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8ISSN 2072-5981 doi: 10.26907/mrsej
aänetic Resonance in Solids
Electronic Journal
Volume 23 Issue 2 Article No 21202
1-6 pages 2021
doi: 10.26907/mrsej-21202
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"Magnetic Resonance in Solids. Electronic Journal" (MRSey) is a
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Yoshio Kitaoka (Osaka University,
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Boris Malkin (KFU, Kazan) Alexander Shengelaya (Tbilisi State University, Tbilisi) Jörg Sichelschmidt (Max Planck Institute for Chemical Physics of Solids, Dresden) Haruhiko Suzuki (Kanazawa University, Kanazava) Murat Tagirov (KFU, Kazan) Dmitrii Tayurskii (KFU, Kazan) Valentine Zhikharev (KNRTU,
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* Address: "Magnetic Resonance in Solids. Electronic Journal", Kazan Federal University; Kremlevskaya str., 18; Kazan 420008, Russia
f In Kazan University the Electron Paramagnetic Resonance (EPR) was discovered by Zavoisky E.K. in 1944.
Short cite this: Magn. Reson. Solids 23, 21202 (2021)
doi: 10.26907/mrsej-21202
Modernization of the X-band EPR spectrometer Bruker ElexSys E580 for dynamic nuclear polarization
M.R. Gafurov*, G.V. Mamin, D.G. Zverev Kazan Federal University, Kremlevskaya 18, Kazan 420008, Russia *E-mail: marat.gafurov@kpfu.ru
(Received September 16, 2021; revised October 23, 2021; accepted October 29, 2021; published ???? ??, 2021)
To study the effects of dynamic nuclear polarization (DNP) in the X-band (microwave frequency of 9 GHz), using the capabilities provided by commercial EPR equipment, a part of the EPR spectrometer associated with the excitation and detection of double electron-nuclear resonance signals (ENDOR) has been modernized. Using the developed preamplifier of NMR signals, a homemade "Kazan Nova II"NMR spectrometer was implemented into the radio frequency path of the EPR spectrometer. The tuning and matching circuits made it possible to observe the NMR and DNP signals on protons in the frequency range 14.5-15.2 MHz. The performance of the DNP equipment was tested for a solution of the stable nitroxyl radical TEMPOL in benzene and a crude oil sample. The DNP effects caused by the Overhauser and solid effects were observed. The modernization of the existing EPR equipment creates a basis for further expanding its capabilities to study DNP effects in various systems at different conditions (in the pulsed mode of saturation of the EPR lines, with the temperature lowering, under the action of optical excitation, etc).
PACS: 76.30.-v, 76.60.-k, 76.60.Lz, 07.57.Pt, 75.78.-n, 89.30.aj
Keywords: electron paramagnetic resonance, nuclear magnetic resonance, dynamic nuclear polarization
1. Introduction
The history of dynamic nuclear polarization (DNP) dates from 1953 when A. Overhauser pro-
posed that irradiation of electron paramagnetic resonance (EPR) transitions could result in
the enhancement of the polarization of coupled nuclei [1]. In 1958 E. Poindexter showed the
enhancement of proton nuclear magnetic resonance (NMR) signal in the low-viscosity oil of fac-
tor 15 in the magnetic field of Bo = 1.8mT [2]. In recent decades, the development of DNP
technology and the areas of application of DNP approaches for both fundamental research and
for amplifying NMR signals has been experiencing their renaissance [3-5] including investiga-
tions of oil systems [6-10]. As it was reviewed in details in the recent paper [11], oils (crude
oil, bitumen, their constituents and products) can contain up to 1020 native paramagnetic cen-
ters (PCs) per gram of substance. The study of both hydrocarbon and other types of PCs
can provide additional information about the presence and concentration of hydrocarbons in
petroleum-containing rocks, the structure and properties of crude oils and their constituents. It
is of practical need for effective exploration, production, transportation and deeper processing of
hard-to-recover hydrocarbon reservoirs, heavy (high-viscous) oil, bitumen, etc. [9,10,12-14]. The
purpose of this work was to build a DNP spectrometer based on a commercial EPR spectrometer
operating at the microwave (MW) frequency of about 9 GHz (X-band) for investigations of crude
oil as well as other liquid systems containing PCs. As it was shown quite recently by applying
field-cycling (FC) DNP/NMR techniques [15,16], perceptible DNP effects can be achieved in
crude oils at X-band even by applying a moderate MW power.
2. Materials and methods
In this work, the ElexSys E580 X-band EPR spectrometer (Bruker, Germany) located at Kazan Federal University equipped with an X-band pulsed resonator for conducting experiments on electron-nuclear double resonance (ENDOR) was modernized. The comercial ENDOR-resonator (EN 4118X-MD4) is designed on the basis of a dielectric (sapphire) ring (position 1 in Figure 1), around which the magnetic component of the microwave field BMW is excited. The direction BMw is directed perpendicular to the vector of the constant magnetic field Bo (Figure 1). A wire-wound coil (position 2 in Figure 1) generates the magnetic component of the radiofrequency RF field; its magnetization vector BRF is perpendicular to both BMW and B0. The homogeneity
Figure 1. The scheme of the ENDOR resonator. Dielectric (sapphire) ring of the resonator (1) with the RF coil (2) and sample holder (3) inside. Relative orientations of the external (Bo), and alternating magnetic fields (BRF and BMW) are depicted
of the magnetic field B0 can be estimated as 0.1 mT/cm. The inner diameter of quartz ampule is 3.5 mm and height of the resonator active zone is 2 cm. The volume of the test samples inside the active zone of the resonator was about 0.2 mL. In our work, the comercial resonator unit was not disassembled or altered.
To observe the NMR signals, LC-circuit was formed by connection of a series of capacitors (SGM 250V, USSR) to the RF-coil of the resonator for tuning and matching (Figure 2). It gave an opportunity to tune the RF resonance frequency in the range 14.5-15.2 MHz for proton (*H) NMR. The Q-factor of the LC-circuit is 25. The capabilities of the homebuilt nuclear mag-netic/quadrupole resonance spectrometer "Kazan Nova II" [17] to excite the NMR transitions and detect the NMR signals were exploited. A broadband power amplifier (300 W, 1-250 MHz) attached to the EPR spectrometer for ENDOR experiments was used as a transmitter with the (n/2) RF pulse duration of 20 ^s. In present paper free induction decay (FID) signal was acquired.
For the signal amplification, a broadband preamplifier based on low-noise transistors was assembled, with a gain of about 8 in 5-50 MHz frequency range. Its scheme is shown in Figure 3. The dead time of the RF tract was about 6 microseconds at 15 MHz. Two serial amplifiers are used with a common gain of about 50.
In the work, only the stationary (CW) mode of operation of the EPR spectrometer for pump-
From e 1 A '"'i i , - Prea m p LIf i e r To
1K a z a n - N ova 1 k azan-Nova
Pp
lOOpF
HOpF
-j-lOOpF 75j,f|
ENDOR coil
Figure 2. Diagram of the NMR path of the spectrometer for XH NMR/DNP
GND
U1
AMS1117-5.0
Figure 3. The RF preamplifier circuit
ing the EPR transitions was tested. There is a potential possibility of using the pulse mode of the EPR spectrometer (like in paper [7]) when triggering the synchronization pulse from the NMR spectrometer. As well, the temperature insert of the EPR spectrometer to cool down the samples can be potentially exploited.
3. Results of the test measurements
The DNP equipment was tested on the liquid samples of the stable nit.roxyl radical 4-Hydroxy-2,2,6,6-tet. r amet.hylpiperidine 1-oxyl (TEMPOL, Merck, Germany, CAS Number 2226-96-2) dissolved in benzene at concentration of 2mM (2mmole/L) [18] and viscous oil samples from Ashalchinskoe oil deposits (Russia) which were investigated in details in paper [15] with EPR, NMR, and FC-DNP. For the both types of the investigated specimens the EPR quality factor for the resonator Qmw was about 900. The EPR pumping and NMR measurements were performed at room temperature. Figure 4 demonstrates the EPR spectrum of 2 mM TEMPOL solution against the corresponding DNP spectrum by changing Bo along the EPR spectrum. The amplitude of the DNP spectrum (DNP enhancement, e) was calculated as [18].
^MW
£ =
^NMR
"I,
(1)
where 2mw and /nmr are the integrated intensities of the Fourier-transformed (FT) FIDs with (/mw) and without (/nmr) EPR irradiation. The 3-line EPR spectrum is due to the hyperfine interaction with the 14N isotope (I = 1). The lines are broadened due to the exchange interaction with Heisenberg exchange constant fcexch °f 3-3 mM-1s-1 [19]. Maximal DNP enhancement was negative and of about, t = —4.35(4) observed at central line of the EPR spectrum by apply-
- _ TEMPOL + benzene
2 mM A
CO
"co c CD
CO \ / ¡1 /
tr Q. LU \J \ / ■
0
-1 - \ / \ I 7 \ 1 "
w -2 f 1 / "
-3 - j ^
-4 . i ^^ • -
e0(mT)
Figure 4. EPR spectrum for 2 mM TEMPOL in bezene (upper panel) and DNP spectrum of the same sample for PMW=200 mW and QMW=900 (lower panel, the experimental points are connected with the spline line)
Pmw (mW)
Figure 5. The DNP enhancement as measured at the central EPR line (upper panel) and amplitude of the central EPR line (lower panel) as a function of the microwave power for 2mM TEMPOL in benzene at (Bo=342.7mT)
ing the maximal possible in the presented setup microwave power PMW=200mW. It is known that for the TEMPOL-benzene solution the DNP effect is due to the Overhauser mechanism (OE DNP) governed by dipole-dipole electron (nitroxyl radical)-nuclear (protons of benzene) interaction and fast tumbling time t of 20-120 ps [18]. Such fast tumbling time as well as the inhomogeneously broadened EPR lines with the hyperfine splittings do not allow saturating the EPR transitions completely for the effective OE DNP as it is shown in Figure 5. Therefore, the experimental values of e are much less than the theoretically predicted of -330 [18] or experimentally achieved of about -180 at PMW=10W [20].
340 ^ 341 342 343 344
B0 (mT)
Figure 6. Upper panel: central part of the EPR spectrum of crude oil sample (full EPR spectrum acquired in the pulse mode is shown as an insert). Lower panel: DNP spectrum of the same sample for PMW=200 mW and QMW = 900 (the experimental points are connected with the spline line); Larmor frequencies (fL) of 14.56 and 14.65MHz for VO and FR lines, respectively, are depicted. Magnetic fields for the corresponding forbidden EPR transitions (±fL) are shown by dash-dot lines.
EPR and DNP spectra of the crude oil sample with the viscosity of 2400mPa-s are shown in Figure 6. It is known that the majority of paramagnetic centers in the oil sample exist in the form of (1) stable carbonaceous "free"radicals (FR) - unpaired electrons delocalized over many conjugated or aromatic chemical bonds - and (2) vanadyl cation (VO, VO2+) functional groups coordinated mainly with porphyrins [11]. Their concentrations were measured to be of 1017 spin/g for FR and 1018 spin/g for VO [16]. Though the oil sample is considered to be a viscous liquid [15], the EPR pattern for VO (due to the anisotropic hyperfine interaction of axial symmetry with I = 7/2 for the 51V nuclei) is typical for the powder (solids) emphasizing that the majority of paramagnetic centers are concentrated in the high-molecular components of oil like asphaltenes and resins [11]. OE DNP is mostly observed in highly diluted, low viscous liquids and solutions. Leblond with co-authors [21] showed that the solid effect (SE) can be observed in viscous liquids at temperatures above 100 K. The enhancement of the NMR signal by the SE DNP mechanism is maximum when the MW and EPR frequencies differ by the nuclear NMR Larmor frequency. These conditions lead to excitation of second order zero and double quantum electron-nuclear spin transitions which cause NMR signal enhancements with inverted or noninverted signal phase at the corresponding frequency. The presented in Figure 6 DNP spectrum (acquired for the most intensive EPR lines) proves that SE DNP is the main cross-polarization mechanism for the oil sample. Indeed, negative and positive DNP enhancements are observed along the DNP spectrum with the differences equal to the proton Larmor frequencies at the corresponding values of the Bo.
Acknowledgments
The work is financially supported by Russian Science Foundation (grant #19-12-00332) Magnetic Resonance in Solids. Electronic Journal. 2021, Vol. 23, No 2, 21202(6 pp.) 5
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