Terminal Radical Research Articles

(Invited) Resolving Catalytic Mechanism at an Electrode Surface: Thermodynamics & Kinetics of Reaction Steps

Computationally, often the energetics of intermediate reaction steps differentiate the efficiency of heterogeneous catalysts for product evolution. Yet, when compared to experiment, kinetic models are applied. For example, a material’s activity measured by one rate of product evolution is plotted as a function of the calculated formation energies of intermediate chemical forms. A critically important reaction for which this dichotomy between experiment and theory exists is the oxygen evolution reaction (OER) from water. In the laboratory group, we employ time-resolved optical and vibrational spectroscopy to deconstruct OER into its individual reaction steps on an electrode surface. In the presentation, I will describe the experimental methodology and the chosen model system, the n-doped SrTiO3/aqueous interface. I’ll show how these experiments identify both the rates and energetics of the first reaction step, the release of a proton and electron from an absorbed water species, denoted by OH* or O*. A recent success was to isolate a Langmuir isotherm of the intermediate population arising within < 2 ps on the SrTiO3 surface. The intermediate population is of trapped holes measured by emission from the conduction band into unoccupied states in the middle of the semi-conductor bandgap. The isotherm is tuned by the pH of the electrolyte. The pH-dependent formation of these intermediates are connected to their pH-dependent decay at microsecond timescales, presumably related to later reaction steps of OER. I'll also present work on the side of vibrational spectroscopy that looks at how the potential energy surface of a particular trapped hole, the terminal oxyl radical, arises in time through the spectral kinetics of its normal mode. A growing area is to apply the particular experimental methodology to a surface of similar electronic structure but diverse crystal geometries, namely polymorphs of TiO2.

Deciphering the Anomalous Acidic Tendency of Terminal Water at Rutile(110)–Water Interfaces

Understanding the mechanism of the oxygen evolution reaction (OER) is essential to improve the efficiency of photocatalysis for TiO2. Previous studies have highlighted the importance of terminal hydroxide radical (TiOH•) in the OER. Ab initio molecular dynamics simulations (AIMD) with hybrid functional have revealed that this radical readily loses its proton, creating the key intermediate, oxygen radical anion (Ti5cO•–). Herein, we combine machine-learning-accelerated molecular dynamics with density functional theory calculations to demonstrate that the Ti5cO•– can alternatively be generated through the trapping of a hole in a terminal oxygen anion (Ti5cO2–) at rutile(110)–water interfaces. Further examination reveals that the Ti5cO2– results from the deprotonation of Ti5cOH– and remains stable at the charge-neutral interfaces for a transient time period of ca. 100 ps. The AIMD-based free energy perturbation method predicts that the acidity constant of Ti5cOH– is even smaller than that of Ti5cOH2, thereby rationalizing the stability of Ti5cO2–. Structural analyses show that this anomalous acidic tendency of terminal water originates from the decrease of Ti–O bond length and the transition of Titanium’s coordination from octahedral to pyramidal in Ti5cO2–. Our findings provide valuable insights into the surface acid–base chemistry and a potential explanation for the pH-dependent behavior of photogenerated holes for TiO2.

Uranium-Mediated Peroxide Activation and a Precursor toward an Elusive Uranium cis-Dioxo Fleeting Intermediate.

The activation of chalcogen-chalcogen bonds using organometallic uranium complexes has been well documented for S-S, Se-Se, and Te-Te bonds. In stark contrast, reports concerning the ability of a uranium complex to activate the O-O bond of an organic peroxide are exceedingly rare. Herein, we describe the peroxide O-O bond cleavage of 9,10-diphenylanthracene-9,10-endoperoxide in nonaqueous media, mediated by a uranium(III) precursor [((Me,AdArO)3N)UIII(dme)] to generate a stable uranium(V) bis-alkoxide complex, namely, [((Me,AdArO)3N)UV(DPAP)]. This reaction proceeds via an isolable, alkoxide-bridged diuranium(IV/IV) species, implying that the oxidative addition occurs in two sequential, single-electron oxidations of the metal center, including rebound of a terminal oxygen radical. This uranium(V) bis-alkoxide can then be reduced with KC8 to form a uranium(IV) complex, which upon exposure to UV light, in solution, releases 9,10-diphenylanthracene to generate a cyclic uranyl trimer through formal two-electron photooxidation. Analysis of the mechanism of this photochemical oxidation via density functional theory (DFT) calculations indicates that the formation of this uranyl trimer occurs through a fleeting uranium cis-dioxo intermediate. At room temperature, this cis-configured dioxo species rapidly isomerizes to a more stable trans configuration through the release of one of the alkoxide ligands from the complex, which then goes on to form the isolated uranyl trimer complex.

How Doping Affects the Activity of the Aluminum Oxide Support.

The performance of heteronuclear clusters [AlXO3 ]+ (X=Al, AlO4 , AlMg2 O2 , AlZnO, AlAu2 , Mg, Y, VO, NbO, TaO) in activating methane has been explored by a combination of high-level quantum calculations with reported and supplementary gas-phase experiments. With different dopants in [AlXO3 ]+ , the mechanism, reactivity and selectivity towards methane activation varies accordingly. The classic HAT competes with PCET, depending on the composition of intramolecular interactions. Although the existence of terminal oxygen radical is beneficial for classic HAT, the Alt -C interaction in the [AlXO3 ]+ clusters as enhanced by the strongly electronegative doping groups (X=Al, AlZnO, Mg, Zn, VO, NbO, TaO) favors the PCET process, facilitating C-H bond breaking. In addition, with different dopants, the destiny of the split methyl group varies accordingly. While strong interaction between Alt and CH3 results in the formation of the Alt -C bond, dopants with variable valance may promote the formation of deep-oxidation products like formaldehyde. It has been discussed in detail how to regulate the activity and selectivity of the active center of the catalyst via rational doping.

Thermal Methane Conversion to Formaldehyde Mediated by NiAlO3+ in the Gas Phase.

Understanding the active sites and reaction mechanisms of Ni-based catalysts, such as Ni/Al2O3, toward methane is a prerequisite for improving their rational design. Here, the gas-phase reactivity of NiAlO3+ cations toward CH4 is studied using mass spectrometry combined with density functional theory. Similar to our previous study on NiAl2O4+, we find evidence for the formation of both the methyl radical (CH3•) and formaldehyde (CH2O). The first step for methane activation is hydrogen atom abstraction by the terminal oxygen radical Ni(O)2AlO• from methane forming a [Ni(O)2AlOH+, •CH3] complex and leaving the Ni-oxidation state unchanged. The second C-H bond is subsequently activated by the association of a bridged Ni-O2--Al. The oxidation state of the Ni atom is reduced from +3 to +1 during the formation of formaldehyde. Compared to Al2O3+/CH4 and YAlO3+/CH4 systems, the Ni-atom substitution increases the overall reaction rate by roughly an order of magnitude and yields a CH3•/CH2O branching ratio of 0.62/0.38. The present study provides molecular-level insights into the highly efficient gas-phase reaction mechanism contributing to an improved understanding of methane conversion by Ni/Al2O3 catalysts.

Infrared spectroscopic and theoretical investigations of novel iridium oxyfluorides.

The iridium oxyfluorides (OIrF, OIrF2 and FOIrF) were prepared for the first time by the reaction of IR-laser ablated iridium atoms and OF2, isolated in solid neon and argon matrices. The assignments of the main vibrational absorptions of these products were supported by a combined analysis of IR-matrix-isolation spectroscopy with 18OF2 substitution and quantum-chemical calculations. The OIrF molecule exhibits triple bond character. In contrast to terminal oxyl radical species OPtF2 and OAuF2, a much lower spin-density contribution at the oxygen atom was found in OIrF2.

Spin-Gated Selectivity of the Water Oxidation Reaction Mediated by Free Pentameric CaxMn5-xO5+ Clusters.

We report on the first preparation of isolated ligand-free CaMn4O5+ gas-phase clusters, as well as other pentameric CaxMn5-xO5+ (x = 0-4) clusters with varying Ca contents, which serve as molecular models of the natural CaMn4O5 inorganic cluster in photosystem II. Ion trap reactivity studies with D2O and H218O reveal a pronounced cluster composition-dependent ability to mediate the oxidation of water to hydrogen peroxide. First-principles density functional theory simulations elucidate the mechanism of water oxidation, proceeding via formation of a terminal oxyl radical followed by oxyl/hydroxy (O/OH) coupling. The critical coupling reaction step entails a single electron transfer from the oxyl radical to the accommodating cluster core with a concurrent O/OH coupling forming an adsorbed OOH intermediate group. The spin-conserving electron transfer step takes place when the spin of the transferred electron is aligned with the spins of the d-electrons of the Mn atoms in the cuboidal high-spin cluster isomer. The d-electrons provide a ferromagnetically ordered environment that facilitates the spin-gated selective electron transfer process, resulting in parallel-spin-exchange stabilization and a lowered transition state barrier for the coupling reaction involving the frontier orbitals of the oxyl and hydroxy reactant intermediates.

Striking Size and Doping Effects of Ti-Si-O Clusters on Methane Conversion Reactions.

The size and doping effects in methane activation by Ti-Si-O clusters have been explored by using a combination of gas-phase experiments and quantum chemical calculations. All [Tim Sin O2(m+n) ].+ (m+n=2, 3, 8, 10, 12, 14) clusters can extract a hydrogen from methane. The associated energies and structures have been revealed in detail. Moreover, the doping and size effects have been discussed involving generalized Kohn-Sham energy decomposition analysis, natural population analysis, Wiberg bond indexes (WBI), molecular polarity index (MPI) and ionization potential (IP). It suggested that Ti-Si-O clusters with a low Ti : Si ratio is beneficial to adsorbing methane and inclination to the hydrogen atom transfer (HAT) process, while the clusters with a high Ti : Si ratio favors the generation of a terminal oxygen radical and results in high reactivity and turnover frequency. On the other hand, a cluster size of m+n=12 is recommended considering both the ionization potential and the turnover frequency of the reaction. Hopefully, these finding will be instructive for the design of high-performance Ti-Si-O catalyst toward methane conversion.

Superoxide Reduction by Cu‐Amyloid Beta Peptide Complexes: A Density Functional Theory Study

AbstractOxidative stress and copper dyshomeostasis are well established factors in the course of Alzheimer's disease (AD). Indeed, the pathological interaction of copper with the amyloid peptide is able to induce reactive oxygen species (ROS) production through a three‐step cycle in which O2 is gradually reduced to superoxide, oxygen peroxide and finally to highly neurotoxic OH⋅ radicals. The purpose of this study has been to investigate at a computational level, by Density Functional Theory (DFT) and molecular mechanics techniques, the final two steps of O2 reduction to OH− +OH radical i.e the superoxide and hydroperoxide reduction by ascorbate, on different Cu(II) ⋅ Aβ coordination models. At variance with the first step of O2 reduction to O2.− characterized by high energy barrier, the successive O2.− and O2H− reductions are low energy barriers processes and that the OH⋅ radical formation is further favored by the possibility of Cu(II) ⋅ Aβ to form the Asp1 amino terminal radical. We discuss the results obtained from the perspective of the recent findings in the Aβ redox chemistry in AD pathology.

Molecules with a TEMPO-based head group as high-performance organic friction modifiers

High-performance organic friction modifiers (OFMs) added to lubricating oils are crucial for reducing energy loss and carbon footprint. To establish a new class of OFMs, we measured the friction and wear properties of N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide referred to as C12Amide-TEMPO. The effect of its head group chemistry, which is characterized by a rigid six-membered ring sandwiched by an amide group and a terminal free oxygen radical, was also investigated with both experiments and quantum mechanical (QM) calculations. The measurement results show that C12Amide-TEMPO outperforms the conventional OFMs of glyceryl monooleate (GMO) and stearic acid, particularly for load-carrying capacity, wear reduction, and stability of friction over time. The friction and wear reduction effect of C12Amide-TEMPO is also greatly superior to those of C12Ester-TEMPO and C12Amino-TEMPO, in which ester and amino groups replace the amide group, highlighting the critical role of the amide group. The QM calculation results suggest that, in contrast to C12Ester-TEMPO, C12Amino-TEMPO, and the conventional OFMs of GMO and stearic acid, C12Amide-TEMPO can form effective boundary films on iron oxide surfaces with a unique double-layer structure: a strong surface adsorption layer owing to the chemical interactions of the amide oxygen and free radical with iron oxide surfaces, and an upper layer owing to the interlayer hydrogen-bonding between the amide hydrogen and free radical or between the amide hydrogen and oxygen. Moreover, the intralayer hydrogen-bonding in each of the two layers is also possible. We suggest that in addition to strong surface adsorption, the interlayer and intralayer hydrogen-bonding also increases the strength of the boundary films by enhancing the cohesion strength, thereby resulting in the high tribological performance of C12Amide-TEMPO. The findings in this study are expected to provide new hints for the optimal molecular design of OFMs.

Adsorption Behavior of TEMPO-Based Organic Friction Modifiers during Sliding between Iron Oxide Surfaces: A Molecular Dynamics Study.

Organic friction modifiers (OFMs) added to lubricating oils to reduce friction and wear are crucial for reducing energy loss and CO2 emissions. In our previous studies, we have developed N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide, referred to as C12TEMPO, as a new type of OFM and experimentally demonstrated its superior performance to conventional OFMs of stearic acid and glycerol monooleate. However, the behavior of C12TEMPO adsorbing onto solid surfaces from base oil during sliding, which largely dictates the lubrication performance, is yet to be elucidated. Here, we performed molecular dynamics simulations for confined shear of a C12TEMPO solution in poly-α-olefin between hematite surfaces. Unlike conventional OFMs, which typically have one functional group or multiple functional groups of the same type, C12TEMPO features two functional groups of different types: one amide and one terminal free oxygen radical. The results showed that adsorbed boundary films with a double-layer structure form stably during sliding, owing to double- or single-site surface adsorption and interlayer hydrogen bonding via the two functional groups. Additionally, some molecules in each of the first and second layers also form intralayer hydrogen bonding. Such multitype adsorption is unique and favorable for enhancing the strength of boundary films to withstand heavily loaded and prolonged sliding. The velocity distribution indicates that the first and second layers are solid- and liquid-like, respectively. The second layer could act as a buffer for the first layer, which is the last barrier to prevent solid-solid contact, against shear. We also found that the second layer can act as a reservoir to rapidly repair the once depleted region in the first layer because of the interlayer hydrogen bonding. The combination of the high strength and self-repair ability of the C12TEMPO boundary films can rationally explain the experimentally observed properties of high load-carrying capacity, excellent antiwear effect, and high stability of friction over time.

Role of Bridging Oxygen Vacancy on Reduced Anatase TiO2 (101) for Photodegradation of Rhodamine-B

A hydrothermal method is proposed for the synthesis of TiO2 nanoparticles, which were further sintered at 200 °C, 300 °C, 500 °C, and 800 °C in a hot air oven. UV–light-driven photodegradation of Rhodamine-B dye was studied for as-prepared and sintered samples. The low probability of recombination of electron-hole pair and high charge separation rate showed better performance in degradation efficiency for a sample sintered at 300 °C. X-ray diffraction patterns suggest that as the sintering temperature is increased, amorphous nature vanishes and a pure anatase phase is observed. With a further increment of sintering temperature above 300 °C, mixed phase is observed. Band bending phenomenon of anatase and rutile provides a clear view on photocatalytic performance. Kinetics and mechanism of adsorbed water molecules with the bridging oxygen vacancy and 5 fold coordinated titanium (Ti5c) of reduced anatase TiO2 (101) is illustrated schematically, where energy is reduced in the presence of bridging oxygen vacancy. The concept of oxygen vacancies and water molecule adsorption over reduced titania surface is discussed. The reaction mechanism process of the kinetics of the adsorbed water molecules with the bridging oxygen vacancy defects and Ti5c site along the bridging and terminal hydroxyl radical are embedded on reduced TiO2 (101) nanoparticles.

Cryptochrome magnetoreception: four tryptophans could be better than three.

The biophysical mechanism of the magnetic compass sensor in migratory songbirds is thought to involve photo-induced radical pairs formed in cryptochrome (Cry) flavoproteins located in photoreceptor cells in the eyes. In Cry4a—the most likely of the six known avian Crys to have a magnetic sensing function—four radical pair states are formed sequentially by the stepwise transfer of an electron along a chain of four tryptophan residues to the photo-excited flavin. In purified Cry4a from the migratory European robin, the third of these flavin–tryptophan radical pairs is more magnetically sensitive than the fourth, consistent with the smaller separation of the radicals in the former. Here, we explore the idea that these two radical pair states of Cry4a could exist in rapid dynamic equilibrium such that the key magnetic and kinetic properties are weighted averages. Spin dynamics simulations suggest that the third radical pair is largely responsible for magnetic sensing while the fourth may be better placed to initiate magnetic signalling particularly if the terminal tryptophan radical can be reduced by a nearby tyrosine. Such an arrangement could have allowed independent optimization of the essential sensing and signalling functions of the protein. It might also rationalize why avian Cry4a has four tryptophans while Crys from plants have only three.

Structural Characterization of Nickel-Doped Aluminum Oxide Cations by Cryogenic Ion Trap Vibrational Spectroscopy.

Isolated nickel-doped aluminum oxide cations (NiOm)(Al2O3)n(AlO)+ with m = 1-2 and n = 1-3 are investigated by infrared photodissociation (IRPD) spectroscopy in combination with density functional theory and the single-component artificial force-induced reaction method. IRPD spectra of the corresponding He-tagged cations are reported in the 400-1200 cm-1 spectral range and assigned based on a comparison to calculated harmonic IR spectra of low-energy isomers. Simulated spectra of the lowest energy structures generally match the experimental spectra, but multiple isomers may contribute to the spectra of the m = 2 series. The identified structures of the oxides (m = 1) correspond to inserting a Ni-O moiety into an Al-O bond of the corresponding (Al2O3)1-3(AlO)+ cluster, yielding either a doubly or triply coordinated Ni2+ center. The m = 2 clusters prefer similar structures in which the additional O atom either is incorporated into a peroxide unit, leaving the oxidation state of the Ni2+ atom unchanged, or forms a biradical comprising a terminal oxygen radical anion Al-O•- and a Ni3+ species. These clusters represent model systems for under-coordinated Ni sites in alumina-supported Ni catalysts and should prove helpful in disentangling the mechanism of selective oxidative dehydrogenation of alkanes by Ni-doped catalysts.

Kinetics of Aqueous Persulfate-Induced Oxidative Degradation of Heptafluorobutanoate, Pentafluoropropionate, and Trifluoroacetate

The kinetics of reaction of three fluorinated carboxylates (heptafluorobutanoate, pentafluoropropionate, and trifluoroacetate) with ammonium persulfate were studied by in situ 19F nuclear magnetic resonance and Raman spectroscopies in acidic aqueous solutions at temperatures between 323 and 363 K. Oxidative decarboxylation of the carboxylates yields a terminal perfluoroalkyl radical intermediate; the dominant reaction of this intermediate is recombination with hydroxyl radical followed by rearrangement to the acid fluoride and then hydrolysis, but other minor recombination products are observed. Arrhenius parameters for the initial decomposition of ammonium persulfate and the reaction of sulfate radical with the fluorinated carboxylates were derived. Heptafluorobutanoate and pentafluoropropionate were found to have similar reaction rates with sulfate radical, while the reaction with trifluoroacetate was 30–50% slower. Reaction rates did not show a strong dependence on pH in the range of 2 – 7.

Size, Stoichiometry, Dimensionality, and Ca Doping of Manganese Oxide-Based Water Oxidation Clusters: An Oxyl/Hydroxy Mechanism for Oxygen–Oxygen Coupling

Gas-phase ion-trap reactivity experiments and density functional simulations reveal that water oxidation to H2O2 mediated by (calcium) manganese oxide clusters proceeds via formation of a terminal oxyl radical followed by oxyl/hydroxy O-O coupling. This mechanism is predicted to be energetically feasible for Mn2Oy+ (y = 2-4) and the binary CaMn3O4+, in agreement with the experimental observations. In contrast, the reaction does not proceed for the tetramanganese oxides Mn4Oy+ (y = 4-6) under these experimental conditions. This is attributed to the high fluxionality of the tetramanganese clusters, resulting in the instability of the terminal oxyl radical as well as an energetically unfavorable change of the spin state required for H2O2 formation. Ca doping, yielding a symmetry-broken lower-symmetry three-dimensional (3D) CaMn3O4+ cluster, results in structural stabilization of the oxyl radical configuration, accompanied by a favorable coupling between potential energy surfaces with different spin states, thus enabling the cluster-mediated water oxidation reaction and H2O2 formation.

Synthesis, properties and graft polymerization of ionic conjugated polymers with TCNQ anion radical

The reactions of N-(2,4-dinitrophenyl)pyridinium 7,7′,8,8′-tetracyanoquinodimethane anion radical (TCNQ−.), namely, salt(TCNQ−.), with piperazine and 2-methylpiperazines without catalyst addition resulted in the opening of pyridinium ring to yield ionic polymers with 5-piperazinium-2,4-dienylideneammonium (PDA) TCNQ−. and 2-methylpiperazinium-2,4-dienylideneammonium (MPDA) TCNQ−. units, respectively, namely polymer(R;TCNQ−.) (R = H, Me, (R)-(−)-Me, and (S)-(+)-Me). The polymerization of methyl methacrylate (MMA) and styrene (St) initiated by AIBN, followed by the treatment with polymer(R;TCNQ−.) afforded radical coupling between TCNQ−. and the terminal radical of growing PMMA and PSt, resulting in poly(PDA) and poly(MPDA) with the PMMA and PSt graft chains, i.e., polymer(R;TCNQ−;GC) (GC = graft chain; PMMA and PSt). The 1H NMR, UV–vis, and SEM data suggested that the structures of polymer(R;TCNQ−;GC) in solution depended on solvent polarity. The electrochemical, magnetic, optical, and thermal properties of the polymers were also investigated.

The FeIVO• oxyl unit as a key intermediate in water oxidation on the FeIIIhydroxide: DFT predictions

AbstractThe OO coupling process in water oxidation on the gamma FeOOH hydroxide catalyst is simulated by means of density functional theory using model iron cubane cluster Fe4O4(OH)4. A key reactive intermediate is proposed to be the HOFeIVO• oxyl unit with terminal oxo radical. The “initial” vertex FeIII(OH) moiety forms this intermediate at the calculated overpotential of 0.93 V by adding one water molecule and withdrawing two proton–electron pairs. The OO coupling goes via water nucleophilic attack on the oxyl oxygen to form the OO bond with a remarkably low barrier of 11 kcal/mol. This process is far more effective than alternative scenario based on direct interaction of two ferryl FeIVO sites (with estimated barrier of 36 kcal/mol) and is comparable with the coupling between terminal oxo center and three‐coordinated lattice oxo center (12 kcal/mol barrier). The process of hydroxylation of terminal oxygen inhibits the OO coupling. Nevertheless, being more effective for ferryl oxygen, the hydroxylation in fact enhances selectivity of the OO coupling initiated by the oxyl oxygen.

Синтез блок-сополимера бутилакрилата с винилбу-тиловым эфиром с использованием инициирующей сис-темы триэтилборан – кислород компенсационным спо-собом

The synthesis of a copolymer of butyl acrylate with vinyl butyl ether was carried out by a compensatory method in boiling vinyl butyl ether, at its excess, in the presence of triethyl boron isolated from the complex with hexamethylenediamine and an oxidizing agent. The resulting copolymer was used as a macroinitiator by adding a new portion of butyl acrylate to boiling vinyl butyl ether. Several samples of macroinitiators were obtained with different copolymerization times. As the formation of macromolecules upon copolymerization of butyl acrylate with vinyl butyl ether occurs along two centers of chain growth: oligomeric, formed due to the capture of initiating and growing radicals with a small number of copolymer units by boroxyl radicals, and one with low molecular weight, formed by conventional radical chain termination during initiation by alkyl and alkoxyl radicals, the composition of the isolated copolymer contains an oligomer and a low molecular weight copolymer. In the oligomeric fraction of the copolymer, there are more labile bonds with the boroxyl radical at the end of the chain than in the low molecular weight copolymer; as a result, the addition of butyl acrylate to the solution in vinyl butyl ether of the copolymer isolated from the reaction mixture leads to the formation of a block copolymer with a higher molecular weight, while the molecular weight increases after this, due to the addition of new copolymer units most of all to the oligomeric part of the sample. An increase in the molecular weight of the copolymer due to the grafting of new units predominantly onto the oligomer via a labile bond according to the reversible inhibition scheme leads to an increase in the mass of the samples and noticeable changes in the molecular weight distribution curve. If the sample synthesis time significantly exceeds the dosing time, the revealed effects are weaker, which is associated with the recombination of radicals formed according to the scheme of reversible inhibition, due to the usual radical termination. The presented data indicate that the copolymer of butyl acrylate with vinyl butyl ether synthesized in the presence of triethyl boron and an oxidizing agent during compensatory copolymerization in an excess of vinyl butyl ether is a macroinitiator of the block copolymerization of butyl acrylate with vinyl butyl ether, due to the labile bond of the terminal boroxyl radical.

Vanadium, niobium and tantalum complexes with terminal sulfur radical ligands.

Sulfur radicals terminally bound to the metal center can be considered as the one-electron reduction products of complexes with terminal sulfido ligands which serve as the reactive sites in enzymes and precursors. However, there is limited information regarding this kind of metal stabilized sulfur radical, which contrasts the more commonly known metal stabilized thiyl radical. In this work, we report the preparation of vanadium, niobium and tantalum radical complexes in the form of M(O)(S)F2 from the reactions of laser-ablated metal atoms and SOF2 in cryogenic matrixes. Combined with the results from infrared spectroscopy and density functional theory calculations, the sulfur ligand in M(O)(S)F2 is characterized to be a terminally bound radical with the unpaired electron located on the sulfur 3p orbital. Besides this radical complex, calculations also predict the existence of MF2(η2-SO) with a side-on SO ligand, but this less stable isomer is not observed as a result of high exothermicity along with its formation from metal atoms and SOF2 that is large enough to overcome the energy barrier towards the occurrence of M(O)(S)F2.