The role of the glassy dynamics and thermal mixing in the dynamic nuclear polarization and relaxation mechanisms of pyruvic acid

Abstract

The temperature dependence of 1 H and 13 C nuclear spin-lattice relaxation rate 1/T1 has been studied in the 1.6 K -4.2 K temperature range in pure pyruvic acid and in pyruvic acid containing trityl radicals at a concentration of 15 mM. The temperature dependence of 1/T1 is found to follow a quadratic power law for both nuclei in the two samples. Remarkably the same temperature dependence is displayed also by the electron spin-lattice relaxation rate 1/T1e in the sample containing radicals. These results are explained by considering the effect of the structural dynamics on the relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show that below 4 K the 13 C build up rate scales with 1/T1e, in analogy to 13 C 1/T1 and consistently with a thermal mixing scenario where all the electrons are collectively involved in the dynamic nuclear polarization process and the nuclear spin reservoir is in good thermal contact with the electron spin system.

Electron spin dynamics and spin–lattice relaxation of trityl radicals in frozen solutions

“…29 The relaxation rate is reported as 0.23 T 2.17 s −1 and agrees well over that temperature range with the Orbach–Aminov term and the total relaxation rate for 50 mM Finland trityl in our solvent, ESI, † Section S2.…”

“…Some reports indicate that 1/ T 1e is ∼1 s −1 at 10 K 22–24 which extrapolate to considerably slower rates at 1–2 K. 22,23 Other reports suggest 1/ T 1e > 20–200 s −1 at 10 K 24–26 and 0.5–10 s −1 at 1–2 K. 5,24,27–30 These results have been called ‘‘partially contradictory”, 29 which is not surprising because they come from different trityl radicals at concentrations between 0.2 and 45 mM in different solvents with EPR frequencies of 9.5–336 GHz. The two order-of-magnitude variation in these reported rates does highlight the critical need for better data to support quantitative modelling of DNP.…”

Nuclear magnetic resonance studies of DNP-ready trehalose obtained by solid state mechanochemical amorphization

“…glass doped with trityls [22]. As explained in that framework, glasses can be characterized by a distribution of local lattice dynamics which control the behaviour of several physical properties.…”

Evidence of spin-temperature in dynamic nuclear polarization: an exact computation of the EPR spectrum

“…Then, the parameters β s and ω 0 are obtained from Eqs. (27). This approach permits us to achieve much larger system sizes.…”

“…Then, the rates in (10, 11) can be computed using the matrix elements between each pair of eigenstates and the parameters in Table I. This set of parameters is chosen to represent a pyruvic acid sample doped with trityl radical at the temperature β −1 = 1.2 K and an external magnetic field of 3.35 T. These conditions have been studied experimentally in great detail [25][26][27] and represent a good test for our theoretical model because of clear evidences of thermal mixing in nuclear polarizations.…”