Pulsed electron spin nutation spectroscopy of weakly exchange-coupled biradicals: a general theoretical approach and determination of the spin dipolar interaction

Abstract

Weakly exchange-coupled biradicals have attracted much attention in terms of their DNP application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits. Pulse-ESR based electron spin nutation (ESN) spectroscopy applied to biradicals is generally treated as transition moment spectroscopy from the theoretical side, illustrating that it is a powerful and facile tool to determine relatively short distances between weakly exchange-coupled electron spins. The nutation frequency as a function of the microwave irradiation strength ω(1) (angular frequency) for any cases of weakly exchange-coupled systems can be classified into three categories; D(12) (spin dipolar interaction)-driven, Δg-driven and ω(1)-driven nutation behaviour with the increasing strength of ω(1). For hetero-spin biradicals, Δg effects can be a dominating characteristic in the biradical nutation spectroscopy. Two-dimensional pulse-based electron spin nutation (2D-ESN) spectroscopy operating at the X-band can afford to determine small values of D(12) in weakly exchange-coupled biradicals in rigid glasses. The analytical expressions derived here for ω(1)-dependent nutation frequencies are based on only four electronic spin states relevant to the biradicals, while real biradical systems often have sizable hyperfine interactions. Thus, we have evaluated nuclear hyperfine effects on the nutation frequencies to check the validity of the present theoretical treatment. The experimental spin dipolar coupling of a typical TEMPO-based biradical 1, (2,2,6,6-tetra[((2)H(3))methyl]-[3,3-(2)H(2),4-(2)H(1),5,5-(2)H(2)]piperidin-N-oxyl-4-yl)(2,2,6,6-tetra[((2)H(3))methyl]-[3,3-(2)H(2),4-(2)H(1),5,5-(2)H(2),(15)N]piperidin-(15)N-oxyl-4-yl) terephthalate in a toluene glass, with a distance of 1.69 nm between the two spin sites is D(12) = -32 MHz (the effect of the exchange coupling J(12) is vanishing due to the homo-spin sites of 1, i.e.Δg = 0), while 0 < |J(12)|≦ 1.0 MHz as determined by simulating the random-orientation CW ESR spectra of 1. In addition, we have carried out Q-band pulsed ELDOR (ELectron-electron DOuble Resonance) experiments to confirm whether the obtained values for D(12) and J(12) are accurate. The distance is in a fuzzy region for the distance-measurements capability of the conventional, powerful ELDOR spectroscopy. The strong and weak points of the ESN spectroscopy with a single microwave frequency applicable to weakly exchange-coupled multi-electron systems are discussed in comparison with conventional ELDOR spectroscopy. The theoretical spin dipolar tensor and exchange interaction of the TEMPO biradical, as obtained by sophisticated quantum chemical calculations, agree with the experimental ones.

A tailored multi-frequency EPR approach to accurately determine the magnetic resonance parameters of dynamic nuclear polarization agents: application to AMUPol

“…2d) is direct evidence that J has a finite value, a conclusion also reached in a recent publication, showing a similar liquid solution 9 GHz EPR spectrum. 22 Similar lineshapes have been observed by Ayabe et al 34 for other biradicals.…”

Engineering coherent interactions in molecular nanomagnet dimers

“…, organic radicals 10,11 , and molecular magnets 12,13,14 . We proposed exploiting molecular magnets based on heterometallic antiferromagnetic rings 15,16 .…”

“…19 In the context of molecular spin qubits, the simplest conceivable multi-qubit structure is a molecular dimer. This observation has motivated various efforts to synthesise dimers including, for example, N@C 60 -N@C 60, 20,21 radical-radical, 10,11 N@C 60 -molecular magnet (Kaminski D. et al in preparation) and molecular magnet-molecular magnet. [22][23][24][25] However, the design of a dimer specifically to host two-qubit experiments should take account of the importance of three key time scales relative to one another: T 2 , the individual qubit phase relaxation time, must be longest; h/J, which is characteristic of the duration of two-qubit gates (where J is the inter-qubit interaction energy and h is Planck's constant) should be intermediate; and the single-qubit manipulation time should be the shortest.…”

“…4 One of the key challenges in realizing a quantum computer lies in identifying a physical system that hosts quantum states sufficiently coherently, and provides appropriate interactions for implementing logic operations. 5 Among the molecular spin systems that have been proposed as qubit candidates are N@C 60, 6-9 organic radicals 10,11 and molecular magnets. [12][13][14] We proposed exploiting molecular magnets based on heterometallic antiferromagnetic rings.…”

Li/MgO Catalysts Doped with Alio‐valent Ions. Part II: Local Topology Unraveled by EPR/NMR and DFT Modeling

“…[9,10] The principle of the TN-EPR spectroscopy is based on the fact that the transition moment between spin sublevelso nr esonance is proportional to the nutation frequency n of spin magnetization in the rotating frame of an oscillatingM Wf ield. Within specific limits, [11] depending on the relative size of spin Hamiltonian parameters compared to the irradiating MW field amplitude, the nutation frequencyb etween j S, msi$ j S, m 2 + 1i in terms of the reference nutation frequency n ref of a S = 1/2 spin state, is given according to Equation (1):…”