New Effect in Dynamic Polarization

Abstract

A new effect in dynamic polarization is found showing characteristics similar to those of the "solid effect" but with significant difference in the spacing between enhancement peaks. A possible explanation, ascribing it to the electron spin-spin interaction, is discussed.In the theory of dynamic polarization of nuclei by the "solid effect," 1>2 the spacing (D) between the positive and negative nuclear-polarization enhancement peaks (±E max ) is a well-defined quantity for a system with completely resolved forbidden transitions. For systems in which both the electronic and nuclear spins are |, one has D = 2<jo n , where oo n is the nuclear Larmor frequency at a given static magnetic field (H 0 ) This relationship has been verified by Jeffries and Schmugge 3 in their experiments on a lanthanum magnesium double nitrate crystal doped with nominally 1% Nd 142 (Nd-LMN). Jeffries also showed 1 that 2c% represents a minimum value of D, and that D may generally be larger due to the broadening of the epr line and to over saturation.We found D to be appreciably less than 2u) n in the dynamic polarization of protons in chemically doped polystyrene or toluene at 4.2°K and a microwave pumping frequency (oo) of 70 GHz. The methods of sample preparation and polarization measurement are described elsewhere. 4 The doping free radicals used and their respective epr full widths at half-maximum (A# 1/2 ), measured at 9 GHz in dilute form in polystyrene, are diphenyl-picryl-hydrazyl (DPPH), A# 1/2 = 36G; Coppinger's radical 5 ' 6 (galvinoxyl), AZ/ 1/2 = 16G; and Ley's radical, 6 ' 7 AH l/2 = 9 G. Figure 1 shows some typical proton dynamic polarization enhancements (E) as a function of H 0 at constant co and microwave power level (W). Curve a illustrates the pure "solid effect" in Nd-LMN. Curve b shows that E for a sample of 0.25% Ley's radical (by weight) in polystyrene exhibits qualitatively the pure "solid effect" behavior, withZ>«2c%. Withincreasing radical concentration we found D « 1.3co w as shown in curves c and d.The same spacing, D«1.3co w , and curves of E vs H 0 similar in shape to that of Fig. 1(d) were also observed for 5% Ley's radical in frozen toluene at 4.2°K and for 5% galvinoxyl in polystyrene at 4.2 and 1.2°K. A reduced!) is not limited to these two radicals since we also observed D » 1.5w w for 3.3% DPPH in polystyrene at 4.2°K. The absolute value of H 0 was monitored continuously during the experiment by a F 19 nmr spectrometer with the F 19 probe located outside the multimode microwave cavity. In addition, both a> and oo n were also monitored, and we found oo~g e (3Ho and oo n =gn^n^0 with H 0 given by the F 19 nmr spectrometer. Hence we believe that the error in D due to uncertainties in# 0 was negligible. 20 10 0 -10 -20 r r -r-i [ (a)

Dynamic Nuclear Polarization Solid-State NMR in Heterogeneous Catalysis Research

“…One of the first such spectrometers dedicated to materials research has been installed in 2014 at the Ames Laboratory (Figure 2A). The conventional ("MW off") and DNP-enhanced ("MW on") 13 C{ 1 H} CPMAS spectra of a standard reference sample (proline, Figure 2B) exemplify the highest enhancement (ε = 260) that can be achieved at 100 K, at least currently, by using AMUPol as a polarizing agent. The instrument uses the lowest of the fields listed above (9.4 T), which at present appears to be the field of choice for DNP applications in catalysis.…”

“…The acquisition of conventional 2D homo-or heteronuclear correlation spectra is often unfeasible on catalytic systems without isotope enrichment. However, signal enhancements of over 100 approach the benefits obtained from moderate isotopic enrichment when performing 13 C− 13 C homonuclear correlation experiments, while avoiding the complicated, time-consuming, and expensive syntheses associated with labeling. Several reports effectively show that 13 C− 13 C through-space or through-bond 2D spectra can be acquired in a few minutes to hours.…”

“…The resulting signal enhancement is determined by the relative sizes of the Zeeman interaction for the excited and observed spins, i.e., by a factor of ε = γ exc /γ obs . In conventional SSNMR, this strategy is commonly applied to enhance the polarization of lower-γ nuclei (such as 13 C or 15 N) via cross-polarization under MAS conditions (CPMAS), resulting in signal-to-noise ratio (S/N) enhancements per scan of ε = γ 1H /γ 13C ≈ 4 or γ 1H /γ 15N ≈ 10. 4,5 Clearly, given the electron's much larger γ, considerably higher ε values can be achieved by applying a similar principle to the electron−nucleus spin pairs; this is the basic idea behind DNP.…”

Rapid-melt Dynamic Nuclear Polarization

“…The Cross Effect was reported and experimentally verified in the 1960's [17,18]. For the Cross Effect three dipolar coupled spins (2 electron spins and 1 nuclecar spin) are involved while DNP is carried out by saturating an allowed transition of one of the electrons.…”

Time domain DNP with the NOVEL sequence

“…In contrast, in insulating solids, such as glycerol/water glasses and biological samples, the OE was thought to be forbidden, but we have recently observed Overhauser enhancements using polarizing agents with narrow EPR spectra that exhibit strong 1 H−e − hyperfine couplings. 4 Furthermore, in these sorts of samples, DNP processes can also be mediated by three other mechanisms -the solid effect (SE), 5,6 the cross effect, [7][8][9][10][11] and/or thermal mixing. 12 Initially, the primary application of DNP was preparation of polarized targets for neutron scattering experiments; 13 however, over the past decade, DNP has been used extensively to enhance the inherently low sensitivity of nuclear magnetic resonance (NMR) signals.…”