Modeling of the vibrational structure of the vibronic spectrum and an excited state of the dinaphthylethylene molecule

The vibrational structure of the fluorescence spectrum and the structure of a dinaphthylethylene molecule in an excited state are calculated in the first and second approximations of the parametric method employed in the theory of vibronic spectra. The calculated spectra are in quantitative agreement with experimental data. The role of the angular parameters of the parametric method in the quantitative prediction of the vibrational structure of the fluorescence spectrum and changes observed in the geometry of the dinaphthylethylene molecule under excitation is determined. It is demonstrated that the polyene and acene parameters of the parametric method possess a high degree of transferability and that the models obtained are quite adequate to the real structures of the molecules under investigation. The proposed approach permits qualitative predictions and quantitative predictive calculations of the spectra of the studied molecules, as well as the spectral characteristics necessary for simulation of photochemical molecular transformations.     

Calculation of the vibronic spectra and structure of methylstyrene in the excited state by the parametric method

Calculations of the excited-state structure and of the absorption and radiation spectra of trans-β-methylstyrene are performed by the parametric method for models of the first and second approximations. Only three of the parameters, being constant in a series of related compounds, are used for molecular fragments. In the calculation, the main regularities observed in electronic spectra are reproduced, and an analysis and interpretation of their fine vibrational structure are carried out. The use of the model of the second approximation with a single additional parameter allows more accurate reproduction of angular deformations of the molecule under excitation and of the characteristic changes observed in the low-frequency spectral region in the series of diphenyl polyenes, stilbene, styrene, and methylstyrene molecules. It is shown that the parameters of the second approximation (just as of the first one) possess the transference property in the homological series of molecules. In modeling complex molecules in excited states and their vibronic spectra, the parametric method is more efficient than the ab initio one. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 148–153, March–April, 2000.     

Application of the method of selective fluorescence excitation to investigation of complexation and solvation processes of polar organic dye molecules in binary solvents

It is proposed to use the method of selective fluorescence excitation to find absorption spectra (fluorescence excitation spectra) of 1 : 1 primary solvated complexes between polar molecules of an organic dye and the active component of a binary solvent, whose neutral component is a nonpolar or low-polarity liquid. The technique was tested with diluted solutions of 4-dimethylaminochalcone (4-DMC) in mixtures of ethylbenzene with dimethyl formamide at extremely low contents of the latter. It is shown that the experimental absolute shift of the long-wavelength vibronic absorption band of three-component DMC solutions is in a good quantitative agreement with the analogous shift obtained independently based on the semi-empirical theory describing the joint effect of nonlinear (complexation) and linear (solvation) dipole-dipole interactions on the shift of spectral bands.     

Particle swarm optimization in the problem of decomposing two-component spectral profiles

An approach based on stochastic particle swarm optimization is proposed for the mathematical processing of spectral profiles. It is shown that the proposed approach allows accurate decomposition of complex model spectra and determination of the parameters of spectral components.     

On the possibility of interpretation of the spectra of tetrapyrrole compounds and their radical ions on the basis of quantum-chemical calculations by the density functional method

Using the time-dependent formalism of the density functional theory (time-dependent density functional theory (TDDFT)), the energies and intensities of the electronic transitions of radical anions of Mg porphyrin and Mg phthalocyanine are calculated quantum chemically. Based on these calculations, an interpretation of the electronic spectra of these compounds is proposed. It is shown that, for the neutral Mg porphin molecule, the results of the TDDFT calculations are consistent with the generally accepted notions and with the experimental data only if hybrid density functionals are applied. In this case, the best results are obtained for an exchange-correlation functional whose parameters are close to the standard parameters of the B3LYP functional. The spectra of the electronic transitions of radical anions of the considered tetrapyrrole molecules calculated by the TDDFT (B3LYP) method are consistent both with previous semiempirical calculations and with the experimental data.     

Spectroscopic parametrization of the method of PNDO

Special spectroscopic parameterization of the semiempirical PNDO method for calculating the spectral characteristics of polyatomic heterocyclic molecules is proposed. The parameters are calibrated according to the experimental absorption spectra, photoelectronic spectra, and the dipole moments of benzene and its derivatives.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 96–100, November, 1986.     

Calculation and interpretation of the vibronic spectra of pyridine and <Emphasis Type="Italic">trans</Emphasis>-1,2-di(2′-pyridyl)ethylene in the second approximation of the parametric method

The structures of pyridine and dipyridylethylene molecules in an excited state and their vibronic spectra are calculated within the second approximation of the parametric method. The system of parameters obtained, including parameters of the σ and π types, ensures a quantitative agreement between the theoretical and experimental spectra of the pyridine and dipyridylethylene molecules. This agreement indicates that the proposed model is adequate to the real structure of the molecules. The parameterization is sufficiently complete and provides a means for quantitatively modeling the vibrational structure of the spectra of complex molecules containing fragments similar to those characterized by the σπ* and ππ* transitions. It is demonstrated that, after such essential substitutions of atoms in the molecules, the system of parameters of the acene and polyene fragments retains the high stability. The vibrational structure of the electronic spectra of the pyridine and dipyridylethylene molecules is interpreted, and the changes observed in the geometry of these molecules under electronic excitation are analyzed.     

Improvement of the parametric method of the theory of vibronic spectra of polyatomic molecules: Absorption spectra and the structure of butadiene, hexatriene, and octatetraene in the excited state

The structure of the excited states and absorption spectra of butadiene, hexatriene, and octatetraene are calculated by the parametric method of the theory of vibronic spectra using models of the first-and second-order approximations. It is shown that these molecular models adequately reflect the molecular structure and allow one to predict quantitatively the shape and fine vibrational structure of the absorption spectra. When passing to the second-order approximation, only two additional (angular) parameters are used. These parameters are transferable in the series of polyenes. Compared to the first-order approximation model, the second-order approximation model more accurately takes into account the angular deformations of polyenes upon their excitation and describes the intensity distribution in the vibrational spectrum, including weak lines. In addition, the calculations also quantitatively predict spectral variations in the molecular series. The parametric method is more efficient for modeling polyatomic molecules in the excited states and their vibrational spectra compared to other semiempirical and ab initio methods.     

Determination of the electronic-vibrational-rotational terms of diatomic molecules from measured wave numbers

A method is proposed for determining the electronic-vibrational-rotational (rovibronic) terms of diatomic molecules from experimental data on the wave numbers of the rovibronic spectral lines. In contrast to the currently available methods, the proposed method is based only on the Rydberg-Ritz combination principle. It is demonstrated that a correlation between the sets of magnitudes of the rovibronic terms and the wave numbers of the observed spectral lines appears when the spectrum involves three or more electronic-vibrational states related in pairs by radiative transitions. Unlike the traditional methods, the proposed approach (i) does not require additional assumptions regarding the molecular structure; (ii) does not employ intermediate parameters, such as molecular constants in the traditional methods; (iii) makes it possible, within the one-step optimization procedure, to use all the available experimental data obtained for several sets of bands by different authors and in different experiments; (iv) permits one to select effectively experimental data, reject outliers and blunders, correct the identification of the rovibronic spectral lines, and analyze the consistency of different data sets in the interactive mode; and (v) provides a means for determining not only the optimum magnitudes of the rovibronic terms but also their errors associated with the amount and quality of the available experimental data. A necessary condition for the implementation of the new method is the preliminary identification of the lines revealed in the rovibronic spectra under investigation, i.e., the assignment of the observed spectral lines to particular rovibronic transitions. This requires the use of conventional procedures for analyzing the molecular spectra.     

Assignment Strategies and Analysis of Cross-Peak Patterns and Intensities in the Three-Dimensional Homonuclear TOCSY-NOESY of RNA

The application of 3D TOCSY-NOESY to the analysis of RNA is presented, using a TOCSY-NOESY spectrum of the RNA duplex r(5′GGGCUGAAGCCU′). It is shown that for RNA molecules, 3D spectra can be obtained with a digital resolution comparable to that obtained for 2D NMR with full spectral information. The improvement in assignment over 2D methods is shown and discussed on the basis of an assignment strategy presented earlier. A simple and straightforward method for determining sugar puckers and γ backbone torsion angles is presented, which is derived from an analysis of cross-peak intensities originating from the TOCSY coherence transfer among sugar protons and HS′/5″ protons. The stereospecific assignment of the HS′/5″ resonances in 3D TOCSY-NOESY spectra is also discussed.     

Fine-structure spectroscopy of 2-methylnaphthalene cooled in a supersonic jet

The fluorescence and fluorescence excitation spectra of 2-methylnaphthalene molecules cooled in a supersonic jet are measured. The frequencies of vibrations in the S ₀ and S ₁ states, as well as the relative intensities of electronic-vibrational transitions in the fluorescence and fluorescence excitation spectra, are calculated with the semiempirical MO/M8ST method. The intensities are calculated in the Franck-Condon approximation taking into account the mixing of all the 38 totally symmetric normal vibrations. Based on the calculations, most observed spectral lines are assigned. It is shown that the calculation accuracy of the method is high enough for it to be used to interpret the spectra of molecules of aromatic compounds such as substituted naphthalenes. It is found that the main contribution to the fluorescence spectrum is made by four optically active vibrations.     

Statistical analysis of spectra of single impurity molecules and dynamics of disordered solids: I. Distributions of linewidths,moments, and cumulants

A review of recent results of studying the low-temperature dynamics of a disordered solid-state medium that were obtained by the method of single molecule spectroscopy (SMS) is presented. The studies were carried out with the use of the new approach of the statistical analysis of a large number of spectra of single molecules embedded into a medium under study as a microscopic spectral probe. The measurements were performed in the range 2–7 K. The use of SMS allowed us to avoid the information losses connected with the averaging over an ensemble of impurity molecules that is inherent in conventional spectral methods. For the example of the system studied (amorphous polyisobutylene doped with molecules of tetra-tert-butylterrylene), it was shown experimentally that, at low temperatures, the spectra of single impurity molecules in disordered media obey the Levy statistics. New microscopic information on the interaction of two-level systems and quasi-local low-frequency vibrational modes of the matrix with impurity molecules in a disordered medium was obtained.     

Spectroscopic Parametrization of the Indo Method Including Vacant Atomic Orbitals in an Extended Basis

An extended basis for a semi-empirical INDO method including wave functions of valent and vacant atomic orbitals is proposed for calculating molecules whose spectra have Rydberg transitions. The parameters for vacant atomic orbitals of oxygen and hydrogen are calibrated against electronic spectra of water clusters. Electronic spectra of water clusters and hydrogen peroxide are examined by the INDO method using the parameters chosen. The results obtained agree reasonably well with the available theoretical and experimental data.     

Trends in the spectral distribution of the absorption coefficient of complex organic molecules

General trends in formation of the observed vibrational spectra are analyzed by calculating spectral distributions of the absorption coefficient in the IR region for 45 molecules of different classes ignoring the electrooptical paramaters of particular bonds and structural groups. It is shown that for many molecules, only vibrations of peripheral X-H bonds make a prevailing contribution to the total integral absorption in a wide spectral range. The effect of increasing intensities of some bands is clearly observed for skeleton vibrations, when optical parameters of external X-H bonds are zero. V. I. Vernadski Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences, 19, Kosygin St., Moscow, 117975, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 491–496, July–August, 1998.     

Polar properties of Langmuir-Blodgett films of copper phthalocyanines

Langmuir-Blodgett films of amphiphilic copper phthalocyanines are obtained. The Stark effect spectra and the spectra of pyro-and photosensitivity of these films are investigated. The following molecular parameters are determined: changes in the polarizability and the dipole moment for individual absorption bands in the Langmuir-Blodgett films and in an isotropic polymer solution. It is shown that the films obtained have photosensitivity due to the charge transfer in the spectral range 600–900 nm. The pyroelectric effect was observed, which indicates that the films have a polar structure.     

Study of the complexation and solvation of polar molecules in binary solvents by differential spectroscopy

A method of application of differential absorption spectroscopy for determining the absorption spectra of primary 1: 1 solvated complexes between polar molecules of an organic dye and the active component of a binary solvent whose neutral component is a nonpolar liquid is proposed. The method was tested on diluted solutions of 4-dimethylaminochalcone (4-DMC) (which is one of the most efficient spectral-luminescent probes used in present-day medical and biological investigations) in mixtures of n-hexane with acetone at extremely small concentrations of the polar component. It is shown that the experimentally found absolute shift of the long-wavelength absorption band of 4-DMC is in satisfactory quantitative agreement with the analogous value obtained independently on the basis of the theory describing the joint effect of nonlinear (complexation) and linear (solvation) dipole-dipole interactions on the spectral band positions.     

Optical properties of cadmium sulfide nanoparticles on the surface of polytetrafluoroethylene nanogranules

The spectral characteristics in the visible range of synthesized nanocomposites based on cadmium sulfide and ultradispersed polytetrafluoroethylene are investigated experimentally. Local perturbations are found in the long-wavelength regions of the reflection and absorption spectra, which are related to the optical transitions between the critical points of the valence and conduction bands of the nanocomposite. The dispersion relations for the refractive index and the absorption coefficient are obtained for undoped and managanesedoped nanocomposites. The following important parameters of the nanocomposites are obtained from the spectral measurements: the fundamental optical absorption edge (the band gap), the refractive index (ω → 0), and the absorption coefficient. It is shown that doping with managanese affects changes in these parameters.     

Spectroscopic method to measure the formation enthalpy of self-association of indolic molecules in non-polar solvents

Formation enthalpy of self-association of indoles in non-polar solvents is obtained from the behaviour of their fluorescence spectra areas with temperature. Formation enthalpy of skatole self-association thus obtained is in agreement with that found by another method. This supports the hypothesis that the spectral anomalies observed are due to self-association. The method can be extended to other molecules and it is independent of the spectral structure.