For the broadest dissemination of solid-state dynamic nuclear polarization (ssDNP) enhanced NMR as a material characterization tool, the ability to employ generic mono-nitroxide radicals as spin probes is critical. A better understanding of the factors contributing to ssDNP efficiency is needed to rationally optimize the experimental condition for the practically accessible spin probes at hand. This study seeks to advance the mechanistic understanding of ssDNP by examining the effect of electron spin dynamics on ssDNP performance at liquid helium temperatures (4-40 K). The key observation is that bi-radicals and mono-radicals can generate comparable nuclear spin polarization at 4 K and 7 T, which is in contrast to the observation for ssDNP at liquid nitrogen temperatures (80-150 K) that finds bi-radicals to clearly outperform mono-radicals. To rationalize this observation, we analyze the change in the DNP-induced nuclear spin polarization (Pn) and the characteristic ssDNP signal buildup time as a function of electron spin relaxation rates that are modulated by the mono- and bi-radical spin concentration. Changes in Pn are consistent with a systematic variation in the product of the electron spin-lattice relaxation time and the electron spin flip-flop rate that constitutes an integral saturation factor of an inhomogeneously broadened EPR spectrum. We show that the comparable Pn achieved with both radical species can be reconciled with a comparable integral EPR saturation factor. Surprisingly, the largest Pn is observed at an intermediate spin concentration for both mono- and bi-radicals. At the highest radical concentration, the stronger inter-electron spin dipolar coupling favors ssDNP, while oversaturation diminishes Pn, as experimentally verified by the observation of a maximum Pn at an intermediate, not the maximum, microwave (μw) power. At the maximum μw power, oversaturation reduces the electron spin population differential that must be upheld between electron spins that span a frequency difference matching the (1)H NMR frequency-characteristic of the cross effect DNP. This new mechanistic insight allows us to rationalize experimental conditions where generic mono-nitroxide probes can offer competitive ssDNP performance to that of custom designed bi-radicals, and thus helps to vastly expand the application scope of ssDNP for the study of functional materials and solids.

Nanoparticulate manganese oxides, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyze water oxidation under neutral pH conditions with the onset of electrocatalysis occurring at an overpotential of only 150 mV. Although XAS experiments indicate that each complex generates the same material in Nafion, the catalytic activity varied substantially with the manganese precursor and did not correlate with the amount of MnO x present in the films. The XAS and EPR studies indicated that the formation of the nanoparticulate oxide involves the dissociation of the complex into Mn(II) species followed by oxidation on application of an external bias. TEM studies of the most active films, derived from [Mn(Me3TACN)(OMe)3]+ and [(Me3TACN)2MnIII 2(μ-O)(μ-CH3COO)2]2+ (Me3TACN = N,N′,N″-trimethyl-1,4,7-triazacyclononane), revealed that highly dispersed MnO x nanoparticles (10–20 nm and 6–10 nm, respectively) were generated in the Nafion film. In contrast, the use of [Mn(OH2)6]2+ resulted in both a higher manganese oxide loading and aggregated nanoparticles with 30–100 nm approximate size, which were less effective water oxidation catalysts. Much higher turnover frequencies (TOFs) were observed for films derived from the two complexes, viz., ∼20 molecules of O2 per Mn per hour in dark and 40 molecules of O2 per Mn per hour under illumination at an overpotential of 350 mV, when compared with MnO x films made with [Mn(OH2)6]2+. This corresponds to a TOF > 100 molecules of O2 per Mn per second for a 10 nm MnO x nanoparticle. Thus, the catalytic activity is dependent on the ability to generate well-defined, dispersed nanoparticles. Electrochemical and spectroscopic methods have been used to follow the conversion of the molecular precursors into MnO x and to further evaluate the origin of differences in catalytic activity.

(7.111.95) Cationic Pd-complexes modified by dicyclohexyl{(R)-l-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}phos-phine (la)give very active catalytic systems for the regioregular isotactic specific copolymerization of propene with CO. Other alk-I-enes also give stereoregular and regioregular copolymers, even if with lower productivity. The copolymers are isolated as poly(4-alkyl-tetrahydrofuran-2,2,5,5-tetrayl-2-oxy-2-methylenes) B in the solid state and give the isomeric poly(2-alkyl-1 -oxopropane-l,3-diyls) A by dissolution in (CF,),CHOH. Solid polymer A (R = Et) is formed back at least partially when the dissolved material is reprecipitated from MeOH. The use of the related (ferroceny1)diphosphine ligands lbe and 2 as the catalyst modifier shows that the presence of both elements of chirality and of large substituents on the P-atoms of the ligand is necessary to achieve good stereocontrol, and that the large difference in basicity between the two P-atoms is probably the reason for the good catalytic activity.Diphosphine-containing cationic Pd-catalysts of the type [PdX,(P-P)] [I] (X = weakly coordinating or non-coordinating anion, P-P = diphosphine) led to the commercial application of the alternating copolymerization of ethene with CO [2], at least on a pilote-plant scale [3]. To extend the scope of the reaction to other aliphatic olefins, problems related to the regioselectivity [4] and to the stereoselectivity [5] of the copolymerization process must be solved. Even though the requirements of the ligands to achieve the above goals seem to be reasonably well identified [4-71, the catalytic activity of the stereoselective systems investigated remains low, thus hampering potential applications. The most active system for the copolymerization of propene reported so far seemingly contains propane-l,3-diylbis(diethylphosphine) [7] as the modifying ligand. The absence of chirality in this ligand, however, causes formation of a material having a low stereoregularity. Surprisingly, the copolymers of propene [7-101 were not of type A (Fig. I), e.g. poly(2-methyl-l-oxopropane-l,3-diyl) (R = Me), but of type B, e.g. poly-(tetrahydro-4-methylfuran-2,2,5,5-tetrayl-2-oxy-2-methylene (R = Me)).The stability of the spiro structure B for the olefin-carbon monoxide copolymers seems to be influenced by the degree of substitution of the C=C bond of the olefin substrate [l 11 [12]. These results imply that the copolymerization mechanism is less simple [8] [9] than expected [l] [13]. In spite of different working hypothesis that may be

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H), known as the Staebler-Wronski effect, has been studied by time-domain pulsed electron-paramagnetic resonance. Electron-spin echo relaxation measurements in the annealed and light-soaked state revealed two types of defects (termed type I and II), which can be discerned by their electron-spin echo relaxation. Type I exhibits a monoexponential decay related to indirect flip-flop processes between dipolar coupled electron spins in defect clusters, while the phase relaxation of type II is dominated by 1H nuclear spin dynamics and is indicative for isolated spins. We propose that defects are either located at internal surfaces of microvoids (type I) or are isolated and uniformly distributed in the bulk (type II). The concentration of both defect type I and II is significantly higher in the light-soaked state compared to the annealed state. Our results indicate that in addition to isolated defects, defects on internal surfaces of microvoids play a role in light-induced degradation of device-quality a-Si:H.