1H relaxation dispersion in solutions of nitroxide radicals: Effects of hyperfine interactions with 14N and 15N nuclei

Abstract

(1)H relaxation dispersion of decalin and glycerol solutions of nitroxide radicals, 4-oxo-TEMPO-d(16)-(15)N and 4-oxo-TEMPO-d(16)-(14)N was measured in the frequency range of 10 kHz-20 MHz (for (1)H) using STELAR Field Cycling spectrometer. The purpose of the studies is to reveal how the spin dynamics of the free electron of the nitroxide radical affects the proton spin relaxation of the solvent molecules, depending on dynamical properties of the solvent. Combining the results for both solvents, the range of translational diffusion coefficients, 10(-9)-10(-11) m(2)/s, was covered (these values refer to the relative diffusion of the solvent and solute molecules). The data were analyzed in terms of relaxation formulas including the isotropic part of the electron spin - nitrogen spin hyperfine coupling (for the case of (14)N and (15)N) and therefore valid for an arbitrary magnetic field. The influence of the hyperfine coupling on (1)H relaxation of solvent molecules depending on frequency and time-scale of the translational dynamics was discussed in detail. Special attention was given to the effect of isotope substitution ((14)N/(15)N). In parallel, the influence of rotational dynamics on the inter-molecular (radical - solvent) electron spin - proton spin dipole-dipole coupling (which is the relaxation mechanism of solvent protons) was investigated. The rotational dynamics is of importance as the interacting spins are not placed in the molecular centers. It was demonstrated that the role of the isotropic hyperfine coupling increases for slower dynamics, but it is of importance already in the fast motion range (10(-9)m(2)/s). The isotope effects is small, however clearly visible; the (1)H relaxation rate for the case of (15)N is larger (in the range of lower frequencies) than for (14)N. It was shown that when the diffusion coefficient decreases below 5 × 10(-11) m(2)/s electron spin relaxation becomes of importance and its role becomes progressively more significant when the dynamics slows done. As far as the influence of the rotational dynamics is concerned, it was show that this process is of importance not only in the range of higher frequencies (like for diamagnetic solutions) but also at low and intermediate frequencies.

Translational diffusion in paramagnetic liquids by 1H NMR relaxometry: Nitroxide radicals in solution

“…The anisotropic part of this coupling influences the 1 H spin-lattice relaxation by affecting the energy level structure of the electron spin. In our recent paper 25 we have derived an expression for the 1 H spin-lattice relaxation rate R par 1I (ω I ) (ω I = 2πν I , where ν I is proton resonance frequency) in solutions of 14 N radicals including this effect. 25 Thus, the expression is valid for an arbitrary magnetic field (resonance frequency) and it yields 13 ) + e 2 J (ω 46 ) + g 2 J (ω 56 )]…”

“…This parameter is well defined for the force-free hard-sphere model 10,11 which assumes that every molecule contains one spin which is located in the molecular centre; then d is given as a sum of the molecular radii. For polyatomic molecules d is an effective quantity of the order of the molecular sizes, for instance, for decalin solution of 4-oxo-TEMPO-d 16 it has been found d ∼ = 5 Å, 25 (11)). Vertical dashed lines indicate frequencies ν I at which ν S ≈ A/2π (green), ω S τ trans ∼ = 1 (blue), and ω I τ trans ∼ = 1 (red).…”

“…The proton spin relaxation is, however, affected by the hyperfine coupling between the electron and nitrogen spins. [21][22][23][24][25] The isotropic part of this coupling modifies the energy levels of the electron spin, while the anisotropic part (modulated by rotational dynamics of the paramagnetic molecule) leads to electron spin relaxation. In consequence, a description of the PRE effect caused by nitroxide radicals, valid for an arbitrary resonance frequency has proven to be a challenge.…”

“…First the problem has been solved for 15 N-containing radicals 22 (as 15 N nuclei possess spin 1/2, the description is less complicated than for the 14 N case with the nitrogen spin 1); recently we have formulated a theory of PRE for 14 N-containing radicals in solution. 25 22 have been combined, in the same work, with the properties of the translational spectral density with the purpose to link the translational diffusion coefficient with the dependence (in fact, dependencies in this case-we shall explain this in the theory section) of the 1 H spin-lattice relaxation dispersion, in analogy to the case of diamagnetic liquids. [12][13][14][15] As 14 N spin probes are much more frequently used than 15 N radicals, in the present paper we extend this approach to the case of 14 N. We wish to stress that this is possible only on the basis of the recently developed description of PRE effects caused by 14 N-radicals.…”

“…[12][13][14][15] As 14 N spin probes are much more frequently used than 15 N radicals, in the present paper we extend this approach to the case of 14 N. We wish to stress that this is possible only on the basis of the recently developed description of PRE effects caused by 14 N-radicals. 25 The motivation for this undertaking is the success of NMR relaxometry as a method probing translational dynamics in diamagnetic systems and some earlier (few, but in our opinion very interesting) attempts of extracting diffusion coefficient from 1 H inter-molecular relaxation in spin labeled systems, more precisely water diffusion coefficient in the vicinity of protein surface to which radical molecules are bound. 26,27 The attempts were, however, based on a theory of PRE effects 6,8,28,29 which is valid only at high frequencies (referred as Solomon-Bloembergen-Morgan (SBM) expression), as it does not take into account the hyperfine-interactions.…”