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The ultrafast processes caused by photoexcitation of (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile were studied by means of transient absorption (TA) pump-probe spectroscopy and verified by quantum chemical calculations. The primary photochemical process was found to be an inner-sphere electron transfer followed by an escape of an •NO3 radical to the bulk solution. The reaction occurs via the dissociative triplet excited LMCT state of the initial complex. Based on the experimental data and quantum chemical calculations, the mechanism of ultrafast photophysical and photochemical processes is proposed. [ABSTRACT FROM AUTHOR]

Cerium ammonium nitrate (CAN) is an important photolytic source of NO3• radicals in aqueous nitric acid solutions and in acetonitrile. In this work we performed the study of primary photochemical processes for CAN in acetonitrile by means of ultrafast TA spectroscopy and quantum chemical calculations. Photoexcitation of CAN is followed by ultrafast (< 100 fs) intersystem crossing; the vibrationally cooled triplet state decays to pentacoordinated Ce(III) intermediate and NO3• radical with the characteristic time of ca. 40 ps. Quantum chemical (QM) calculations satisfactorily describe the UV–vis spectrum of the triplet state. An important feature of CAN photochemistry in CH3CN is the partial stabilization of the radical complex (RC) [(NH4)2CeIII(NO3)5...NO3•], which lifetime is ca. 2 μs. The possibility of the RC stabilization is supported by the QM calculations. [ABSTRACT FROM AUTHOR]

Photochemistry of the (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile was studied by means of stationary photolysis and nanosecond laser flash photolysis. The primary photochemical process was found to be an intramolecular electron transfer followed by an escape of an •NO3 radical to the solution bulk. The spectra of two successive Pt(III) intermediates were detected in the microsecond time domain, and their spectral and kinetic characteristics were determined. These intermediates were identified as PtIII(NO3)52− and PtIII(NO3)4− complexes. Disproportionation of Pt(III) species resulted in formation of final Pt(II) products. [ABSTRACT FROM AUTHOR]

The work examines the possibilities of generating optically inaccessible exciplexes by radiation chemistry techniques and describing them using quantum chemical methods. Two new target compounds were synthesized, and their luminescent properties were characterized. Using radiation-induced luminescence methods, the formation of exciplexes with N,N-dimethylaniline from these compounds as a result of recombination of a radical ion pair in a nonpolar solution was shown, and their luminescent characteristics were studied. Bound excited states for three possible exciplex structures and their corresponding emission spectra were calculated, and the optimal exciplex structure was shown to agree with experiment. [ABSTRACT FROM AUTHOR]

We demonstrate that a series of perfluorinated para-oligophenylenes C6F5-(C6F4)n-C6F5 (n = 1–3) produce exciplexes with N,N-dimethylaniline (DMA) in degassed X-irradiated n-dodecane solutions. The optical characterization of the compounds shows that their short fluorescence lifetimes (ca. 1.2 ns) and UV-Vis absorption spectra, overlapping with the spectrum of DMA with molar absorption coefficients of 2.7–4.6 × 104 M−1cm−1, preclude the standard photochemical exciplex formation pathway via selective optical generation of the local excited state of the donor and its bulk quenching by the acceptor. However, under X-rays, the efficient assembly of such exciplexes proceeds via the recombination of radical ion pairs, which delivers the two partners close to each other and ensures a sufficient energy deposition. The exciplex emission is completely quenched by the equilibration of the solution with air, providing a lower bound of exciplex emission lifetime of ca. 200 ns. The recombination nature of the exciplexes is confirmed by the magnetic field sensitivity of the exciplex emission band inherited from the magnetic field sensitivity from the recombination of spin-correlated radical ion pairs. Exciplex formation in such systems is further supported by DFT calculations. These first exciplexes from fully fluorinated compounds show the largest known red shift of the exciplex emission from the local emission band, suggesting the potential of perfluoro compounds for optimizing optical emitters. [ABSTRACT FROM AUTHOR]

The photochemistry of the OsIVCl62− complex in ethanol was studied by means of stationary photolysis, nanosecond laser flash photolysis, ultrafast pump–probe spectroscopy and quantum chemistry. The direction of the photochemical process was found to be wavelength-dependent. Irradiation in the region of the d–d and LMCT bands results in the photosolvation (with the wavelength-dependent quantum yield) and photoreduction of Os(IV) to Os(III), correspondingly. The characteristic time of photosolvation is ca. 40 ps. Photoreduction occurs in the micro- and millisecond time domains via several Os(III) intermediates. The nature of intermediates and the possible mechanisms of photoreduction are discussed. We believe that the lability of the photochemically produced Os(IV) and Os(III) intermediates determines the synthetic potential of OsIVCl62− photochemistry. [ABSTRACT FROM AUTHOR]

The first quantum-chemical calculation of the molecules 22, 124 and 200 hexagons by the method MNDO was performed with optimisation of the geometry for all parameters by a standard gradient method. The optimised geometric and electronic structures of 22, 124 and 200 hexagons were obtained and their acid strength was theoretically evaluated (pKa = 33). It was found that the molecules belong to the class of very weak CH-acids (pKa > 14). [ABSTRACT FROM AUTHOR]

The dependence of the ultrafast photoinduced electron transfer dynamics in donor-acceptor complexes on the excitation pulse carrier frequency (spectral effect) has been investigated in the framework of a model involving three electronic state. The spectral effect has been shown to strongly depend on the angle θ between the reaction coordinate directions corresponding to optical and charge transfer transitions. Describing the solvent as a linear homogenous polar medium and accounting for Coulombic interaction of the transferred charge with the medium polarization fluctuations, the angle θ has been found out to be typically in the area 40°–85°. Exactly in this area of θ the spectral effect is predicted to be most pronounced. [ABSTRACT FROM AUTHOR]

The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations (CASSCF and XMCQDPT level of theory, IMCP-SR1 and SBKJC basis sets) of the structures and electronic absorption spectra of the OsIVCl5(H2O)− and OsIVCl5(OH)2− complexes, which are the products of OsIVCl62− photoaquation, were performed. The satisfactory agreement between the experimental and calculated spectra was achieved using both triplet and quintet manifolds. The dissociation of the aquacomplex with the formation of the hydroxocomplex was explained by the thermochemical data. [ABSTRACT FROM AUTHOR]

Two mechanisms of OsIVCl62− photolysis were studied by means of quantum chemical calculations in gas and aqueous phases. The difference between these mechanisms is in the nature of the possible Os(iv) key intermediates (KI). According to calculations, the intermediate is an OsIVCl5− complex of square pyramidal coordination geometry. The calculations do not give an opportunity to make an unambiguous choice between the triplet and quintet multiplicities of OsIVCl5−. The calculated CASSCF/IMCP-SR1 transition energies for 5OsIVCl5− are lower than for 3OsIVCl5−, while the calculated XMC-QDPT2/SBKJC spectra for the triplet state are in better agreement with the experimental absorption spectrum of the KI than for the quintet state. [ABSTRACT FROM AUTHOR]

nalysis of the oxidation mechanism of 6-methyl-2,4-dioxopyrimidine with selenium oxide and selenous acid with the formation of orotic aldehyde has been carried out by the ab initio quantum-chemical method on the 6-31G** basis. The mixed anhydride of acetic and selenous acids is formed with a gain of energy. It possesses high activity and steric accessibility in electrophilic attack on position 5 of the pyrimidine ring. The three-stage mechanism of the oxidation of the methyl group in 6-methyluracil by the mixed anhydride of selenous and acetic acids has been analyzed. [ABSTRACT FROM AUTHOR]

Reactions of 5-benzylidene-2-thioxo-2,3-dihydro-(1 H,5 H)-pyrimidine-4,6-diones with a singlet difluorocarbene afford difluoromethoxy derivatives in 20-34% yield. The quantum-chemical analysis of the reaction mechanism showed that N,N-dimethylformamide is involved into the formation of difluoromethoxy derivatives. [ABSTRACT FROM AUTHOR]

Ribulose 1,5-bisphosphate (RuBP) undergoes enolization to initiate fixation of atmospheric carbon dioxide in the plant carbon cycle. The known model assumes the binding of RuBP to the Rubisco active site with the subsequent formation of 2,3-enediol (2,3,4-trihydroxypent-2-ene-1,5-diyl diphosphate). In the present study, it is assumed that 1,2-enol (2,3,4-trihydroxypent-1-ene-1,5-diyl diphosphate) can be formed in the enolization step to initiate the carboxylation reaction. We have used Kohn–Sham density functional theory on WB97X-D3/Def2-TZVP levels to compare the reaction barriers in the two ways. We considered the pathways of carboxylation of 1/2-ene (mono/di)ol via the C1 and C2 carbons without taking into account the binding of RuBP to the magnesium ion. Calculations of Gibbs free energies confirm the equal probability of the formation of 2,3-enediol and 1,2-enol. Quantum–chemical modeling of enolization and carboxylation reactions supports the important role of the bridging water molecule and diphosphate groups, which provide proton transfer and lower reaction barriers. The results show that carbon dioxide fixation can occur without a magnesium ion, and binding with C1 can have a lower barrier (~12 kcal/mol) than with C2 (~23 kcal/mol). [ABSTRACT FROM AUTHOR]

A letter to the editor is presented regarding the condensation of acid-catalyzed azepan-2-one with Dimethyl Hydrogen Phospite and formaldehyde.

The article presents an experimental report on reactions of ε-caprolactam with carboxylic acids in the presence of acid catalysts. The reaction gave rise to the formation of N-acyl derivatives of the ε-aminocaproic acid oligomers. The initial stages of the reactions were analyzed using quantum-chemical method.

Presents a letter to the editor which offers a discussion on new catalytic synthesis of polyfluoroalkyl chlorosulfites.