Dynamics Of Nuclear Spin Polarization

1
Dynamics Of Nuclear Spin Polarization
investigated by simultaneous NMR and polarized
neutron scattering
J. Kohlbrecher Paul Scherrer Institute CH-5232
Villigen Switzerland
B. van den Brandt, P. Hautle, J. Kohlbrecher,
J.A. Konter, S. Mango (PSI) H. Glättli, E.
Leymarie (CEA-Saclay) I. Grillo, R. May (ILL) H.
Jouve, H.B. Stuhrmann (IBS) O. Zimmer (TU München)
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Dynamics Of Nuclear Spin Polarization
investigated by simultaneous NMR and polarized
neutron scattering
Joachim Kohlbrecher
Contents

• DNP a two step process
• How to measure clusters of polarized protons
• scattering of polarized neutron on polarized
  nuclei
• small angle neutron scattering (SANS)
• model system EHBA-Cr(V)
• time-resolved SANS and NMR experiments

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DNP a two step process
1.
polarization diffuses to the bulk protons by
flip-flop transitions time constant tdiff few
seconds

• protons close to electron polarize first
• time constant tpol lt 1 s

• microwave induced direct interaction
• falls off like r6

paramagnetic centre generates strong local field
close protons ? strongly coupled to the p.c. ?
weakly coupled to the bulk ? poorly visible"
by cw-NMR bulk protons ? weakly coupled to the
p.c ? "visible" by cw-NMR
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How to measure clusters of polarized protons
what do we want to see

1. time-dependent polarization built-up of the
   proton spins around the radical
2. and in the bulk

requirements for experimental method

1. sensitive on polarization of protons
2. sensitive on length scale in nm-range

experimental method

1. cw-NMR (bulk protons)
2. neutron scattering technique (close protons)

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Neutron scattering technique
polarized neutron scattering on polarized nuclei
cold neutrons and their interaction with matter
wavelength 4 Å frequency 1.2 THz energy
5meV wavevector 1.55 1/Å velocity 980 m/s
temperature 58 K
interaction potential for a single atom
refraction index
l range of V(r)
scattering length density
scattering length
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Polarized neutron scattering on polarized nuclei
b0 -0.374 bn 5.8254
proton
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How to measure clusters of polarized protons
what do we want to see

1. time-dependent polarization built-up of the
   proton spins around the radical
2. and in the bulk

requirements for experimental method

1. sensitive on polarization of protons
2. sensitive on length scale in nm-range

experimental method

1. cw-NMR (bulk protons)
2. neutron scattering technique (close protons)

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Small Angle Neutron Scattering
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Formfactor in SANS
shape
height
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Model System EHBA-Cr(V)
sample E2 (solvent 88 D)
sample E5 (solvent 98 D)
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Time resolved data acquisition
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Observation of polarized proton clusters
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Results
Experimental results 200 spectra of neutron
scattering
interpretation of the scattering curves
close protons
Fitting parameters R1, R2, A, IInc and
P Conditions R1 and R2 are constant and
time independent, Incoherent
scattering independent of Q but dependent on
time homogeneous polarization
of the protons in the shell, dependent on time
P(t)
IInc(t)
R1 3.4 Å R2 4.9 Å
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Time evolution of the polarization
characterization sum of two exponential t1
1.1s t2 5.5s
close protons versus bulk protons
time dependence of polarization close protons
exponential (SANS) bulk protons linear
(NMR)
Europhys. Lett. 59 (2002) 62-67
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Influence of the solvent deuteration
Evolution of the close proton polarization for
various solvent deuterations 98, 95 and 92 D
expected effect faster diffusion of spin in
solvent