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The reaction between bis(benzene)chromium(0), Cr0(C6H6)2, and [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (abbreviated as TDTD) formed single crystals of the 1 : 1 salt, [CrI(C6H6)2]+[TDTD]−. The crystal structure of [Cr(C6H6)2][TDTD] belongs to the monoclinic P21/c space group, and involves a CdSO4-type network (or quartz dual net), which is formed by CH…N hydrogen bonds between [Cr(C6H6)2]+ (S = 1/2) and [TDTD]− (S = 1/2). In addition to this network, the two components form an alternating chain crystal with a π–π overlap along the [110] and [11̅0] directions. The theoretical calculations for the pairwise intermolecular magnetic exchange interactions in [Cr(C6H6)2][TDTD] reveal the presence of 3D interactions, ranging from an antiferromagnetic interaction of −8.96 cm−1 to a ferromagnetic one of 1.70 cm−1. The temperature dependence of the paramagnetic susceptibility χp indicates the dominance of an antiferromagnetic interaction with a negative Weiss constant of −4.8 K and a magnetic ordering at 8 K, which can be characterized in terms of weak ferromagnetism. [ABSTRACT FROM AUTHOR]

The [(α-diimine)Re(CO) 3 (Hal)] complexes are able to act as efficient catalysts for electrochemical reduction of CO 2 into energy-rich compounds. Among the α-diimine ligands, the 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) attracted recently increased attention. Reaction of [Re(CO) 5 Br] with dpp-bian leads to formation of [(dpp-bian)Re(CO) 3 Br], which was isolated and characterized by XRD as solvent-free [(dpp-bian)Re(CO) 3 Br] ( 1 ), and two solventomorphs [(dpp-bian)Re(CO) 3 Br]·C 6 H 5 CH 3 ( 1-C 7 H 8 ) and [(dpp-bian)Re(CO) 3 Br]·0.5CH 3 CN ( 1-0.5CH 3 CN ). Electrochemical reduction of 1 in DMF and CH 3 CN has been studied using CV and in situ EPR spectroelectrochemistry. According to the experimental results, complex 1 undergoes in DMF multistep reduction via a number of intermediates, two of which were detected using in situ EPR spectroscopy. A careful theoretical analysis of the multistep reduction mechanism including calculations of the thermodynamics of elementary reactions and electronic structures of proposed intermediates has been performed. General scheme of the electrochemical reduction, which is essentially ligand-centered, has been proposed and supported by full-electron DFT calculations with scalar relativistic DKH2 and ZORA Hamiltonians. [ABSTRACT FROM AUTHOR]

A new approach to the synthesis of fused 1,2,3-thiaselenazoles-rare five-membered heterocycles that contain two different chalcogens-from the corresponding 1,2,3-dithiazoles and SeO2 was accomplished by selective exchange of S and Se atoms. The fused carbo- and heterocyclic units were indene, naphthalenone, cyclohexadienone, cyclopentadiene, benzoannulene, and benzoxazine. The molecular structures of two of the thiaselenadiazole products and one of the dithiazole precursors were confirmed by single-crystal X-ray diffraction. The reaction is highly solvent selective; it only takes place in solvents that contain a C=O group (e.g., DMF or tetramethylurea). According to DFT calculations, the reaction is thermodynamically favorable. Based on the DFT calculations and 77Se NMR spectroscopy, two tentative mechanisms that feature isomeric transition states and intermediates are suggested for the reaction via ring-opening addition of SeO2 to the S−X dithiazole bond (X=N or S). The DFT-calculated first adiabatic electron affinities of the compounds were chalcogen independent and positive in all cases, which assumes formation of thermodynamically stable radical anions (RAs). These calculated RAs featured either normal or abnormal elongation of the S1−X2 (X=S or Se) bond relative to their neutral precursors and possessed π* or σ* SOMOs, respectively. [ABSTRACT FROM AUTHOR]

Molecular magnets based on copper(ii) ions and stable nitroxide radicals exhibit promising switchable behavior triggered by a number of external stimuli; however, their spin-state-correlated optical properties vital for photoinduced switching have not been profoundly investigated to date. Herein, the electronic absorption spectra of single crystals of three representatives of this unique family are studied experimentally and theoretically in the visible and near-IR regions. We established that the color of the complexes is mainly determined by optical properties of the nitroxide radicals, whereas the Cu(hfac)2 fragment contributes to the near-IR range with the intensity smaller by an order of magnitude. The thermochromism of these complexes evident upon thermal spin state switching is mainly caused by a spectral shift of the absorption bands of the nitroxides. The vibrational progression observed in the visible range for single crystals as well as for solutions of pure nitroxides is well reproduced by DFT calculations, where the C–C stretching mode governs the observed progression. The analysis of the spectra of single crystals in the near-IR region reveals changes in the energy and in the intensity of the copper(ii) d–d transitions, which are well reproduced by SOC-NEVPT2 calculations and owe to the flip of the Jahn–Teller axis in the coordination environment of copper. Further strategies for designing bidirectional magnetic photoswitches using these appealing compounds are discussed. [ABSTRACT FROM AUTHOR]

The design and synthesis of novel charge-transfer (CT) complexes are of interest for fundamental chemistry and applications to materials science. In addition to the recently described first CT complex with both electron acceptor (A) and donor (D) groups belonging to the 1,2,5-thiadiazole series ( 1; A: 4-nitro-2,1,3-benzothiadiazole; D: 4-amino-2,1,3-benzothiadiazole), herein novel CT complexes 2 and 3 with 1,2,5-thiadiazoles as both A (4,6-dinitro-2,1,3-benzothiadiazole and [1,2,5]thiadiazolo[3,4- c][1,2,5]thiadiazole) and D (4-amino-2,1,3-benzothiadiazole) were synthesized. The series is completed by complex 4 with [1,2,5]thiadiazolo[3,4- c][1,2,5]thiadiazole as A and phenoxatellurine as D. Structures of complexes 2- 4 were characterized by single-crystal X-ray diffraction (XRD), as well as solution and solid-state UV/Vis spectroscopy. Thermodynamics of their formation were obtained by density functional theory (DFT) calculations, their bonding situations were analyzed by quantum theory of atoms in molecules (QTAIM) calculations and dimer model energies of interactions quantified in the framework of the Hirshfeld surface (HS) analysis. With DFT calculations, the largest value of CT between D and A was found for complex 2, with 0.027 e in the XRD structure and 0.150 e in the optimized structure in MeCN. In the UV/Vis spectra, the λmax of the CT bands of 2- 4 varied in the range λ=517-705 nm. Model energy calculations for 1- 4 revealed the importance of both dispersion interactions and hydrogen bonding between D and A as contributors to CT in the crystalline state. In an attempt to enlarge the CT value with bis[1,2,5-thiadiazolo][3,4- b;3′,4′- e]pyrazine as A and 4-amino-2,1,3-benzoselenadiazole as D, an unprecedented 1:1 addition reaction was observed upon formation of a C−N bond between atom C7 of D and pyrazine atom N4 of A, accompanied by hydrogen atom transfer from C7 to another pyrazine atom N8 (compound 5). According to DFT calculations, the reaction is a multistep process featuring diradical intermediates and hydrogen atom intramolecular migration over four positions. Molecular and crystal structures of 5 (solvate with toluene) were elucidated by XRD and the crystal structure revealed a rather unusual porous framework. [ABSTRACT FROM AUTHOR]

Previously unknown mono-, di- and in some cases tri- and tetra- carbocycle-substituted quinoxalines ( 2 – 8 ), 2,1,3-benzothiadiazoles ( 11 , 12 , 14 – 17 ) and 2,1,3-benzoselenadiazoles ( 20 - 25 ) were synthesized by nucleophilic substitution of fluorine in 5,6,7,8-tetrafluoroquinoxaline ( 1 ) and 4,5,6,7-tetrafluoro-2,1,3-benzothia/selenadiazoles ( 10 and 19 , respectively) with methoxide and dimethylamine. In the 1:1 reactions, the nucleophiles attacked selectively the position 6 of 1 or the position 5 of 10 and 19 . The regioselective nature of the 1:1 reactions was confirmed by the DFT calculations at the M06-2X/6-31+G(d,p) level of theory. Disubstituted quinoxaline ( 28 ), thia- ( 29 ) and selena- ( 30 ) diazoles bearing two different substituents, i.e. MeO- and Me 2 N-, were synthesized in a similar way. New substituted 1,2-diaminobenzenes ( 31 – 33 ) were prepared by reduction of corresponding thiadiazoles ( 12 , 14 , 15 ) and isolated in the form of hydrochlorides. Compound 33 was converted into a new quinoxaline ( 34 ) by reaction with (PhCO) 2 . Compounds 5 , 7 and 14 were studied for cytotoxicity toward the human cancer cells and effects on the cytochrome P450 mRNA expression. They did not cause any significant modulations in the expression of several cytochrome P450 genes, and 7 was weakly toxic for the Hep2 (carcinoma) and U937 (leukemia) cells, particularly, apoptosis was observed. [ABSTRACT FROM AUTHOR]

Abstract: (6H-1,2,3-Benzodithiazol-6-ylidene)malononitrile (4) was electrochemically and chemically reduced with tetrakis(dimethylamino)ethylene (TDAE) and CrTol2 into the title radical anion (5). The electrochemically generated 5 and its salts [TDAE]2+[5]2 (7) and [CrTol2]+ [5] (8) were characterized by solution EPR in combination with UV–Vis spectroscopy. Salts 7 and 8 were isolated and characterized by elemental analysis and solid-state EPR, and 8 by magnetic measurements in the temperature range 2–300K. The value of the effective magnetic moment of the salt 8 at 300K, μ eff =1.64μB, indicates π-dimerization of 5 in the solid state. For the molar magnetic susceptibility χ of 8, the temperature dependences of χT and 1/χ in the range 2–300K can be approximated by the Curie–Weiss law with the Curie–Weiss constant C and temperature Tc equal to 0.33±0.01cm3 Kmol−1 and −1.0±0.1K, respectively. In the mean-field approximation, the effective value of the exchange interaction of [CrTol2]+ with its neighboring cations can be estimated as zJ =−1.4сm−1. All the experimental results were supported by DFT calculations. [Copyright &y& Elsevier]

Polymer-chain complexes Cu(hfac)2LR represent an interesting type of molecular magnets exhibiting thermally induced and light-induced magnetic switching, in many respects similar to a spin crossover. In the majority of these compounds the polymer chain consists of alternating oneand threespin units composed of copper(ll) ions and nitronyl nitroxides. The principal one-dimensional structure of the complexes has previously been assumed to play a key role in the observed magnetic anomalies. Using Q-band electron paramagnetic resonance (EPR) spectroscopy, we have reliably demonstrated that these complexes are indeed one-dimensional in the sense of the topology of their exchange channels; however, the magnetic chains spread across the structural polymer chains and consist solely of spin triads of nitroxide-copper(ll)-nitroxide. Using four selected examples of complexes Cu(hfac)2LR, we have found the exchange coupling values between spin triads of neighboring polymer chains to range from <1 to ca. 10 cm-1. This conclusion could only be reached due to the selective probing of oneand three-spin units by EPR and correlates perfectly with both previous magnetic susceptibility data and quantum chemical calculations performed in this work. These findings give new insights into the cooperativity effects and mechanisms of magnetic anomalies in the Cu(hfac)2LR family of molecular magnets. [ABSTRACT FROM AUTHOR]

Ferrocenophanes, Nitroxide diradicals, Wittig reactions, EPR spectroscopy, DFT calculations Keywords: Ferrocenophanes; Nitroxide diradicals; Wittig reactions; EPR spectroscopy; DFT calculations EN Ferrocenophanes Nitroxide diradicals Wittig reactions EPR spectroscopy DFT calculations 1 1 1 02/21/22 20220218 NES 220218 B The Cover Feature b shows a beautiful sunrise metaphorically representing the boundary between chemistry and physics, the interaction of which has paved the way for the synthesis of a biradical with a ferrocene-derived inflexible bridging moiety. Cover Feature: Multispin Systems with a Rigid Ferrocene-1,1'-diyl-Substituted 1,3-Diazetidine-2,4-diimine Coupler: A General Approach (Eur. J. Org. [Extracted from the article]

Knowledge of the formation of radicals from sulfites, in vivo, is of interest in understanding the allergic and inflammatory responses to environmental sulfur dioxide exposure. Sulfite anion trioxide (SO3−•) radical formation was measured in mice, preloaded with the spin trap, 5-(diethoxy-phosphoryl)-5-methyl-pyrrolidine-N-oxide, (DEPMPO). Based on spin trapping NMR, a surprising quantity of reduced SO3−•-adduct was observed that did not depend on co-administration of oxidizing agents, suggesting a possible nonradical reaction between (bi)sulfite and DEPMPO. The products of the reversible nucleophylic addition of (bi)sulfite to the nitrone functional group were identified using 31P-NMR, 1H-NMR, and 13C-NMR spectroscopy as cis- and trans- stereoisomers of hydroxylamine and confirmed by quantum chemical calculations. Oxidation of the hydroxylamines results in the formation of two corresponding cis- and trans-isomeric nitroxides, only one of which has been earlier described as the paramagnetic adduct of genuinely trapped SO3−• radical. The results demonstrate that SO3−• detection using nitrone spin traps such as DEPMPO and DMPO may involve nonradical addition reactions except in cases when the required controls unambiguously prove a radical mechanism. [Copyright &y& Elsevier]

A long-lived π -heterocyclic radical anion of the hybrid 1,2,5-thiadiazolidyl / 1,2,3-dithiazolidyl type was electrochemically generated and characterized by EPR spectroscopy and DFT calculations. [ABSTRACT FROM AUTHOR]

The reaction of the redox active 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-BIAN) and iron(II) iodide in acetonitrile led to a new complex [(dpp-BIAN)FeIII2] (1). Molecular structure of 1 was determined by the single crystal X-ray diffraction analysis. The spin state of the iron cation in complex 1 at room temperature and the magnetic behavior of 1 in the temperature range of 2–300 K were studied using Mossbauer spectroscopy and magnetic susceptibility measurements, respectively. The neutral character of dpp-BIAN in 1 was confirmed by IR and UV spectroscopy. The electrochemistry of 1 was studied in solution and solid state using cyclic voltammetry. The generation of the radical anion form of the dpp-BIAN ligand upon reduction of 1 in a CH2Cl2 solution was monitored by EPR spectroscopy. [ABSTRACT FROM AUTHOR]

The bridged systems containing pyrenyl and aryl azido residues have been synthesized for the first time, and the quenching of pyrene fluorescence on a picosecond time scale has been observed. [Copyright &y& Elsevier]

• New materials are synthetized introducing an TCNQ moiety to anthra[2,3- b ]thiophene core. • Fused aromatic TCNQs are potential non-fullerene acceptors for organic photovoltaics. • Vacuum-free fabricated OPV devices based on thiophene-fused TCNQs shows PCE of 0.71%. Three novel molecules based on thiophene-fused 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) units linked through functionalized π-conjugated arene bridges are synthesized by three-step sequence of Sonagashira coupling, double thioannulation and Knoevenagel condensation. The energy of their frontier orbitals shows that these molecules are suitable for replacing fullerenes as electron acceptors in composites with conjugated donor polymers used in organic photovoltaics (OPV). The composite of polymer donor PCDTBT with fluorene-linked thiophene-fused TCAQs demonstrates good solubility in organic solvents and bulk heterojunction morphology with characteristic donor and acceptor domain size of several tens of nanometers, favourable for photoelectric conversion. This composite also exhibits prominent light-induced EPR signal indicating efficient photoinduced free charge generation. OPV devices based on this composite were fabricated by vacuum-free method. They show photoelectric conversion with maximum PCE of 0.71%. [ABSTRACT FROM AUTHOR]

High-spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but examples reported to date exhibit limited stability and processability. In this work, we designed the first tetraradical based on an oxoverdazyl core and nitronyl nitroxide radicals and successfully synthesized it using a palladium-catalyzed cross-coupling reaction of an oxoverdazyl radical bearing three iodo-phenylene moieties with a gold(I) nitronyl nitroxide-2-ide complex in the presence of a recently developed efficient catalytic system. The molecular and crystal structures of the tetraradical were confirmed by single crystal X-ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼125 °C in an inert atmosphere; in a toluene solution upon prolonged heating at 90 °C in air, no decomposition was observed. The resulting unique verdazyl-nitroxide conjugate was thoroughly studied using a range of experimental and theoretical techniques, such as SQUID magnetometry of polycrystalline powders, EPR spectroscopy in various matrices, cyclic voltammetry, and high-level quantum chemical calculations. All collected data confirm the high thermal stability of the resulting tetraradical and quintet multiplicity of its ground state, which makes the synthesis of this important paramagnet a new milestone in the field of creating high-spin systems.

We have prepared a bis(compartmental) Mannich base ligand H 4 L (1,4,8,11-tetraaza-1,4,8,11-tetrakis(2-hydroxy-3-methoxy-5-methylbenzyl)cyclotetradecane) specifically designed to obtain bis(TM II Ln III ) tetranuclear complexes (TM = transition metal). In this regard, we have succeeded in obtaining three new complexes of the formula [Zn 2 (μ-L)(μ-OAc)Dy 2 (NO 3 ) 2 ]·[Zn 2 (μ-L)(μ-OAc)Dy 2 (NO 3 )(OAc)]·4CHCl 3 ·2MeOH (1) and [TM 2 (μ-H 2 L) 2 (μ-succinate)Ln 2 (NO 3 ) 2 ] (NO 3 ) 2 ·2H 2 O·6MeOH (TM II = Zn, Ln III = Dy (2); TM II = Co, Ln III = Dy (3)). Compound 1 contains two different bis(ZnDy) tetranuclear molecules that cocrystallize in the structure, in which acetato bridging ligands connect the Zn II and Dy III ions within each ZnDy subunit. This compound does not exhibit slow magnetic relaxation at zero field, but it is activated in the presence of an applied dc magnetic field and/or by Dy/Y magnetic dilution, showing two relaxation processes corresponding to each of the two different bis(ZnDy) units found in the structure. As revealed by the theoretical calculations, magnetic relaxation in 1 is single-ion in origin and takes place through the first excited state of each Dy III ion. When using the succinato dicarboxylate bridging ligand instead of acetate, compounds 2 and 3 were serendipitously formed, which have a closed structure with the succinate anion bridging two ZnDy subunits belonging to two different ligands. It should be noted that only compound 2 exhibits slow relaxation of magnetization in the absence of an external magnetic field. According to experimental and theoretical data, 2 relaxes through the second excited Kramers doublet ( U eff = 342 K). In contrast, 3 displays field-induced SMM behaviour ( U eff = 203 K). However, the Co/Zn diluted version of this compound 3Zn shows slow relaxation at zero field ( U eff = 347 K). Ab initio theoretical calculations clearly show that the weak ferromagnetic coupling between Co II and Dy III ions is at the origin of the lack of slow relaxation of this compound at zero field. Compound 2 and its diluted analogues 2Y and 3Zn show hysteresis loops at very low temperature, thus confirming their SMM behaviour. Finally, compounds 1 and 2 show Dy III based emission even at room temperature that, in the case of 2, allows us to extract the splitting of the ground 6 H 15/2 term, which matches reasonably well with theoretical calculations.

High-spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but those synthesized to date possess limited stability and processability. In this work, we have designed a tetraradical based on the Blatter's radical and nitronyl nitroxide radical moieties and successfully synthesized it by using the palladium-catalyzed cross-coupling reaction of a triiodo-derivative of the 1,2,4-benzotriazinyl radical with gold(I) nitronyl nitroxide-2-ide complex in the presence of a newly developed efficient catalytic system. The molecular and crystal structure of the tetraradical was confirmed by X-ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼150 °C under an inert atmosphere and exhibits reversible redox waves at -0.54 and 0.45 V versus Ag/AgCl. The magnetic properties of the tetraradical were characterized by SQUID magnetometry of polycrystalline powders and EPR spectroscopy in various matrices. The collected data, analyzed by using high-level quantum chemical calculations, confirmed that the tetraradical has a triplet ground state and a nearby excited quintet state. The unique high stability of the prepared triazinyl-nitronylnitroxide tetraradical is a new milestone in the field of creating high-spin systems., (© 2023 Wiley-VCH GmbH.)

In the present work, the study of the unusual interaction between copper hexafluoroacetylacetonate and the diacetyliminoxyl radical resulted in two discoveries from different fields: the determination of the oxime radical spatial structure and the introduction of an oxime radical into the field of molecular magnetic material design. Oxime radicals are key plausible intermediates in the processes of oxidative CH-functionalization and in the synthesis of functionalized isoxazolines from oximes. Due to the lack of X-ray diffraction data for oxime radicals, the knowledge about their structure is based mainly on indirect approaches, spectroscopic methods (electron paramagnetic resonance and IR), and quantum chemical calculations. The structure of the oxime radical was determined for the first time by stabilizing the diacetyliminoxyl radical in the form of its complex with copper (II) hexafluoroacetylacetonate (Cu(hfac) 2 ), followed by single-crystal X-ray diffraction analysis. Although oxime radicals are known to undergo oxidative coupling with acetylacetonate ligands in transition-metal complexes, a complex is formed with intact hfac ligands. X-ray diffraction studies have shown that the oxime radical is coordinated with copper ions through the oxygen atoms of the carbonyl groups without the direct involvement of the C═N-O • radical moiety. The structure of the coordinated diacetyliminoxyl is in good agreement with the density functional theory (DFT) prediction for free diacetyliminoxyl due to the very weak interaction of the radical molecule with copper ions. Remarkably, both weak ferromagnetic and antiferromagnetic interactions between Cu (II) and oxime radicals have been revealed by modeling the temperature dependence of magnetic susceptibility and confirmed by DFT calculations, rendering diacetyliminoxyl a promising building block for the design of molecular magnets.

Heterometallic complexes [Cp* 2 Ln(μ- iso CO) 2 {Cr 2 (μ-P)Cp 2 (CO) 2 }] [Ln = Yb ( 1 ), Sm ( 2 )] were obtained in reactions of [Cp* 2 Ln(thf) 2 ] (Ln = Sm, Yb) with [{CpCr(CO) 2 } 2 (μ,η 2:2 -P 2 )] ( 4 ). An analogous yttrium compound [Cp* 2 Y(μ- iso CO) 2 {Cr 2 (μ-P)Cp 2 (CO) 2 }] ( 3 ) was synthesized using a three-component reaction between [Cp* 2 Y(BPh 4 )], 4 , and KC 8 . Compounds 1 - 3 were isolated as solvent-free crystalline phases; in the case of 2 , the 2·0.5C 7 CpCr═P═CrCp(CO) H 8 solvate was also obtained. The structures of all crystalline phases were determined by single-crystal X-ray diffraction analysis. All compounds contain a unique {((CO) 2 CpCr═P═CrCp(CO) 2 )} - unit, which is linked to Ln 3+ ions through CO ligands in the isocarbonyl mode. Compounds 1 and 3 have a molecular structure, while compound 2 contains polymeric chains of triangular [Cp* 2 (μ-P)Cp iso CO) 2 {Cr 2 (μ-P)Cp 2 P NMR studies demonstrated similar dramatic downfield shifts for complexes 2 }] units linked by μ- iso CO-ligands. 31 P NMR studies demonstrated similar dramatic downfield shifts for complexes 1 - 3 P shift, quantum chemical calculations were performed both for 1 - 3 and to elucidate the nature of the high downfield chemical 31 P shift, quantum chemical calculations were performed both for 1 are due to the anisotropic effect of the Cr═P double bonds.3 and for related Cr- and Fe-phosphido complexes. Calculations show that the anomalously high downfield chemical shifts for 1 - 3 are due to the anisotropic effect of the Cr═P double bonds.

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin-spin exchange interaction parameter ( J ), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1'-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.

2-Methyl-9 H -thioxanthene-9-one (1) and its S,S-dioxide (2) are the precursors of pendant groups that determine the reduction potentials of electro-active polyimides, which exhibit electrochromic behavior and are used in organic electronics. Electrochemical reduction of 1 and 2 leads to the formation of the corresponding persistent radical anions and dianion (for S , S -dioxide). Using 3D spectroelectrochemistry, all anions have been shown to exhibit strong absorption in the UV-VIS-NIR wavelength region. Electronic absorption spectra of 1 and 2 and their negative ions were interpreted using time-dependent DFT. According to the calculations, the most intense electronic transitions of the dianions 12- and 22- in the visible region exhibit hypsochromic shift compared to the intense transitions of the corresponding radical anions and have much higher oscillator strengths, which was confirmed experimentally for 2. An empirical kinetic model was proposed based on the analysis of the total charge passed through the cell during electrolysis and on the established mechanism of electrochemical reduction. This model perfectly described the UV-VIS-NIR optical density time dependences observed on 3D spectroelectrochemical surfaces for both compounds 1 and 2. This made it possible to explain the differences in the electrochromic behaviour of ambibolar electro-active polyimides with pendant groups based on 1, 2.

Thermally resistant air-stable organic triradicals with a quartet ground state and a large energy gap between spin states are still unique compounds. In this work, we succeeded to design and prepare the first highly stable triradical, consisting of oxoverdazyl and nitronyl nitroxide radical fragments, with a quartet ground state. The triradical and its diradical precursor were synthesized via a palladium-catalyzed cross-coupling reaction of diiodoverdazyl with nitronyl nitroxide-2-ide gold(I) complex. Both paramagnetic compounds were fully characterized by single-crystal X-ray diffraction analysis, superconducting quantum interference device magnetometry, EPR spectroscopy in various matrices, and cyclic voltammetry. In the diradical, the verdazyl and nitronyl nitroxide centers demonstrated full reversibility of redox process, while for the triradical, the electrochemical reduction and oxidation proceed at practically the same redox potentials, but become quasi-reversible. A series of high-level CASSCF/NEVPT2 calculations was performed to predict inter- and intramolecular exchange interactions in crystals of di- and triradicals and to establish their magnetic motifs. Based on the predicted magnetic motifs, the temperature dependences of the magnetic susceptibility were analyzed, and the singlet-triplet (135 ± 10 cm -1 ) and doublet-quartet (17 ± 2 and 152 ± 19 cm -1 ) splitting was found to be moderate. Unique high stability of synthesized verdazyl-nitronylnitroxide triradical opens new perspectives for further functionalization and design of high-spin systems with four or more spins.

The first example of a triply bridging (μ 3 -P) phosphine ligand has been discovered in the crown-shaped [Cu 3 (μ 2 -Hal) 3 L] (Hal=Cl, Br, or I) complexes supported by tris[2-(2-pyridyl)ethyl]phosphine (L). Theoretical analysis completely confirms the observed μ 3 -P-bridging pattern, revealing the interaction of the same lone pair of phosphorus with three valence 4s-orbitals of Cu atoms. The presented complexes exhibit outstanding blue phosphorescence (λ em =442-465 nm) with the quantum efficiency reaching 100 %. The complex [Cu 3 (μ 2 -I) 3 L] also exhibits remarkable thermo- and mechanochromic luminescence resulting in a sharp change in the emission colour upon external stimuli. These findings essentially contribute to coordination chemistry of the pnictine ligands., (© 2021 Wiley-VCH GmbH.)

triangulo -Trirhenium nonaiodide Re 3 I 9 reacts with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) to produce the novel 13-electron paramagnetic cluster Re 3 I 8 (IMes) 2 , which was characterized by means of X-ray diffraction analysis, ESR spectroscopy, magnetometry, and quantum chemistry.

Azobis tetrazole and triazole derivatives containing long catenated nitrogen atom chains are of great interest as promising green energetic materials. However, these compounds often exhibit poor thermal stability and high impact sensitivity. Kinetics and mechanism of the primary decomposition reactions are directly related to these issues. In the present work, with the aid of highly accurate CCSD(T)-F12 quantum chemical calculations, we obtained reliable bond dissociation energies and activation barriers of thermolysis reactions for a number of N-rich heterocycles. We studied all existing 1,1'-azobistetrazoles containing an N 10 chain, their counterparts with the 5,5'-bridging pattern, and the species with hydrazo- and azoxy-bridges, which are often present energetic moieties. The N 8 -containing azobistriazole was considered as well. For all compounds studied, the radical decomposition channel was found to be kinetically unfavorable. All species decompose via the ring-opening reaction yielding a transient azide (or diazo) intermediate followed by the N 2 elimination. In the case of azobistetrazole derivatives, the calculated effective activation barriers of decomposition are ∼26-33 kcal mol -1 , which is notably lower than that of tetrazole (∼40 kcal mol -1 ). This fact agrees well with the low thermal stability and high impact sensitivities of the former species. The activation barriers of the N 2 elimination were found to be almost the same for the azobis compounds and the parent tetrazole, and the effective decomposition barrier is determined by the thermodynamics of the tetrazole-azide rearrangement. In comparison with 1,1'-azobistetrazole, the hydrazo-bridged compound is more stable kinetically due to the lack of pi-conjugation in the azide intermediate. In turn, the azoxy-bridged compounds are entirely unstable due to tremendous azide stabilization by the hydrogen bond formation. In general, the 5,5'-bridged species are more thermally stable than their 1,1'-counterparts due to a much higher barrier of the N 2 elimination. Apart from this, the highly accurate gas-phase formation enthalpies were calculated at the W1-F12 level of theory for all species studied.

A series of red-emissive {Cu4I6} clusters have been synthesized from alkyl-tris(2-pyridyl)phosphonium halides, [R-PPy3]Hal, and CuI. The size of the alkyl substituent (R) has a dramatic impact on the structure of the clusters assembled. [Me-PPy3]I salt reacts with CuI (1 : 2) to give the ionic [Cu(Me-PPy3)I]2Cu2I4 complex consisting of the scorpionate [Cu(N,N',N''-Me-PPy3)I]+ cation. Under similar conditions, [Pr-PPy3]I forms the zwitterionic [Cu4I6(Pr-TPP)2] complex containing an unusual stepwise [Cu4I6] cluster core. The use of [Bu-PPy3]I or [Bn-PPy3]I in this reaction leads to zwitterionic [Cu4I6(R-TPP)2] complexes, in which a linear-shaped [Cu4I6] module appears. Photophysical studies supported by TD-DFT computations have revealed that the title complexes in the solid state at 298 K exhibit a red photoluminescence (λemmax = 620-650 nm) with short lifetimes (0.04-2.10 μs), which are assigned to the thermally activated delayed fluorescence (TADF) mixed with the cluster centered (3CC) phosphorescence. The compounds synthesized are the first red-emitting representatives of the recently discovered family of zwitterionic CuI-based complexes (so-called "AIO" structures).

The thermal stability of energetic materials, being of the utmost importance for safety issues, is often considered in terms of kinetics, e.g., the Arrhenius parameters of the decomposition rate constant. The latter, in turn, are commonly determined using conventional thermoanalytical procedures with the use of simple Kissinger or Ozawa methods for kinetic data processing. However, thermal decomposition of energetic materials typically occurs via numerous exo- and endothermal processes including fast parallel reactions, phase transitions, autocatalysis, etc. This leads to numerous drawbacks of simple approaches. In this paper, we proposed a new methodology for characterization of the thermochemistry and thermal stability of melt-cast energetic materials, which is comprised of a complementary set of experimental and theoretical techniques in conjunction with a suitable kinetic model. With the aid of the proposed methodology, we studied in detail a novel green oxidizer, tetranitroacetimidic acid (TNAA). The experimental mass loss kinetics in the melt was perfectly fitted with a model comprised of zero-order reaction (sublimation or evaporation) and first-order thermal decomposition of TNAA with the effective Arrhenius parameters Ea = 41.0 ± 0.2 kcal mol-1 and log(A/s-1) = 20.2 ± 0.1. We rationalized the experimental findings on the basis of highly accurate CCSD(T)-F12 quantum chemical calculations. Computations predict that thermolysis of TNAA involves an intricate interplay of multiple decomposition channels of the three tautomers, which are equilibrated via either monomolecular reactions or concerted double hydrogen atom transfer in the H-bonded dimers; the calculated Arrhenius parameters of the effective rate constant coincide well with experiment. Most importantly, calculations provide detailed mechanistic evidence missing in the thermoanalytical experiment and explain formation of the experimentally observed primary products N2O and NO2. Along with the kinetics and mechanism of decomposition, the proposed approach yields accurate thermochemistry and phase change data of TNAA.

Donor-acceptor (D-A) complexes between 3,4-dicyano-1,2,5-chalcogenadiazoles [chalcogen=Te (1 a), Se (1 b), S (1 c)] and the pseudohalides CN - and XCN - (X=O, S, Se, Te) were studied experimentally and theoretically. For 1 a, they were isolated as [K(18-crown-6)][1 a-CN] (2), [K(18-crown-6)][1 a-NCO] (3), [K(18-crown-6)][1 a-SCN] (4), [K(18-crown-6)][1 a-SeCN] (5), and [K][1 a-NCSe] (6) and characterized by X-ray diffraction (XRD), UV/Vis and NMR spectroscopy, and DFT and QTAIM calculations. For 1 b and 1 c, the complexes were not isolated due to unfavorable thermodynamics. In all isolated complexes, the D-A bonds, stabilized by negative hyperconjugation, were longer than the sum of the covalent radii and shorter than the sum of the van der Waals radii of the bonded atoms. In mixtures of 1 a, F - , and SeCN - , the complex [1 a-F] - was selectively formed in accordance with thermodynamics. The reaction of 1 a with SeCN - and the cyclic trimeric perfluoro-ortho-phenylene mercury afforded the complex [K(18-crown-6)][SCN]⋅(o-C 6 F 4 Hg) 3 , which was characterized by XRD., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Thermochemistry, kinetics, and mechanism of thermal decomposition of 1,5-diaminotetrazole (DAT), a widely used "building block" of nitrogen-rich energetic compounds, were studied theoretically at a high and reliable level of theory (viz., using the explicitly correlated CCSD(T)-F12/aug-cc-pVTZ procedure). Quantum chemical calculations provided detailed insight into the thermolysis mechanism of DAT missing in the existing literature. Moreover, several contradictory assumptions on the mechanism and key intermediates of thermolysis were resolved. The unimolecular primary decomposition reactions of the seven isomers of DAT were studied in the gas phase and in the melt using a simplified model of the latter. The two-step reaction of N 2 elimination from the diamino tautomer was found to be the primary decomposition process of DAT in the gas phase and melt. The effective Arrhenius parameters of this process were calculated to be E a = 43.4 kcal mol -1 and log( A/s -1 ) = 15.2 in a good agreement with the experimental values. Contrary to the existing literature data, all other decomposition channels of DAT isomers turned out to be kinetically unimportant. Apart from this, a new primary decomposition channel yielding N 2 , cyanamide, and 1,1-diazene was found for some H-bonded dimers of DAT. We also determined a reliable and mutually consistent set of thermochemical values for DAT (Δ f H solid 0 = 74.5 ± 1.5 kcal·mol -1 ) by combining theoretically calculated (W1 multilevel procedure along with an isodesmic reaction) gas phase enthalpy of formation (Δ f H gas 0 = 100.7 ± 1.0 kcal·mol -1 ) and experimentally measured sublimation enthalpy (Δ sub H 0 = 26.2 ± 0.5 kcal·mol -1 ).

The first lanthanide complexes with a redox-active sulfur diimide ligand, [LnCp* 2 (Me 3 SiN=) 2 S] (Ln=Sm, Eu, Yb; Cp*=η 5 -C 5 Me 5 ), are reported. The complexes were synthesized by using [LnCp* 2 (THF) 2 ] and (Me 3 SiN=) 2 S and have been thoroughly characterized by single-crystal X-ray diffraction, EPR spectroscopy, UV/Vis/NIR electronic absorption spectroscopy and SQUID magnetometry. The results, as interpreted by CASSCF/SOC-RASSI calculations providing a non-perturbative treatment of spin-orbit coupling, indicate that these paramagnetic complexes are best described as Ln 3+ and [(Me 3 SiN=) 2 S] - . adducts. As such, these complexes contain the first isolated and structurally characterized acyclic [(RN=) 2 S] - . radical anions., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Low-temperature interaction of [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (1) with MoMes2 (Mes = mesitylene/1,3,5-trimethylbenzene) in tetrahydrofuran gave the heterospin (S1 = S2 = (1)/2) radical-ion salt [MoMes2](+)[1](-) (2) whose structure was confirmed by single-crystal X-ray diffraction (XRD). The structure revealed alternating layers of the cations and anions with the Mes ligands perpendicular, and the anions tilted by 45°, to the layer plane. At 300 K the effective magnetic moment of 2 is equal to 2.40 μB (theoretically expected 2.45 μB) and monotonically decreases with lowering of the temperature. In the temperature range 2-300 K, the molar magnetic susceptibility of 2 is well-described by the Curie-Weiss law with parameters C and θ equal to 0.78 cm(3) K mol(-1) and -31.2 K, respectively. Overall, the magnetic behavior of 2 is similar to that of [CrTol2](+)[1](-) and [CrCp*2](+)[1](-), i.e., changing the cation [MAr2](+) 3d atom M = Cr (Z = 24) with weak spin-orbit coupling (SOC) to a 4d atom M = Mo (Z = 42) with stronger SOC does not affect macroscopic magnetic properties of the salts. For the XRD structure of salt 2, parameters of the Heisenberg spin-Hamiltonian were calculated using the broken-symmetry DFT and CASSCF approaches, and the complex 3D magnetic structure with both the ferromagnetic (FM) and antiferromagnetic (AF) exchange interactions was revealed with the latter as dominating. Salt 2 is thermally unstable and slowly loses the Mes ligands upon storage at ambient temperature. Under the same reaction conditions, interaction of 1 with MoTol2 (Tol = toluene) proceeded with partial loss of the Tol ligands to afford diamagnetic product.

Bis(toluene)chromium(0), Cr(0)(η(6)-C7H8)2 (3), readily reduced [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (1) and [1,2,5]thiadiazolo[3,4-b]pyrazine (2) in a tetrahydrofuran solvent with the formation of heterospin, S1 = S2 = ½, radical-ion salts [3](+)[1](-) (4) and [3](+)[2](-) (5) isolated in high yields. The salts 4 and 5 were characterized by single-crystal X-ray diffraction (XRD), solution and solid-state electron paramagnetic resonance, and magnetic susceptibility measurements in the temperature range 2-300 K. Despite the formal similarity of the salts, their crystal structures were very different and, in contrast to 4, in 5 anions were disordered. For the XRD structures of the salts, parameters of the Heisenberg spin Hamiltonian were calculated using the CASSCF/NEVPT2 and broken-symmetry density functional theory approaches, and the complex magnetic motifs featuring the dominance of antiferromagnetic (AF) interactions were revealed. The experimental χT temperature dependences of the salts were simulated using the Van Vleck formula and a diagonalization of the matrix of the Heisenberg spin Hamiltonian for the clusters of 12 paramagnetic species with periodic boundary conditions. According to the calculations and χT temperature dependence simulation, a simplified magnetic model can be suggested for the salt 4 with AF interactions between the anions ([1](-)···[1](-), J1 = -5.77 cm(-1)) and anions and cations ([1](-)···[3](+), J2 = -0.84 cm(-1)). The magnetic structure of the salt 5 is much more complex and can be characterized by AF interactions between the anions, [2](-)···[2](-), and by both AF and ferromagnetic (FM) interactions between the anions and cations, [2](-)···[3](+). The contribution from FM interactions to the magnetic properties of the salt 5 is in qualitative agreement with the positive value of the Weiss constant Θ (0.4 K), whereas for salt 4, the constant is negative (-7.1 K).

Polymer-chain complexes Cu(hfac)(2)L(R) represent an interesting type of molecular magnets exhibiting thermally induced and light-induced magnetic switching, in many respects similar to a spin crossover. In the majority of these compounds the polymer chain consists of alternating one- and three-spin units composed of copper(II) ions and nitronyl nitroxides. The principal one-dimensional structure of the complexes has previously been assumed to play a key role in the observed magnetic anomalies. Using Q-band electron paramagnetic resonance (EPR) spectroscopy, we have reliably demonstrated that these complexes are indeed one-dimensional in the sense of the topology of their exchange channels; however, the magnetic chains spread across the structural polymer chains and consist solely of spin triads of nitroxide-copper(II)-nitroxide. Using four selected examples of complexes Cu(hfac)(2)L(R), we have found the exchange coupling values between spin triads of neighboring polymer chains to range from <1 to ca. 10 cm(-1). This conclusion could only be reached due to the selective probing of one- and three-spin units by EPR and correlates perfectly with both previous magnetic susceptibility data and quantum chemical calculations performed in this work. These findings give new insights into the cooperativity effects and mechanisms of magnetic anomalies in the Cu(hfac)(2)L(R) family of molecular magnets.

Different uni- and bimolecular reactions of hydroxymethylene, an important intermediate in the photochemistry of formaldehyde, as well as its halogenated derivatives (XCOH, X = H, F, Cl, Br), have been considered using high-level CCSD(T)/CBS quantum chemical methods. The Wentzel-Kramers-Brillouin (WKB) and Eckart approximations were applied to estimate the tunneling rate constant of isomerization of trans-HCOH to H(2)CO, and the WKB procedure was found to perform better in this case. In agreement with recent calculations and experimental observations [Schreiner et al., Nature 2008, 453, 906], the half-life of HCOH at the low temperature limit in the absence of bimolecular processes was found to be very long (approximately 2.1 h). The corresponding half-life at room temperature was also noticeable (approximately 35 min). Bimolecular reactions of trans-hydroxymethylene with parent formaldehyde yield primarily more thermodynamically favorable glycolaldehyde via the specific mechanism involving 5-center transition state. The most preferable reaction of cis-hydroxymethylene with formaldehyde yields carbon monoxide and methanol. Due to very low activation barriers, both processes occur with nearly a collision rate. If the concentration of HCOH (and its halogenated analogues XCOH as well) is high enough, the bimolecular reactions of this species with itself become important, and H(2)CO (or X(H)CO) is then formed with a collision rate. The singlet-triplet energy separation of trans-HCOH is confirmed to be approximately -25 kcal/mol.

UV irradiation of aqueous 4-hydroxyquinoline (4HQN) solutions results in the formation of the triplet states with the quantum yields 30%, 35%, and 7.5% in acidic, neutral, and basic solutions, respectively. In neutral solutions, the keto form is the major tautomeric structure for ground, excited singlet and triplet states of 4HQN. Triplet 4HQN reacts with amino acids tryptophan and tyrosine and with antioxidant ascorbate via the mechanism of electron transfer. The individual experimental and calculated UV-Vis spectra of different acid-base forms of 4HQN in the ground and triplet excited states, as well as reduced and oxidized forms of 4HQN, are reported. Under intense laser irradiation 4HQN undergoes biphotonic ionization, the excited singlet state being the precursor for photoionization.

The primary reactions of the thermal decomposition of hydrazinium nitroformate (HNF) as well as ammonium nitroformate (ANF) were investigated theoretically using the G3 multilevel procedure. Calculations were performed for the reactions in the gas phase and in the melt using a simplified model of the latter. The influence of the melt on the reaction barriers was taken into account by calculation of the solvation free energies using a PCM model. In contrast with many other energetic salts, the ionic salt structures of the HNF and ANF were found to be minima on the PES. However, the most energetically favorable structures of the HNF and ANF in the gas phase are H-bonded complexes. In the melt, on the contrary, the ionic structure is lowest in free energy because of solvation effects. In both the gas phase and melt, the HNF decomposes preferably to nitroform and hydrazine. This fact agrees well with the experimentally observed absence of HNF among the gas-phase decomposition products. Thermolysis of HNF occurs mainly through the intermediacy of nitroform; computations do not support the suggestion that the aci-nitroform is an important intermediate of HNF thermolysis. The primary dissociation reactions of ANF resemble those of the HNF.

To elucidate the multifaceted but poorly understood chemistry of the pivotal polysulfur-nitrogen heterocycle, tetrasulfur tetranitride (S(4)N(4), 1), its photochemistry was studied in Ar matrices. Thereby two primary intermediates and a secondary one (2-4) were detected, and their UV-vis and IR spectra were identified through specific interconversions of 1-4 that can be induced by selective irradiations. The structures associated with these spectra were assigned with the help of DFT calculations. From these assignments it follows that, under the conditions of the present experiments, the cage structure of 1 transforms into isomeric structures 2-4, one of which is a boat-shaped 8-membered cycle (2), and the two other are novel 6-membered S(3)N(3) cycles carrying exocyclic (N)S[triple bond]N (3) or (S)N=S (4) groups, respectively, which have not been previously described. These three intermediates probably play a pivotal role in the formation of the diverse products that are observed in the reactions of S(4)N(4) even under mild reaction conditions.

The thermal decomposition of 5-aminotetrazole was studied theoretically using the G3 multilevel procedure and DFT B3LYP technique. The unimolecular primary decomposition reactions of the three most stable isomers of 5-ATZ were studied in the gas phase and in the melt using a simplified model of the latter. The influence of the melt on the elementary reaction barrier was taken into account by the calculation of the solvation free energies using the PCM model. In contrast to all previous publications, we considered the bimolecular reactions of 5-ATZ and demonstrated that they are very important especially in the condensed phase. It was found that the imino form undergoes fast isomerization to the amino form in the H-bonded dimers and does not participate in the 5-ATZ thermolysis. On the contrary, amino and, probably, the 2H isomer are the main isomers of 5-ATZ in the melt and gas phase. The N(2) elimination reaction was found to be the dominant unimolecular channel of the amino and 2H isomer decomposition in both the gas phase and melt. The significant lowering of the activation barriers of decomposition reactions in H-bonded dimers was found. In agreement with the existing experimental data, HN(3) elimination dominates for some of the considered complexes. It was concluded that the initial stages of thermolysis of 5-ATZ cannot be satisfactory described by the simple unimolecular reactions proposed in the literature.

The gas phase enthalpies of formation of mono-, di-, tri-, tetranitromethane and nitroethane, as well as of their nitrite and aci-form isomers were calculated using different multilevel (G2, G3, G2M(CC5)) and density functional theory (DFT)-based (B3LYP, MPW1B95 and MPWB1K) techniques. The enthalpies of the C-N bond dissociation and isomerization of these nitroalkanes were also calculated. The calculated values of the formation and reaction enthalpies were compared with the experimental data when these data were available. It was found that only the G3 procedure gave accurate (within 1 kcal/mol) results for the formation enthalpy of nitroalkanes, their isomers, and radical products. The G3 procedure and two new hybrid meta DFT methods proposed by Truhlar's group (Zhao, Y.; Truhlar, D. J. Phys. Chem. A 2004, 108, 6908) showed good results for the reaction enthalpies of the nitromethane isomerization and the C-N bond dissociation. Our calculation results were used to analyze thermodynamics of the dissociation and isomerization reactions of the poly nitro-substituted methanes.

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.

The degenerate electron exchange (DEE) reaction involving radical cations (RCs) of n-nonane, n-dodecane, and n-hexadecane in n-hexane solution was studied over the temperature range 253-313 K using the method of time-resolved magnetic field effect in recombination fluorescence of spin-correlated radical ion pairs. In the dilute solutions the rate constant of DEE was found to be 200 times slower than the diffusion limit. Using n-nonane as an example, we showed that two reasons are responsible for the low value of the RC self-exchange rate: (1) conformational variability of molecules and RCs and (2) the activation barrier of DEE reaction. The calculations of the reaction enthalpy performed by the B3LYP/6-31G(d) method indicated that electron transfer can be effective only upon collision of RC with a neutral molecule either in the all-trans conformation or in the conformation differing from the latter by rotation of the end ethyl fragment. The activation barrier of the DEE reaction was estimated using the reorganization energy of the internal degrees of freedom calculated at the B3LYP level and was found to be about 6 kcal/mol. A possible influence of the interaction between RC and a neutral molecule in an encounter complex on DEE rate constant is also discussed.

Photolysis of 1,3,2,4-benzodithiadiazine and its derivatives in hydrocarbon solutions yields dinitrogen and stable 1,2,3-benzodithiazolyls (Herz radicals) whose interaction with dioxygen leads finally to O==S==N-substituted diphenyl disulfides via a self termination-like process with an effective second-order rate constant depending linearly on the concentration of dissolved O2.

The dynamics of singlet 3,5-dichloro-2-biphenylnitrene and the products of its rearrangement were monitored by pico- and nanosecond laser flash photolysis and the results were consistent with the predictions of DFT and ab initio molecular orbital calculations.

The photolysis of 1-aryloxy-2-azidoanthraquinones (3) in benzene is described herein which gave 1-hydroxy-2-arylaminoanthraquinones (4) and two types of 5H-naphtho[2,3-c]phenoxazine-8,13-diones (5 and 6). Thermolysis of 3 yielded only one of phenoxazines 5 and small amount of 4. On the other hand thermolysis of 3 in the presence of phenols gave phenoxazine 6 as a major product. The mechanism of the photolysis and thermolysis of 2-azido-1-aryloxyanthraquinones (3) is proposed and supported by the results from semiempirical calculations. A relative contribution of the primary photoreactions-azido group dissociation and aryl group migration was estimated to be 3:1. Photolysis and thermolysis of 4-azido-1-(p-tert-butylphenoxy)-9,10-anthraquinone (8) gave 3-(p-tert-butylphenoxy)-anthra[1,9-cd]-izoxazole-6-one (9) as the only product.

Knowledge of the formation of radicals from sulfites, in vivo, is of interest in understanding the allergic and inflammatory responses to environmental sulfur dioxide exposure. Sulfite anion trioxide (SO(3)(-*)) radical formation was measured in mice, preloaded with the spin trap, 5-(diethoxy-phosphoryl)-5-methyl-pyrrolidine-N-oxide, (DEPMPO). Based on spin trapping NMR, a surprising quantity of reduced SO(3)(-*)-adduct was observed that did not depend on co-administration of oxidizing agents, suggesting a possible nonradical reaction between (bi)sulfite and DEPMPO. The products of the reversible nucleophylic addition of (bi)sulfite to the nitrone functional group were identified using (31)P-NMR, (1)H-NMR, and (13)C-NMR spectroscopy as cis- and trans- stereoisomers of hydroxylamine and confirmed by quantum chemical calculations. Oxidation of the hydroxylamines results in the formation of two corresponding cis- and trans-isomeric nitroxides, only one of which has been earlier described as the paramagnetic adduct of genuinely trapped SO(3)(-*) radical. The results demonstrate that SO(3)(-*) detection using nitrone spin traps such as DEPMPO and DMPO may involve nonradical addition reactions except in cases when the required controls unambiguously prove a radical mechanism.