TY - JOUR

T1 - A tailored multi-frequency EPR approach to accurately determine the magnetic resonance parameters of dynamic nuclear polarization agents

T2 - Application to AMUPol

AU - Gast, P.

AU - Mance, D.

AU - Zurlo, E.

AU - Ivanov, K. L.

AU - Baldus, M.

AU - Huber, M.

PY - 2017/1/1

Y1 - 2017/1/1

N2 - To understand the dynamic nuclear polarization (DNP) enhancements of biradical polarizing agents, the magnetic resonance parameters need to be known. We describe a tailored EPR approach to accurately determine electron spin-spin coupling parameters using a combination of standard (9 GHz), high (95 GHz) and ultra-high (275 GHz) frequency EPR. Comparing liquid- and frozen-solution continuous-wave EPR spectra provides accurate anisotropic dipolar interaction D and isotropic exchange interaction J parameters of the DNP biradical AMUPol. We found that D was larger by as much as 30% compared to earlier estimates, and that J is 43 MHz, whereas before it was considered to be negligible. With the refined data, quantum mechanical calculations confirm that an increase in dipolar electron-electron couplings leads to higher cross-effect DNP efficiencies. Moreover, the DNP calculations qualitatively reproduce the difference of TOTAPOL and AMUPol DNP efficiencies found experimentally and suggest that AMUPol is particularly effective in improving the DNP efficiency at magnetic fields higher than 500 MHz. The multi-frequency EPR approach will aid in predicting the optimal structures for future DNP agents.

AB - To understand the dynamic nuclear polarization (DNP) enhancements of biradical polarizing agents, the magnetic resonance parameters need to be known. We describe a tailored EPR approach to accurately determine electron spin-spin coupling parameters using a combination of standard (9 GHz), high (95 GHz) and ultra-high (275 GHz) frequency EPR. Comparing liquid- and frozen-solution continuous-wave EPR spectra provides accurate anisotropic dipolar interaction D and isotropic exchange interaction J parameters of the DNP biradical AMUPol. We found that D was larger by as much as 30% compared to earlier estimates, and that J is 43 MHz, whereas before it was considered to be negligible. With the refined data, quantum mechanical calculations confirm that an increase in dipolar electron-electron couplings leads to higher cross-effect DNP efficiencies. Moreover, the DNP calculations qualitatively reproduce the difference of TOTAPOL and AMUPol DNP efficiencies found experimentally and suggest that AMUPol is particularly effective in improving the DNP efficiency at magnetic fields higher than 500 MHz. The multi-frequency EPR approach will aid in predicting the optimal structures for future DNP agents.

KW - SOLID-STATE NMR

KW - 275 GHZ

KW - NITROXIDE BIRADICALS

KW - POLARIZING AGENTS

KW - CONTINUOUS-WAVE

KW - HIGH-FREQUENCY

KW - AQUEOUS-MEDIA

KW - PULSED EPR

KW - SPECTROSCOPY

KW - DNP

UR - http://www.scopus.com/inward/record.url?scp=85026995982&partnerID=8YFLogxK

U2 - 10.1039/c6cp05864g

DO - 10.1039/c6cp05864g

M3 - Article

C2 - 28098267

AN - SCOPUS:85026995982

VL - 19

SP - 3777

EP - 3781

JO - Physical Chemistry Chemical Physics

JF - Physical Chemistry Chemical Physics

SN - 1463-9076

IS - 5

ER -