Chemically Induced Formation of Monovalent Cd⁺ Ions and Reversible O₂ Activation in Cadmium-Loaded ZSM‑5 Zeolite

We have studied the nature of monovalent Cd⁺ (4d¹⁰ 5s¹) species generated in a Cd-loaded HZSM-5 zeolite by reaction with molecular O₂. The open-shell species formed in the different steps of thereversiblereaction are characterized by electron paramagnetic resonance and hyperfine sublevel correlation (HYSCORE) spectroscopies at X- and Q-band frequencies. The same Cd⁺ species are obtained via a photochemical pathway through the UV irradiation of the Cd-loaded zeolite. Unambiguous evidence for the formation of mononuclear Cd⁺ species is obtained by the detection of the hyperfine interaction associated to the naturally abundant ¹¹¹Cd and ¹¹³Cd magnetic isotopes (<i>I</i> = 1/2 natural abundance 12.8 and 12.22%, respectively). The full ¹¹¹Cd <b>A</b> tensor (<i>A</i>_<i>x</i> = 10 620, <i>A_y</i> = 10 639, <i>A</i>_<i>z</i> = 10 790 MHz) is resolved, indicating that 84% of the unpaired electron spin density is localized on the Cd ion. The small spin density delocalization on the zeolite framework is observed through the detection of ²⁷Al hyperfine interactions by means of 6 pulse HYSCORE experiments, allowing for a detailed description of the geometric and electronic structure of the monovalent cadmium species and the zeolite stabilizing sites

Structure of the Catalytic Active Sites in Vanadium-Doped Aluminophosphate Microporous Materials. New Evidence from Spin Density Studies

Electron spin resonance and hyperfine sublevel correlation (HYSCORE) spectroscopy at X- and Q-band frequencies have been employed, in conjunction with DFT modeling, to determine the location of V­(IV) ions in AlPO-5 zeotype materials. Two EPR-active species are detected, whose spin Hamiltonian parameters are in accord with vanadyl ions (VO²⁺) experiencing slightly different local environments. Interactions of the unpaired electrons of the paramagnetic VO²⁺ species with all relevant nuclei (¹H, ³¹P, ²⁷Al, and ⁵¹V) could be resolved, allowing for the first detailed structural analysis of the VO²⁺ paramagnetic ions in AlPO materials. Dehydration treatments indicate that the observed ¹H hyperfine couplings stem from structural protons in the first coordination sphere of the VO²⁺ species, strongly pointing to charge compensating mechanisms associated with isomorphous framework substitution at Al³⁺ sites, in good agreement with the large ³¹P hyperfine couplings. Detection of fairly large ²⁷Al couplings point to the presence of VO²⁺–O–Al linkages associated with a different structural arrangement, in agreement with the presence of two EPR-active species. The interpretation of the experimental results is corroborated by DFT modeling, which affords a microscopic description of the system investigated. The two EPR-active species are found to be consistent with isolated VO²⁺ species isomorphously substituted in the AlPO framework at Al³⁺ sites and extraframework VO²⁺ species docked in the center of the 6-membered rings that line up the main channel of the AFI structure

Mechanism of the Photoactivity under Visible Light of N‑Doped Titanium Dioxide. Charge Carriers Migration in Irradiated N‑TiO₂ Investigated by Electron Paramagnetic Resonance.

The generation of surface charge carriers in N-doped TiO₂ under various types of irradiation has been investigated by electron paramagnetic resonance using an approach consisting in scavenging surface migrated electrons and holes using oxygen and hydrogen, respectively. N-doped TiO₂, which is moderately active in photocatalytic processes under visible light, forms surface electrons and, at lower extent, surface holes due to the synergistic effect of visible components (around 400 nm) and near-infrared ones. The visible radiation excites electrons from intra band gap NO^<i>x</i>– states to the conduction band, while NIR frequencies excite electrons from the valence band to the NO^<i>x</i>– centers. The limited concentration of such centers explains the moderate efficiency of the whole process and, consequently, of the photocatalytic activity of N-TiO₂ in visible light with respect to the case of UV light irradiation. Despite the mentioned limits this material remains a fundamental starting point for a new generation of photocatalytic systems exploiting solar light

Intimate Binding Mechanism and Structure of Trigonal Nickel(I) Monocarbonyl Adducts in ZSM‑5 ZeoliteSpectroscopic Continuous Wave EPR, HYSCORE, and IR Studies Refined with DFT Quantification of Disentangled Electron and Spin Density Redistributions along σ and π Channels

Interaction of tetracoordinated nickel­(I) centers generated inside the channels of ZSM-5 zeolite with carbon monoxide (^12,13CO, <i>p</i>_CO < 1 Torr) led to the formation of T-shaped, top-on monocarbonyl adducts with a unique trigonal nickel core, supported by two oxygen donor ligands. The mechanism of the formation of the {Ni^I–CO}­ZSM-5 species was accounted for by a quantitative molecular orbital correlation diagram of CO ligation. Detailed electronic and magnetic structure of this adduct was obtained from comprehensive DFT calculations, validated by quantitative reproduction of its continuous wave electron paramagnetic resonance (CW-EPR), hyperfine sublevel correlation (HYSCORE), and IR fingerprints, using relativistic Pauli and ZORA-SOMF/B3LYP methods. Molecular analysis of the stretching frequency, ν_CO = 2109 cm^–1, <i><b>g</b></i> and <i><b>A</b></i>(¹³C) tensors (<i>g</i>_<i>xx</i> = 2.018, <i>g</i>_<i>yy</i> = 2.380, <i>g</i>_<i>zz</i> = 2.436, <i>A</i>_<i>xx</i> = +1.0 ± 0.3 MHz, <i>A</i>_<i>yy</i> = −3.6 ± 0.9 MHz, <i>A</i>_<i>zz</i> = −1.6 ± 0.3 MHz) and <i><b>Q</b></i>(²⁷Al) parameters (e²<i>Qq</i>/h = −13 MHz and η = 0.8) supported by quantum chemical modeling revealed that the Ni–CO bond results from the π overlap between the low-laying π­(2p) CO states with the 3d_<i>xz</i> and 3d_<i>yz</i> orbitals, with a small σ contribution due to the overlap of σ­(2p+2s) orbital and a protruding lobe of the in-plane 3d_<i>xz</i> orbital. Two types of orbital channels (associated with the σ and π overlap) of the electron and spin density flows within the {Ni^I–CO} unit were identified. A bathochromic shift of the ν_CO stretching vibration was accounted for by resolving quantitatively the separate contributions due to the σ donation and π back-donation, whereas the ¹³C hyperfine coupling was rationalized by incongruent α and β spin flows via the σ and π channels. As a result the very nature of the carbon–metal bond in the Ni^I–CO adduct and the molecular backbone of the corresponding spectroscopic parameters were revealed with unprecedented accuracy

Room-Temperature Quantum Coherence and Rabi Oscillations in Vanadyl Phthalocyanine: Toward Multifunctional Molecular Spin Qubits

Here we report the investigation of the magnetic relaxation and the quantum coherence of vanadyl phthalocyanine, VOPc, a multifunctional and easy-processable potential molecular spin qubit. VOPc in its pure form (<b>1</b>) and its crystalline dispersions in the isostructural diamagnetic host TiOPc in different stoichiometric ratios, namely VOPc:TiOPc 1:10 (<b>2</b>) and 1:1000 (<b>3</b>), were investigated via a multitechnique approach based on the combination of alternate current (AC) susceptometry, continuous wave, and pulsed electron paramagnetic resonance (EPR) spectroscopy. AC susceptibility measurements revealed a linear increase of the relaxation rate with temperature up to 20 K, as expected for a direct mechanism, but τ remains slow over a very wide range of applied static field values (up to ∼5 T). Pulsed EPR spectroscopy experiments on <b>3</b> revealed quantum coherence up to room temperature with <i>T</i>_m ∼1 μs at 300 K, representing the highest value obtained to date for molecular electronic spin qubits. Rabi oscillations are observed in this nuclear spin-active environment (¹H and ¹⁴N nuclei) at room temperature also for <b>2</b>, indicating an outstanding robustness of the quantum coherence in this molecular semiconductor exploitable in spintronic devices

Electronic Structure of Ti³⁺–Ethylene Complexes in Microporous Aluminophosphate Materials. A Combined EPR and DFT Study Elucidating the Role of SOMO Orbitals in Metal–Olefin π Complexes

The interaction of tetrahedrally coordinated Ti³⁺ ions generated in the framework of TiAlPO-5 microporous materials with ^12,13C₂H₄ leads to the formation of side-on η² {Ti³⁺C₂H₄} complexes with a unique 5-fold coordination of titanium, supported by four oxygen donor ligands of the framework. The detailed electronic and magnetic structure of this adduct is obtained by the combination of advanced EPR techniques (HYSCORE and SMART-HYSCORE) in conjunction with periodic and cluster model DFT calculations. The binding of C₂H₄ results from the σ overlap of low lying C₂H₄ filled π orbitals with the 3<i>d</i>_<i>z</i>² empty orbital of titanium, enhanced by a small contribution due the π overlap between the semioccupied 3<i>d</i>_<i>yz</i> orbital of titanium and the empty π* orbital of ethylene. The spin density repartition over the ethylene molecule, obtained experimentally, allows probing directly the entity of the metal-to-substrate π*-back-donation, highlighting an asymmetry in the spin density delocalization. This interesting feature is supported by parallel theoretical calculations, which cast the role of the oxygen donor ligands in driving this bonding asymmetry. As a consequence, the interesting structural feature of potential and actual inequality in the electronic spin states (α,β) on the two ethylene carbon atoms of the π coordinated ethylene molecule is produced. The underlying electronic effects associated with the π coordination of ethylene to an early transition metal in paramagnetic state are thus revealed with an unprecedented accuracy for the first time

Electronic and Geometrical Structure of Zn⁺ Ions Stabilized in the Porous Structure of Zn-Loaded Zeolite H‑ZSM-5: A Multifrequency CW and Pulse EPR Study

Electron spin resonance and hyperfine sublevel correlation (HYSCORE) spectroscopy at X- and Q-band frequencies have been employed, in conjunction with X-ray absorption spectroscopy (XAS), to determine the geometric and electronic structure of Zn⁺ ions in H-ZSM-5 zeolite. Zn⁺ ions were generated by the direct exposure of dehydrated acid H-ZSM-5 to Zn vapors. The number of Zn⁺ ions is found to increase substantially upon UV irradiation. A single EPR active species is detected, indicating a single site of adsorption, characterized by the presence of an Al³⁺ site, as revealed by superhyperfine interactions. The full <b>g</b> (<i>g</i>_<i>x</i> = 1.9951, <i>g</i>_<i>y</i> = 1.9984, <i>g</i>_<i>z</i> = 2.0015) and ²⁷Al |<b>A</b>| (<i>A</i>_<i>x</i> = 2.8, <i>A</i>_<i>y</i> = 2.7, <i>A</i>_<i>z</i> = 4.6) MHz tensors were resolved, allowing for a detailed description of the geometric and electronic structure of Zn⁺ ions stabilized in the cages of H-ZSM-5 zeolite. The dispersion and nuclearity of Zn species formed during the sublimation/irradiation process was assessed by means of XAS spectroscopy, which indicates the absence of metal or metal oxide particles in a significant amount

Nature of Reduced States in Titanium Dioxide as Monitored by Electron Paramagnetic Resonance. II: Rutile and Brookite Cases

We have systematically used electron paramagnetic resonance (EPR) to understand the nature of excess electron centers in titanium dioxide and to classify their spectroscopic features. Excess electrons in TiO₂ (probably the most important photoactive oxide) have been generated either by photoinduced charge separation or by reductive treatments and are stabilized in the solid by titanium ions which reduce to paramagnetic Ti³⁺. These are monitored by EPR and classified on the basis of their <b>g</b> tensor values in order to amend a certain confusion present in the literature about this subject. In the previous paper of this series (S. Livraghi et al. <i>J. Phys. Chem. C</i> <b>2011</b>, <i>115</i>, 25413–25421), excess electron centers in anatase were investigated while the present one is devoted to rutile and brookite, the two other TiO₂ polymorphs, in the aim of providing a thorough and consistent guideline to researchers working in the wide area of titanium dioxide applications

Quantum Coherence Times Enhancement in Vanadium(IV)-based Potential Molecular Qubits: the Key Role of the Vanadyl Moiety

In the search for long-lived quantum coherence in spin systems, vanadium­(IV) complexes have shown record phase memory times among molecular systems. When nuclear spin-free ligands are employed, vanadium­(IV) complexes can show at low temperature sufficiently long quantum coherence times, <i>T</i>_m, to perform quantum operations, but their use in real devices operating at room temperature is still hampered by the rapid decrease of <i>T</i>₁ caused by the efficient spin–phonon coupling. In this work we have investigated the effect of different coordination environments on the magnetization dynamics and the quantum coherence of two vanadium­(IV)-based potential molecular spin qubits in the solid state by introducing a unique structural difference, i.e., an oxovanadium­(IV) in a square pyramidal versus a vanadium­(IV) in an octahedral environment featuring the same coordinating ligand, namely, the 1,3-dithiole-2-thione-4,5-dithiolate. This investigation, performed by a combined approach of alternate current (ac) susceptibility measurements and continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopies revealed that the effectiveness of the vanadyl moiety in enhancing quantum coherence up to room temperature is related to a less effective mechanism of spin–lattice relaxation that can be quantitatively evaluated by the exponent <i>n</i> (ca. 3) of the temperature dependence of the relaxation rate. A more rapid collapse is observed for the non-oxo counterpart (<i>n</i> = 4) hampering the observation of quantum coherence at room temperature. Record coherence time at room temperature (1.04 μs) and Rabi oscillations are also observed for the vanadyl derivative in a very high concentrated material (5 ± 1%) as a result of the additional benefit provided by the use of a nuclear spin-free ligand

Toward Understanding the Catalytic Synergy in the Design of Bimetallic Molecular Sieves for Selective Aerobic Oxidations

Structure–property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and <i>in situ</i> FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO²⁺ ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems