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A procedure was developed for the synthesis of the kinetically stable nitroxide 2-(N-tert-butyl-N-hydroxyamino)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl containing the sterically hindered hydroxylamino group. The crystal and molecular structure of this compound was determined. The isolation of this nitroxide hydroxylamine in the individual state enabled a new procedure for the preparation of its oxidation product, viz., the corresponding nitroxide biradical. The results of quantum chemical calculations of the electron exchange in the biradical molecule are in good agreement with the experimental data on the triplet ground state of the molecule in the crystal obtained earlier.

The pulsed method of chemically induced dynamic nuclear polarization (CIDNP) with the microsecond time resolution was used to study the nuclear polarization kinetics of Trp-Trp (tryptophyl-tryptophan) dipeptide protons in the photoreaction between the dipeptide and 2,2′-dipyridyl in aqueous solutions at pH from 2 to 10. The dependence of the selectivity of the reversible photoinduced oxidation of the dipeptide by 2,2′-dipyridyl in the triplet-excited state was found. When the pH of the solutions was below pKa of the terminal amino group, polarization was observed only for the protons of the C-terminal residue. The threshold effect of protonation of the terminal amino group of the dipeptide on the selectivity of both the deactivation of the 2,2′-dipyridyl triplet state and the degenerate intermolecular electron exchange was revealed. No intramolecular electron migration in the oxidized dipeptide was detected. At the pH exceeding pKa of the terminal amino group of the dipeptide, the quenching of the triplet excited state of 2,2′-dipyridyl is not selective. This is manifested as equal intensities of the nuclear polarization signals of the both residues of tryptophan in the 1H NMR spectrum of the dipeptide containing the CIDNP signals of the geminate reaction products. The rate constant of the degenerate electron exchange and nuclear paramagnetic relaxation times of the protons of the neutral radical and radical cation of the dipeptide were determined by the numerical simulation of the nuclear polarization kinetics.

The review concerns methods of synthesis, as well as the molecular, electronic, and crystal structures and the magnetic properties of a novel class of paramagnetic compounds, namely, chalcogen-nitrogen π-heterocyclic radical anion salts derived from 1,2,5-chalcogenadiazole.

This paper gives a brief overview of a new approach to chalcogen–nitrogen heterocyclic π-radicals, particularly 1,2,3- and 1,3,2-benzodithiazolyls, based on thermolysis or photolysis of closed-shell precursors (such as 1,3,2,4-benzodithiadiazines and their 1-Se congeners, 1,2,4,3,5- and 1,3,5,2,4-benzotrithiadiazepines, and some other sulfur–nitrogen rings and cages). The approach allows obtaining radicals hardly accessible or inaccessible in other ways, especially, fluorinated radicals. The identity of radicals prepared is confirmed by electron paramagnetic resonance spectroscopy in combination with density functional theory calculations.

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 enthalpy of formation in the standard state for the promising novel energetic material [1,2,5]oxadiazolo[3,4-e][1,2,3,4]-tetrazine-4,6-di-N-dioxide (furazano-1,2,3,4-tetrazine-1,3-dioxide) was calculated using a theoretically calculated value of the heat of formation in the gas phase and am experimentally measured value of the heat (enthalpy) of sublimation. The theoretical calculations were performed using the G2, G3, and CBS-QB3 high-accuracy multilevel quantum chemical techniques.

The primary physical and chemical processes in the photochemistry of 1-(trideuteromethyl)-2,3,4-trideutero (1) and 1-acetoxy-2-methoxy-(2) 9,10-anthraquinones were studied using femtosecond transient absorption spectroscopy and computational chemistry. Excitation of 1 and 2 at 270 nm results in the population of a set of highly excited singlet states which decay within the laser pulse by internal conversion and vibrational energy redistribution. The transient absorption spectra of the lowest singlet and triplet excited states of substituted anthraquinones 1 and 2 as well as the triplet excited and ground states of the products were detected. The assignments of the transient absorption spectra were performed on the basis of quantum chemical calculations of the electronic absorption spectra of the intermediates. Time-dependent density functional theory or CASSCF/CASPT2 procedure were used to calculate the spectroscopic properties of the intermediates.

The singlet—triplet energy splitting (ΔE ST = E S − E T ) was calculated for formylnitrene (5) and for the syn- and anti-rotamers of carboxynitrene HOC(O)N (6) by the CCSD(T) method. Extrapolation of ΔE ST to a complete basis set was calculated to be negative for 5 and strongly positive for 6. Similar results were obtained by the G2 procedure. The reason for the dramatic stabilization of the singlet state appeared to be a special bonding interaction between the nitrogen and oxygen atoms, which results in the structure intermediate between those of nitrene and oxazirene. It was found that the B3LYP/6-31G(d) method overestimates ΔE ST by ∼9 kcal mol−1 for 5 and by ∼7 kcal mol−1 for 6. Taking into account this overestimation and the results of DFT calculations, it was concluded that benzoylnitrene has a singlet ground state. It was proved experimentally using photolysis of benzoyl azide in an argon matrix at 12 K that benzoylnitrene has a singlet ground state and its structure is similar to that of oxazirene. Nevertheless, these singlet intermediates have low barrier to the aziridine formation, which is traditionally considered to be a typical singlet nitrene reaction.

The rate constant k CO of decarbonylation of phenylacetyl radicals generated by photolysis of dibenzyl ketone was measured by laser flash photolysis technique in six solvents in a wide temperature range. The pre-exponential factors A and activation energies E a of decarbonylation were found for all solvents. The k CO value decreases with an increase in the dielectric constant e of the solvent, whereas an increase in the ability of the solvent for hydrogen bonding increases k CO. The results of quantum-chemical calculations confirm the mutual compensation of the contributions of specific and nonspecific solvations to the activation energy of decarbonylation in alcohols.

Thermolysis of 1-arylthio-2-azidoanthraquinones in DMSO at 150 °C afforded substituted 5H-naphtho[2,3-c]phenothiazine-8,13-diones. These compounds were also formed upon photolysis of azides at 77 K and were prepared in quantitative yields by the reactions of 1-arylthio-2-azidoanthraquinones with KOH in DMSO at 20 °C.

Photolysis of 12π-electron, formally antiaromatic, 1,3,2,4-benzodithiadiazine (1a) and its tetrafluoro-substituted derivative (1b) was studied in glassy matrices at 77 K and in an argon matrix at 14 K by UV, IR, and ESR spectroscopy. In these matrices, a diamagnetic species (3a,b) is formed as a persistent species. It is product of the intramolecular transformations of heterocycles 1a,b and the precursor of 1,2,3-benzodithiazolyl radicals (2a,b). The nitrenoid structure, intermediate between the structures of singlet 1,2,3-benzodithiazol-2-ylnitrene RS—N: and the corresponding thiazyl RS≡N, was ascribed to intermediate 3a,b in agreement with the data of IR spectroscopy.

The kinetics and mechanisms of the reactions of singlet perfluoro-4-biphenylnitrene and N -propyl- 4-nitreno-2,3,5,6-tetrafluorobenzylamide with various amines, pyridine, and dimethylsulfoxide were studied by laser flash photolysis. The reactions of singlet arylnitrenes with amines are two-step processes. The primary step of the process is adduct formation; the rate constant of this reaction is high and lies within the range 4 〈 10 7 -2 〈 10 8 l mol -1 s -1 for the tested secondary amines. The second step (1,2-hydrogen shift) was acceler- ated in the presence of water.

Photochemical transformations of 2-arylamino-1-(4-tert-butylphenoxy)-9,10-anthraquinones involve migration of thetert-butylphenyl group either to theperi-located carbonyl oxygen to give 2-arylamino-9-(4-tert-butylphenoxy)-1,10-anthraquinones or to the nitrogen atom to give 2-aryl(4-tert-butylphenyl)amino-1-hydroxy-9,10-anthraquinones (typical products of the Smiles rearrangement).

The geometries and formation enthalpies of reagents, products, and triplet intermediates in the reaction of photoenolization of o- methylacetophenone and 1- methylanthraquinone are calculated by semiempirical quantum chemical methods. It is shown that the triplet intermediate is a triplet Σπ- biradical. The geometries and formation enthalpies of the transition state of the thermal transfer of the hydrogen atom in 9- hydroxy- 1,10- anthraquinone- 1- methide are calculated. The MNDO semiempirical methods give significantly exaggerated activation enthalpies in the thermal transfer of the hydrogen atom.

Photochemical transformations of 1-arylcyanomethyl-9,10-anthraquinones were studied. It was established that under irradiation, the hydrogen atom of the substituted methyl group is transferred to aperi-quinoid oxygen atom to form the corresponding 9-hydroxy1, 10-anthraquinone-1-arylcyanomethides. Dark transformations of photoinduced quinonemethides result from three competing parallel processes: intramolecular transfer of the hydrogen atom, a reaction with a solvent (alcohol), and oxidation by dissolved oxygen. The kinetics of these reactions were studied.

The kinetics of the reactions of 9-aryloxy-1,10-anthraquinone derivatives with aliphatic and aromatic amines was studied. The limiting stage of the reaction is the nucleophilic 1,4-addition. Electron withdrawing substituents in anthraquinone increase and electron donor substituents considerably decrease the rate constant, stabilizing photoinduced 1,10-anthraquinones. The geometric and electronic structures of the amine also affect the rate constant. On going from alkylamines to aniline, the rate constant decreases by more than an order of magnitude. The introduction of electron donor substituents into the aniline molecule leads to an increase in the rate constant, while the introduction of electron withdrawing substituents leads to its decrease. The significant negative activation entropy of the reaction between 1,10-anthraquinone derivatives andn-propylamine leads one to the conclusion that the transition state is highly ordered. The reaction occurs easily due to the low activation enthalpy (ΔH# = 2÷7 kcal mol−1).

Photochemical conversions of O-acyl derivatives of 1-hydroxy-2-methoxy-9,10-anthraquinone have been studied by low temperature and pulsed photolysis. Acyl group structure has a substantial effect on the rate of thermal migration. Replacement of methyl by ethoxy decreases the rate constant by two orders of magnitude; replacement by diethylamino reduces it nearly five orders of magnitude. The log rate constant of the aroyl rearrangement is linearly dependent on the electrophilic constant of the substituents in the phenyl nucleus of the migrant.

A stepwise mechanism for the reaction of 9-aryloxy-1,10-anthraquinones with alcohols has been established, starting with nucleophilic 1,4-addition of a molecule of alcohol. A quantitative reactivity study has been carried out. Introduction of acceptor substituents into the anthraquinone nucleus substantially increases the reaction rate, while donor substituents reduce it. A qualitative correlation between the rate constants and the electron density at the C and O atoms of the reaction site has been established.

Laser flash photolysis of perfluoro-4-biphenyl azide andN-propyl-4-azido-2,3,5,6-tetrafluorobenzylamide in acetonitrile, water-acetonitrile mixtures, and HCl-containing solutions was studied. The absorption spectra of primary intermediates, singlet arylnitrenes(2a(S) and2b(S), respectively), were recorded. The absolute rate constants of their intersystem crossing in MeCN were measured, and the corresponding Arrhenius parameters were found from the temperature dependences of the rate constants of isomerization of singlet arylnitrenes2a(S) and2b(S) to azirines. Protonation of singlet arylnitrenes2a(S) and2b(S) was observed, the rate constants of their protonation were measured, and the transient absorption spectra of arylnitrenium ions were recorded. It was shown by quantum-chemical calculations (the hybrid B3LYP method) that the arylnitrenium ions that formed have the singlet ground state and the singlet-triplet gap is ∼20 kcal mol−1.

By means of flash photolysis and low-temperature spectrophotometry, the formation of a complex between a Cu(I) ion and a peroxy radical of the solvent has been detected in ethanol, isopropanol, and dimethylformamide. The peroxy radical is generated in a reaction of a solvent radical with a molecule of dissolved oxygen. The solvent radical appears as a result of photoreduction of chloride complexes of Cu(II). The radical complex has a band in the optical absorption spectrum with a maximum at 415–420 nm in ethanol and isopropanol. The rate of formation of this complex is determined mainly by the reaction of the radical of the matrix (R.) with complexes of bivalent copper. The rate constant of this process in isopropanol at room temperature is (2–3)·108 liters/ mole·sec. Disappearance of the radical complex Cu(I)...RO2. takes place in a reaction with complexes Cu2+solv and CuCl+ with a rate constant of 2.3·107 liters/mole·sec at room temperature.

The optical and ESR spectra have been examined for complexes of Cu(I) with various radicals, which contain various numbers of Cl ions in the central-atom coordination sphere. The spin-Kamiltonian parameters have been determined for all these radical complexes, and the observed ESR spectra have been compared with those calculated with allowance for second-order effects. The observed values for the isotropic and anisotropic components of the HFI constant from the central ion have been used to estimate the contributions from the 4s and 3d2z orbitals of the copper ion to the unpaired-electron MO. Quantum-chemical calculations have been performed by the INDO method on the electronic structures and geometries of complexes formed by CH2OH with Cu(I) for various Cl contents in the coordination sphere. The radical is coordinated by the π orbital on the carbon atom, and the stabilities of the radical complexes decrease as the number of Cl ions in the coordination sphere increases. A geometry close to planar is proposed for the CuCl4−3 fragment in a complex containing four Cl ions.

1. The effect of the solvent on the photolysis quantum yield of 2-amino-1,4-naphthoquinone derivatives was studied. A linear correlation was established between the function of the quantum yield log[(1−ϕ)/ϕ] and the position of thelaπ*-type long-wavelength band. The major effect on the quantum yield is related to the dielectric properties of the medium and the formation of O...H hydrogen bonds. 2. The temperature effect on the photolysis quantum yield of 2-morpholino-1,4-naphthoquinone was studied and the effective activation energy of this reaction was determined. 3. The photolysis of 2-amino-1,4-naphthoquinone derivatives was studied at 77°K and the formation of precursors of the product of photolysis at room temperature was established. 4. A mechanism was proposed for the transformation of the products of the photolysis at 77°K to the product of the photolysis at ∼ 20°C.

1. The effect of substituants on the rate constant and activation energy of the dark stage in the photochromic transformations of 1-methylanthraquinone derivatives was studied. It was found that there is a correlation between the π-electron density at the carbon atom of the methide group and the rate constant. 2. A quantum-chemical calculation of the rate constant for the dark stage of the process was made on the basis of the proposed mechanism for the photochromic transformations of 1-hydroxy-4-methylanthraquinone. The calculated rate constant agrees satisfactorily with the experimental value. 3. The effect of the nature of the solvents on the rate constant and on the activation energy of the dark stage in the photochromic transformations of 1-hydroxy-4-methylanthraquinone was studied. The nonspecific effect of the solvent on the process rate predominates.

Steady-state and pulsed methods have been applied to static phosphorescence and fluorescence quenching for phenanthrene caused by oxygen and nitric oxide. The characteristic fluorescence quenching distances have been determined for oxygen (8.7±0.5 A) and nitric oxide (8.9±0,5 A); the phosphorescence quenching involves an exchange mechanism, and measurements have been made on the characteristic quenching distances R0 and exchange-interaction attenuation parameters L. Estimates have been made on the rate constants for dynamic quenching of the triplet excited states of phenanthrene in low-viscosity liquids, which agree well with the measured values for hexane.

1. The photolysis of p-azidoaniline in organic aprotic solvents was investigated in the absence of O2; the main product of photolysis at room temperature is 4,4′-diaminoazobenzene, but when samples irradiated at 77°K are thawed, the main products are 4,4′-diaminoazobenzene and p-phenylenediamine. The precursor of the end products is p-aminophenylnitrene in the triplet state. 2. In the presence of O2, the main products of the photolysis of p-azidoaniline in aprotic solvents are p-nitroaniline and p-nitrosoaniline. The quantum and chemical yields of their formation were measured. 3. p-Aminophenylnitrene reacts with O2 to form an adduct; subsequently, a dimer is obtained, which breaks down to the end products.

In an earlier paper [i] we have described the photochromic conversion of l-phenoxy-4acetoxy-9,10-anthraquinone(I) to 9-phenoxy-10-acetoxy-l,4-anthraquinone(II), which proceeds through the intermediate formation of a derivative of 1,10-anthraquinone, and it was established that during this conversion two successive migration processes occur: an initial photochemical migration of the phenyl group and a subsequent dark migration of the acety! group [i]. Recently [2, 3] we have discovered a photoinitiated, thermally reversible migration of the acetyl group for a number of derivatives of l-acetoxy-9,10-anthraquinone and 9-acetoxy-l,4-anthraquinone. From these results one can assume that for compounds I and II containing two different groups able to migrate under the action of light, besides the photoinitiated migration of the phenyl group, the possibility potentially exists of photoinitiated migration of the acetyl group. In the present work a detailed study has been made of the mechanism of the photochromic conversions of l-phenoxy-4-acetoxy-9,10-anthraquinone (utilizing the new data in [2, 3]), the nature of the intermediate products have been established and the values of the photochemical quantum yields and of the rate constants of the dark stages of the photochromic cycle have been determined.

When exposed to UV and visible light, 2-N,N-dialkylamino-l,4-naphthoquinones in EtOH. hexane, and benzene are isomerized to substituted 2,3-dihydronaphtho[2,l-d]oxazoles [i]. We have observed that acetylene derivatives of aminonaphthoquinones (I) undergo photocycloisomerization to 3-alkylidene-2,3-dihydronaphtho[2,3-b]pyrrole-4,9-diones (III). The reactions include two photochemical steps and may be carried out in a stepwise fashion