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A series of heterometallic carboxylate 1D polymers of the general formula [LnIIICd2(piv)7(H2O)2]n·nMeCN (LnIII = Sm (1), Eu (2), Tb (3), Dy (4), Ho (5), Er (6), Yb (7), piv = anion of trimethylacetic acid) was synthesized and structurally characterized. The use of CdII instead of ZnII under similar synthetic conditions resulted in the formation of 1D polymers, in contrast to molecular trinuclear complexes with LnIIIZn2 cores. All complexes 1–7 are isostructural. The luminescent emission and excitation spectra for 2–4 have been studied, the luminescence decay kinetics for 2 and 3 was measured. Magnetic properties of the complexes 3–5 and 7 have been studied, 4 and 7 exhibited the properties of field-induced single-molecule magnets in an applied external magnetic field. Magnetic properties of 4 and 7 were modelled using results of SA-CASSCF/SO-RASSI calculations and SINGLE_ANISO procedure. Based on the analysis of the magnetization relaxation and the results of ab initio calculations, it was found that relaxation in 4 predominantly occurred by the sum of the Raman and QTM mechanisms, and by the sum of the direct and Raman mechanisms in the case of 7.

Quartet verdazyl-nitronylnitroxide triradical was synthesized via palladium-catalyzed cross-coupling reaction of the corresponding diiodoverdazyls with a nitronyl nitroxide-2-ide gold(I) complex. The triradical is air-stable and possess good thermal stability with decomposition onset at ∼160 °C in an inert atmosphere. X-ray diffraction analysis of single crystals confirmed the presence of verdazyl and nitroxide radical centers. Magnetic properties were characterized by a SQUID magnetometry of polycrystalline powders and by EPR spectroscopy in different matrices. Collected data analyzed using of the high-level quantum chemical calculations confirmed that the triradical has high-spin ground states.

A new synthetic pathway to diradical organic systems is proposed. The effectiveness of this approach was exemplified by the synthesis of a new nitroxide diradical. An interaction of perfluorobiphenyl with lithium tert-butylamide, followed by oxidation of the thusly formed N4,N4&prime, di-tert-butyl-2,2&prime, 3,3&prime, 5,5&prime, 6,6&prime, octafluorobiphenyl-4,4&prime, diamine with meta-chloroperoxybenzoic acid, led to the polyfluorinated nitroxide diradical, N,N&prime, (perfluorobiphenyl-4,4&prime, diyl)bis(N-tert-butyl(oxyl)amine), with a good total yield. The polyfluorinated diradical is stable and can be isolated in free form and completely characterized. The structure of the nitroxide diradical was proved by single-crystal X-ray diffraction analysis. According to the X-ray diffraction data, the diradical is considerably twisted: dihedral angles between the planes of the nitroxide groups and aromatic cycles are 65.1&deg, and 69.5&deg, and between aromatic cycles 52.6&deg, Quantum chemical calculations predict well-balanced size of both intramolecular and intermolecular exchange interactions with J from &minus, 2.65 to &minus, 1.14 cm&minus, 1.

Verdazyl-nitroxide diradicals were synthesized using the palladium-catalyzed cross-coupling reaction of the corresponding iodoverdazyls with a nitronyl nitroxide-2-ide gold(I) complex with high yields (up to 82%). The synthesized diradicals were found to be highly thermally stable and have a singlet (DEST » -64 cm–1) or triplet ground state (DEST ³ 25 and 100 cm–1), depending on which canonical hydrocarbon diradical type they belong to. Upon crystallization, triplet diradicals form unique one-dimensional (1D) spin S = 1 chains of organic diradicals with intrachain ferromagnetic coupling of J′/kB from 3 to 6 K.

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.

By means of cyclic voltammetry (CV) and DFT calculations, it was found that the electron-acceptor ability of 2,1,3-benzochalcogenadiazoles 1-3 (chalcogen: S, Se, and Te, respectively) increases with increasing atomic number of the chalcogen. This trend is nontrivial, since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [1].- and [2].- , RA [3].- was not detected by EPR spectroscopy under CV conditions. Chemical reduction of 1-3 was performed and new thermally stable RA salts [K(THF)]+ [2].- (8) and [K(18-crown-6)]+ [2].- (9) were isolated in addition to known salt [K(THF)]+ [1].- (7). On contact with air, RAs [1].- and [2].- underwent fast decomposition in solution with the formation of anions [ECN]- , which were isolated in the form of salts [K(18-crown-6)]+ [ECN]- (10, E=S; 11, E=Se). In the case of 3, RA [3].- was detected by EPR spectroscopy as the first representative of tellurium-nitrogen π-heterocyclic RAs but not isolated. Instead, salt [K(18-crown-6)]+2 [3-Te2 ]2- (12) featuring a new anionic complex with coordinate Te-Te bond was obtained. On contact with air, salt 12 transformed into salt [K(18-crown-6)]+2 [3-Te4 -3]2- (13) containing an anionic complex with two coordinate Te-Te bonds. The structures of 8-13 were confirmed by XRD, and the nature of the Te-Te coordinate bond in [3-Te2 ]2- and [3-Te4 -3]2- was studied by DFT calculations and QTAIM analysis.

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.

A novel β-ketoimine-functionalized 2,1,3-benzothiadiazole (4-(2,1,3-benzothiadiazole-4-ylamino)pent-3-en-2-one, L2H) was synthesized and used as chelating ligand for the complexation with Zn(II) (complex ZnL2). The spectroscopic and photophysical properties of L2H and ZnL2 as well as those of previously described 4-amino-2,1,3-benzothiadiazole (L1) and its complex ZnL1 have been thoroughly studied in the crystalline state and solution. The UV-Vis absorption spectra of free ligands L1, L2H and its deprotonated form L2−, and complexes ZnL1 and ZnL2 were assigned on the basis of TD-DFT calculations. In contrast to ZnL2 having conventional single-band fluorescence, ZnL1 demonstrates a broad double-band purple-white fluorescence spanning the entire visible region. On the basis of careful spectroscopic studies the second band of this unusual fluorescence spectrum was attributed to the presence of traces of free ligand L1 in the crystals of complex ZnL1. The energy transfer from the excited state of ZnL1 to a free ligand L1 was revealed.

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 N2 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 N2, 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 solid0 = 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 gas0 = 100.7 ± 1.0 kcal·mol-1) and experimentally measured sublimation enthalpy (Δ sub H0 = 26.2 ± 0.5 kcal·mol-1).

The authors are grateful to the Presidium of the Russian Academy of Sciences (Project 8.14), the Royal Society (RS International Joint Project 2010/R3), the Leverhulme Trust (Project IN-2012-094), the Siberian Branch of the Russian Academy of Sciences (Project 13), the Ministry of Education and Science of the Russian Federation (Project of Joint Laboratories of Siberian Branch of the Russian Academy of Sciences and National Research Universities), and the Russian Foundation for Basic Research (Projects 13-03-00072 and 15-03-03242) for financial support of various parts of this work. N.A.S. thanks the Council for Grants of the President of Russian Federation for postdoctoral scholarship (grant MK-4411.2015.3). B.E.B. is grateful for an EaStCHEM Hirst Academic Fellowship. A.V.Z. thanks the Foundation named after D. I. Mendeleev, Tomsk State University, for support of his work. 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 cm3⋅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. Postprint Postprint

The first alkyl chain-anchored heteropentacene, dithieno[2,3-d;2′,3′-d′]benzo-[1,2-b;3,4-b′]dithiophene (DTmBDT), mono- or disubstituted with a nitronyl nitroxide group has been prepared through a cross-coupling synthetic procedure of the corresponding dibromo-derivative (Br 2 -DTmBDT) with a nitronyl nitroxide-2-ide gold(I) complex. The synthesized nitroxides possess high kinetic stability, which allowed us to investigate their structure and thermal, optical, electrochemical, and magnetic properties. Single-crystal X-ray diffraction of both mono- and diradicals revealed that the nitronyl nitroxide group lies almost in the same plane as the nearest side thiophene ring. Such arrangement favors formation of edge-to-edge dimers, which then form close π-stacks surrounded by interdigitating alkyl chains. Before melting, these nitronyl nitroxide radical substituted molecules undergo at least two different phase transitions (PTs): for the monoradical, PTs are reversible, accompanied by hysteresis, and occur near 13 and 83 °C; the diradical upon heating shows a reversible PT with hysteresis in the temperature range 2–11 °C and an irreversible PT near 135 °C. PTs of this type are absent in Br 2 -DTmBDT. Therefore, the step-by-step substitution of bromine atoms by nitronyl nitroxide groups changes the structural organization of DTmBDT and induces the emergence of PTs. This knowledge may facilitate crystal engineering of π-stacked paramagnets and related molecular spin devices.

New nitronyl nitroxides, namely, 2-(4,5-dimethylimidazol-2-yl)- and 2-(4,5-dichloroimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl, were prepared in crystalline form. According to single-crystal X-ray data, intra- and intermolecular hydrogen bonds are formed between NH groups of the imidazole cycles and O atoms of the nitroxide moieties. The intermolecular H-bonds contribute to the alignment of molecules into chains along the a-axis, this alignment causes short intrachain contacts between O and C atoms carrying spin density of opposite signs. Such an arrangement of nitroxides induces ferromagnetic intrachain interactions (J &asymp, 10 cm&minus, 1) between neighboring radicals.

The reactions of O((1)D) atoms with VF(5) at room temperature have been studied by time-resolved laser magnetic resonance at the buffer gas (SF(6)) pressure of 6 Torr. The O((1)D) atoms were produced by the photodissociation of ozone using an excimer laser (KrF, 248 nm). By monitoring the kinetics of FO radical formation, the bimolecular rate constant of O((1)D) consumption in collisions with VF(5) has been determined to be k(VF(5)) = (7.5 ± 2.2) × 10(-11) cm(3) s(-1). The branching ratio for the channel producing FO radicals (k(8a)) has been found to be k(8a)/k(VF(5)) = 0.11 ± 0.02. Quantum chemical calculations at the CCSD(T)/CBS level of theory give evidence that the reactions of O((1)D) with VF(5) proceed via the VF(4)OF intermediate. The enthalpy of the reaction leading to this intermediate formation was calculated to be -245.8 kJ/mol. In qualitative agreement with the experimental results, the reaction channel O((1)D) + VF(5) → FO + VF(4) (8a) turned out to be 72.9 kJ/mol energetically more favorable than the channel O((1)D) + VF(5) → F + OVF(4) (8b). The dissociation enthalpy of the OVF(4) radical was calculated to be very low (18.1 kJ/mol); hence, the decay of OVF(4) to F + OVF(3) should proceed very fast. The molecular channel O((1)D) + VF(5) → F(2) + VF(3)O, though being most favorable thermodynamically, is kinetically unimportant.

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.

Radical cations of heptane and octane isomers, as well as several longer branched alkanes, were detected in irradiated n-hexane solutions at room temperature by the method of time-resolved magnetic field effect (TR MFE). To identify radical cations, the hyperfine coupling constants as determined by simulation of the TR MFE curves were compared to the constants calculated using the density functional theory (DFT) approach. The g-values of the observed radical cations were close to that of the 2,2,3,3-tetramethylbutane radical cation studied earlier by optically detected electron spin resonance (ESR) and TR MFE techniques. No evidence of the decay of the radical cations of branched alkanes to produce olefin radical cations was found, which was further supported by the observation of positive charge transfer from the observed radical cations to cycloalkane molecules. The lifetimes of the radical cations of the branched alkanes were found to be longer than tens of nanoseconds.

The mutual interconversion and decomposition reactions of four tetrazole isomers (1H-TZ, 2H-TZ, 5H-TZ, and an N-heterocyclic carbene 14H) have been studied theoretically using the W1 high-level procedure. Computations allowed resolution of the existing discrepancies in the mechanism and key intermediates of TZ thermolysis. The tautomeric equilibria between 1H-TZ, 2H-TZ, and 14H turned out to play a very important role in the mechanism of thermal decomposition. Although the barriers of monomolecular tautomeric transformations were found to be high (∼50-70 kcal/mol), the concerted double H atom transfer reactions in the H-bonded complexes of TZ tautomers have profoundly lower barriers (∼18-28 kcal/mol). These reactions lead to fast interconversion between 1H-TZ, 2H-TZ, and 14H. The carbene 14H has never been considered before; however, it was predicted to be a key intermediate in the mechanism of thermal decomposition of TZ. For all species considered, the unimolecular reactions of N(2) elimination were predicted to dominate over the elimination of hydrazoic acid. In agreement with existing experimental data, the effective activation energy of thermolysis was calculated to be 36.2 kcal/mol.

Spin-labelled compounds are widely used in chemistry, physics, biology, and material sciences, but the directed synthesis of some functionalized organic radicals is still a challenge. We succeeded in the preparation of a tetrazolyl-substituted nitronyl nitroxide radical in pure crystalline form. According to the single-crystal X-ray data, intra- (NH…O, 2.43 Å) and inter-molecular hydrogen bonds (NH…O, 1.91 Å) are formed between NH groups of the tetrazole cycles and O atoms of the paramagnetic moieties. The intermolecular H-bonds connect the molecules forming chains along the a-axis. Moreover, there are short intermolecular contacts between the O atoms (3.096 Å) and between the O and C atoms (3.096 Å) of the nitronyl nitroxide moieties within the chain. The spin-unrestricted broken-symmetry calculations performed at the BS-UB3LYP/def2-TZVP level of theory predicted a sufficient ferromagnetic interaction (J ≈ 20 cm–1) between the adjacent radicals inside the chain, but a weak antiferromagnetic interaction (−J ≤0.2 cm−1) between the nearest radicals belonging to the different chains. Thus, a rare case when stable radicals, the tetrazolyl-substituted nitronyl nitroxides, are ordered into ferromagnetic chains was revealed; an investigation of the magneto-structural correlations inherent in the nitroxide radical will demand a special experiment in the sub-Kelvin regime.

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.

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 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 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.

Solvent radical cations (RCs) are the primary positive charge carriers that emerge upon ionizing irradiation of alkane solutions. Conductivity measurements of irradiated alkanes show that the rate and mechanism of solvent RC (hole) transport can differ radically in different types of alkanes [1, 2]. For example, the solvent RC mobility in cyclohexane is two orders of magnitude higher than the mobility of molecular ions. Such a high mobility is associated with fast degenerate electron exchange (DEE) between the solvent molecules and the holes

Synthetic, structural, and thermodynamic aspects of the recently discovered new reaction, donor-acceptor coordination of anions (A–) by chalcogen atoms (E) of 1,2,5-chalcogenadiazoles, are considered. According to the quantum chemical calculations, the charge transfer from A– to the heterocycle via the mechanism of negative hyperconjugation (i.e., from the MO of the lone pair of A– to the virtual σ* orbital of the E—N bond of chalcogenadiazole) depends on the nature of E and A–, being 0.42—0.52 and 0.30—0.44 e in terms of the Mulliken and NBO methods, respectively. According to the X-ray diffraction data, the E—A– coordinate bond is always longer than the sum of the covalent radii but shorter than the sum of the van der Waals radii of the atoms forming the bond. The E—A– bond energy varies in a wide range, from ~25 kcal mol–1 comparable to the energy of weak covalent bonds (e.g., internal N—N bond in organic azides) to ~86 kcal mol–1 comparable to the C—C bond energy in organic compounds. The quantum chemical estimations of the thermodynamics of the donor-acceptor coordination of the anions by the chalcogen atoms of 1,2,5-chalcogenadiazoles indicate that for E = Te and Se this reaction may be of general character also covering E = S in some cases.

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.

The primary thermolysis reactions of a promising insensitive explosive 1,1-diamino-2,2-dinitroethylene (DADNE, FOX-7) have been studied in the gas phase at a high level of theory (CCSD(T)-F12/aVTZ). Our calculations revealed that none of the conventional reactions (C-NO2 bond fission, nitro-nitrite and nitro-aci-nitro rearrangements) dominate thermolysis of FOX-7. On the contrary, two new decomposition pathways specific for this particular species that commenced with enamino-imino isomerization and intramolecular cyclization were found instead to be more feasible energetically. The activation barriers of these primary isomerization reactions were calculated to be 48.4 and 28.8 kcal/mol, while the activation energies of the overall decomposition pathways are predicted to be ∼49 and ∼56 kcal/mol, respectively. The new pathways can also be relevant for a wide series of unsaturated hydrocarbons substituted with both nitro- and amino-groups (e.g., triaminotrinitrobenzene, TATB).

The photochemistry of several para-substituted aryl azides was studied by laser flash photolysis (LFP) techniques in pentane. Para F, Cl, Br, I, CH3 ,C F 3, COCH3, OCH3, and NO2 substituted aryl azides were studied. LFP of aryl azides generates singlet aryl nitrenes which were detected in absorption with maxima between 300 and 365 nm in pentane. The nitrene decay can be fit to an exponential function and analyzed to yield observed rate constant, kobs, which was measured as a function of temperature. The value of kobs ) kR + kISC where kR and kISC are the rate constants of nitrene rearrangement to azirine intermediates and intersystem crossing to the lower energy triplet states, respectively. Values of kobs decrease as the temperature is reduced and reach a limiting value attributed to kISC. Assuming that kISC is temperature independent allows values of kR to be deduced along with the Arrhenius parameters to cyclization for para CH3 ,C F 3, COCH3, F, Cl, and Br singlet phenyl nitrenes. The Arrhenius parameters to rearrangement closely resemble those of parent singlet phenyl nitrene with the exception of the para bromo compound. For para I and OCH3, kISC is large and prevents measurement of kR. The increased rate of intersystem crossing in these two singlet nitrenes is due to heavyatom and to electron-donation effects, respectively. No transient absorptions were detected upon LFP of p-nitrophenyl azide.

The transient absorption spectrum of the tert-butoxyl radical in the UV region was obtained by the laser flash photolysis technique. The rate constants for β-scission and self-termination reactions of tert-butoxyl radicals were measured in five solvents; the Arrhenius parameters of the rate constant for β-scission kβ were determined. It was shown that both the solvent polarity and ability for hydrogen bonding accelerate the reaction of β-scission. The solvent effect on the rate constant of the β-scission reaction is discussed in terms of a simple Onzager−Betcher model, a point dipole model, and a model of the H-bonded complex of the radical with the solvent molecule.

Laser flash photolysis (LFP) of perfluorophenyl azide and perfluoro-4-biphenyl azide produces the corresponding singlet nitrenes which were detected by their transient absorptions at 330 and 350 nm, respectively. The absolute rate constants of the fundamental processes that consume the singlet nitrenes (intersystem crossing, kisc, rearrangement, kR; reaction with pyridine, kpyr) were determined by monitoring the decay of the singlet nitrene and by the growth of its reaction products (ketenimine, triplet nitrene, or pyridine ylide). In the case of singlet 4-perfluorobiphenylnitrene in CH2Cl2 kisc = (2.2 ± 0.1) × 106 s-1, kR = 1013.2±0.2 exp[−(9400 ± 400)/RT] s-1, and kpyr = 109.06±0.15 exp[−(2400 ± 200)/RT] M-1 s-1. In the case of singlet perfluorophenylnitrene in CH2Cl2 kisc = (1.05 ± 0.05) × 107 s-1, kR = 1013.8±0.3 exp[−(8800 ± 400)/RT] s-1, and kpyr = 109.00±0.13 exp[−(1600 ± 160)/RT] M-1 s-1.

Nanosecond laser flash photolysis has been used to study the primary processes in the photochromic reaction of five O-acylic derivatives of 1-hydroxy-2-methoxyanthraquinone with methyl (I), tolyl (II), phenyl (III), ethoxyl (IV), and diethylamino (V) groups in the migrating acyl. The triplet-triplet absorption spectra of the reactive triplet states of quinones IV and V were detected, and the rate constants of the primary photochemical step were measured. The temperature dependence of the rate constants of acyl migration in the triplet states of quinones IV and V was studied, and the Arrhenius parameters were determined. The rate constants of thermal acyl migration and their Arrhenius parameters were measured for compounds I-V. It was found that migrant nature significantly influences the activation energy of both thermal and photochemical reactions of acyl migration. The activation energy of thermal migration increases from 37.9 ( 0.2 kJ/mol for compound I up to 66.5 ( 0.7 kJ/mol for compound V. The photochemical process is characterized by considerably lower values of the activation energy (15.9 ( 0.8 and 26.0 ( 1.7 kJ/mol for compounds IV and V, respectively). It was confirmed in the present work that photochemical migration of acyl groups is an adiabatic process occurring on the triplet potential energy surface.

The authors are grateful to the Royal Society (RS International Joint Project 2010/R3), Deutsche Forschungsgemeinschaft (project 436 RUS 113/967/0-1 R), the Russian Foundation for Basic Research (project 10-03-00735), the Presidium of the Russian Academy of Sciences (projects 7.17, 8.14 and P-8), and to the Siberian Branch of the Russian Academy of Sciences (project 105) for funding. The first charge-transfer complexes of tetrathiafulvalene (1) with 1,2,5-chalcogenadiazole derivatives, i.e. with [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (2) and 3,4-dicyano-1,2,5-telluradiazole (3), were designed, prepared in the form of air and thermally stable single crystals and structurally defined by X-ray diffraction as 1-2 and 1.3(2), respectively. Starting compound 2 (effective electron acceptor with potentially broad application in the field) was synthesized by a new efficient one-pot method from 3,4-diamino-1,2,5-oxadiazole and disulfur dichloride. The electronic structure of complexes 1.2 and 1.3(2) and thermodynamics of their formation were studied by means of DFT and QTAIM calculations and UV-Vis spectroscopy. The electrical properties of single crystals of the complexes were investigated revealing semiconductor properties with an activation energy of 0.34 eV for 1.2 and 0.40 eV for 1.3(2). Polycrystalline films of the complexes displayed photoconductive effects with increased conductivity under white-light illumination. Postprint

According to the DFT calculations, [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (4), [1,2,5]selenadiazolo[3,4-c][1,2,5]thiadiazole (5), 3,4-dicyano-1,2,5-thiadiazole (6), and 3,4-dicyano-1,2,5-selenadiazole (7) have nearly the same positive electron affinity (EA). Under the CV conditions they readily produce long-lived π-delocalized radical anions (π-RAs) characterized by EPR. Whereas 4 and 5 were chemically reduced into the π-RAs with thiophenolate (PhS(-)), 6 did not react and 7 formed a product of hypercoordination at the Se center (9) isolated in the form of the thermally stable salt [K(18-crown-6)][9] (10). The latter type of reactivity has never been observed previously for any 1,2,5-chalcogenadiazole derivatives. The X-ray structure of salt 10 revealed that the Se-S distance in the anion 9 (2.722 Å) is ca. 0.5 Å longer than the sum of the covalent radii of these atoms but ca. 1 Å shorter than the sum of their van der Waals radii. According to the QTAIM and NBO analysis, the Se-S bond in 9 can be considered a donor-acceptor bond whose formation leads to transfer of ca. 40% of negative charge from PhS(-) onto the heterocycle. For various PhS(-)/1,2,5-chalcogenadiazole reaction systems, thermodynamics and kinetics were theoretically studied to rationalize the interchalcogen hypercoordination vs reduction to π-RA dichotomy. It is predicted that interaction between PhS(-) and 3,4-dicyano-1,2,5-telluradiazole (12), whose EA slightly exceeds that of 6 and 7, will lead to hypercoordinate anion (17) with the interchalcogen Te-S bond being stronger than the Se-S bond observed in anion 9.

Photolysis (254 nm) of CF3N3 in pentane at 6−10 K produced a persistent electron paramagnetic resonance spectrum typical of a triplet nitrene centered at 8620 G. The spectrum (|D/hc| = 1.736 cm-1) is attributed to triplet CF3−N, and is very similar to that of triplet NH (|D/hc| = 1.863 cm-1) and CH3-N (|D/hc| = 1.720 cm-1) which were reported previously. CF3N3 was also deposited in an argon matrix at 14 K. The azide (λmax = 257 nm) was decomposed by exposure to 254 nm light with the concurrent formation of sharp, structured absorption bands at 342, 347.5, and 354 nm. The UV bands are assigned to the triplet nitrene CF3N on the basis of its similarity to the spectrum of CH3N and time-dependent density functional theory (B3LYP) calculations.

The X- and W-band electron paramagnetic resonance (EPR) spectroscopies were employed to investigate a series of imidazolidine nitroxide radicals with different number of ethyl and methyl substituents at positions 2 and 5 of a heterocycle in liquid and frozen solutions. The influence of the substituents on the line shape and width was studied experimentally and analyzed using quantum chemical calculations. Each pair of the geminal ethyl groups in the positions 2 or 5 of the imidazolidine ring was found to produce an additional hyperfine splitting (hfs) of about 0.2 mT in the EPR spectra of the nitroxides. The effect was attributed to the hfs constant of only one of four methylene hydrogen atoms of two geminal ethyl substituents not fully averaged by ethyl group rotation and ring puckering. In accordance with this assumption, the substitution of hydrogen atoms of CH(2) groups in 2,2,5,5-tetraethyl-substituted imidazolidine nitroxides by deuterium leads to the substantial narrowing of EPR lines which could be useful for many biochemical and biomedical applications, including pH-monitoring. W-band EPR spectra of 2,2,5,5-tetraethyl-substituted imidazolidine nitroxide and its 2,2,5,5-tetraethyl-d(8) deuterium-substituted analog measured at low temperatures demonstrated high sensitivity of their g-factors to pH, which indicates their applicability as spin labels possessing high stability.

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.

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.

Photolysis of 1,3,2,4-benzodithiadiazine (1) at ambient temperature yields stable 1,2,3-benzodithiazolyl radicals. In order to reveal the mechanism of this unusual transformation, the photochemistry of 1 was studied in argon matrices using IR and UV-vis spectroscopy. A series of intermediates, including four- and five-membered heterocyclic and o-quinoid acyclic species, were characterized spectroscopically with the help of quantum chemical calculations. With selective irradiation, these intermediates can be mutually interconverted as well as converted back to the starting compound 1.

The thermally reversible phototransfer of the acyl group of 9-acetoxy-1,4-anthraquinone and a series of 1-acyloxy-9,10-anthraquinone derivatives was discovered and investigated using low temperature and flash photolysis techniques. The relationship between the reactivity of these compounds and their chemical structure was studied. The correlation diagrams of the energy levels of the initial compounds and the photolysis products were analysed, and an adiabatic excitation-conserving mechanism is proposed for the photochemical migration of the acyl group. Formation of triplet-excited 9-acetoxy-2-methoxy-1,10-anthraquinone was detected experimentally during the course of photochemical isomerization of 1-acetoxy-2-methoxy-9,10-anthraquinone. The influence of temperature, medium and nature of the substituents in the anthraquinone ring and migrant on the rate constant of the thermal migration of the acyl group was studied. Donor substituents in the migrant led to a substantial decrease (up to five orders of magnitude) in the rate constant of migration and acceptor substituents to a substantial increase. The irreversible photochemical reactions of the 1-acetoxy-9,10-anthraquinone 2-dialkylamino derivatives were investigated. A two-stage mechanism involving the participation of excited molecules caused by the action of one photon is proposed.

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.

Two novel applications of functionalized 2,1,3-benzothiadiazoles for metal coordination chemistry and crystal engineering of organic solids are presented. 4-Amino-2,1,3-benzothiadiazole (1) forms a 2 : 1 complex with ZnCl2 (complex 2) and a 1 : 1 complex with 4-nitro-2,1,3-benzothiadiazole (3) (complex 4). The structures of compounds 1–4 were confirmed by single-crystal X-ray diffraction and studied by UV-Vis and IR spectroscopy, and DFT and QTAIM calculations. Complex 2 is the first structurally defined Zn complex with 2,1,3-benzothiadiazole ligands. In this complex, both molecules 1 are coordinated to Zn only by amino groups, thus revealing a novel type of coordination of this ligand to the metal center. According to 1H NMR data, complex 2 dissociates in CHCl3, THF or DMSO solutions. There are only a few examples of similar complexes, which are also considered to dissociate in solutions. In crystalline complex 4, molecules 1 and 3 form infinite alternating π-stacks connected by lateral S⋯N interactions between the neighboring stacks. Intermolecular S⋯N interactions are also observed in the crystals of individual 1 and 3 but the packing motifs are different from those in 4. DFT calculations predict a small charge transfer (CT, ∼0.02e at B97-D3 level) from 1 to 3 upon the formation of 4, which therefore is an unprecedented CT complex where both donor and acceptor moieties are the derivatives of the 2,1,3-benzothiadiazole ring system. This finding creates some new prospects for the crystal engineering of organic solids. Crystalline complex 4 is characterized by an intense CT absorption band with a maximum at ∼550 nm. However, according to DFT and QTAIM calculations the complex is weakly bonded and its formation is not observed in CH2Cl2 solution with 1H NMR and UV-Vis techniques.