Phenomenological Model for the New Effect in Dynamic Polarization

Abstract

We propose a phenomenological model involving the cross-coupling of the allowed and forbidden epr transitions between electrons belonging to neighboring packets of an inhomogeneously broadened epr spectrum. This model yields all the observed qualitative features of the new effect in dynamic polarization previously reported.In a previous communication 1 we reported the empirical evidence of a new effect in the dymanic polarization of protons in chemically doped plastics at liquid-helium temperatures. 2 We there attributed this new effect to some mechanism of spin-spin interaction, which described the qualitative behavior of our results. This Letter presents a phenomenological model for a specific form of spin-spin interaction.We assume essentially the Portis 3 picture of the structure of inhomogeneously broadened epr due primarily to hfs. The epr line then consists of many narrow packets of electrons e$ (j = 1, 2,3, • • •), with populations iV-7, in proximity to each other. Each packet of electrons couples dipolarly with nJ protons. Consider three such packets whose resonant frequencies satisfywhere v n is the nmr frequency of the protons at a given applied external magnetic field H', ^\oc * s ^e l oca * magnetic field for thejth packet, and y e is the gyromagnetic ratio of the electron. Figure 1 shows the energy levels for representative electrons in weak dipolar coupling with a proton for each of these three packets together with the relevant stimulated transition rates Wj through W 3 J and the relaxation rates wj through w s J. The direct excitation of transitions WJ and W 3 J by an applied rf magnetic field gives rise to the "solid effect" discussed in great detail by many investigators. 4 " 6 For our phenomenological model, we propose here two additional transition rates Wj'and W 6 J which represent the crosscoupling between W^ and WJ-1, Wj and Wj-1 (WJ); or between Wj and WJ + 1 WJ and Wj + lThe energy levels and pertinent connecting rates for the packets of Eq. (1). The A z are total electron splittings; W^ couples transitions W^-1 to W£ and Wj> to w£-l, W Q i couples W^+l to W$ and wf> to W£+\ The proton rates W} and 40/ are not shown.(W Q l ). An essentially identical form of interaction was first implied by Kessenikh, Manenkov, 7 and Pyatnitskii in their treatment of dynamic polarization of protons in irradiated polyethylene. However, their results are not valid for the very high electron polarizations germane to our case. Moreover, our treatment of the microwave power dependence is simpler, allowing us to approximate the power dependence in closed form. We assume specifically that transition rates W 5 J and W Q J are effectively much larger than the forbidden transition rates W 2 j and W Z J 9 because of the spin-spin interaction among the electrons. The enhanced transition rates could be visualized as caused by the increased virtual photon density over the real applied photon density on account of the saturation of the allowed transitions Wj. This assumption is the essence of our phenomenological mode...

Time domain DNP with the NOVEL sequence

“…The deuteration of the sample suppresses the contribution from matrix protons, which permits the observation of all ten pairs of sharp resonances from ten protons of each DPNO molecule. Figure 3(a) presents the EPR spectrum and the 1 H DNP field profile from a sample with 94% 13 C carbonyl labeled BzP-h 10 . In this case, the crystal was orientated such that B 0 is aligned with the crystallographic b-axis.…”

“…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.…”

Analysis of sensitivity enhancement by dynamic nuclear polarization in solid-state NMR: a case study of functionalized mesoporous materials

“…12 This facilitates an efficient three-spin ''cross-effect'' involving a flip-flop process of the two unpaired electrons and a flip of the nucleus. [12][13][14] Significant signal enhancements can be achieved, not only in biological systems 8,[15][16][17][18][19][20] but also in microcrystalline organic solids 21,22 or on surfaces and subsurfaces of silicates, aluminates, nanomaterials, and metal-organic frameworks (MOFs). [23][24][25][26] Several studies have addressed the quantification of the sensitivity enhancement in these experiments.…”

“…21,[26][27][28][29][30] The DNP enhancement factor is typically determined by comparing the spectra measured with mw irradiation ''on'' and ''off'', under the same conditions of static field B 0 and sample temperature T, using sufficiently long recycle delays so that relaxation effects can be neglected. In a study of DNP-enhanced 13 C CP-MAS NMR of a solvent-free peptide with covalently attached TOTAPOL 28 a more general enhancement factor was described which also accounts for effects of the radicals on the spin-lattice relaxation times T 1 H of the protons and on paramagnetic broadening ('quenching') of the signals. The influence of TOTAPOL concentration (in water/glycerol) on T 1 H and…”

“…T 1r H relaxation of protons, and the performance of 13 C DNP-CP-MAS NMR in proline was studied by Lange et al 30 Rossini et al 29 quantified sensitivity enhancements of 29 Si DNP-CP-MAS NMR of passivated hybrid mesoporous silica. In addition to quenching and T 1 H relaxation, they studied the effect of TOTAPOL on T Here, we set out to systematically assess the contributions of various experimental factors to the global sensitivity enhancement in DNP-CP-MAS NMR of both 13 C and 29 Si nuclei.…”