14 research outputs found
Chemically Induced Formation of Monovalent Cd<sup>+</sup> Ions and Reversible O<sub>2</sub> Activation in Cadmium-Loaded ZSM‑5 Zeolite
We have studied the
nature of monovalent Cd<sup>+</sup> (4d<sup>10</sup> 5s<sup>1</sup>) species generated in a Cd-loaded HZSM-5
zeolite by reaction with molecular O<sub>2</sub>. The open-shell species
formed in the different steps of theî—¸reversibleî—¸reaction
are characterized by electron paramagnetic resonance and hyperfine
sublevel correlation (HYSCORE) spectroscopies at X- and Q-band frequencies.
The same Cd<sup>+</sup> species are obtained via a photochemical pathway
through the UV irradiation of the Cd-loaded zeolite. Unambiguous evidence
for the formation of mononuclear Cd<sup>+</sup> species is obtained
by the detection of the hyperfine interaction associated to the naturally
abundant <sup>111</sup>Cd and <sup>113</sup>Cd magnetic isotopes (<i>I</i> = 1/2 natural abundance 12.8 and 12.22%, respectively).
The full <sup>111</sup>Cd <b>A</b> tensor (<i>A</i><sub><i>x</i></sub> = 10 620, <i>A<sub>y</sub></i> = 10 639, <i>A</i><sub><i>z</i></sub> = 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 <sup>27</sup>Al 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 VÂ(IV) ions in AlPO-5
zeotype materials. Two EPR-active species are detected, whose spin
Hamiltonian parameters are in accord with vanadyl ions (VO<sup>2+</sup>) experiencing slightly different local environments. Interactions
of the unpaired electrons of the paramagnetic VO<sup>2+</sup> species
with all relevant nuclei (<sup>1</sup>H, <sup>31</sup>P, <sup>27</sup>Al, and <sup>51</sup>V) could be resolved, allowing for the first
detailed structural analysis of the VO<sup>2+</sup> paramagnetic ions
in AlPO materials. Dehydration treatments indicate that the observed <sup>1</sup>H hyperfine couplings stem from structural protons in the
first coordination sphere of the VO<sup>2+</sup> species, strongly
pointing to charge compensating mechanisms associated with isomorphous
framework substitution at Al<sup>3+</sup> sites, in good agreement
with the large <sup>31</sup>P hyperfine couplings. Detection of fairly
large <sup>27</sup>Al couplings point to the presence of VO<sup>2+</sup>–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 VO<sup>2+</sup> species isomorphously substituted in
the AlPO framework at Al<sup>3+</sup> sites and extraframework VO<sup>2+</sup> 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‑TiO<sub>2</sub> Investigated by Electron Paramagnetic Resonance.
The generation of surface charge carriers in N-doped
TiO<sub>2</sub> 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 TiO<sub>2</sub>, 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<sup><i>x</i>–</sup> states to the conduction band, while NIR frequencies excite electrons
from the valence band to the NO<sup><i>x</i>–</sup> centers. The limited concentration of such centers explains the
moderate efficiency of the whole process and, consequently, of the
photocatalytic activity of N-TiO<sub>2</sub> 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 nickelÂ(I)
centers generated inside
the channels of ZSM-5 zeolite with carbon monoxide (<sup>12,13</sup>CO, <i>p</i><sub>CO</sub> < 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<sup>I</sup>–CO}ÂZSM-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<sub>CO</sub> = 2109 cm<sup>–1</sup>, <i><b>g</b></i> and <i><b>A</b></i>(<sup>13</sup>C) tensors (<i>g</i><sub><i>xx</i></sub> = 2.018, <i>g</i><sub><i>yy</i></sub> =
2.380, <i>g</i><sub><i>zz</i></sub> = 2.436, <i>A</i><sub><i>xx</i></sub> = +1.0 ± 0.3 MHz, <i>A</i><sub><i>yy</i></sub> = −3.6 ± 0.9
MHz, <i>A</i><sub><i>zz</i></sub> = −1.6
± 0.3 MHz) and <i><b>Q</b></i>(<sup>27</sup>Al)
parameters (e<sup>2</sup><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 Ï€Â(2p) CO states with the 3d<sub><i>xz</i></sub> and 3d<sub><i>yz</i></sub> orbitals, with a small
σ contribution due to the overlap of σÂ(2p+2s) orbital
and a protruding lobe of the in-plane 3d<sub><i>xz</i></sub> orbital. Two types of orbital channels (associated with the σ
and π overlap) of the electron and spin density flows within
the {Ni<sup>I</sup>–CO} unit were identified. A bathochromic
shift of the ν<sub>CO</sub> stretching vibration was accounted
for by resolving quantitatively the separate contributions due to
the σ donation and π back-donation, whereas the <sup>13</sup>C 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<sup>I</sup>–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 Ï„
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><sub>m</sub> ∼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
(<sup>1</sup>H and <sup>14</sup>N 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 Ti<sup>3+</sup>–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 Ti<sup>3+</sup> ions
generated in the framework of TiAlPO-5 microporous materials with <sup>12,13</sup>C<sub>2</sub>H<sub>4</sub> leads to the formation of side-on
η<sup>2</sup> {Ti<sup>3+</sup>C<sub>2</sub>H<sub>4</sub>} 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 C<sub>2</sub>H<sub>4</sub> results from the σ overlap of low lying
C<sub>2</sub>H<sub>4</sub> filled π orbitals with the 3<i>d</i><sub><i>z</i></sub><sup>2</sup> empty orbital
of titanium, enhanced by a small contribution due the π overlap
between the semioccupied 3<i>d</i><sub><i>yz</i></sub> 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
Ï€*-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<sup>+</sup> 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<sup>+</sup> ions in H-ZSM-5 zeolite.
Zn<sup>+</sup> ions were generated by the direct exposure of dehydrated
acid H-ZSM-5 to Zn vapors. The number of Zn<sup>+</sup> 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 Al<sup>3+</sup> site, as revealed by superhyperfine
interactions. The full <b>g</b> (<i>g</i><sub><i>x</i></sub> = 1.9951, <i>g</i><sub><i>y</i></sub> = 1.9984, <i>g</i><sub><i>z</i></sub> =
2.0015) and <sup>27</sup>Al |<b>A</b>| (<i>A</i><sub><i>x</i></sub> = 2.8, <i>A</i><sub><i>y</i></sub> = 2.7, <i>A</i><sub><i>z</i></sub> = 4.6)
MHz tensors were resolved, allowing for a detailed description of
the geometric and electronic structure of Zn<sup>+</sup> 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
TiO<sub>2</sub> (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 Ti<sup>3+</sup>. 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
TiO<sub>2</sub> 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,
vanadiumÂ(IV) complexes have shown record phase memory times among
molecular systems. When nuclear spin-free ligands are employed, vanadiumÂ(IV)
complexes can show at low temperature sufficiently long quantum coherence
times, <i>T</i><sub>m</sub>, 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><sub>1</sub> 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 vanadiumÂ(IV)-based
potential molecular spin qubits in the solid state by introducing
a unique structural difference, i.e., an oxovanadiumÂ(IV) in a square
pyramidal versus a vanadiumÂ(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 VO<sup>2+</sup> 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