Energies Of Excited Electronic States Research Articles

Bioconcentration Factor of Polychlorinated Biphenyls and Its Correlation with UV- and IR-Spectroscopic data: A DFT based Study

Polychlorinated biphenyls (PCBs) are important class of persist organic pollutants that were used as a component of paints especially in printings, as plastificator of plastics and insulating materials in transformers and capacitors, heat transfer fluids, additives in hydraulic fluids in vacuum and turbine pumps. There is always a need to establish reliable procedures for predicting the bioconcentration potential of chemicals from the knowledge of their molecular structure, or from readily measurable properties of the substance. Hence, correlation and prediction of bio-concentration factors (BCFs) based on λmax and vibration frequencies of various bonds viz υ(C-H) and υ(C=C) of biphenyl and its fifty-seven derivatives have been made. For the study, the molecular modeling and geometry optimization of the PCBs have been performed on workspace program of CAChe Pro 5.04 software of Fujitsu using DFT method. UV-visible spectra for each compound were created by electron transition between molecular orbitals as electromagnetic radiation in the visible and ultraviolet (UV-visible) region is absorbed by the molecule. The energies of excited electronic states were computed quantum mechanically. IR spectra of transitions for each compound were created by coordinated motions of the atoms as electromagnetic radiation in the infrared region is absorbed by the molecule. The force necessary to distort the molecule was computed quantum mechanically from its equilibrium geometry and thus frequency of vibrational transitions was predicted. Project Leader Program associated with CAChe has been used for multiple linear regression (MLR) analysis using above spectroscopic data as independent variables and BCFs of PCBs as dependent variables. The reliability of correlation and predicting ability of the MLR equations (models) are judged by R2, R2adj, se, q2L10O and F values. This study reflected clearly that UV and IR spectroscopic data can be used to predict BCFs of a large number of related compounds within limited time without any difficulty.

A theoretical study on the spectroscopy, structure, and stability of C2H3NS molecules

Here we report the results of a theoretical study devoted to the family of methyl thiocyanate (CH3–SCN) isomers. From among 14 species sharing the C2H3NS stoichiometry, the most thermodynamically stable of these are methyl isothiocyanate (CH3–NCS), methyl thiocyanate (CH3–SCN), and mercaptoacetonitrile (HS–CH2–CN). Energies were reliably predicted using the CCSD(T) variant of coupled-cluster calculations making use of a quadruple zeta-quality basis set. Minor contributions to the total energy, including scalar relativistic effects and extrapolation to the complete basis set limit, were obtained using second-order many-body perturbation theory. The three most stable isomers feature similar energy values (differing by few kJ/mol) that are much lower than those of the remaining C2H3NS species (more than 85 kJ/mol). Spectroscopic properties including rotational constants, anharmonic vibrational frequencies, infrared absorption intensities (harmonic), Raman activities, and the energies of excited electronic states have been derived using coupled-cluster or density functional theory for the whole set of C2H3NS molecules. Additionally, infrared absorption intensities and frequencies of overtone and combination bands are given for the three lowest energy isomers.

The Single-Determinant Approximation with a Local Potential for Excited States

The specific features of the calculations of the electronic structure in the approximation of a local exchange potential that is identical for all the electrons involved are considered. An optimized effective potential method is proposed for calculating the energies of excited electronic states of the same symmetry. A single-particle Schro dinger equation is derived for an excited state whose orbitals are described by a single-determinant wave function orthogonal to the ground state. The equations determining the local potential for excited states are obtained within the variational approach. The solution to these equations is analyzed in the framework of the parameterized representation of the effective potential. The efficiency of the proposed method is demonstrated by calculating the energies of three excited states of the same symmetry for a HeH molecule. The difference between the results obtained by the Hartree-Fock method and the method proposed in this paper is equal, on average, to 0.05%. A comparison with the results obtained from precise calculations based on the configuration interaction method shows that the accuracy in determining the energy of the excited states by the optimized effective potential method is comparable to the accuracy in calculating the energy of the ground state.

An approximate ab initio method based on the DIM model

Using a second quantized formulation, an approximate diatomics in molecules (DIM) theory is presented in which all three- and four-centered electronic integrals are neglected. To ameliorate the effects of this approximation, the DIM one electron operator is constructed so that the true ab initio first-order density matrix and total energy are reproduced at the Hartree–Fock level. The resulting model was extensively tested for a variety of basis sets for its capability of capturing both the dynamic and nondynamic components of the electron correlation energy as well as the energies of excited electronic states. A modified method in which the DIM one-electron operator is formed from the initial extended Hückel guess of the Hartree–Fock orbitals was also found to produce excellent results.Key words: DIM, electron correlation energy, excited states, semiempirical.

Photoionisation mass-spectrometric study of fragmentation of SiBr4and GeBr4in the range 400–1220 Å

The non-radiative decay channels of the valence electronic states of SiBr+4 and GeBr+4 have been studied in the range 1220–400 A(10–31 eV) by photoionisation mass spectrometry. Ion-yield curves for the parent ions and for MBr+3, MBr+2, MBr+, M+ and Br+(M = Si, Ge) have been obtained, as well as the relative photoionisation branching ratios. The appearance thresholds for SiBr+3 and GeBr+3 occur at 11.31 and 10.97 eV, respectively. They lie within the Franck–Condon region of the ground state of SiBr+4 and GeBr+4, and are at the thermodynamic thresholds for SiBr+3+ Br and GeBr+3+ Br. The smaller fragment ions have appearance thresholds which relate to energies of excited electronic states of SiBr+4, and GeBr+4, and not to the lower-lying thermodynamic energy of the fragment ion. The results are discussed with reference to our earlier work on radiative decay from excited states of SiBr+4 and GeBr+4(J. Chem. Soc., Faraday Trans., 1990, 86, 2021). We have obtained a new value for the ionisation potential of SiBr3 of 7.6 ± 0.4 eV, and we suggest that the previously accepted value for SiBr2(12 ± 1 eV) is ca. 3.5 eV too high.

Induced birefringence as a method for studying the electronic transitions and polarizability of molecules in excited electronic states

The measurement of excited state polarizabilities and the determination of both the symmetries and energies of excited electronic states using an induced birefringence method is described. This technique is applied to study the triplet states of naphthalene - d 8.