The primary physical and chemical processes which follow photoexcitation of the covalently linked... more The primary physical and chemical processes which follow photoexcitation of the covalently linked systems, 1-((4-azido-2,3,5,6-tetrafluorobenzoyloxy)methyl)pyrene (2) and 1-(3-(4-azido-2,3,5,6-tetrafluorobenzoyloxy)- propyl)pyrene (3), have been studied using femto- and nanosecond transient absorption spectroscopy and computational chemistry. Excitation of 2 and 3 at 336 nm results in the population of the second excited singlet state of the pyrene moiety (S2). Internal conversion to the
The journal of physical chemistry. A, Jan 11, 2014
The primary thermolysis reactions of a promising insensitive explosive 1,1-diamino-2,2-dinitroeth... more 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).
Photolysis of 12π-electron, formally antiaromatic, 1,3,2,4-benzodithiadiazine (1a) and its tetraf... more 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 Ê and in an argon matrix at 14 Ê 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 RSN: and the corresponding thiazyl RS≡N, was ascribed to intermediate 3a,b in agreement with the data of IR spectroscopy.
The mutual interconversion and decomposition reactions of four tetrazole isomers (1H-TZ, 2H-TZ, 5... more 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.
The reactions of O( 1 D) atoms with VF 5 at room temperature have been studied by time-resolved l... more 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.
The flame structure of the premixed H 2 /O 2 /Ar mixture (13%/14.5%/72.5% vol.) with and without ... more The flame structure of the premixed H 2 /O 2 /Ar mixture (13%/14.5%/72.5% vol.) with and without 0.12 vol.% of titanium tetraisopropoxide (Ti(OC 3 H 7 ) 4 ) has been studied experimentally using the flamesampling molecular beam mass-spectrometry and the microthermocouple techniques. The flame was stabilized on a flat burner at 1 atm. The temperature and concentration profiles for H 2 , O 2 , H 2 O, and Ti(OC 3 H 7 ) 4 have been measured. The mass peak intensity profiles of the titanium-containing species TiO 2 , HTiO 2 , TiO, HTiO, Ti, TiH, Ti 2 O 3 , TiO 3 have been measured as well. The experimental results were analyzed using kinetic modeling and quantum chemical calculations. It was established that hydrolysis of Ti(OC 3 H 7 ) 4 is the dominating primary reaction of its decomposition in the flame. The rate constant of hydrolysis was estimated to be k = 2 Â 10 12 exp(À6160/T) mol À1 cm 3 s À1 . The results of quantum chemical computations support this conclusion. On the basis of concentration profiles of the Ti-containing intermediates, the schematic mechanism of the Ti(OC 3 H 7 ) 4 conversion in the flame has been proposed.
The first steps of electrochemical oxidation and reduction of the title compounds were studied by... more The first steps of electrochemical oxidation and reduction of the title compounds were studied by cyclic voltammetry, and electrochemically generated long-lived radical cations were characterised by ESR measurements and DFT calculations at the UB3LYP level of theory.
This review highlights the most important advances in the heteroatom chemistry of 1,3,2,4-benzodi... more This review highlights the most important advances in the heteroatom chemistry of 1,3,2,4-benzodithiadiazines (π-excessive and formally antiaromatic heterocycles), covering methods for synthesis, nontrivial features of the molecular and π-electronic structure, spectral properties, and reactivity, in particular the transformations into persistent π-radicals.
Chemical reduction of 2,thiadiazolo [3,4-c][1,2,5]thiadiazole (2) with 1,1,2,2-tetrakis(dimethyla... more Chemical reduction of 2,thiadiazolo [3,4-c][1,2,5]thiadiazole (2) with 1,1,2,2-tetrakis(dimethylamino)ethene (3) leads to the [3] 2+ [1 2 ] 2salt (4) whose X-ray crystal structure reveals diamagnetic π-dimers of the [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazolidyl radical anion (1). The interplanar separation of 324.5 pm in the [1 2 ] 2dimers is significantly shorter than the sum of the isotropic van der Waals radii of two S atoms of
1,2,5]Selenadiazolo[3,4-c][1,2,5]thiadiazole and [1,2,5]Selenadiazolo-[3,4-c][1,2,5]thiadiazolidy... more 1,2,5]Selenadiazolo[3,4-c][1,2,5]thiadiazole and [1,2,5]Selenadiazolo-[3,4-c][1,2,5]thiadiazolidyl -A Synthetic, Structural, and Theoretical Study Keywords: Density functional calculations / Magnetic properties / Nitrogen heterocycles / Radical anions / Sulfur / Selenium A new heterocyclic system, namely [1,2,5]selenadiazolo[3,4c][1,2,5]thiadiazole , has been prepared in 60 % yield by the 1:1 condensation of 3,4-diamino-1,2,5-thiadiazole with SeCl 4 in the presence of pyridine. The structures of 1 and its 1:1 complex with pyridinium chloride (5) have been elucidated by X-ray diffraction (XRD). The reversible electrochemical reduction of 1, as well as its chemical reduction with PhSanion, provide the long-lived [1,2,5]selenadiazolo [3,4-c][1,2,5]thiadiazolidyl radical anion (2), which has been isolated in the form of the thermally stable salt [K(18crown-6)][2] (3) in 75 % yield. The experimental structural and spectral characteristics of 1 and 2 agree fairly well with
Cobaltocene, [Co II (η 5 -C 5 H 5 ) 2 ], readily reduced [1,2,5]thiadiazolo [3,4-c][1,2,5]thiadia... more Cobaltocene, [Co II (η 5 -C 5 H 5 ) 2 ], readily reduced [1,2,5]thiadiazolo [3,4-c][1,2,5]thiadiazole (2) to the [1,2,5]thiadiazolo [3,4c][1,2,5]thiadiazolidyl radical anion (1) which was isolated in the form of the thermally stable salt [Co III (η 5 -C 5 H 5 ) 2 ][1] (5) in 93 % yield. The XRD structure of 5 displays layers of radical anions 1, with S···N contacts of 358.2 and 366.0 pm between neighboring species. The salt was further characterized by ESR spectroscopy in the solid state as well as in solution. The temperature dependence of its magnetic susceptibility (χ) in the range of 2-300 K with a maximum at (9.7 Ϯ 0.5) K indicates antiferromagnetic ordering of the spin system. The magnetic structure of 5 was analyzed in terms of dimeric exchange integrals calculated by a spin-unrestricted broken-[a] Institute
Breathing crystals" based on copper(II) hexafluoroacetylacetonates and pyrazolyl-substituted nitr... more Breathing crystals" based on copper(II) hexafluoroacetylacetonates and pyrazolyl-substituted nitronyl nitroxides comprise the exchange-coupled clusters within the polymeric chains. Owing to an interplay of exchange interaction between copper(II) and nitroxide spins and Jahn−Teller nature of copper(II) complex, the breathing crystals demonstrate thermally and light-induced magnetostructural transitions in many aspects similar to the classical spin crossover. Herewith, we report the first application of variable temperature (VT) far/mid Fourier transform infrared (FTIR) spectroscopy and mid FTIR microscopy to breathing crystals. This VT-FTIR study was aimed toward clarification of the transitions mechanism previously debated on the basis of superconducting quantum interference device, X-ray diffraction, and electron paramagnetic resonance data. VT-FTIR showed the onset of new vibrational bands during phase transitions occurring at the expense of several existing ones, whose intensity was significantly reduced. The most pronounced spectral changes were assigned to corresponding vibrational modes using quantum chemical calculations. A clear-cut correlation was found between temperature-dependent effective magnetic moment of studied compounds and the observed VT-FTIR spectra. Importantly, VT-FTIR confirmed coexistence of two types of copper(II)−nitroxide clusters during gradual magnetostructural transition. Such clusters correspond to weakly coupled and strongly coupled spin states, whose relative contribution depends on temperature. The pronounced difference in the VT-FTIR spectra of two states in breathing crystals is a fingerprint of magnetostructural transition, and understanding of these characteristics achieved by us will be useful for future studies of breathing crystals as well as their diamagnetic analogues.
A procedure was developed for the synthesis of the kinetically stable nitroxide 2 (N tert butyl N... more 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 con taining 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.
Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences... more Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences but the synthesis of stable high-spin organic molecules is still a challenge. We succeeded in synthesising heteroatom analogues of the 1,1,2,3,3-pentamethylenepropane (PMP) diradicals with two nitronyl nitroxide (DR 1 ) and with two iminonitroxide (DR 2 ) fragments linked through the C(sp 2 ) atom of the nitrone group. According to magnetic susceptibility measurements, EPR data and ab initio calculations at the (8,6)CASSCF and (8,6)NEVPT2 levels, DR 1 and DR 2 have singlet ground states. The singlet-triplet energy splitting (2J) is low (J/k = À7.4 for DR 1 and À6.0 K for DR 2 ), which comes from the disjoint nature of these diradicals. The reaction of [Cu(hfac) 2 ] with DR 1 gives rise to different heterospin complexes in which the diradical acts as a rigid ligand, retaining its initial conformation. For the [{Cu(hfac) 2 } 2 (DR 1 )(H 2 O)] complex, sufficiently strong ferromagnetic interactions (J 1 /k = 42.7 and J 2 /k = 14.1 K) between two coordinating Cu II ions and DR 1 were revealed. In [{Cu(hfac) 2 } 2 (DR 1 )(H 2 O)][Cu(hfac) 2 -(H 2 O)], the very strong and antiferromagnetic (J/k = À416.1 K) exchange interaction between one of the coordinating Cu II ions and DR 1 is caused by the very short equatorial CuÀO bond length (1.962 ).
The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resol... more The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UVÀvis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S 1 ) was observed by both time-resolved techniques. Three processes are involved in the decay of the S 1 excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S 1 state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S 1 state. Nitrene formation correlates with the decay of the relaxed S 1 state only upon 350 nm excitation (S 0 f S 1 excitation). When S n (n g 2) states are populated upon excitation (λ ex = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S 1 and higher singlet excited states (S n ) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S 1 state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies. 9752 dx.
The primary physical and chemical processes which follow photoexcitation of the covalently linked... more The primary physical and chemical processes which follow photoexcitation of the covalently linked systems, 1-((4-azido-2,3,5,6-tetrafluorobenzoyloxy)methyl)pyrene (2) and 1-(3-(4-azido-2,3,5,6-tetrafluorobenzoyloxy)- propyl)pyrene (3), have been studied using femto- and nanosecond transient absorption spectroscopy and computational chemistry. Excitation of 2 and 3 at 336 nm results in the population of the second excited singlet state of the pyrene moiety (S2). Internal conversion to the
The journal of physical chemistry. A, Jan 11, 2014
The primary thermolysis reactions of a promising insensitive explosive 1,1-diamino-2,2-dinitroeth... more 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).
Photolysis of 12π-electron, formally antiaromatic, 1,3,2,4-benzodithiadiazine (1a) and its tetraf... more 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 Ê and in an argon matrix at 14 Ê 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 RSN: and the corresponding thiazyl RS≡N, was ascribed to intermediate 3a,b in agreement with the data of IR spectroscopy.
The mutual interconversion and decomposition reactions of four tetrazole isomers (1H-TZ, 2H-TZ, 5... more 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.
The reactions of O( 1 D) atoms with VF 5 at room temperature have been studied by time-resolved l... more 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.
The flame structure of the premixed H 2 /O 2 /Ar mixture (13%/14.5%/72.5% vol.) with and without ... more The flame structure of the premixed H 2 /O 2 /Ar mixture (13%/14.5%/72.5% vol.) with and without 0.12 vol.% of titanium tetraisopropoxide (Ti(OC 3 H 7 ) 4 ) has been studied experimentally using the flamesampling molecular beam mass-spectrometry and the microthermocouple techniques. The flame was stabilized on a flat burner at 1 atm. The temperature and concentration profiles for H 2 , O 2 , H 2 O, and Ti(OC 3 H 7 ) 4 have been measured. The mass peak intensity profiles of the titanium-containing species TiO 2 , HTiO 2 , TiO, HTiO, Ti, TiH, Ti 2 O 3 , TiO 3 have been measured as well. The experimental results were analyzed using kinetic modeling and quantum chemical calculations. It was established that hydrolysis of Ti(OC 3 H 7 ) 4 is the dominating primary reaction of its decomposition in the flame. The rate constant of hydrolysis was estimated to be k = 2 Â 10 12 exp(À6160/T) mol À1 cm 3 s À1 . The results of quantum chemical computations support this conclusion. On the basis of concentration profiles of the Ti-containing intermediates, the schematic mechanism of the Ti(OC 3 H 7 ) 4 conversion in the flame has been proposed.
The first steps of electrochemical oxidation and reduction of the title compounds were studied by... more The first steps of electrochemical oxidation and reduction of the title compounds were studied by cyclic voltammetry, and electrochemically generated long-lived radical cations were characterised by ESR measurements and DFT calculations at the UB3LYP level of theory.
This review highlights the most important advances in the heteroatom chemistry of 1,3,2,4-benzodi... more This review highlights the most important advances in the heteroatom chemistry of 1,3,2,4-benzodithiadiazines (π-excessive and formally antiaromatic heterocycles), covering methods for synthesis, nontrivial features of the molecular and π-electronic structure, spectral properties, and reactivity, in particular the transformations into persistent π-radicals.
Chemical reduction of 2,thiadiazolo [3,4-c][1,2,5]thiadiazole (2) with 1,1,2,2-tetrakis(dimethyla... more Chemical reduction of 2,thiadiazolo [3,4-c][1,2,5]thiadiazole (2) with 1,1,2,2-tetrakis(dimethylamino)ethene (3) leads to the [3] 2+ [1 2 ] 2salt (4) whose X-ray crystal structure reveals diamagnetic π-dimers of the [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazolidyl radical anion (1). The interplanar separation of 324.5 pm in the [1 2 ] 2dimers is significantly shorter than the sum of the isotropic van der Waals radii of two S atoms of
1,2,5]Selenadiazolo[3,4-c][1,2,5]thiadiazole and [1,2,5]Selenadiazolo-[3,4-c][1,2,5]thiadiazolidy... more 1,2,5]Selenadiazolo[3,4-c][1,2,5]thiadiazole and [1,2,5]Selenadiazolo-[3,4-c][1,2,5]thiadiazolidyl -A Synthetic, Structural, and Theoretical Study Keywords: Density functional calculations / Magnetic properties / Nitrogen heterocycles / Radical anions / Sulfur / Selenium A new heterocyclic system, namely [1,2,5]selenadiazolo[3,4c][1,2,5]thiadiazole , has been prepared in 60 % yield by the 1:1 condensation of 3,4-diamino-1,2,5-thiadiazole with SeCl 4 in the presence of pyridine. The structures of 1 and its 1:1 complex with pyridinium chloride (5) have been elucidated by X-ray diffraction (XRD). The reversible electrochemical reduction of 1, as well as its chemical reduction with PhSanion, provide the long-lived [1,2,5]selenadiazolo [3,4-c][1,2,5]thiadiazolidyl radical anion (2), which has been isolated in the form of the thermally stable salt [K(18crown-6)][2] (3) in 75 % yield. The experimental structural and spectral characteristics of 1 and 2 agree fairly well with
Cobaltocene, [Co II (η 5 -C 5 H 5 ) 2 ], readily reduced [1,2,5]thiadiazolo [3,4-c][1,2,5]thiadia... more Cobaltocene, [Co II (η 5 -C 5 H 5 ) 2 ], readily reduced [1,2,5]thiadiazolo [3,4-c][1,2,5]thiadiazole (2) to the [1,2,5]thiadiazolo [3,4c][1,2,5]thiadiazolidyl radical anion (1) which was isolated in the form of the thermally stable salt [Co III (η 5 -C 5 H 5 ) 2 ][1] (5) in 93 % yield. The XRD structure of 5 displays layers of radical anions 1, with S···N contacts of 358.2 and 366.0 pm between neighboring species. The salt was further characterized by ESR spectroscopy in the solid state as well as in solution. The temperature dependence of its magnetic susceptibility (χ) in the range of 2-300 K with a maximum at (9.7 Ϯ 0.5) K indicates antiferromagnetic ordering of the spin system. The magnetic structure of 5 was analyzed in terms of dimeric exchange integrals calculated by a spin-unrestricted broken-[a] Institute
Breathing crystals" based on copper(II) hexafluoroacetylacetonates and pyrazolyl-substituted nitr... more Breathing crystals" based on copper(II) hexafluoroacetylacetonates and pyrazolyl-substituted nitronyl nitroxides comprise the exchange-coupled clusters within the polymeric chains. Owing to an interplay of exchange interaction between copper(II) and nitroxide spins and Jahn−Teller nature of copper(II) complex, the breathing crystals demonstrate thermally and light-induced magnetostructural transitions in many aspects similar to the classical spin crossover. Herewith, we report the first application of variable temperature (VT) far/mid Fourier transform infrared (FTIR) spectroscopy and mid FTIR microscopy to breathing crystals. This VT-FTIR study was aimed toward clarification of the transitions mechanism previously debated on the basis of superconducting quantum interference device, X-ray diffraction, and electron paramagnetic resonance data. VT-FTIR showed the onset of new vibrational bands during phase transitions occurring at the expense of several existing ones, whose intensity was significantly reduced. The most pronounced spectral changes were assigned to corresponding vibrational modes using quantum chemical calculations. A clear-cut correlation was found between temperature-dependent effective magnetic moment of studied compounds and the observed VT-FTIR spectra. Importantly, VT-FTIR confirmed coexistence of two types of copper(II)−nitroxide clusters during gradual magnetostructural transition. Such clusters correspond to weakly coupled and strongly coupled spin states, whose relative contribution depends on temperature. The pronounced difference in the VT-FTIR spectra of two states in breathing crystals is a fingerprint of magnetostructural transition, and understanding of these characteristics achieved by us will be useful for future studies of breathing crystals as well as their diamagnetic analogues.
A procedure was developed for the synthesis of the kinetically stable nitroxide 2 (N tert butyl N... more 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 con taining 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.
Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences... more Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences but the synthesis of stable high-spin organic molecules is still a challenge. We succeeded in synthesising heteroatom analogues of the 1,1,2,3,3-pentamethylenepropane (PMP) diradicals with two nitronyl nitroxide (DR 1 ) and with two iminonitroxide (DR 2 ) fragments linked through the C(sp 2 ) atom of the nitrone group. According to magnetic susceptibility measurements, EPR data and ab initio calculations at the (8,6)CASSCF and (8,6)NEVPT2 levels, DR 1 and DR 2 have singlet ground states. The singlet-triplet energy splitting (2J) is low (J/k = À7.4 for DR 1 and À6.0 K for DR 2 ), which comes from the disjoint nature of these diradicals. The reaction of [Cu(hfac) 2 ] with DR 1 gives rise to different heterospin complexes in which the diradical acts as a rigid ligand, retaining its initial conformation. For the [{Cu(hfac) 2 } 2 (DR 1 )(H 2 O)] complex, sufficiently strong ferromagnetic interactions (J 1 /k = 42.7 and J 2 /k = 14.1 K) between two coordinating Cu II ions and DR 1 were revealed. In [{Cu(hfac) 2 } 2 (DR 1 )(H 2 O)][Cu(hfac) 2 -(H 2 O)], the very strong and antiferromagnetic (J/k = À416.1 K) exchange interaction between one of the coordinating Cu II ions and DR 1 is caused by the very short equatorial CuÀO bond length (1.962 ).
The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resol... more The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UVÀvis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S 1 ) was observed by both time-resolved techniques. Three processes are involved in the decay of the S 1 excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S 1 state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S 1 state. Nitrene formation correlates with the decay of the relaxed S 1 state only upon 350 nm excitation (S 0 f S 1 excitation). When S n (n g 2) states are populated upon excitation (λ ex = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S 1 and higher singlet excited states (S n ) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S 1 state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies. 9752 dx.
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Papers by Nina Gritsan