EPR Study of NO radicals encased in modified open C60 Fullerenes (original) (raw)
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Applied Magnetic Resonance, 1997
A sedes of C60 fullerene derivatives containing a nitroxide group has been photoexcited by short LASER pulses in the microwave cavity of a cw-EPR spectrometer. Strongly spin polarized signals have been observed, in glassy matrix as well as in liquid sotution, for both the ground electronic st.ate and the excited quartet state. In the quartet state the excitation resides in the fullerene part and the molecule constitutes a triplet-radical pair with the partner covalently linked. The absorptive or emissive character of the transitions is explained in terms of the mechanism of radicaltriplet interaction producing spin polarization. Opposite initial sign and polarization pattems are observed for molecules with different spacer between nitroxide and fullerene. The time evolution of the relevant sublevel populations is fitted by a kinetic model taking into account quartet deeay constants, quartet and doublet spin-lattice relaxation rates and branching ratios.
The Journal of Physical Chemistry A, 2001
Nitroxide free radical/[60]fullerene derivative liquid solutions photoexcited by visible laser pulses are investigated by time-resolved EPR. Both radical and triplet excited fullerene spin-polarized EPR signals are observed. Their time evolution is examined in terms of CIDEP effects due to electron spin polarization transfer from the initially polarized triplet to the nitroxide and to spin polarization generated by the radical triplet pair mechanism. Radical and triplet spin relaxation times and rate constants for the processes of polarization transfer and radical triplet pair mechanism are obtained.
Structure-Magnetism Relationships inα-Nitronyl Nitroxide Radicals
Chemistry-a European Journal, 1999
oxidized species of the compounds gave rise to only one sub-gap electronic transition near 1.25 (pentamer) and 1.15 eV (hexamer). More detailed experimental conditions of electronic spectral measurements can be found elsewhere [9b,14].
The sign of the exchange interaction between triplet excited fullerene and nitroxide free radicals
Applied Magnetic Resonance, 2000
The sign of the exchange interaction J in a series of radical triplet pairs (RTPs), formed by a nitroxide free radical and a triplet excited fullerene, has been determined from the spin polarization of time-resolved electron paramagnetic resonance spectra. Radical and fullerene are linked together by covalent bonds in different geometries. It is shown that the sign of J depends on the overlap between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly occupied in the excited triplet state. When the overlap does not vanish, a negative contribution to J arises from the admixing of a charge transfer structure in the wave function of the excited doublet state D of the RTP, which does not take place in the excited quartet state Q. The mixing of D* and Q' states lowers the energy of the former spin state and gives antiferromagnetic coupling.
Journal of Physical Chemistry A, 2008
The redox behavior of Sc 3 N@C 68 is studied systematically by means of electrochemistry, in situ ESR/Vis-NIR spectroelectrochemistry, and detailed theoretical treatment. Formation of the negatively and positively charged paramagnetic species for the same trimetallic nitride endohedral fullerene is demonstrated for the first time. The electrochemical study of Sc 3 N@C 68 exhibits two electrochemically irreversible but chemically reversible reduction steps and two reversible oxidation steps. A double-square reaction scheme is proposed to explain the observed redox reaction at cathodic potentials involving the reversible dimerisation of the Sc 3 N@C 68 monoanion. The spin state of the radical cation and the radical anion is probed by ESR spectroscopy, indicating that in both states, the large part of the unpaired spin is delocalized on the fullerene cage. The charged states of the non-isolated pentagon rule fullerene are characterized furthermore by in situ absorption spectroscopy. The interpretation of experimental data is supported by the density functional theory (DFT) calculations of the spin distribution in the anion and cation radicals of Sc 3 N@C 68 and time-dependent DFT calculations of the absorption spectra of the charged species.
Journal of the American Chemical Society, 2004
A nitronyl-nitroxide (NIT) biradical D-NIT2 linked by a single double bond has been engineered and investigated in the solid state by a combination of X-ray diffraction, magnetic susceptibility measurement, EPR, as well as solid-state 1 H and 13 C NMR spectroscopies, and experimental electron density distribution. All techniques reveal that a double bond is a very efficient coupling unit for exchange interactions between two radical moieties. Using a Bleaney-Bowers model dimer (H)-JS1S2), a singlet-triplet energy gap of J)-460 K was found with the singlet state being the ground state. This very strong intramolecular interaction was confirmed by EPR measurements in CH2Cl2 solution (6 10-4 M) or dispersed in a polymer matrix at low concentration. In keeping with these unusual interactions, solid-state NMR signals of the biradical were found to be considerably less shifted than those found for related monoradicals. Temperaturedependent solid-state 13 C NMR spectra of D-NIT2 confirmed the very strong intramolecular coupling constant (J)-504 K). The electron density distribution of D-NIT2 was measured by high resolution X-ray diffraction, which also revealed that this biradical is an ideally conjugated system. The in-depth characterization includes the deformation maps and the observed electron density ellipticities, which exhibit a pronounced σ-π character of the OsNsCdCsNsO cores in keeping with an efficient electronic delocalization along the alkene spacer.
Environmental effects on electron spin relaxation in N@C60
Physical Review B, 2007
We examine environmental effects of surrounding nuclear spins on the electron spin relaxation of the N@C60 molecule (which consists of a nitrogen atom at the centre of a fullerene cage). Using dilute solutions of N@C60 in regular and deuterated toluene, we observe and model the effect of translational diffusion of nuclear spins of the solvent molecules on the N@C60 electron spin relaxation times. We also study spin relaxation in frozen solutions of N@C60 in CS2, to which small quantities of a glassing agent, S2Cl2 are added. At low temperatures, spin relaxation is caused by spectral diffusion of surrounding nuclear 35,37 Cl spins in the S2Cl2, but nevertheless, at 20 K, T2 times as long as 0.23 ms are observed.
Journal of The American Chemical Society, 2008
DFT calculations of Sc 3 N@C 80 in the neutral and anionic states are performed which revealed that in the neutral state of the nitride clusterfullerene the lowest energy structure has C 3 symmetry, while in the anionic and dianionic states the C 3v conformer has the lowest energy. Barriers to the cluster rotation inside the cage are also found to increase in the charge states. The 45 Sc hyperfine slitting constant, a(Sc), in Sc 3 N@C 80 anion radical is calculated by different theoretical approaches and in different conformations of Sc 3 N cluster. It is found that a(Sc) is strongly dependent on the cluster orientation with respect to the cage, covering a range form −10 to +25 Gauss at the B3LYP/6-311G*//PBE/TZ2P level of theory. A thorough analysis of the computed values as well as comparison of unrestricted and orbital-restricted calculations revealed that the polarization contribution to a(Sc) is about −10 Gauss and does not depend on the cluster orientation. Dependence of the predicted a(Sc) values on the density functional form (LSDA, BP, PBE, BLYP, OLYP, TPSS, B3LYP, and TPSSh), the basis set, as well as on the scalar-relativistic and spin-orbit corrections were investigated. The analysis of the charge distribution in the Sc 3 N@C 80 − anion radical revealed an interesting peculiarity of its electronic structure: while the spin density mostly resides on the cluster, only a slight decrease of its charge is found using both Bader and Mulliken definitions of atomic charges. A set of other endohedral
Spin relaxation of muonium-substituted ethyl radicals (MuCH2C-dot H2) in the gas phase
Chemical Physics, 1996
The spin relaxation of the muonium-substituted ethyl radical ͑MuCH 2 Ċ H 2 ͒ and its deuterated analog ͑MuCD 2 Ċ D 2 ͒ has been studied in the gas phase in both transverse and longitudinal magnetic fields spanning the range ϳ0.5-35 kG, over a pressure range from ϳ1-16 atm at ambient temperature. The Mu 13 CH 2 13 Ċ H 2 radical has also been investigated, at 2.7 atm. For comparison, some data is also reported for the MuCH 2 Ċ ͑CH 3 ͒ 2 ͑Mu-t-butyl͒ radical at a pressure of 2.6 atm. This experiment establishes the importance of the SR technique in studying spin relaxation phenomena of polyatomic radicals in the gas phase, where equivalent ESR data is sparse or nonexistent. Both T 1 ͑longitudinal͒ and T 2 ͑transverse͒ SR relaxation rates are reported and interpreted with a phenomenological model. Relaxation results from fluctuating terms in the spin Hamiltonian, inducing transitions between the eigenstates assumed from an isotropic hyperfine interaction. Low-field relaxation is primarily due to the electron, via both the nuclear hyperfine ͑S-A-I͒ and the spin rotation interactions ͑S-J͒, communicated to the muon via the isotropic muon-electron hyperfine interaction. At the highest fields, direct spin flips of the muon become important, due to fluctuations in the anisotropic part of the muon-electron hyperfine interaction. In the intermediate field region a muon-electron ''flip-flop'' relaxation mechanism dominates, due partly to the anisotropic hyperfine interaction and partly to modulation of the isotropic muon-electron hyperfine coupling. In the case of the T 2 rates, electron relaxation mechanisms dominate over a much wider field range than for the T 1 rates, and inhomogeneous line broadening also contributes. The fluctuations that induce both the T 1 and T 2 relaxation rates are described by a single correlation time, c , inversely proportional to the pressure. An effective spin-reorientation cross section is deduced from this pressure dependence, J ϳ100Ϯ20 Å 2 , for all isotopically substituted ethyl radicals. This is similar to the geometrical cross section, but about a factor of 4 larger than values of J found for similar-sized diamagnetic molecules by gas phase NMR, primarily reflecting the longer range of the electron-induced intermolecular potential.