Search

A model for simulating the transient electronic absorption spectra of donor–acceptor dyads undergoing ultrafast intramolecular charge transfer in solution has been developed. It is based on the stochastic multichannel point-transition approach and includes the reorganization of high-frequency intramolecular modes (treated quantum mechanically) and of low frequency intramolecular and solvent modes (described classically). The relaxation of the slow modes is assumed to be exponential with time constants taken from experiments. The excited-state dynamics is obtained by simulating the population distribution of each quantum state after optical excitation and upon electronic and vibrational transitions. This model was used to simulate the transient electronic absorption spectra measured previously with a pyrylium phenolate in acetonitrile. A very good agreement between the simulated and measured spectra was obtained assuming a three-level model including the ground state, the optically excited state, and a dark state with large charge-transfer character and a substantially different geometry relative to that of the optically excited state. The merit of this approach to disentangle the contributions of both population changes and relaxation processes to the ultrafast spectral dynamics will be discussed.

Nitro derivatives of xanthione, 2,7-dinitro-9H-xanthene-9-thione and 2,4,7-trinitro-9H-xanthene-9-thione, have been first synthesized and their stationary and transient spectra have been measured. The stationary spectra show that the attachment of the nitro groups to the xanthione scaffold leads to strong quenching of S2→ S0fluorescence and the decrease of the oscillator strength of the S2← S0electronic transition. Analysis of the transient absorption spectra uncovers the ultrafast stimulated emission quenching from the second excited state, S2, in the both derivatives. A kinetic scheme has been suggested to rationalize the complex spectral dynamics of the transient absorption signal. The kinetic scheme is deduced from the analysis of the transient spectra and supported by the quantum-chemical calculations, which predict the existence of a dark state and S2state splitting into two close levels. The ultrafast transitions between S2state sublevels and the transition into the dark state play a crucial role in spectral dynamics. These new features discovered in the nitro derivatives of xanthione distinguish essentially their spectral dynamics from that observed in xanthione.

The photochemistry of sodium thiosulfate (S2O32–) in aqueous solutions is rather complicated. Several sulfur-containing radical anions are formed upon photoexcitation. Any of them are rather common (SO3•–, SO2•–, and SO5•–); others are rare (S2O3•–, •S4O63–, and S•–) or never documented (S2O5•–). In order to support the identification of intermediate radical anions, quantum-chemical (QM─quantum mechanical) calculations of the geometric and electronic structures of S2O3•–, S2O5•–, and •S4O63–were performed. Two different approaches, time-dependent density functional theory and complete active space self-consistent field, were applied to identify the method optimal for the reproduction of the experimental electronic absorption spectra. Several of the most commonly used functionals were considered. The best agreement with the experimentally observed spectra of reference compounds (common sulfur-containing anions and radical anions) was achieved for the WB97X-D3 functional. Using this approach, satisfactory agreement between experimental and calculated spectra of S2O3•–, S2O5•–, and •S4O63–was achieved. It was shown that S2O5•–and •S4O63–can exist in two isomeric forms with different spectral properties. These isomers are S2O3O2•–; SO3SO2•–for the case of S2O5•–and (S2O3)2•3–; (S3O32–...SO3•–) for the case of •S4O63–.

The synthesis of N-benzoyl [N-(m-nitrobenzoyl)] derivatives of ɛ-aminocaproic acid oligomers by the reaction of ɛ-caprolactam with benzoic (nitrobenzoic) acid in the presence of p-toluenesulfonic acid as catalyst was studied.The synthesis of N-benzoyl [N-(m-nitrobenzoyl)] derivatives of ɛ-aminocaproic acid oligomers by the reaction of ɛ-caprolactam with benzoic (nitrobenzoic) acid in the presence of p-toluenesulfonic acid as catalyst was studied.

A complex for remote-controlled contact-free diagnostics systems for performing correlated measurements of the main parameters and characteristics of fast electronuclear and transmutation processes (t= 1 μsec) in a nuclear-power system is described. The complex contains γ-ray, x-ray, and infrared radiation detectors, which operate in real time in line with a computer.

Polycaproamide is modified with polyfluorinated compounds to increase the resistance of the material to heat and aggressive media and the elasticity of the fibre. The mechanism of the modification process is examined.

The calculation of complexes RnBF3-n. HF (there is R = CH3 and C2H5) has been carried out by the quantum-chemical semi-empirical MNDO method in Dewar and Teel parameterization. The geometrical and electronic structure of these complexes was obtained. On an example of simple H-acids we estimated their acid strength. We found that irrespective of the ligand surrounding of B atom the complexes RnBF3-n HF have rather high acid strength (pKa = 17.9 14.9).

The theoretical evaluation of acid strength reactive fuels, through quantum-chemical calculation by the method CNDO/2 of the charges on hydrogen atoms, which correlate with the universal index of acidity is presented. Correlations defining relationship of some parameters of combustion with the accounting parameters of acidic strength of the fuels are established to oxygen.

Abstract: A system is described for detecting optical radiation in order to measure the transverse dimensions of beams and their particle density distribution. The system is based on hodoscope photomultipliers, scintillators, fiber-optic transmission lines, and electronic units. The system was designed for monitoring beams of elementary particles in the K4-K10 c-τ-factory accelerator-storage systems, the synchrophasotron, and the nuclotron, with energies of 20 MeV to 10 GeV and intensities of 103–108 particles/cm2. A brief description is given of the optical part, the design of the detector, and the hardware and software.