Electronic and molecular structures of C60-based polyanionic high-spin molecular clusters: Direct spin identification and electron spin transient nutation spectroscopy for high-spin chemistry

Nakazawa, Shigeaki; Sato, Kazunobu; Shiomi, Daisuke; Franco, M. Luisa T. M. B.; Lazana, M. Celina R. L. R.; Shohoji, M. Cândida B. L.; Itoh, Koichi; Takui, Takeji

doi:10.1016/j.ica.2008.03.048

Cited by 14 publications

(9 citation statements)

References 33 publications

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“…All these experimental results demonstrate that the extensive spin delocalization onto the corannulene moiety (Figure 3, see below) results in significant electronic‐spin communication via the curved and nonalternant π‐surface of corannulene 23. Furthermore, in terms of the curved π‐conjugated structure, we believe that diradical 2 has its own place as an intermediate case between thus‐far reported planar π‐conjugated systems and completely 3D π‐conjugated species such as neutral C 60 in the triplet state, noting that spin–orbit contributions are relevant to the particular canonical resonance structures (see Figure S12 in the Supporting Information) 2d,f…”

Section: Methodsmentioning

confidence: 90%

Three‐Dimensional Intramolecular Exchange Interaction in a Curved and Nonalternant π‐Conjugated System: Corannulene with Two Phenoxyl Radicals

et al. 2010

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In recent years, curved p-conjugated molecules such as fullerenes and carbon nanotubes have attracted much attention not only in chemistry but also in materials science. [1] Their intra/intermolecular interactions within/between threedimensional (3D) curved p-electron networks play intrinsically vital roles in their unique properties and functionalities. Among them, intramolecular magnetic interaction between electronic spins on a curved p surface was extensively studied for ionic species of C 60 such as C 60 2À and C 60 3À .[2] Their electronic structures are greatly influenced by not only the dynamic spin polarization of electrons but also the negative charges on the spherical p-conjugated system and the countercation.Thus, we have focused on neutral diradical systems, which are known to be the most useful probes for studying intramolecular magnetic interactions in organic molecules. [3] While many neutral diradical derivatives relevant to planar pconjugated systems have so far been investigated, studies on curved p-conjugated neutral diradicals are limited to a single C 60 -based system in which [60]fullerene is linked to two nitroxide radicals.[4] However, their intramolecular exchange interaction J through the C 60 skeleton was very weak (j Jk À1 B j < 0.1 K)[4b] because of the small spin delocalization onto the C 60 p network from the nitroxide radicals with spin-localized nature on the NO moieties. Therefore, in order to evaluate an intramolecular exchange interaction in a curved p-conjugated system in a quantitative manner, synthesis and isolation of a stable neutral diradical derivative with extensively spindelocalized nature on its curved p-conjugated system have been the focus of current attention in molecular magnetism and open-shell chemistry. [2,5] Recently, we studied corannulene [6] -based stable neutral monoradical systems, [7] such as a phenoxyl radical derivative 1[7d] with highly spin-delocalized nature on the intrinsically 3D bowl-shaped and nonalternant p-conjugated network. These studies inspired us to propose a 3D intramolecular exchange interaction in this class of curved p radicals: in terms of geometry, they are intermediate between a planar p radical such as a phenalenyl system [8] and a tetrahedral s radical. We have now, for the first time, synthesized and isolated a corannulene-based neutral diradical, namely, 2, which has two phenoxyl radical moieties, as air-stable crystals. Due to its highly spin-delocalized nature, 2 shows strong intramolecular exchange interaction (Jk À1 B = À405 AE 2 K) through the 3D corannulene p-electron network. Bond-length analyses and DFT calculations showed that 2 has contributions from diradical canonical forms a-g and closed KekulØ structure d (Scheme 1). Furthermore, in the crystalline state the presence of three crystallographically independent diradical molecules with different curvature enabled us to study the relationship between the curvature of the corannulene p skeleton and the

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Section: Methodsmentioning

confidence: 90%

Three‐Dimensional Intramolecular Exchange Interaction in a Curved and Nonalternant π‐Conjugated System: Corannulene with Two Phenoxyl Radicals

et al. 2010

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“…ESN spectroscopy applied to systems with good spin quantum numbers S has been well established. 32,34,[39][40][41][42][43][44][45][46][47] In the weak extreme limit of the irradiation power, the nutation frequency o n for the |M S i -|M S + 1i allowed transition of the isolated spin state S is described by a rather simple expression in first order, 39,44,47…”

Section: A General Treatment Of Pulse-based Electron Spin Nutation Sp...mentioning

confidence: 99%

“…[35][36][37][38] In addition, twodimensional (2D) pulse-based electron spin transient nutation (2D-ESN or 2D-ESTN) spectroscopy can differentiate different spin multiplicities in a mixture straightforwardly, to yield zerofield splitting parameters by reliable spectral simulations. 32,34,[39][40][41][42][43][44][45][46][47] The nutation experiments in non-oriented media can be carried out with conventional X-band pulsed ESR spectrometers equipped with 1 kW TWT microwave amplifiers. 32,[40][41][42][43] Thus, in high spin chemistry, even if exchange interactions are as small as ca.…”

Section: Introductionmentioning

confidence: 99%

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

Ayabe

Sato

Nishida

et al. 2012

Phys. Chem. Chem. Phys.

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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.

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“…Due to the spherical symmetry, the electronic states for a single C 60 may be compared to the multiplet states of a single atom in the second row with incompletely filled 2p shell. The 2s 2 2p 2 configuration of carbon produces the states: 3 P, 1 D, 1 S. According to Hund's rule, the ground state has the highest possible spin, therefore 3 P. In the C 60 anions the energy difference between high spin and low spin states is very small [26][27][28]. For C 60 2− , the reason is that the exchange integrals are numerically small, of about the same size as the relevant integrals for the t 1u 2 → t 2g 2 correlation effect [29][30][31].…”

Section: Molecular Electronic and Vibrational Structurementioning

confidence: 99%

Phonons in Lattice and Structural Dynamics

Larsson

2010

Advances in Condensed Matter Physics

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The critical temperature (T C ) of superconductivity in A 3 C 60 compounds is generally lower smaller with alkali atoms (A). Furthermore T C decreases with applied pressure. In the BCS model, these trends are explained by the lower density of states at the Fermi level for a decreased lattice constant (R). There is more than one counterexample, however, suggesting that BCS does not give the whole truth. The most important one is that the compound with the largest lattice constant, Cs 3 C 60 , is not superconducting at all at ambient pressure. In this paper we derive a novel model where a negative lattice contribution to Hubbard U, proportional to 1/R, is taken into account. It is possible to explain why A 3 C 60 compounds with A = Li, and Na have a low T C or are not superconducting at all, and why Cs 3 C 60 is superconducting only at applied pressure and then with the highest T C of all C 60 alkali fullerides. It is concluded that the density of states mechanism derived in the BCS model is in doubt. Nevertheless superconductivity in A 3 C 60 depends on electron-phonon coupling. The dominating phonon is the bond stretching A g phonon, a breathing phonon for the whole fullerene molecular ion.

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Electronic and molecular structures of C60-based polyanionic high-spin molecular clusters: Direct spin identification and electron spin transient nutation spectroscopy for high-spin chemistry

Cited by 14 publications

References 33 publications

Three‐Dimensional Intramolecular Exchange Interaction in a Curved and Nonalternant π‐Conjugated System: Corannulene with Two Phenoxyl Radicals

Three‐Dimensional Intramolecular Exchange Interaction in a Curved and Nonalternant π‐Conjugated System: Corannulene with Two Phenoxyl Radicals

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

Phonons in Lattice and Structural Dynamics

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