EPR Line Shifts and Line Shape Changes Due to Heisenberg Spin Exchange and Dipole–Dipole Interactions of Nitroxide Free Radicals in Liquids: 9. An Alternative Method to Separate the Effects of the Two Interactions Employing ¹⁵N and ¹⁴N

Abstract

A method to separate the effects of Heisenberg spin exchange (HSE) and dipole–dipole (DD) interactions on EPR spectra of nitroxide spin probes in solution by employing 15N and 14N nitroxide spin probes in parallel experiments is developed theoretically and tested experimentally. Comprehensive EPR measurements are reported of 4-oxo-2,2,6,6-tetramethylpiperidine-d16;1-15N-1-oxyl (perdeuterated 15N Tempone; 15pDT), in 70 wt % aqueous glycerol as functions of concentration and temperature. The method, termed the relative broadening constant method (RBCM), is demonstrated by using the present results together with those in the literature that employed perdeuterated 14N Tempone (14pDT) under identical conditions. In principle, the separation of DD and HSE is dependent on the model of diffusion and molecular-kinetic parameters; however, within present day experimental uncertainties, the RBCM method turns out to be insensitive to the model. The earlier methods to separate DD and HSE by measuring the dispersion component introduced by the two interactions shows general agreement with the RBCM; however, there are discrepancies larger than estimated uncertainties due to random errors. Thus, further support is found for Salikhov’s recent theory of the effects of DD and HSE on EPR spectra (Appl. Magn. Reson.201038237); however, detailed confirmation is still lacking. The RBCM affords a possible approach to separate HSE and DD in spectra complicated by slow motion and/or overlap with other resonance lines, allowing the method to be used in situations more complicated than low-viscosity simple liquids.

2D Arrays of Organic Qubit Candidates Embedded into a Pillared-Paddlewheel Metal–Organic Framework

“…Moving from top to bottom in Figure B as the samples change from low to high nitroxide concentrations, the spectra become increasingly broadened. This broadening is due to Heisenberg spin exchange as neighboring electrons with identical spin states are close enough to swap their spin information at a rate that is much faster than the rate of the inverse of the hyperfine interactions . In going from lower to higher temperatures at intermediate concentrations, the line shapes sharpen at higher temperatures as a result of motional narrowing.…”

Continuous Diffusion Model for Concentration Dependence of Nitroxide EPR Parameters in Normal and Supercooled Water

“…Standard relations imply that the HSE and DDI contributions to the coefficient W j can be separated by defining H j = ( b j W j + V j )/(1/ j + b j ) and D j = W j – H j , where b 1 = 2/7 for 15 N-pDTEMPONE and b 2 = 4/19 for 14 N-pDTEMPONE . It can be easily shown that H j = W j HSE and D j = W j DD when eq holds.…”

EPR Line Shifts and Line Shape Changes Due to Spin Exchange Between Nitroxide Free Radicals in Liquids 10. Spin-Exchange Frequencies of the Order of the Nitrogen Hyperfine Interaction: A Hypothesis

“…[1, 6–8, 11–13] Here, the results are far more interesting than for 15 N, because there are differences between the exact and perturbation results for every parameter. We have not been able to find any simple relationships between the exact and perturbation results similar to that in Eq.…”

“…[1, 6–8, 11–13] All entries given as fractions In Tables 2 – 5 are exact to within ±1 in the least significant of 16 digits. Comparing Tables 2 and 3 highlights the fact that values of V pp± 1 and Δ H pp± 1 are identical for the absorption components and their perturbation counterparts, but only to 5 significant figures; in fact, the largest difference is 5 x 10 −16 .…”