NMR-Paramagnetic Relaxation Due to the High-Spin d³ Electron Configuration:  Cr(III)−TSPP

Abstract

Sulfonated metalloporphyrins (Me-TSPP, where Me = Cr(III), Mn(III), Fe(III), and Mn(II)) comprise a well-characterized series of water-soluble paramagnetic complexes with electron spins of S = 3/2, 2, 5/2, and 5/2, respectively, which provide important model systems for mechanistic studies of paramagnetic NMR relaxation in solution. Previous studies of Mn(III), Fe(III), and Mn(II)-TSPP have uncovered relaxation mechanisms which differ qualitatively from each other and exhibit numerous unexpected features. In this study, Cr(III)-TSPP was examined as a model system for the d3 S = 3/2 electron configuration. Magnetic relaxation dispersion (MRD) profiles of the water proton R1 were measured as a function of pH between pH 1 and pH 9. In acid samples, R1 results from acid-catalyzed prototropic chemical exchange involving the Cr(III)-TSPP x 2 H2O. In neutral and basic solution, this species deprotonates, and base-catalyzed prototropic exchange becomes important. The pH 1 data were analyzed quantitatively using theory that accounts for the role of the permanent zero field splitting (zfs) tensor and for the effects of Brownian reorientation. Two levels of theory were employed: (1) spin dynamics simulation, which accurately describes the effects of Brownian reorientation on the spin wave functions, and (2) the "constant H(S)" approximation, which incorporates the effects of multiexponential electron spin relaxation and facilitates the physical interpretation of the relaxation mechanism. It was found that neither level of theory alone provides a fully satisfactory quantitative description of the data due to the fact that both reorientational modulation of the spin wave functions and multiexponential electron spin relaxation are important. The zero field splitting parameter, D = 0.27 cm(-1), is well defined by the data and was measured.

Application of the electronic spectra of porphyrins for analytical purposes: The effects of metal ions and structural distortions

“…Therefore, the length of the M-N bonds (and probably the spin multicipity) increases, the ruffled (or saddle), as well as the overall distortion decreases, together with the large redshift, e.g. in the reaction between Cr III TSPP 3-and hydroxide [105]. If the investigated molecule as potential axial ligand is a bulky group, it sterically hinders the axial coordination of other ligands to the metal center from the same side of the porphyrin plane.…”

NMR Paramagnetic Relaxation of the Spin 2 Complex MnIIITSPP: A Unique Mechanism

“…The success of the simulations reported below for both gel and solution data indicate that the principal aspects of the relaxation mechanism are now satisfactorily understood. A recent study of Fe III TSPP ( S = 5/2) shows that the 4th-order tetragonal zfs interaction is the critical determinant of the form of the MRD profile for this complex as well, although the role of the 4th-order zfs term in the relaxation mechanism is rather different for integer and half-integer spins.…”

Four complementary theoretical approaches for the analysis of NMR paramagnetic relaxation

“…During the past 10 years, our laboratory has studied various aspects of these phenomena theoretically and experimentally [1][2][3][4][5][6][7][8][9][10][11][12][13] with the objective of understanding the relationship of the NMR-PRE phenomenon to the electron spin level diagram and the electron spin motion. Systematic studies of this relationship have also been published by the laboratories of Kowalewski and Westlund in Sweden [14][15][16] and Kruk in Germany [17].…”