V. I. Baranov

Parametric method in the theory of vibronic spectra of polyatomic molecules: Absorption and fluorescence spectra and structure of styrene in the excited state

The excited states and absorption and fluorescence spectra of styrene and β-d2-styrene are calculated by the parametric method. It is shown that parametric models of these molecules in the excited states adequately describe their real structure and predict the fine structure of their electronic spectra at the quantitative level, which is sufficient for its detailed interpretation and the refinement of parameters of the molecular model by solving inverse problems. In a second approximation (using only one additional parameter), the method provides a more exact calculation of angular deformations of the molecule upon excitation and, in particular, describes characteristic differences in the intensities of low-frequency spectral components in a series of diphenylpolyene-stilbene-styrene. The number of parameters for molecular fragments is small (2 and 3 in the first and second approximations, respectively), and they are the same for a series of related compounds. Compared to direct ab initio calculations, the parametric method yields substantially better results already in the first approximation, and it is more efficient for modeling molecules in excited states and description of their vibronic spectra.

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.

Modeling vibronic spectra and excited states of polyenes with a parametric method

The structural-dynamic models of excited states and vibronic structure of absorption spectra of linear polyenes (R-(CH=CH)n-R, R=H, CH3, n=4, 5, 7) are calculated using the parametric method of the theory of vibronic spectra of such molecules. Good agreement is obtained between the calculated and experimental spectra. It is shown that the system of parameters of the method for the polyenes has a high degree of transferability in the series of related polyenes. The constructed models adequately reflect the real structure of the molecules in excited states and allow one to predict quantitatively the fine vibrational structure of the spectra, including relatively weak effects related to the methyl substitution. The dynamic of structural changes of the molecules upon their excitation is studied in the series of polyenes.

Modeling of the Time-Resolved Vibronic Spectra of Polyatomic Molecules: The Formulation of the Problem and Analysis of Kinetic Equations

A semiempirical parametric method is proposed for modeling three-dimensional (time-resolved) vibronic spectra of polyatomic molecules. The method is based on the use of the fragment approach in the formation of molecular models for excited electronic states and parametrization of these molecular fragments by modeling conventional (one-dimensional) absorption and fluorescence spectra of polyatomic molecules. All matrix elements that are required for calculation of the spectra can be found by the methods developed. The time dependencies of the populations of a great number (>10^3) of vibronic levels can be most conveniently found by using the iterative numerical method of integration of kinetic equations. Convenient numerical algorithms and specialized software for PC are developed. Computer experiments showed the possibility of the real-time modeling of three-dimensional spectra of polyatomic molecules containing several tens of atoms.A semiempirical parametric method is proposed for modeling three-dimensional (time-resolved) vibronic spectra of polyatomic molecules. The method is based on the use of the fragment approach in the formation of molecular models for excited electronic states and parametrization of these molecular fragments by modeling conventional (one-dimensional) absorption and fluorescence spectra of polyatomic molecules. All matrix elements that are required for calculations of the spectra can be found by the methods developed. The time dependences of the populations of a great number (>103) of vibronic levels can be most conveniently found by using the iterative numerical method of integration of kinetic equations. Convenient numerical algorithms and specialized software for PC are developed. Computer experiments showed the possibility of the real-time modeling three-dimensional spectra of polyatomic molecules containing several tens of atoms.

Calculation and analysis of the excited states and fine structure of vibronic spectra of anthracene, anthracene- d 10 , and tetracene

Structural dynamic models of the excited states of some molecules of the acene series (anthracene, anthracene-d10, and tetracene) have been developed and the vibronic structure of the fluorescence and absorption spectra of these molecules has been calculated by the parametric method of the theory of vibronic spectra. Good agreement is obtained between the calculated and experimental spectra. It is shown that the system of parameters determined for these acenes in the second-order approximation of the method is highly transferable in the acene series. The models proposed adequately describe the real structure of the excited molecules and make it possible to predict quantitatively the fine vibrational structure of the spectra, including relatively weak effects (weak bands, the spectral variations in the series of these molecules, and the variations upon deuterium substitution). The specific features of the structural variations in the investigated acene molecules upon their excitation and the dynamics of these variations in the series of these molecules are studied.

Analysis of Electronic-Vibrational Spectra of Uracil, Thymine, and Cytosine

A theoretical analysis of absorption spectra of uracil, thymine, and cytosine—nucleic acid bases— is carried out. Structural dynamic models of these molecules in their electronically excited states are constructed. On the basis of the calculated vibrational structure of the electronic spectra, different tautomeric forms of these molecules are determined. The possibility of modeling the influence of hydrogen bonds on the electronic-vibrational spectra is shown.

Interpretation of the vibrational spectra of polycrystalline adenine

The infrared and Raman spectra of the tetramer of the adenine N9H are calculated and analyzed. The vibrational spectra of polycrystalline adenine are interpreted. It is demonstrated that the method for calculating the vibrational spectra of molecular complexes formed by hydrogen bonds can be used for interpreting the vibrational spectra of polyatomic molecules in the solid state.

Theoretical analysis of the tautomer composition of cytosine isolated in an Ar matrix

The IR spectra of cytosine and its five tautomers are calculated and analyzed in the B3LYP/6-31G(d) approximation. Comparison of calculated and experimental spectra showed that, in the isolated state, there occur four tautomeric forms, cis, trans, trans-amino-hydroxy, and cis-imino-oxo, as well as one canonical form of cytosine.

Modeling of the vibrational structure of the vibronic spectra of diphenylpolyenes by the parametric method

The structures of the electronic-vibrational spectra and of the excited states of a number of diphenylpolyene molecules are determined within the framework of the second approximation of the parametric method. The system of parameters of the structural fragments of molecules is improved and good agreement with spectral experiment is obtained. It is shown that there is a high degree of transferability of the polyene and acene parameters of the method and that the models obtained are adequate to the real structure of molecules. It is also shown that the method proposed makes it possible to perform predictive qualitative and quantitative calculations of the spectra of these molecules, as well as of the spectral characteristics necessary for modeling photochemical molecular transformations. In the series of diphenylpolyene molecules, an interpretation of the vibrational structure of the spectra is proposed and the specific features of variation of the geometry upon excitation of molecules are considered.

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.