At low temperature, methyl groups act as hindered quantum rotors exhibiting rotational quantum tunneling, which is highly sensitive to a local methyl group environment. Recently, we observed this ...effect using pulsed electron paramagnetic resonance (EPR) in two dimethylammonium-containing hybrid perovskites doped with paramagnetic Mn2+ ions. Here, we investigate the feasibility of using an alternative fast-relaxing Co2+ paramagnetic center to study the methyl group tunneling, and, as a model compound, we use dimethylammonium zinc formate (CH3)2NH2Zn(HCOO)3 hybrid perovskite. Our multifrequency (X-, Q- and W-band) EPR experiments reveal a high-spin state of the incorporated Co2+ center, which exhibits fast spin-lattice relaxation and electron spin decoherence. Our pulsed EPR experiments reveal magnetic field independent electron spin echo envelope modulation (ESEEM) signals, which are assigned to the methyl group tunneling. We use density operator simulations to extract the tunnel frequency of 1.84 MHz from the experimental data, which is then used to calculate the rotational barrier of the methyl groups. We compare our results with the previously reported Mn2+ case showing that our approach can detect very small changes in the local methyl group environment in hybrid perovskites and related materials.
The structure of paramagnetic surface species is notoriously difficult to determine. For TiIII centers related to Ziegler–Natta catalysis, we demonstrate here that detailed structural information can ...be obtained by advanced EPR spectroscopy and DFT computations, benchmarked on molecular analogs. The hyperfine sublevel correlation (HYSCORE) spectra obtained after reaction with 13C‐labeled ethylene provides information about the coupling with a proton in the first coordination sphere of TiIII as well as significant 13C hyperfine coupling and thereby allows structural assignment of the surface species.
High score: 2D HYSCORE EPR spectroscopy augmented with DFT calculations makes it possible to detect and characterize TiIII alkyl species in silica‐supported ethylene polymerization catalysts.
Divalent complexes of vanadium were synthesized employing bulky silyl(aryl)amido ligands −N(Si i Pr3)DIPP and −N(Si t Bu2Me)DIPP (DIPP = 2,6- i Pr2C6H3). Solid-state structural characterization ...revealed that although the ligand −N(Si i Pr3)DIPP supports a monomeric, bis(amido) complex of vanadium, its constitutional isomer −N(Si t Bu2Me)DIPP affords a homoleptic complex in which the vanadium center is sandwiched between the arene rings, an unusual binding mode for arylamido ligands. Magnetometry studies indicate that VN(Si i Pr3)DIPP2 and V(η5-DIPP)N(Si t Bu2Me)2 have similar high-spin d3 electron configurations. However, spectroscopic methods, including electron paramagnetic resonance, nuclear magnetic resonance, infrared, and UV–visible spectroscopies, in addition to cyclic voltammetry and reactivity studies, suggest that VN(Si i Pr3)DIPP2 is stereochemically nonrigid in solution, while V(η5-DIPP)N(Si t Bu2Me)2 is not. This nonrigidity explicitly impacts the reactivity of VN(Si i Pr3)DIPP2, which can be used to access both amido-bound and arene-bound complexes. Moreover, treatment of VN(Si i Pr3)DIPP2 with single and multielectron oxidants reveals a range of transformations including an intramolecular sp3 C–H bond activation.
It is well known that disordered relaxor ferroelectrics exhibit local polar correlations. The origin of localized fields that disrupt long‐range polar order for different substitution types, however, ...is unclear. Currently, it is known that substituents of the same valence as Ti4+ at the B‐site of barium titanate lattice produce random disruption of TiOTi chains that induces relaxor behavior. On the other hand, investigating lattice disruption and relaxor behavior resulting from substituents of different valence at the B‐site is more complex due to the simultaneous occurrence of charge imbalances and displacements of the substituent cation. The existence of an effective charge mediated mechanism for relaxor behavior appearing at low (<10%) substituent contents in heterovalent modified barium titanate ceramics is presented in this work. These results will add credits to the current understanding of relaxor behavior in chemically modified ferroelectric materials and also acknowledge the critical role of defects (such as cation vacancies) in lattice disruption, paving the way for chemistry‐based materials design in the field of dielectric and energy storage applications.
In BaTiO3 solid solutions, relaxor behavior is caused by lattice disorder disrupting long‐range ferroelectricity. In this work, it is shown that introducing donor substituents at the B‐site of BaTiO3 leads to the formation of clusters of charged defects (involving Ti vacancies) as a charge‐compensation mechanism, which leads to an earlier onset of relaxor behavior compared to homovalent‐substituted systems.
This research addresses one of the most compelling issues in the field of photosynthesis, namely, the role of the accessory chlorophyll molecules in primary charge separation. Using a combination of ...empirical and computational methods, we demonstrate that the primary acceptor of photosystem (PS) I is a dimer of accessory and secondary chlorophyll molecules, Chl2A and Chl3A, with an asymmetric electron charge density distribution. The incorporation of highly coupled donors and acceptors in PS I allows for extensive delocalization that prolongs the lifetime of the charge-separated state, providing for high quantum efficiency. The discovery of this motif has widespread implications ranging from the evolution of naturally occurring reaction centers to the development of a new generation of highly efficient artificial photosynthetic systems.
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•Electron-nuclear hyperfine couplings indicate that A0·– is a chlorophyll dimer•DFT calculations reveal electron delocalization over both Chl2 and Chl3 in A0·–•Photosystem I utilizes Chl dimers as both the primary donor (P700) and acceptor (A0)•A0 is predicted to be a dimer in type I RCs and a monomer in type II RCs
Chemistry; Inorganic chemistry; Molecular inorganic chemistry; Theoretical photochemistry
The photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that comprises the Mn4CaO5 cluster, with participation of the redox-active tyrosine ...residue (YZ) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between YZ and D1-His190 likely renders YZ kinetically and thermodynamically competent leading to highly efficient water oxidation. However, a detailed understanding of the proton-coupled electron transfer (PCET) at YZ remains elusive owing to the transient nature of its intermediate states involving YZ⋅. Herein, we employ a combination of high-resolution two-dimensional 14N hyperfine sublevel correlation spectroscopy and density functional theory methods to investigate a bioinspired artificial photosynthetic reaction center that mimics the PCET process involving the YZ residue of PSII. Our results underscore the importance of proximal water molecules and charge delocalization on the electronic structure of the artificial reaction center.
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•Structural factors are critical in the design of artificial photosynthetic systems•Correlation between hyperfine couplings of the N atoms and electron spin density•Spin density distribution affected by charge delocalization and explicit waters•Spin density modulation by electronic coupling as observed with P680 and YZ in PSII
Spectroscopy; Organic Reaction; Computational Chemistry
Cu2S-CdS junctions of the polycrystalline material layers have been examined by combining the capacitance deep level transient spectroscopy technique together with white LED light additional ...illumination (C-DLTS-WL) and the photo-ionization spectroscopy (PIS) implemented by the photocurrent probing. Three types of junction structures, separated by using the barrier capacitance characteristics of the junctions and correlated with XRD distinguished precipitates of the polycrystalline layers, exhibit different deep trap spectra within CdS substrates.
In the present work, copper whitlockite (Cu-WH, Ca
18
Cu
2
(HPO
4
)
2
(PO
4
)
12
) was successfully synthesized and comprehensively characterized, founding the base knowledge for its future studies ...in medicine, particularly for bone regeneration. This material is a copper-containing analog of the well-known biomineral magnesium whitlockite (Mg-WH, Ca
18
Mg
2
(HPO
4
)
2
(PO
4
)
12
). The synthesis of powders was performed by a dissolution-precipitation method in an aqueous medium under hydrothermal conditions. Phase conversion from brushite (CaHPO
4
·2H
2
O) to Cu-WH took place in an acidic medium in the presence of Cu
2+
ions. Optimization of the synthesis conditions in terms of medium pH, temperature, time, Ca/Cu molar ratio and concentration of starting materials was performed. The crystal structure of the synthesized products was confirmed by XRD, FTIR and Raman spectroscopy,
1
H and
31
P solid-state NMR, and EPR. Morphological features and elemental distribution of the synthesized powders were studied by means of SEM/EDX analysis. The ion release in SBF solution was estimated using ICP-OES. Cytotoxicity experiments were performed with MC3T3-E1 cells. The study on thermal stability revealed that the synthesized material is thermally unstable and gradually decomposes upon annealing to Cu-substituted β-Ca
3
(PO
4
)
2
and Ca
2
P
2
O
7
.
In this work, copper whitlockite (Cu-WH, Ca
18
Cu
2
(HPO
4
)
2
(PO
4
)
12
) was successfully synthesized and comprehensively characterized. This material is a copper-containing analog of the well-known biomineral magnesium whitlockite (Ca
18
Mg
2
(HPO
4
)
2
(PO
4
)
12
).
Hybrid methylammonium (MA) lead halide perovskites have emerged as materials exhibiting excellent photovoltaic performance related to their rich structural and dynamic properties. Here, we use ...multifrequency (X-, Q-, and W-band) electron paramagnetic resonance (EPR) spectroscopy of Mn
impurities in MAPbCl
to probe the structural and dynamic properties of both the organic and inorganic sublattices of this compound. The temperature dependent continuous-wave (CW) EPR experiments reveal a sudden change of the Mn
spin Hamiltonian parameters at the phase transition to the ordered orthorhombic phase indicating its first-order character and significant slowing down of the MA cation reorientation. Pulsed EPR experiments are employed to measure the temperature dependences of the spin-lattice relaxation
and decoherence
times of the Mn
ions in the orthorhombic phase of MAPbCl
revealing a coupling between the spin center and vibrations of the inorganic framework. Low-temperature electron spin echo envelope modulation (ESEEM) experiments of the protonated and deuterated MAPbCl
analogues show the presence of quantum rotational tunneling of the ammonium groups, allowing to accurately probe their rotational energy landscape.
At low temperature, methyl groups act as hindered quantum rotors exhibiting rotational quantum tunneling, which is highly sensitive to a local methyl group environment. Recently, we observed this ...effect using pulsed electron paramagnetic resonance (EPR) in two dimethylammonium-containing hybrid perovskites doped with paramagnetic Mnsup.2+ ions. Here, we investigate the feasibility of using an alternative fast-relaxing Cosup.2+ paramagnetic center to study the methyl group tunneling, and, as a model compound, we use dimethylammonium zinc formate (CHsub.3)sub.2NHsub.2Zn(HCOO)sub.3 hybrid perovskite. Our multifrequency (X-, Q- and W-band) EPR experiments reveal a high-spin state of the incorporated Cosup.2+ center, which exhibits fast spin-lattice relaxation and electron spin decoherence. Our pulsed EPR experiments reveal magnetic field independent electron spin echo envelope modulation (ESEEM) signals, which are assigned to the methyl group tunneling. We use density operator simulations to extract the tunnel frequency of 1.84 MHz from the experimental data, which is then used to calculate the rotational barrier of the methyl groups. We compare our results with the previously reported Mnsup.2+ case showing that our approach can detect very small changes in the local methyl group environment in hybrid perovskites and related materials.
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