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Owing to the key role of the lead dimer (Pb 2 ) as a heavy element benchmark for the Group IV-A dimers the assignment of its spectroscopic properties and chemical bonding is an important undertaking. To meet this demand, the present work provides comprehensive and detailed information on electronic structure and properties comprising a wide set of Pb 2 states. Calculations are performed by a high-level ab initio approach. Firstly, the potential energy curves (PECs) of 19 Λ-S states as well as those of 24 ungerade Ω states are calculated by utilizing the multi-reference configuration interaction plus Davidson correction (MRCI + Q) method taking into account core-valence correlation (CV) and spin-orbit coupling (SOC) effect, where Ω is a quantum number of the total (Λ + S) angular momentum projection. Secondly, interactions between the bound F 3 Σ-u, 2 3 Σ+u states and repulsive 1 5 Π u state induced by strong SOC are discussed based on the PECs analysis and calculated SOC matrix, which also indicates that the F 3 Σ-u state is predissociative. Thirdly, based on the calculated electric dipole transition moments and energy gaps between the 0+u(III), F0+u(II), C0+u(I) and X0+g states, the intense absorption bands of Pb 2 due to these transitions are interpreted. Our results indicate that the trends in intensity of absorption spectra (F0+u(II), C0+u(I) ← X0+g) in the range of 12 600-13 600 and 22 200-23 800 cm -1 are consistent with the previously observed spectra of Pb 2 in the qualitatively similar regions (15 200-16 200 and 19 800-21 800 cm -1 ). Finally, the calculated intensity of the weak magnetic-dipole transitions from the singlet excited b 1 Σ+g and a 1 Δ g states to the triplet ground X 3 Σ-g state and their electric quadrupole components are presented for the Pb 2 molecule in terms of SOC perturbations for the calculated Ω states expressed in Λ-S state notation. Based on our theoretical assignment, we predict that the weak emission a 1 Δ g2 → X 3 Σ-g1 bands could be observed experimentally. The present work provides comprehensive electronic structure information and sheds new light on the absorption and emission spectra of the Pb 2 dimer.

The ground and excited states of Sn 2 are calculated using the multireference configuration interaction method combined with Davidson correction (MRCI+Q). The influence of the spin-orbit coupling (SOC) effect on the electronic structure is also considered by the state interaction method of Breit-Pauli Hamiltonian. In the calculations, the potential energy curves and spectroscopic constants of 23 Λ-S states and 31 Ω states of Sn 2 are obtained. The prominent spectral features in the visible region, new constants, and potential energy curves are discussed. The intensity of weak magnetic and quadrupole transitions in the near IR spectra is also calculated. From a computational point of view, we predict that the weak v' (0-2)- v″ (0-5) bands of the magnetic b 1 Σ g,0 + + -X 3 Σ g,1(Ms=±1) - transition may be detected experimentally; the sub-bands (0, 0), (1, 0), and (2, 0) of the a 1 Δ g,2 -X 3 Σ g,1(Ms=±1) - transition also may be observed in experiments since they are not overlapped by the strong electric dipole transition in the same IR region. According to the SOC matrix elements and contributions of the 1 5 Π u0 + , 1 5 Π u1 (|Σ| = 0), and 1 5 Π u1 (|Σ| = 2) states to the predissociation line width of the 1 3 Σ u - -X 3 Σ g1 - transition, the broading and other predissociation features of the 1 3 Σ u - state are analyzed. From our calculations, it follows that the strong coupling between the bound 1 3 Σ u - state and the repulsive 1 5 Π u state causes the predissociation of the 1 3 Σ u - state at the vibrational levels v ' ≥ 8. In addition, our results suggest that the previously observed bands of Sn 2 in the visible range of 19000-20000 cm -1 should be reassigned into the mixing transitions among the X 3 Σ g,1 - -2 3 Σ u,0 - + and X 3 Σ g,0 + - -2 3 Σ u,1 + manifold. The results are expected to provide new comprehensive information for better understanding the spectra and dynamics of the electronic excited states of the Sn 2 molecule.

High level ab initio calculations on the electronic states of a silicon dimer (Si 2 ) have been carried out by employing a multi-reference configuration interaction plus Davidson correction (MRCI + Q) approach with the aug-cc-pVQZ basis set. The scalar relativistic correction is taken into consideration by the second-order Douglas-Kroll-Hess approximation. In the present work, the transition properties (oscillator strength, Einstein spontaneous emission coefficient and radiative lifetime) of the singlet-singlet, triplet-triplet, and quintet-quintet transitions of Si 2 are discussed. We emphasize the triplet-triplet emission bands H 3 Σ-u-X 3 Σ-g, K 3 Σ-u-X 3 Σ-g and D 3 Π u -L 3 Π g which are dominant for 0-11 (18 822 cm -1 ), 0-0 (30 672 cm -1 ), and 0-0 (28 881 cm -1 ) transitions, respectively. In addition, the strong experimentally observed b 1 Π u -d 1 Σ+g band around 4184 cm -1 corresponds to the second 1 Σ+g-b 1 Π u combination in the infrared region. The calculated oscillator strengths of the singlet-singlet transitions (f 1 Π g -e 1 Σ-u, 2 1 Π g -b 1 Π u , b 1 Π u -d 1 Σ+g and g 1 Δ u -a 1 Δ g ) are in the order of 10 -3 . From a theoretical point of view, the 0-0 sub-band for the f 1 Π g -e 1 Σ-u transition, 0-7 for 2 1 Π g -b 1 Π u , 0-0 for b 1 Π u -d 1 Σ+g and the 0-7 vibronic bands for the g 1 Δ u -a 1 Δ g transition may be observed experimentally. It is expected that the present results could provide theoretical support for a deeper understanding of the experimental Si 2 spectra providing further applications in astrophysics.

Triplet excited states of the N 2 molecule play an important role in electric discharges through air or liquid nitrogen accompanied by various afterglows. In the rarefied upper atmosphere, they produce aurora borealis and participate in other energy-transfer processes connected with atmospheric photochemistry and nightglow. In this work, we present spin-orbit coupling calculations of the intensity of various forbidden transitions, including the prediction of the electric dipole transition moment of the new 1 3 Σ g - ←   A 3 Σ u + band, which is strongly prohibited by the (+|-) selection rule, the new spin-induced magnetic B ' 3 Σ u - ←   A 3 Σ u + transition, magnetic and electric quadrupole transitions for the B 3 Π g ← X 1 Σ g + Wilkinson band, and the Lyman-Birge-Hopfield a 1 Π g ← X 1 Σ g transition. Also, two other far-UV singlet-singlet quadrupole transitions are calculated for the first time, namely, the Dressler-Lutz a" 1 Σ g + -X 1 Σ g + and the less studied z 1 Δ g -X 1 Σ g + weak transitions., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Minaev, Panchenko, Minaeva and Ågren.)

It is a challenge to acquire, realize, and comprehend highly emissive phosphorescent molecules. Herein, we report that, using persulfurated benzene compounds as models, phosphorescence can be strongly enhanced through the modification of molecular conformation and crystal growth conditions. By varying the peripheral groups in these compounds, we were able to control their molecular conformation and crystal growth mode, leading to one- (1D), two- (2D), and three-dimensional (3D) crystal morphologies. Two kinds of typical molecular conformations were separately obtained in these crystals through substituent group control or the solvent effect. Importantly, a symmetrical 3,3-conformer exhibits that a planar central benzene ring prefers a 3D-type crystal growth mode, demonstrating high phosphorescence efficiency. Such outcome is attributed to the strong crystal protection effect of the 3D crystal and the bright global minimum (GM) boat-like T 1 state of the symmetrical 3,3-conformer. The conformation studies further reveal small deformation of the inner benzene ring in both singlet and triplet states. The GM boat-like T 1 state is indicated by theoretical calculations, which is far away from the conical intersection (CI) point between the S 0 and T 1 potential energy surfaces. Meanwhile, the small energy gap between S 1 and T 1 states and the considerable spin-orbit coupling matrix elements allow an efficient population of the T 1 state. Combined with the crystal protection and conformation effect, the 3,3-conformer crystal shows high phosphorescence efficiency. The unsymmetrical 2,4-conformer conformation with the twisted central benzene ring leads to 1D or 2D crystal growth mode, which has a weak crystal protection effect. In addition, the unsymmetrical conformation has a dark GM T 1 state that is very close to the T 1 -S 0 CI point, implying an efficient nonradiative T 1 -S 0 quenching. Thus, weak phosphorescence was observed from the unsymmetrical conformation. This study provides an insight for the development of highly emissive phosphorescent materials.

Dioxygen in the quintet O 2 ( 5 Π g ) state is a weakly bound species near the entrance of the O( 3 P) + O( 3 P) recombination channel. It was predicted by ab initio calculations in 1977 and detected experimentally in 1999. Meantime, the O 2 ( 5 Π g ) species was tentatively assumed as intermediate in transport properties calculations for the rarefied gases of the Earth's upper atmosphere, though its potential energy curve is still debated. Besides six other strongly bound low-lying states of dioxygen, the O 2 ( 5 Π g ) state is an important potential candidate for modeling energy transfer and airglow of the upper atmosphere. A number of photochemical kinetic schemes designed to simulate energy flow upon atomic and molecular oxygen collisions in the rarefied mesosphere take into account a participation of the O 2 ( 5 Π g ) state in energy relaxation processes responsible for terrestrial nightglow. All mechanisms of energy redistribution are based on the hard-sphere collision models. The possibility of chemical interactions between the quintet excited state of dioxygen and other atmospheric components has not been considered so far in photochemistry of the upper atmosphere. In the present paper, the chemical reactivity of the quintet O 2 ( 5 Π g ) species is calculated for the first time in the framework of the density functional theory. Definitely, O 2 ( 5 Π g ) is the most reactive species among all other metastable dioxygen states below 5.1 eV. Quintet products of the O 2 ( 5 Π g ) state association with heavy inert gases, H 2 O, N 2 , and CO 2 are predicted to be chemically significant, while the complexes with abundant H 2 and He species are rather weak and not important even in the mesopause low-temperature region. The complex with N 2 molecule is unexpectedly stable with dissociation energy 4 kJ/mol, which can strongly influence the abundant termolecular association O + O + N 2 → O 2 + N 2 process. Reaction with meteoritic ablated Mg atom produces metastable 5 A 1 excited state of MgO 2 being more strongly bound than the ground 3 A 2 state of magnesium peroxide.

A new method for calculating internal conversion rate constants (k[combining low line]IC), including anharmonic effects and using the Lagrangian multiplier technique, is proposed. The deuteration effect on k[combining low line]IC is investigated for naphthalene, anthracene, free-base porphyrin (H2P) and tetraphenylporphyrin (H2TPP). The results show that anharmonic effects are important when calculating k[combining low line]IC for transitions between electronic states that are energetically separated (ΔE) by more than 20 000-25 000 cm-1. Anharmonic effects are also important when ΔE < 20 000-25 000 cm-1 and when the accepting modes are X-H stretching vibrations with a frequency larger than 2000 cm-1. The calculations show that there is mixing between the S1 and S2 states of naphthalene induced by non-adiabatic interactions. The non-adiabatic interaction matrix element between the S1 and S2 states is 250 cm-1 and 50 cm-1 for the normal and fully deuterated naphthalene structure and this difference significantly affects the estimated fluorescence quantum yield. Besides aromatic hydrocarbons H2P and H2TPP, the k[combining low line]IC rate constant is also calculated for pyrometene (PM567) and tetraoxa[8]circulene (4B) with a detailed analysis of the effect of the vibrational anharmonicity.

Fully aromatic [ n ]circulenes have only been known to encompass up to eight aromatic rings ( n = 8), with no reports of endeavors in the synthesis of higher-order analogues ( n > 8). Herein we present the first [9]circulene, formally a diazatrioxa[9]circulene, along with a tetrahydro-diazatetraoxa[10]circulene. The key transformation, for construction of the macrocyclic framework, is a simple high-yielding dimerizing condensation between 3,6-dihydroxycarbazole and glyoxal. Single crystal X-ray analysis reveals the [9]circulene to be perfectly planar and containing elongated benzene rings, which is induced by strain to accommodate planarity. Alternating bond lengths in the solid state indicate contribution from a [9]radialene resonance structure in the [9]circulene π-system. The central nonaromatic rings of both circulenes have paratropic ring currents, as evident by nucleus independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations, which can be attributed to induced paratropicity from the surrounding aromatic rings.

Ultralong organic phosphorescence strongly depends on the formation of aggregation, while it is difficult to obtain in dilute environments on account of excessive internal and external molecular motions. Herein, ultralong single-molecule phosphorescence (USMP) at room temperature was achieved in the monomer state by coassembling biphenyl and naphthalene derivatives at low density with poly(vinyl alcohol) (PVA), where PVA provides a confined environment to stabilize the triplet state. Various factors that affect the USMP were studied, including aggregation, conformation, temperature, and moisture. In these systems, the formation of aggregates through intermolecular stacking and hydrogen bonding interactions in the film or crystal phases completely suppresses the USMP. However, the fluorescence is enhanced when coassembling these compounds at high concentration with PVA and becomes stronger in their powder state, indicating that the intersystem crossing process is blocked by the aggregation. Theoretical calculations suggest that the aggregation depresses spin-orbit coupling between the excited singlet and triplet states and enhances the nonradiative quenching process. Moreover, a relatively twisted conformation is more conducive to the occurrence of intersystem crossing than planar conformation. The USMP shows delicate and reversible sensitivity to the changes of temperature and moisture, rendering them with the applicability as smart organic optoelectronic materials.

The first examples of phosphorescent platinum complexes bearing 2- and 3-(2-pyridyl)benzo[b]selenophenes (PyBSe) were synthesized and fully characterized. Almost identical ionization potential values (5.6 and 5.58 eV) of the solid samples of the Pt complexes were obtained by electron photoemission spectroscopy. Having slightly different molecular design, the solid solutions of the complexes emitted efficient green and red phosphorescence with absolute quantum yields of 52% (for green) and 11.6% (for red). It is demonstrated that the platinum complexes synthesized can be used as phosphorescent dopants for hybrid solution-processable OLEDs.

Motivated by evidence that silver nanoparticles have found numerous technological applications we have explored in this work utilization of polythiocyanatohydroquinone as a new efficient reducing and stabilizing agent for the preparation of such nanoparticles. The formation of silver nanoparticles has been confirmed by the UV-Vis spectroscopy, X-ray powder diffraction and by transmission electron microscopy. The potentiometric and spectroscopy kinetic measurements during the nanoparticles growth are also presented. Thermodynamic activation parameters for the silver nanoparticle formation have been determined from the reaction kinetic studies at variable temperatures. On the ground of observations using these techniques, a mechanism for silver nanoparticle growth has been proposed. The narrow size (20-40 nm) and spherical shape distribution of the fabricated nanoparticles together with the high stability of colloids for sedimentation provide a firm basis for applications of the polythiocyanatohydroquinone polymer as a reducing and stabilizing material for the metal nanoparticles preparation and storage.

The electronic structure, absorption and emission spectra, aromaticity and photophysical behavior of the recently synthesized tetrasilatetrathia[8]circulene and tetragermatetrathia[8]circulene compounds have been studied computationally. Both compounds demonstrate a specific bifacial aromaticity, which is unusual for hetero[8]circulenes; the inner eight-membered core sustains an expected strong paratropic magnetically-induced ring current, while the outer perimeter contains saturated Si(Et)2 and Ge(Et)2 moieties which break the conjugation between the thiophene rings. The overall magnetically-induced ring current for both studied circulenes is close to zero because of the strong local diatropic currents in each thiophene ring that compensate the paratropic counterpart. The electronic absorption and emission spectra of tetrasilatetrathia[8]circulene and tetragermatetrathia[8]circulene demonstrate a clear visible vibronic progression. The 0-0 band is the most active one in the absorption spectra, while in the fluorescence spectra the 0-1 band composed of several normal vibrations is more intense compared with the 0-0 band in excellent agreement with experiment. Accounting for spin-orbit coupling effects, an analysis of the photophysical constants for the two compounds demonstrates: (1) a clear manifestation of the internal heavy atom effect on the inter-system crossing efficiency; (2) one to two order domination of non-radiative rates over the fluorescence rate; and (3) that the S1-S0 internal conversion is extremely slow and can not compete with the fluorescence, while the S1-Tn inter-system crossing is a main deactivation channel of the S1 excited state. These results provide new insight into the electronic structure and photophysics of tetrasilatetrathia[8]circulene and tetragermatetrathia[8]circulene as novel standalone representatives of hetero[8]circulenes - tetraannelated derivatives of tetrathienylene.

The recently synthesized thiazolylazo dye, 1-[5-benzyl-1,3-thiazol-2-yl)diazenyl]naphthalene-2-ol called shortly BnTAN, is studied by density functional theory (DFT) in three tautomeric forms in order to explain the available 1 H NMR, UV-Vis and FTIR spectra. An experimentally observed IR band at 1678 cm -1 , assigned to the CO bond stretching vibration, supports the notion that BnTAN retains in the less stable keto-form even in the solid state due to an ultrafast single-coordinate intramolecular proton transfer. This finding is also in a good agreement with an X-ray crystallography analysis which indicates an intermediate position of the proton between the -OH and -N=N- groups. Calculations also show that some experimentally observed 1 H NMR signals could be considered as being averaged values between theoretically calculated chemical shifts for the corresponding protons in the keto- and enol-tautomers. At the same time the UV-Vis spectra are almost insensitive to the tautomerization processes as the main single band absorption at 500 nm is present in all tautomers according to our TD DFT simulations. The minor differences in spectral features of the long-wavelength visible region are also noted and discussed with respect to the manifestation of the less stable tautomer form., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Singlet oxygen, O 2 (a 1 Δ g ), the lowest excited electronic state of molecular oxygen, is an omnipresent part of life on earth. It is readily formed through a variety of chemical and photochemical processes, and its unique reactions are important not just as a tool in chemical syntheses but also in processes that range from polymer degradation to signaling in biological cells. For these reasons, O 2 (a 1 Δ g ) has been the subject of intense activity in a broad distribution of scientific fields for the past ∼50 years. The characteristic reactions of O 2 (a 1 Δ g ) kinetically compete with processes that deactivate this excited state to the ground state of oxygen, O 2 (X 3 Σ g - ). Moreover, O 2 (a 1 Δ g ) is ideally monitored using one of these deactivation channels: O 2 (a 1 Δ g ) → O 2 (X 3 Σ g - ) phosphorescence at 1270 nm. Thus, there is ample justification to study and control these competing processes, including those mediated by solvents, and the chemistry community has likewise actively tackled this issue. In themselves, the solvent-mediated radiative and nonradiative transitions between the three lowest-lying electronic states of oxygen [O 2 (X 3 Σ g - ), O 2 (a 1 Δ g ), and O 2 (b 1 Σ g + )] are relevant to issues at the core of modern chemistry. In the isolated oxygen molecule, these transitions are forbidden by quantum-mechanical selection rules. However, solvent molecules perturb oxygen in such a way as to make these transitions more probable. Most interestingly, the effect of a series of solvents on the O 2 (X 3 Σ g - )-O 2 (b 1 Σ g + ) transition, for example, can be totally different from the effect of the same series of solvents on the O 2 (X 3 Σ g - )-O 2 (a 1 Δ g ) transition. Moreover, a given solvent that appreciably increases the probability of a radiative transition generally does not provide a correspondingly viable pathway for nonradiative energy loss, and vice versa. The ∼50 years of experimental work leading to these conclusions were not easy; spectroscopically monitoring such weak and low-energy transitions in time-resolved experiments is challenging. Consequently, results obtained from different laboratories often were not consistent. In turn, attempts to interpret molecular events were often simplistic and/or misguided. However, over the recent past, increasingly accurate experiments have converged on a base of credible data, finally forming a consistent picture of this system that is resonant with theoretical models. The concepts involved encompass a large fraction of chemistry's fundamental lexicon, e.g., spin-orbit coupling, state mixing, quantum tunneling, electronic-to-vibrational energy transfer, activation barriers, collision complexes, and charge-transfer interactions. In this Account, we provide an explanatory overview of the ways in which a given solvent will perturb the radiative and nonradiative transitions between the O 2 (X 3 Σ g - ), O 2 (a 1 Δ g ), and O 2 (b 1 Σ g + ) states.

A new ambient-pressure metastable single-bonded 3D nitrogen allotrope (TrigN) of trigonal symmetry (space group R3[combining macron]) was calculated using density functional theory (DFT). A comprehensive characterization of this material, comprising thermodynamic, elastic, and spectral (vibrational, UV-vis absorption, and nuclear magnetic resonance) properties, was performed. Using high-throughput band structure calculation, the TrigN phase was characterized as an insulator with an indirect band gap of 2.977 eV. Phonon dispersion calculations justified that this structure is vibrationally stable at ambient pressure. The calculated Raman activities at the Γ-point demonstrated a rich pattern, whereas no relatively intense transitions were observed in its IR absorption spectrum. The TrigN material is almost transparent to visible light as well as to ultraviolet A and B. The main absorption peaks appeared within the range of 50-200 nm. The electron arrangement of the nitrogen nuclei in the studied nitrogen allotrope is much denser compared to that of the molecular nitrogen, which is in agreement with the calculated magnetic shielding tensor values. Robust mechanical stability is revealed from the elastic constants calculation. Due to strong anisotropy, the values of the Young's moduli vary from 281 to 786 GPa. A huge amount of internal energy is enclosed in the TrigN material. Upon decomposition to molecular nitrogen, the energy release is expected to be 11.01 kJ g -1 compared to the value of 10.22 kJ g -1 for the cubic gauche form of nitrogen. The TrigN allotrope possesses unique detonation characteristics with a detonation pressure of 146.06 GPa and velocity of 15.86 km s -1 .

A series of three star-shaped compounds containing both donor (carbazole) and acceptor (2,4,6-triphenyl-1,3,5-triazine) moieties linked through various linking bridges was studied theoretically at the linear response TD-DFT level of theory to describe their absorption and fluorescence spectra. The concept of a localized charge-transfer excited state has been applied successfully to explain the observed strong solvatochromic effect in the emission spectra of the studied molecules, which can be utilized for the fabrication of color tunable solution-processable OLEDs. The concept is in particularly applicable to donor-acceptor species with a C 3 symmetry point group where the static dipole moment changes dramatically upon electronic excitation. An important peculiarity of the studied molecules is that they are characterized by non-zero values of the HOMO and LUMO orbitals in the same common part of molecular space that provides a large electric dipole transition moment for both light absorption and emission. Graphical abstract Star-shaped C 3 symmetry point group derivatives for color tunable OLEDs.

In the present work, we studied the synergetic effect of benzoannelation and NH/O-substitution for enhancing the absorption intensity in a series of novel designed benzoannelated aza- and oxa[8]circulenes. Semi-empirical estimations of the fluorescence rate constants allowed us to determine the most promising fluorophores among all the possible benzoannelated aza-, oxa- and mixed azaoza[8]circulenes. Among them, para-dibenzoannelated [8]circulenes demonstrated the most intense light absorption and emission due to the prevailing role of the linear acene chromophore. Calculated φ fl values are in complete agreement with experimental data for a number of already synthesized circulenes. Thus, we believe that the most promising circulenes designed in this study can demonstrate an intensive fluorescence in the case of their successful synthesis, which in turn could be extremely useful for the fabrication of future blue OLEDs. Special attention is devoted to the aromaticity features and peculiarities of the absorption spectra for the two highly-symmetrical (D 4h ground state symmetry) π-isoelectronic species as well as the so-called tetrabenzotetraaza[8]circulene and tetrabenzotetraoxa[8]circulene molecules. Both of them are characterized by rich electronic spectra, which can be assigned only by taking into account the vibronic coarse structure of the first electronic absorption band; the 0-1 and 0-2 transitions were found to be active in the absorption spectrum in complete agreement with experimental data obtained for both energy and intensity. The corresponding promotive vibrational modes have been determined and their vibronic activity estimated using the Franck-Condon approximation.

The electric dipole transitions between pure spin and mixed spin electronic states are calculated at the XMC-QDPT2 and MCSCF levels of theory, respectively, for different intermolecular distances of the C6H6 and O2 collisional complex. The magnetic dipole transition moment between the mixed-spin ground ("triplet") and the first excited ("singlet") states is calculated by quadratic response at MCSCF level of theory. The obtained results confirm the theory of intensity borrowing and increasing the intensity of electronic transitions in the C6H6 + O2 collision. The calculation of magnetically induced current density is performed for benzene molecule being in contact with O2 at the distances from 3.5 to 4.5 Å. The calculation shows that the aromaticity of benzene is rising due to the conjugation of π-MOs of both molecules. The C6H6 + O2 complex becomes nonaromatic at the short distances (r < 3.5 Å). The computation of static polarizability in the excited electronic states of the C6H6 + O2 collisional complex at various distances supports the theory of red solvatochromic shift of the a → X band. Graphical abstract The C6H6+ O2 collisional complex.

Magnetically induced current densities and current pathways have been calculated for a series of fully annelated dicationic and dianionic tetraphenylenes, which are also named [8]circulenes. The gauge including magnetically induced current (GIMIC) method has been employed for calculating the current density susceptibilities. The aromatic character and current pathways are deduced from the calculated current density susceptibilities showing that the neutral [8]circulenes have two concentric pathways with aromatic and antiaromatic character, respectively. The inner octatetraene core (the hub) is found to sustain a paratropic (antiaromatic) ring current, whereas the ring current along the outer part of the macrocycle (the rim) is diatropic (aromatic). The neutral [8]circulenes can be considered nonaromatic, because the sum of the ring-current strengths of the hub and the rim almost vanishes. The aromatic character of the doubly charged [8]circulenes is completely different: the dianionic [8]circulenes and the OC-, CH-, CH2-, SiH-, GeH-, SiH2-, and GeH2-containing dicationic species sustain net diatropic ring currents i.e., they are aromatic, whereas the O-, S-, Se-, NH-, PH- and AsH-containing dicationic [8]circulenes are strongly antiaromatic. The present study also shows that GIMIC calculations on the [8]circulenes provide more accurate information about the aromatic character than that obtained using local indices such as nuclear-independent chemical shifts (NICSs) and (1)H NMR chemical shifts.

The FTIR, Raman and UV-vis spectra of the recently synthesized tetrathia[8]circulene and tetraselena[8]circulene compounds have been measured and interpreted in details by the density functional theory (DFT) calculations taking into account the molecular symmetry constrains. The structural and electronic features of the studied compounds have also been discussed in connection with the observed spectroscopic characteristics. Particularly, we have found that despite a slightly non-planar conformation the neutral tetrathia[8]circulene and tetraselena[8]circulene molecules demonstrate bifacial aromatic/antiaromatic nature. The inner octatetraene core is characterized by the presence of paratropic ("antiaromatic") ring currents, whereas the outer macrocycle constructed of benzene, thiophene or selenophene rings possesses the strong magnetically-induced diatropic ("aromatic") ring current. This fact suggests the general electronic and magnetic similarity of the tetrathia- and tetraselena[8]circulenes with the strictly planar isoelectronic tetraoxa[8]circulene and related azaoxa-derivatives discussed earlier. However, the vibrational and UV-vis absorption spectra of the studied circulenes are rather different from those of the parent tetraoxa[8]circulene which indicates a clear manifestation of the symmetry selection rules., (Copyright © 2015 Elsevier B.V. All rights reserved.)

A series of heterocyclic and hydrocarbon [8]circulenes (also named completely annelated tetraphenylenes) were studied by the NICS and GIMIC methods in order to describe their aromatic properties from the magnetic criterion point of view. According to calculations all the hetero[8]circulene molecules demonstrate the bifacial aromatic/antiaromatic nature. The inner octatetraene core of the studied [8]circulenes is characterized by the presence of paratropic ("antiaromatic") ring currents, whereas the outer macrocycle constructed from the five- and six-membered rings possesses the magnetically-induced diatropic ("aromatic") ring current. The hydrocarbon [8]circulenes studied in this work consist of a similar planar cyclooctatetraene core but they exhibit a rather different balance of magnetically-induced ring currents.

Novel sulfur and selenium-bridged [8]circulenes were prepared from octabromotetraphenylene. Structures of these compounds were unambiguously confirmed by X-ray crystallographic analyses. Photophysical and electrochemical investigations of these [8]circulenes suggest their potential applications as electronic materials. The antiaromatic nature of tetrathio[8]circulene and tetraselenium[8]circulene was studied by computational methods, and the NICS computational results reveal that the central eight-membered ring has highly antiaromatic character., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Absorption and fluorescence spectra of the chromene 3 and chromene 13 dyes are studied experimentally and by density functional theory (DFT) including vibronic structure analysis. Vertical electronic absorption spectra are also calculated with the ab initio multiconfiguration method XMC-QDTP2. The vibronic progression for the S0 → S1 electronic transition is calculated within the Franck-Condon approximation including Dushinsky effect and promoting modes are analyzed. The laser-active solid-state media with high efficiency and long operation time are created implementing the studied dyes. The results of investigation indicate that the studied compounds can be used as effective laser dyes in the red range of visible light.

Structural, topological, optical, energetic, and magnetic properties and reactivity parameters of benzidine, its radical cation, and its dication as well as molecular complexes of the benzidine dication with the F(-), Cl(-), Br(-), I(-), NO3(-), HSO4(-), and H2PO4(-) anions were calculated at the B3LYP/6-311++G(2d,2p) level of theory in the CH2Cl2 medium. The CAM-B3LYP functional (as the most reliable one) and the 6-311++G(3df,3pd) basis set were used for the UV-vis absorption spectra prediction. The obtained spectral results are in a good agreement with available experimental data. A number of the calculated global and local molecular properties, including several recently developed ones, (in general, more than 20 parameters), namely, λmax, the bond lengths and orders (l and LA,B), adiabatic ionization energy (IEad), global electrophilicity index (ω), condensed electrophilic Fukui functions (f(+)) and dual descriptor (ΔfA), van der Waals molecular volume, nuclear independent chemical shifts (NICS) and QTAIM topological parameters were estimated in the critical points of the C(1)-C(1'), C(2)-C(3), and C(4)-N bonds as well as at the ring critical point. These quantities were found to be in a strong linear dependence (R(2) > 0.99 in most cases) with the number of detached electrons (Nel) from the benzidine molecule up to formation of the dication (Nel = 2). On one hand, a position of the long-wave absorption band (λCT) corresponding to the anion-to-cation charge transfer in the neutral complexes of the benzidine dication with anions, correlates with the Mulliken electronegativity of the anion (R(2) = 0.8646) and its adiabatic ionization energy (R(2) = 0.8054). On the other hand, the correlations with the anion charge in the complexes and the anion isotropic polarizability are rather poor (R(2) = 0.6392 and 0.3470, respectively). On the ground of the obtained strong relationships, one may recommend the calculated molecular properties as potentially preferable descriptors for the benzidine-based compounds in terms of the QSAR methodology.

A series of planar hetero[8]circulenes and their doubly charged ions are studied by the NICS and GIMIC methods to interpret the aromatic properties of these high symmetry species. In accordance with the performed calculations all studied hetero[8]circulenes are found to be nonaromatic compounds because paratropic and diatropic ring-currents are completely canceled yielding almost zero net current. In great contrast, the dicationic and dianionic hetero[8]circulenes demonstrate the predominant contribution of diatropic ring currents resulting in the total aromatic character of the studied doubly charged ions. This fact allows us to predict the high stability of dianionic hetero[8]circulenes and explains the extremely high stability of dicationic species observed in the mass-spectra.

Secondary electron emission is the most important stage in the mechanism of radiation damage to DNA biopolymers induced by primary ionizing radiation. These secondary electrons ejected by the primary electron impacts can produce further ionizations, initiating an avalanche effect, leading to genome damage through the energy transfer from the primary objects to sensitive biomolecular targets, such as nitrogenous bases, saccharides, and other DNA and peptide components. In this work, the formation of positive and negative ions of purine bases of nucleic acids (adenine and guanine molecules) under the impact of slow electrons (from 0.1 till 200 eV) is studied by the crossed electron and molecular beams technique. The method used makes it possible to measure the molecular beam intensity and determine the total cross-sections for the formation of positive and negative ions of the studied molecules, their energy dependences, and absolute values. It is found that the maximum cross section for formation of the adenine and guanine positive ions is reached at about 90 eV energy of the electron beam and their absolute values are equal to 2.8 × 10(-15) and 3.2 × 10(-15) cm(2), respectively. The total cross section for formation of the negative ions is 6.1 × 10(-18) and 7.6 × 10(-18) cm(2) at the energy of 1.1 eV for adenine and guanine, respectively. The absolute cross-section values for the molecular ions are measured and the cross-sections of dissociative ionization are determined. Quantum chemical calculations are performed for the studied molecules, ions and fragments for interpretation of the crossed beams experiments.

Two alternative approaches—vertical and adiabatic—are used to estimate global and local electrophilicity (ω and ωk+) indexes for a series of aryl cations in both the ground and first excited electronic states using the well-known Parr scheme. The energy parameters used in these methods are obtained by the B3LYP/6-311++G(2d,2p) calculations of the aryl cations and of their oxidized and reduced forms in acetonitrile medium. The ground state ω values are lower than those for the excited state, which is in accord with the maximum hardness principle. Analysis of the ω indexes calculated with more reliable adiabatic approach reveals a dependence of the ground and first excited state ω indexes on the singlet–triplet energy gap of the aryl cations. A plot of the above dependence has a hyperbola-like shape; thus, the maximum (ground state) and minimum (first excited state) ω indexes correspond to the aryl cation, for which the singlet–triplet splitting is close to zero. Moreover, the ωk+ index distribution at the ipso-carbon atoms does not obey the maximum hardness principle, since it depends on spin multiplicity, not on the electronic state spatial type. For many singlet ground state aryl cations, the ωk+ indexes at the ipso-carbon atom are lower when calculated in the excited triplet state; that is due to a strong ω delocalization onto two electrophilic centers. This explains a higher chemoselectivity of the triplet aryl cations in reactions with the π-nucleophiles compared to the corresponding singlet arylium species. Applicability of the adiabatic approach for calculation of the ω and ωk+ indexes is supported by the experimental data on the nucleophile-independent parameter E for the singlet and triplet state of the p-Me2NC6H4+ cation.

Nanotubes and two-dimensional sheet-like compounds containing tetraoxa[8]circulene core units are theoretically predicted. The large cavities with a diameter of 6 Å in the 2D sheets could be useful for hydrogen storage. The designed compounds are predicted to adsorb strongly in the visible region and to be a promising material for nanophotonics because of their electron-donor properties (p-type semiconductors).

The single crystal architecture of the high-symmetry octathio[8]circulene and sym-tetraselenatetrathio[8]circulene is studied at the density functional theory (DFT) level with the quantum theory of atoms in molecules (QTAIMs) approach to the electron density distribution analysis. The presence of stabilizing intermolecular C---C, C---S and C---Se contacts in the longitudinal and transversal projections of the single crystals is postulated on the grounds of the previous high-resolution X-ray data for octathio[8]circulene; it is supported by the present QTAIM calculations and also predicted in some new details for both circulenes. We suggest that the appearance of the observed red color for the monocrystalline octathio[8]circulene is caused by strong intermolecular interactions between the molecules in the single crystal. However, the intermolecular interactions for the sym-tetraselenatetrathio[8]circulene crystal fragment are weaker and molecular layers are more friable in comparison to octathio[8]circulene crystal structure. These lead to the absence of visible absorption for the sym-tetraselenatetrathio[8]circulene crystal.

In this present study, silver nanoparticles were synthesized using synthetic humic substances (HSs) as reducing and stabilizing agents. Preference of synthetic HSs over natural humic matter is determined by a standardization problem resolution of the product due to the strict control of conditions of the synthetic HSs formation. It allows to receive the silver nanoparticles with the standardized biologically-active protective shell that is very important for their use, mainly in medicine. The concentration of sodium hydroxide, synthetic HSs, silver nitrate and temperature employed in the synthesis process are optimized to attain better yield, controlled size and stability by means of UV-visible technique. In the optimal reaction conditions the concentrated silver colloids (55 mM) with 99.99% yield are obtained which were stable for more than 1 year under ambient conditions. The received silver nanoparticles are characterized by UV-visible spectroscopy, X-ray diffraction (XRD), FT-IR spectroscopy and transmission electron microscopy (TEM). The antimicrobial activity of silver nanoparticles against fungal and bacterial strains is also shown., (Copyright © 2013 Elsevier B.V. All rights reserved.)

We describe herein the first synthesis of a new class of anti-aromatic planar cyclooctatetraenes: the azatrioxa[8]circulenes. This was achieved by treating a suitably functionalised 3,6-dihydroxycarbazole with 1,4-benzoquinones or a 1,4-naphthoquinone. We fully characterised the azatrioxa[8]circulenes by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The results of a computational study (NICS) suggest that the central planar cyclooctatetraene is anti-aromatic when the molecules are in neutral or oxidised states (2+), and that the corresponding dianions are aromatic. We discuss the aromatic/anti-aromatic nature of the planar cyclooctatetraenes and compare them with the isoelectronic tetraoxa[8]circulenes., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Recently synthesized π-extended symmetrical tetraoxa[8]circulenes that exhibit electroluminescent properties were calculated at the density functional theory (DFT) level using the quantum theory of atoms in molecules (QTAIM) approach to electron density distribution analysis. Nucleus-independent chemical shift (NICS) indices were used to characterize the aromaticity of the studied molecules. The tetraoxa[8]circulene molecules were found to consist of two antiaromatic perimeters (according to the Hückel "4n" antiaromaticity rule) that include 8 and 24 π-electrons. Conversely, NICS calculations demonstrated the existence of a common π-extended system (distributed like a flat ribbon) in the studied tetraoxa[8]circulene molecules. Thus, these symmetrical tetraoxa[8]circulene molecules provide examples of diatropic systems characterized by the presence of induced diatropic ring currents.

Photochemical reaction of the electronically excited NO(2)* species with the water molecule is studied in terms of a new version of density functional theory by selecting the specific (2)A'' symmetry of the whole system, which is different from the ground-state pattern. The excited C(2)A(2) state of the NO(2) molecule is found to be distorted to the equilibrium structure O=N-O(.), which poses the (2)A'' symmetry in the C(s) point group. With the B3LYP functional it is shown that such an electronically excited NO(2)* molecule, generated by visible light (λ=420 nm), can react with water vapor to produce OH+HONO species, an important source of tropospheric hydroxyl radicals. This photochemical process can be considered as a possible mechanism of atmosphere self-cleaning.

Rational explanation of the mechanisms of bioactivation of molecular oxygen by enzymes is impossible without understanding more simple mechanisms of the O2 photoactivation in collision complexes of gases and solvents. Production of peroxides in oxidases and more complicated oxidation processes by molecular oxygen are spin forbidden reactions and bioactivation of dioxygen is connected with enzymatic spin-catalysis by acceleration of the triplet-singlet (T-S) quantum transitions. Internal magnetic perturbations in the free oxygen molecule and in O2 complexes with solvents or with coenzyme in biopolymers bear characteristic entirely similar features and removal of spin prohibition on T-S transitions is quantified by some common physical mechanisms. An account of specific spin-orbit coupling (SOC) in the open pi(g)-shell of dioxygen permits to explain the T-S transitions intensity of the red atmospheric band in the O2, molecule and the selective enhancement of the radiative alpha1delta(g)-->X3Sigma(g)- transition intensity in various solvents (P). Charge transfer contribution P+ O2- leads to increase of SOC between the T-S oxygen states that enhances the O2 (alpha1delta(g)) quenching. Similar T-S transitions mechanisms and SOC enhancement is realized upon dioxygen activation by enzymes of the glucoseoxidase type. Three electronic mechanisms of reductive bioactivation of O2 by oxidases coenzymes are considered on the basis of physical mechanisms of the O2 photoactivation. They include intermediate stages of the superoxide and peroxide ion formation. Study of the O2 activation mechanisms by T-S transitions in enzymatic complexes of dioxygen binding open new potentialities in biotechnology and medicine.

The structure of estrogen receptors and their interaction with 17beta-estradiol and estriol are of particular interest today because the treatment of breast cancer and the cause of the disease are intricately linked to the activity of the estrogen receptor and the normal blood serum level of these hormones. Molecular geometry and vibration frequencies of these steroid hormones are calculated by density functional theory with the B3LYP/ 6-31G** approximation. Intensities of infrared absorption and Raman spectra for estradiol are in an agreement with the experiment data. The assignments of all vibrational bands in the spectra of these hormones are presented on the basis of quantum chemical calculations of frequencies and normal modes. For the large number of bands such an assignment is made for the first time. The analysis of infrared spectra of both hormones indicates some nontrivial structure-spectra correlations. A series of specific vibrations is predicted in the low-frequency region of the IR spectra; their role in hormone-receptor interaction and in energy transfer processes are discussed. The search of the optimized geometrical structure by minimization of the total energy gradient is accompanied by the second derivatives calculation; diagonalization of the Hessian matrix leads finally to solution of vibration problem. The 17beta-estradiol molecule consists of 44 atoms and has 126 normal modes of internal vibrations. All these normal modes are presented together with their analysis and comparison with experimental data. From this comparison we have obtained an assignment of all absorption IR bands of 17beta-estradiol recorded earlier in KBr. For all stretching vibrations our assignment is in agreement with the previous one, obtained on the basis of the empirical rules. Direct DFT calculation of vibrational frequencies cannot provide a 100% agreement with the experimental IR spectra and scaling factors in the range of 0.95-0.97 are used in order to fit theoretical and experimental data.

The electric and magnetic properties of the ground state of oxygen molecule are calculated by multiconfiguration self-consisted field (MCSCF) method and compared with experimental data: the quadrupole moment, polarizability, the 17O nuclear quadrupole coupling constant, magnetizability tensor, nuclear spin-rotation coupling constant and rotational g factor. The last two constants are calculated for all possible isotope modifications. The rotational, ESR and NMR spectra are discussed. Fermi-contact hyperfine coupling parameter is additionally estimated by different methods. The NMR chemical shielding tensor for 17O16O species at high temperature limit (without electron spin contribution) is predicted. Potential energy curves for 10 excited bound states and the internuclear distance dependence of the studied properties are also presented., (Copyright 2003 Elsevier B.V.)

The Einstein coefficient for the singlet oxygen emission a1Deltag-->X3Sigmag- at lambda=1270 nm and b1Sigmag+-->X3Sigmag- emission at lambda=750 nm were calculated by quadratic response (QR) multiconfiguration self-consisted field (MCSCF) method for a number of collision complexes O2+M, where M=He, Ne, Ar. Interaction with He clusters was studied in order to simulate cooperative effect of the environment on the oxygen emission. Calculations of the dipole transition moment for the Noxon band, b1Sigmag+-a1Deltag, by linear response (LR) MCSCF method were also performed for a number of collision complexes. Spin-orbit coupling (SOC) between the b1Sigmag+ and X3Sigmag- (MS=0) states does not change much upon collisions, thus the a-X transition borrows intensity mostly from the collision-induced Noxon band b-a. The a-X intensity borrowing from the Schumann-Runge transition is negligible. The calculations show that the b-a and a-X transition probabilities are enhanced approximately by 10(5) and 10(3) times by O2+M collisions. An order of magnitude differences occur for both transitions for noble gases with large difference in polarizability. A strong cooperative effect is obtained when few He atoms perturb the oxygen molecule. Depending on mutual orientation of the partners it can be a complete quenching of the a-->X emission or strong non-additive enhancement. Collision-induced infrared vibrational transitions in a number of molecular oxygen excited states were studied and shown to be state selective.

Glucose oxidase (GO) and copper amine oxidase (CAO) catalyze the reduction of molecular oxygen to hydrogen peroxide. If a closed-shell cofactor (like FADH(2) in GO and topaquinone (TPQ) in CAO) is electron donor in dioxygen reduction, the formation of a closed-shell species (H(2)O(2)) is a spin forbidden process. Both in GO and CAO, formation of a superoxide ion that leads to the creation of a radical pair is experimentally suggested to be the rate-limiting step in the dioxygen reduction process. The present density functional theory (DFT) studies suggest that in GO, the creation of the radical pair induces a spin transition by spin orbit coupling (SOC) in O(2)(-)(rad), whereas in CAO, it is induced by exchange interaction with the paramagnetic metal ion (Cu(II)). In the rate-limiting step, this spin-transition is suggested to transform the O(2)(-)(rad)-FADH(2)(+)(rad) radical pair in GO and the Cu(II)-TPQ (triplet) species in CAO, from a triplet (T) to a singlet (S) state. For CAO, a mechanism for the O[bond]O cleavage step in the biogenesis of TPQ is also suggested.

Electronic mechanisms of reductive activation of O2 by oxidases coenzymes (K) are considered. Activation is induced by triplet-singlet (T-S)-transitions at the charge-transfer stage and K+...O2- radical pair (RP) formation. For flavoproteins the spin reversion is induced by spin-orbit coupling (SOC) in superoxide ion, for metal-oxidases--by exchange interaction with paramagnetic metal ion. Another type of spin activation is connected with the singlet peroxide ion O(2)2- formation in cytochromes (structures of the type Fe(3+)-O- -O-Cu2+).