Optical Nuclear Spin Polarization (ONP) Studies of Short‐Lived Excited Triplet States in Anthracene Crystals Doped with Phenazine. Structure and Mechanism of Reversible Photochemical H‐Transfer

Buntkowsky, Gerd; Nack, Melanie; Stehlik, D.; Vieth, Hans‐Martin

doi:10.1002/ijch.198900015

Cited by 13 publications

(31 citation statements)

References 33 publications

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“…Thus the maxima in the curve can be identified as a typical level crossing effect. Similar level crossing effects are also known from other types of spectroscopy, as for example optical nuclear polarization [51,52]. This result is not only important on its own, but it also helps us to draw conclusions on the fate of a para-water molecule which is sublimating on or evaporating from an ice crystal.…”

Section: Discussionsupporting

confidence: 61%

Mechanisms of Dipolar Ortho/Para-H₂O Conversion in Ice

Buntkowsky¹,

Limbach

Walaszek³

et al. 2008

Zeitschrift Für Physikalische Chemie

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In this paper a possible explanation for an unexpected ortho/para-water ratio in the gas clouds of comets is given. The description is based on the quantum-mechanical density matrix formalism and the spin temperature concept. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a four spin system, created by two nearest neighbour water molecules, spin eigenstates and their dynamics under the influence of their mutual dipolar interactions are studied. It is shown that a fast conversion between ortho- and para-states occurs on a msec time scale, caused by the intermolecular homonuclear magnetic dipolar interaction. Moreover the spin eigenstates of water in an ice crystal are determined by magnetic dipolar interactions and are not given by normal ortho- and para-H2O states of gaseous water. As a result of this the spin temperature of gaseous water evaporated from ice depends strongly on its evaporation history and the ortho/para-ratio of water molecules are only an indirect measure of the temperature of ice crystals from where they descend. This result could explain the unexpected experimentally observed ortho/para-ratios in the clouds of comets.

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Section: Discussionsupporting

confidence: 61%

Mechanisms of Dipolar Ortho/Para-H₂O Conversion in Ice

Buntkowsky¹,

Limbach

Walaszek³

et al. 2008

Zeitschrift Für Physikalische Chemie

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“…CIDNP is formed in diamagnetic products of chemical reactions having radical pair (RP) intermediates 1 . Such photo-induced RP reactions (H-transfer) have been observed by Stehlik et al [2][3][4] in monocrystalline matrices doped with dye molecules. They proceed via a triplet precursor followed by spin-selective decay back to the singlet ground state and lead to efficient nuclear polarization mainly in avoided crossing regions.…”

Section: Introductionmentioning

confidence: 79%

“…Ideally, analysis of CIDNP in such cases should be performed in single crystals, which is possible in some cases. [2][3][4]32,33 Such a study is, however, beyond the scope of the present work.…”

Section: Discussionmentioning

confidence: 99%

Magnetic field and orientation dependence of solid-state CIDNP

Sosnovsky

Lukzen

Vieth

et al. 2019

The Journal of Chemical Physics

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The magnetic field dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) in solid-state systems is analyzed theoretically with the aim to explain the puzzling sign change of polarization found at low fields [Sci. Rep., 7 (2017) 12111]. We exploit the analysis of polarization in terms of level crossings and level anti-crossings trying to identify the positions of features in the CIDNP field dependence with specific crossings between spin energy levels of the radical pair. Theoretical treatment of solid-state CIDNP reveals a strong orientation dependence of polarization due to the spin dynamics conditioned by anisotropic spin interactions. Specifically, different anisotropic CIDNP mechanisms become active at different magnetic fields and different molecular orientations. Consequently, the field dependence and orientation dependence of polarization need to be analyzed together in order to rationalize experimental observations. By considering both magnetic field and orientation dependence of CIDNP, we are able to explain the previously measured CIDNP field dependence in photosynthetic reaction centers and to obtain a good qualitative agreement between experimental observations and theoretical results.

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“…First experiments 40,41 on pure anthracene single crystals showed an increase of the proton polarization up to 3 orders of magnitude as compared to thermal polarization. Later investigations on several other molecular crystals (for example refs , , , , , , ) showed that ONP is a rather common phenomenon in molecular crystals. In a typical ONP experiment, optically active molecules are photoexcited into a short-lived paramagnetic triplet state the polarization of which is used as source of spin order.…”

Section: Optical Nuclear Polarizationmentioning

confidence: 97%

Application of Optical Nuclear Polarization Enhanced ¹³C NMR

et al. 1998

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A gain in detection sensitivity of more than 3 orders of magnitude has been achieved in high-resolution solid-state 13C NMR of monocrystalline fluorene doped with acridine by applying optical nuclear polarization (ONP) via excited triplet states to protons and transferring this proton polarization to the 13C nuclei. This sensitivity gain was utilized to measure the angular dependence (rotation pattern) of the 13C NMR lines. In this way the principal values and orientations of all 13C chemical shielding tensors were determined. While the 13C shielding tensor of the bridging methylene group exhibits only a small anisotropy, at the aromatic carbon positions the typical strong anisotropy is observed. All tensors belonging to the same molecule have one principal axis, perpendicular to the molecular plane, in common, showing that in the crystal lattice the fluorene molecule is in a planar configuration. The experimental data are compared to ab initio calculations employing optimized geometries and gauge included atomic orbitals density functional theory (GIAO DFT). The orientations of all calculated tensors are in excellent agreement with the experimental data. On an absolute scale the calculated shielding parameters reproduce the experimental values reasonably well. A significant improvement of the calculated eigenvalues is achieved by shifting the tensors employing data from calculations of benzene and methane.

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Optical Nuclear Spin Polarization (ONP) Studies of Short‐Lived Excited Triplet States in Anthracene Crystals Doped with Phenazine. Structure and Mechanism of Reversible Photochemical H‐Transfer

Cited by 13 publications

References 33 publications

Mechanisms of Dipolar Ortho/Para-H₂O Conversion in Ice

Mechanisms of Dipolar Ortho/Para-H₂O Conversion in Ice

Magnetic field and orientation dependence of solid-state CIDNP

Application of Optical Nuclear Polarization Enhanced ¹³C NMR

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Optical Nuclear Spin Polarization (ONP) Studies of Short‐Lived Excited Triplet States in Anthracene Crystals Doped with Phenazine. Structure and Mechanism of Reversible Photochemical H‐Transfer

Cited by 13 publications

References 33 publications

Mechanisms of Dipolar Ortho/Para-H2O Conversion in Ice

Mechanisms of Dipolar Ortho/Para-H2O Conversion in Ice

Magnetic field and orientation dependence of solid-state CIDNP

Application of Optical Nuclear Polarization Enhanced 13C NMR

Contact Info

Product

Resources

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Mechanisms of Dipolar Ortho/Para-H₂O Conversion in Ice

Mechanisms of Dipolar Ortho/Para-H₂O Conversion in Ice

Application of Optical Nuclear Polarization Enhanced ¹³C NMR