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Title: Correlation of the EPR properties of perchlorotriphenylmethyl radical and their efficiency as DNP polarizers
Author: Banerjee, Debamalya
Paniagua, Juan Carlos
Mugnaini, Verónica
Veciana, Jaume
Feintuch, Akiva
Pons Vallès, Miquel
Goldfarb, Daniella
Keywords: Aigua
Polarització (Física nuclear)
Ressonància magnètica nuclear
Magnetisme nuclear
Química física
Spin (Física nuclear)
Polarization (Nuclear physics)
Nuclear magnetic resonance
Nuclear magnetism
Physical and theoretical chemistry
Nuclear spin
Issue Date: 22-Sep-2011
Publisher: Royal Society of Chemistry
Abstract: Water soluble perchlorinated trityl (PTM) radicals were found to be effective 95 GHz DNP (dynamic nuclear polarization) polarizers in ex situ (dissolution) 13C DNP (Gabellieri et al., Angew Chem., Int. Ed. 2010, 49, 3360). The degree of the nuclear polarization obtained was reported to be dependent on the position of the chlorine substituents on the trityl skeleton. In addition, on the basis of the DNP frequency sweeps it was suggested that the 13C NMR signal enhancement is mediated by the Cl nuclei. To understand the DNP mechanism of the PTM radicals we have explored the 95 GHz EPR characteristics of these radicals that are relevant to their performance as DNP polarizers. The EPR spectra of the radicals revealed axially symmetric g-tensors. A comparison of the spectra with the 13C DNP frequency sweeps showed that although the solid effect mechanism is operational the DNP frequency sweeps reveal some extra width suggesting that contributions from EPR forbidden transitions involving 35,37Cl nuclear flips are likely. This was substantiated experimentally by ELDOR (electron-electron double resonance) detected NMR measurements, which map the EPR forbidden transitions, and ELDOR experiments that follow the depolarization of the electron spin upon irradiation of the forbidden EPR transitions. DFT (density functional theory) calculations helped to assign the observed transitions and provided the relevant spin Hamiltonian parameters. These results show that the 35,37Cl hyperfine and nuclear quadrupolar interactions cause a considerable nuclear state mixing at 95 GHz thus facilitating the polarization of the Cl nuclei upon microwave irradiation. Overlap of Cl nuclear frequencies and the 13C Larmor frequency further facilitates the polarization of the 13C nuclei by spin diffusion. Calculation of the 13C DNP frequency sweep based on the Cl nuclear polarization showed that it does lead to an increase in the width of the spectra, improving the agreement with the experimental sweeps, thus supporting the existence of a new heteronuclear assisted DNP mechanism.
Note: Versió postprint del document publicat a:
It is part of: Physical Chemistry Chemical Physics, 2011, vol. 13, p. 18626-18637
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ISSN: 1463-9076
Appears in Collections:Articles publicats en revistes (Química Inorgànica i Orgànica)

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