Energetics and structural mechanisms of photochemical processes in molecules of aldonitrone vinylogs

Depending on the multiplicity of the excited state, alternative photoreactions are realized in methoxy and acetoxy derivatives of aldonitrone vinylogs: diabatic nitrone-oxazypyridone rearrangement in the first excited singlet state and E-Z isomerization involving the C=N bond in the triplet state. In the case of aldonitrone vinylogs with an o-hydroxy group in the -aryl ring one observed adiabatic proton transfer from the hydroxy group to the azotoxido group with a subsequent diabatic reaction involving cyclization to the 2H-chromene structure. The same transfer is also realized in the triplet state. The quantum yields were determined, and the energies of activation of the photoreactions and the reverse thermal processes were evaluated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1373–1378, October, 1990.     

Modified electron transfer model for excitation functions of the type M + RX → MX + R and M+ + RX− (M = alkali,R = alkyl group,X = halogen) and M′ + N2O → MO* + N2 (M′= Ba OR Sm)

A one-dimensional model is described for the excitation functions of reactions that are initiated by an electron transfer at close range. The process is governed by a barrier in the entrance channel, the abortive reflection of trajectories at higher energies and by the competition of an adiabatic and a diabatic channel for the reactive flux. The model is fitted to measured cross sections for the (K.Rb)+CH₃I, K+C₂H₅Br and (Ba.Sm)÷ N₂O reactions and the electron transfer cross section for K+CH₃I → K⁺ + CH₃I^- is successfully predicted from the fitting parameters of the reactive channel.     

Analyzing the efficiency of proton transfer to carbon in Kirby’s enzyme model—a computational approach

A mechanistic study using ab initio and DFT calculation methods on the intramolecular ring-closing of enol ethers 1Z, 1E, and 2 (Kirby’s enzyme models for aldolase and isomerase) has revealed that proton transfer is the rate determining step and involves two stages: re-organization of the global minimum structure (having the lowest enthalpic energy and a relatively long distance between the two reactive centers, r) to a more organized ground state conformation (having the smallest r distance among all the conformations), and a second stage by which the proton is transferred from the organized state to the corresponding transition state. The energy needed for the first stage to occur is dependent on the rotation barrier for the proton to be in proximity to the CC double bond carbon (proximity effects), whereas, that needed for the second step is largely affected by the strain energies of the reactant and the corresponding product. In addition, it was found that the oxocarbocation intermediate involved in the proton transfer is unstable and undergoes ring-closing to the corresponding product with zero activation energy. Further, the calculated DFT effective molarities (EM) for 1Z, 1E, and 2 were found to correlate strongly with experimental EM values.     

From torsional spectra to Hamiltonians and dynamics: effects of coupled bright and dark states of 9-(N-carbazolyl)-anthracene

The torsional dynamics of the 9-(N-carbazolyl)-anthracene (C9A) molecule is investigated by means of time-independent (1) and time-dependent (2) quantum-mechanical simulations in a diabatic representation. The study includes effects of surface crossing of the brightS ₁ state with a dark state. (1) The intensity pattern of theS ₀ S ₁ fluorescence excitation spectrum is used to fit an effective one-dimensional Hamiltonian with a single-minimum potential for the dark state together with diabatic couplings to the double well potential of the bright state. (2) Based on this Hamiltonian, first predictions for a pump-probe scheme are made. In the pump process the molecules are excited to theS ₁ state followed by competing torsions in the bright state and diabatic curve crossings to the dark state, depending on the pump frequency. Assuming the probe process to be an ionization from the bright state, the interfering effects of the dark state on the dynamics in the bright state can be monitored in a directly time-dependent way on a fs-ps time scale.     

Predissociation lifetime of the 1σ g 2 2σ g diabatic state of He 2 + : a one-channel approach

Summary This work is concerned with the application of a one-channel model to obtaining predissociation lifetimes and transition rates in a system of crossing diabatic states. The calculation focuses on the first shape resonance of the 1 _g ² 2 _g diabatic state of He ₂ ⁺ , which is relatively stable with respect to tunneling. This resonance predissociates as a result of the 1 _g ² 2 _g state being crossed by the 1 _g 1 _u ² dissociative diabatic state near the resonance level. We have estimated its predissociation lifetime to be of the order of 10^–11 s.     

Importance of Radical Transfer in Precipitation Polymerization: The Case of Vinyl Chloride Suspension Polymerization

The importance of radical transfer between the reactive phases in precipitation polymerization processes is investigated with the vinyl chloride suspension polymerization as an example. A two‐film model that accounts for a mass transfer resistance in both the monomer‐rich and the polymer‐rich phase is constructed and applied. Equilibrium is assumed at the interphase boundary. Based on model calculations using intrinsic rate coefficients obtained by regression to experimental data the effect of accounting for radical transfer between the reactive phases on the simulated monomer conversion and total moments of the molar mass distribution is demonstrated. It is found that the effect of radical transfer between the reactive phases is most pronounced at low polymerization temperatures.

     

Modelling charge transfer reactions with the frozen density embedding formalism

The frozen density embedding (FDE) subsystem formulation of density-functional theory is a useful tool for studying charge transfer reactions. In this work charge-localized, diabatic states are generated directly with FDE and used to calculate electronic couplings of hole transfer reactions in two π-stacked nucleobase dimers of B-DNA: 5'-GG-3' and 5'-GT-3'. The calculations rely on two assumptions: the two-state model, and a small differential overlap between donor and acceptor subsystem densities. The resulting electronic couplings agree well with benchmark values for those exchange-correlation functionals that contain a high percentage of exact exchange. Instead, when semilocal GGA functionals are used the electronic couplings are grossly overestimated.     

Microscopic Models for Proton Transfer in Water and Strongly Hydrogen-Bonded Complexes with a Single-Well Proton Potential

A new mechanism and formalism for proton transfer in donor-acceptor complexes with long hydrogen bonds introduced recently [1], is applied to a proton transfer in liquid water. Structural diffusion of hydroxonium ions is regarded as totally adiabatic process, with synchronous hindered translation of two closest water molecules to and from the reaction complex as crucial steps. The water molecules induce a gated shift of the proton from the donor to the acceptor in the double-well potential with simultaneous breaking/formation of hydrogen bonds between these molecules and the proton donor and acceptor. The short-range and long-range proton transfer as structural diffusion of Zundel complexes is also considered. The theoretical formalism is illustrated with the use of Morse, exponential, and harmonic molecular potentials. This approach is extended to proton transfer in strongly hydrogen-bonded donor-acceptor complexes. In contrast to the above model [1], the short hydrogen bond between the donor and acceptor moieties, however, completely erodes the barrier along the proton transfer mode. This introduces some physical pattern differences from proton transfer reactions in truly double-well potentials with a finite proton transfer barrier at the transition configuration with respect to the environmental nuclear coordinates. The differences apply particularly to the origin of the kinetic isotope effect. We discuss explicitly details of the excess proton conductivity in aqueous solution, but the concepts and formalism apply broadly to acid-base reactions, proton conduction channels, and other strongly hydrogen-bonded O- and N-proton donor-acceptor systems.     

The charge-carrier mobility in disordered organic materials: the long-range one-dimensional diffusion with the memory effect

The transport of charge carriers in disordered organic materials is considered based on the techniques of generalized Langevin equation. We simulate the one-dimensional diffusion of a charge in the ensemble of molecular chains interacting with the acoustic phonon subsystem of bulk environment. The random local charge transitions between chain links are mutually correlated. The full computation of the zero-field charge mobility for the N, N-di(1-naphthyl)-N, N-diphenyl-(1,1-biphenyl)-4,4-diamine (\(\alpha \)-NPD) is performed as an illustration. Several models for the probabilities of local transitions are tested. The individual local diffusion constants are randomly varied along a molecular chain within several orders of magnitude. The stationary diffusion regime establishes for every chain the temperature-dependent partial charge mobility as a frequency-dependent complex-valued response function. It is averaged over the chain ensemble. The computational scheme is simple and efficient. The importance of the memory effect depends on specific properties of a given material. This dependence in terms of the system parameters is discussed.     

Zur Farbigkeit der Pseudooxo-Krokonsäurebisamide

The colour of oxo- und pseudooxo croconic acid bisamides (1) in solution is due to two intense * transitions in the visible region. These transitions are related to those of the Klessinger-Lüttke basic indigo chromophore. The first band corresponds to the second absorption band of this basic chromophore red-shifted by the intramolecular charge transfer excitation between the molecular subsystems of1. The second band of1 is closely related to the colour band of the basic indigo chromophore. According to the results of various analysis and projection techniques the interpretation of the colour of1 in terms of the basic indigo chromophore is superior to any other one in terms of alternative chromophoric subsystems.

     

Concept of polarity of free radicals in reactions of addition to olefins

The concept of polarity (nucleo- and electrophilicity) of free radicals in processes of addition to unsaturated substrates was developed within the framework of the method of qualitative potential energy surfaces on the basis of an analysis of the interaction of diabatic surfaces for reactions of free-radical addition to olefins. A formulation is given for the concept of the polarity (philicity) of the attacking free radical; the theoretical index of philicity and the criterion of polarity of the radicals are proposed; the concept of ambiphilicity of a radical is introduced. The chemical aspects of the philicity of free radicals — the influence of a change in it on the formation and height of the activation barrier of the reaction and the strength of the bond formed — are discussed. A parallel formulation of the concept of polarity of radicals in terms of boundary orbital interactions in the free-radical-unsaturated substrate system is given.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 22, No. 5, pp. 562–571, September–October, 1986.     

A probe of dynamical models using functional sensitivity densities with application to He++Ne(2p 6)→He++Ne(2p 53s) and Li+I→Li++I−

Summary The functional sensitivity densities ln ₁₂(E)/ lnV _ii (R) for He⁺+Ne(2p ⁶)He⁺+Ne(2p ⁵3s) reveal that the collisional excitation cross section ₁₂(E) is insensitive to the additional diabatic curveV ₃₃ included in some models. The negligible sensitivity of ₁₂(E) toV ₃₃ offers a quantitative validation of the more popular two state model for collisional excitation of Ne by He⁺. The sensitivity profiles for the collisional ionization Li+ILi⁺+I^– modeled by crossing diabatic curvesV ₁₁ (covalent) andV ₂₂ (ionic) shows that the ionization cross section does not depend on inner crossings even when these stem from large distortions in the underlying potential energy curves. The lack of sensitivity to inner crossings establishes the predominant role of the outermost crossing in triggering nonadiabatic transitions.     

Micellization of St/MMA Gradient Copolymers: A General Picture of Structural Transitions in Gradient Copolymer Micelles

In this work, a gradient copolymer of styrene (St) and methyl methacrylate (MMA) is synthesized via reversible addition–fragmentation chain transfer living radical polymerization and its micellization behaviors in an acetone and water mixture are investigated by transmission electron microscopy, light scattering, and NMR spectroscopy. Three different kinds of transitions were found to coexist in a single system for the first time: a unimers to micelles transition, a star‐like micelles to crew‐cut micelles transition resulting from the shrinkage of micelles, and morphological transitions from spherical micelles to cylindrical micelles to vesicles. Our findings provide a general picture of structural transitions and relaxation processes in gradient copolymer micelles, which can lead to the development of novel materials and applications based on gradient copolymers.

     

A DFT/TDDFT Study on Excited State Process of a Novel Probe 4′-Fluoroflavonol

A novel fluorescent probe 4′-fluoroflavonol (4F) was reported by Serdiuk et al. (RSC Adv 6:42532, 2016) in a previous paper. Spectroscopic studies on excited-state proton transfer (ESPT) of 4F was mentioned, while the mechanism of ESPT for 4F isdeficiency. In this present work, based on the time dependent density functional theory (TDDFT), we investigated the excited-state intramolecular proton transfer (ESIPT) mechanism of 4F theoretically. The primary bond lengths, bond angles and the infrared (IR) vibrational spectra involved in the formation of hydrogen bonds vertified the intramolecular hydrogen bond was strengthened, which manifests the tendency of excited state proton transfer. According to the results of calculated potential energy curves along O–H coordinate, an about 13.18 kcal/mol barrier has been found in the S₀ state. However, a barrier of 3.29 kcal/mol was found in the S₁ state, which demonstrates that the proton transfer process is more likely to occur in the excited state. In other words, the proton transfer was facilitated by photoexcitation. Particularly, the study about ESIPT mechanism of 4F should be helpful for further understanding property of fisetin.     

The dynamics of nonadiabatic transitions in collisions between the I<Subscript>2</Subscript>(<Emphasis Type="Italic">E</Emphasis>) and I<Subscript>2</Subscript>(<Emphasis Type="Italic">X</Emphasis>) molecules

The paper presents the results of a theoretical study of the dynamics of nonadiabatic transitions between the ion-pair states E0 _g ⁺ and D0 _u ⁺ of the I₂ molecule induced by collisions with the I₂ molecule in the ground electronic state X0 _g ⁺ . The potential energy surfaces and diabatic coupling matrix elements of electronic states were obtained using a model based on the diatomics-in-molecule approximation. Special perturbation theory for intermolecular interaction was used to show that the large transition dipole moment between the E0 _g ⁺ and D0 _u ⁺ states caused the appearance of additional long-range corrections, an electrostatic dipole-quadrupole correction to the diabatic coupling matrix elements and induction dipole-dipole correction to the potential energy surface. The influence of these corrections on nonadiabatic dynamics was studied at the level of the semiclassical approximation. The electrostatic correction was found to sharply increase the contribution of resonance (accompanied by minimum kinetic energy changes) vibronic transitions at large distances between the colliding molecules. The induction correction had the opposite effect because of the high transition probability at short distances. The results obtained were in qualitative agreement with experimental data. The conclusion was drawn that obtaining quantitative agreement required a more balanced inclusion of interactions at short and long distances.     

Structure and spectra of 1,3,2-dioxaphospholenes. 1. Microwave spectra of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene and its isotopomers in the ground and excited vibrational states

The microwave spectrum of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene (1) was studied in the frequency range from 7 to 53 GHz. Rotational transitions of the parent molecule in the ground and eleven excited vibrational states and those of its mono-substituted ³⁷Cl, ¹³C_Me, and ¹³C_Cycle isotopomers in the ground vibrational state were identified. Rotational constants and partial r _s-structure were obtained. The quartic centrifugal distortion constants, dipole moment components _a = 3.8D and _c = 0.24D (the total dipole moment is 3.81D), and the ³⁵Cl quadrupole coupling constants were determined for the parent molecule. The fine structure of the microwave transitions in the parent molecule was analyzed under the assumption of noninteracting methyl groups. The height of the barrier to internal rotation (V ₃₀ = V ₀₃ = 665 cm^–1) and the frequency of torsional vibrations ( = 167 cm^–1) were found. The frequencies of two lowest vibrational modes corresponding to deformation vibrations of the five-membered ring were estimated (100 cm^–1) from the relative intensities of rotational transitions for different vibrational states.     

Total reactive cross section for K + Br2 in the energy range of 0–4 eV

3-D classical trajectory calculations were performed using diabatic as well as adiabatic potential energy surfaces. Non-adiabatic transitions were allowed and localized at the avoided crossing of the two adiabatic surfaces. The transition probability was calculated according to the Landau—Zener formalism. The total cross sections for the reaction K + Br₂ → KBr + Br were calculated and compared with experimental data. The total cross sections, calculated with the aid of adiabatic potential energy surfaces, were, contrary to those with diabatic surfaces, in very good agreement with the measured total reactive cross sections over the whole energy range of 0–4 eV.     

Calculation of the ultraviolet and visible spectra of copper acetylacetonate

The calculation of the acetylacetonate complex of copper was performed by the Wolfsberg-Helmholz method. An interpretation of the observed absorption bands in the ultraviolet (UV) and visible regions is given. It is shown that the bands at 200, 250, and 390 mµ belong to spectra with a charge transfer of the ligand-metal type; the 300 mµ band belongs to interligand transition of the ^* type; the 560 and 670 mµ bands are associated with d-d transitions. The effective charges at the atoms and, for some transitions, the oscillator strengths, are calculated.