diNOsar)]³⁺ (2; diNOsar = dinitrosarcophagine) to mixtures of H₂O and D₂O produces distributions of [Co(en)₃]³⁺-d_n (n = 0-12) and [Co(diNOsar)]³⁺-d_n (n = 0-6) isotopomers all resolvable by ⁵⁹Co NMR. Variable-temperature ⁵⁹Co NMR analyses reveal a temperature dependence of the ⁵⁹Co chemical shift, Δδ/ΔT, on deuteration of the N-donor atoms. For 1, deuteration amplifies Δδ/ΔT by 0.07 ppm/°C. Increasing degrees of deuteration yield an opposing influence on 2, diminishing Δδ/ΔT by -0.07 ppm/°C. Solution-phase Raman spectroscopy in the low-frequency 200-600 cm^-1 regime reveals a red shift of Raman-active Co-N₆ vibrational modes by deuteration. Analysis of the normal vibrational modes shows that Raman modes produce the largest variation in ⁵⁹Co δ. Finally, partition function analysis of the Raman-active modes shows that increased populations of Raman modes predict greater Δδ/ΔT, representing new experimental insight into the thermometry mechanism.

Intrinsic 37/35Cl and 18/16O isotope shifts in 195Pt and 103Rh NMR of purely inorganic Pt and Rh complexes as unique spectroscopic fingerprints for unambiguous assignment of structure

Well-resolved intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope shifts (where M = ¹⁹⁵Pt or ¹⁰³Rh) are visible in the ¹⁹⁵Pt NMR peak profiles of relatively kinetically inert [PtCl_n(H₂O)_6-n]^4-n (n = 1-6) complexes, their corresponding hydroxido [PtCl_6-n(OH)_n]^2- (n = 1-5/6) anions, and [RhCl_n(H₂O)_6-n]^3-n (n = 3-6) complexes in aqueous solutions at ca. 293 K. Although some such isotope effects have been previously reported, there are very limited published data in the open literature, and the first systematic studies of such intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope effects are reviewed in this perspective. In high magnetic-field NMR spectrometers, the ¹⁹⁵Pt and ¹⁰³Rh NMR peak profiles acquired within a relatively narrow temperature range (288-300 K) constitute unique 'spectroscopic fingerprints', which allow unambiguous structural assignment in solution. Available data for Pt(iv) and Rh(iii) complexes give rise to intrinsic isotope ¹Δδ¹⁹⁵Pt/¹⁰³Rh(^37/35Cl) profiles, which are extraordinarily sensitive to the structure of a particular complex or its geometric isomer. The profiles of aquated Pt(iv) and Rh(iii) complexes in acidic solutions may be resolved at either an isotopologue level only or at both an isotopologue and an isotopomer level depending on the structure. By contrast, in the series of [PtCl_6-n(OH)_n]^2- (n = 1-6) anions, ¹Δδ¹⁹⁵Pt(^37/35Cl) isotope shifts are resolved only at an isotopologue level. Relatively larger ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts obtained by the partial ¹⁸O enrichment of both the [PtCl_n(H₂O)_6-n]^4-n (n = 1-6) and [PtCl_6-n(OH)_n]^2- (n = 1-6) series give rise to remarkable ¹⁹⁵Pt NMR peak profiles showing both ^37/35Cl and ^18/16O shifts. In the [PtCl_6-n(OH)_n]^2- (n = 1-5/6) anions a typical NMR peak profile spanning ∼2 ppm only may be resolved at both the isotopologue and isotopomer levels, depending on whether ^18/16OH^- ions are coordinated trans to chloride ions or not. The potential utility of such ¹Δ¹⁹⁵Pt(^37/35Cl) and ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts in selected practical applications involving such complexes is briefly illustrated.

Atomic under-coordination fascinated catalytic and magnetic behavior of Pt and Rh nanoclusters

Atomic under-coordination fascinated catalytic and magnetic properties of Pt and Rh nanoclusters have been studied by DFT calculations, and consistency with the calculation and experimental results confirmed predications based on BOLS correlation.