K. Ya. Burshtein

Molecular simulation of the complexation effects on conformations and electronic absorption spectra of crown ether styryl dyes

Abstract The structure and spectra of new styryl dyes containing a crown ether moiety and a sulfoalkyl group along with their complexes with metal cations are investigated using spectroscopic methods and computer simulation. Large differences between the absorption spectra of the cis form and its complexes with alkaline earth metals are found. The absorption maximum of the high intensity longwave band is at 429 nm for the cis form and at 321–328 nm for the complexes. We show that this difference results from a large distortion of the chromophore geometry, which arises from the formation of an intramolecular coordination bond between the sulfo group and the metal cation captured in the crown cavity. For the distorted geometry, the first electronic transition is symmetry forbidden. The disappearance of the longwave band results in a large hypsochromic shift, observed experimentally.

Radical anions of halosubstituted methanes Structure and chemical reactivity

Abstract The structure, as well as the formation and decomposition reactions, of radical anions of halosubstituted methanes are investigated by the MNDO, AM1 and PM3 (UHF and RHF) methods. It is shown that the electron addition leads to a lowering of the T d symmetry of CF 4 and CI 4 . The geometry of other radical anions is similar to that of neutral parent molecules. After an electron transfer to halogen substituted methanes, the forming radical anions decompose by the cleavage of the C-Hal bond. An increase in the number of Hal atoms, as well as a decrease in the solvent polarity, enhance the radical anion stability. Decomposition with cleavage of the C-H bond is also possible, but only in a special environment, in particular, in electrochemical reactions on a Pt electrode, which may adsorb the H atoms.

Atom-atom potential calculations on Langmuir-Blodgett films: Structure of stearate films

Abstract The atom-atom potential method is used to determine the microstructure of the Langmuir-Blodgett stearate monolayer and Y-type bilayer. It is shown that the two-dimensional crystalline lattice of stearate films has no hexagonal symmetry; however, the deviation from the hexagonal structure is small. The packing of the molecules is determined by the electrostatic interaction as well as by the hydrocarbon tail size. The structural parameters of monolayer and bilayer are similar, but the bilayer lattice is more rigid.

Calculation of thermochemical parameters for CHnX3−n−→: CHnX2−n+X−, n = 0,1 and 2, X = Cl and Br. MO study of solvent and counter-ion effects

Abstract The dissociation of anions (CCl 3 − , CHCl 2 − , CH 2 Cl − , CBr 3 − , CHBr 2 − , CH 2 Br − ) to carbene products (:CCl 2 , :CHCl, :CBr 2 , :CHBr, :CH 2 ) was studied within the MNDO technique. Increments were added to the calculated energies to reproduce the experimental gas-phase values. The solvent effect was introduced within the point dipole model. The counter ion was simulated by the unity positive point charge. It is found that the reaction under consideration can occur only in solution. The increase in the solvent polarity and the substitution of the H atoms by Cl or Br accelerate the reaction. Interaction with the small counter ion may also accelerate the reaction, this effect depending on both the solvent polarity and the counter-ion size.

Quantum-chemical investigation of allyl compounds of group IV elements. 2. Electron absorption spectra

Conclusions1.A breach is coplanarity in allyl compounds of silicon, germanium, tin, and lead results in a modification and mixing of the upper occupied MO of the π and σ types, and to an increase in the energy gap between them. The same situation occurs with two lower vacant MO. The energy of quasilocal electron transitions, π → π* and σ → σ*, remains practically unchanged in this case, and the energy of the transition with charge transfer π-σ* decreases significantly. The maximum longwave displacement of this absorption band takes place in the gauche conformer.2.The root cause of the intensification degree of the modification of MO and of electron absorption spectra when passing to allyl compounds with heavier atoms is the closer approach in the position on the scale of energies of boundary a and σ* MO of the CH2-XH3 group.

Shapes of absorption bands in solutions. Organic chromophores

Abstract The shapes of absorption bands for polyenes, cyanine dyes, and some aromatic hydrocarbons are simulated using a combination of empirical force field for the ground state with quantum chemical calculations of excitation energies as a function of nuclear coordinates. The results are in good agreement with experimental spectra of solutions under normal conditions. Three factors of the band broadening are outlined: 1. (1) high-frequency progression from stretch vibrations (1600-1300 cm −1 ); 2. (2) low-frequency progression from bend vibrations (about 400 cm −1 ); and 3. (3) inhomogeneous and rotational effects. The bandwidth of the absorption band arises from the first factor. The action of the second factor simultaneously with the third may transform the absorption spectrum into a wide band with poorly pronounced or without vibrational structure.

Revised semiempirical parameters for Br, I, Sn, Hg, and Pb in the MNDO method

The semiempirical MNDO parameters have been modified for Br, I, Sn, Hg, and Pb. Expressions are given for rho/sub 1/ and rho/sub 2/. The ..delta..H/sub f/ calculated with them are no less reliable or accurate than the results obtained by Dewar et al., and the same applies to the bond lengths, bond angles, ionization potentials, and dipole moments

Factors which affect the direction of the reaction of nucleophilic addition of CH2XCO2Et toα,β-unsaturated aldehydes in conditions of two-phase catalysis

Conclusions1.The general mechanisms of the direction of addition of CH acids of the XCH2CO2Et and BrCH(CO2Et)2 type to enals in conditions of two-phase catalysis were formulated: a) CH acids (X=CO2Et, COCH3, CN) are added to enals unsubstituted in position 3 only at the C=C bond; b) enals monosubstituted in position 3 add CH acids (X=CO2Et, COCH3) at the C=C bond and CH acids (X=CN) at the C=O bond; c) enals disubstituted at position 3 add CH acids (X=CO2Et, COCH3, CN) exclusively at the C=O bond.2.Quantum-chemical calculations showed that the direction of the reaction of addition of CH acids to enals cannot be explained based on concepts concerning the competition of orbital and charge interactions.3.The change in the direction of addition in methylation and chlorination of the enal is due to steric hindrances which arise when the structure of the enal is altered when the reagents converge.

Crown-ether styryl dyes: 20. Synthesis, spectral properties, and complex formation of photochromic dyes based on 15-crown-5-substituted benzothiazole

Novel crown-containing styryl dyes (CSD) based on 15-crown-5-substituted benzothiazole with various electron-donating groups in thepara-position of the benzene ring were synthesized. Spectral and photochemical properties ofcis- andtrans-isomers of CSD and their complexes with Mg2+ ions in solutions were studied. By analysis of spectral parameters and the magnitudes and directions of the shifts of absorption (fluorescence) maxima, the effects of substituents and complex formation on the photochromism of CSD were elucidated. Using quantum-chemical calculations, the possibility of controlling the degree of participation of either of the two chromophores in the long-wave electron transition by complex formation was analyzed.

Computer simulation of band shapes in electronic absorption spectra of dimers of organic dyes

The band shapes in the absorption spectra of dimers of cyanine dyes were simulated using a combination of an empirical molecular force field for the ground state with quantum-chemical calculations of the electron excitation energy as a function of normal nuclear coordinates. The shape and the width of an absorption band strongly depend on the mutual arrangement of the monomers. If the monomers are located one directly above the other, the sublevels arising from intramolecular vibrations disappear in the spectrum, and a large hypsochromic shift of the 0-0-transition band is observed, which results mainly from through-space interaction of monomer orbitals. If the monomers are strongly shifted relative to each other, the sublevels mentioned are also absent in the spectrum, but the bathochromic shift of the 0-0-transition band is small and results from interaction of dipole moments of electron transitions. A rather broad region of intermediate structures is found between these dimer forms, where the interaction of dipole moments of electron transitions in monomers is low, and the shapes of absorption bands are similar to those of the monomers.