Transition energies and intensities

Calculations have been made, first by a semiempirical treatment due to Parisier and Parr, and to Pople, and then by a simplified version of this method, of the transition energies and intensities of the 77— 77 bands in pyridopyrimidines (cf. Table I). 

Firstly, we focus on cofacial dimers formed by stilbene molecules in such conformations, the amplitude of interchain interactions is expected to be maximized [57], Table 4-1 collects the INDO/SCl-calculated transition energies and intensities of the lowest two excited states of stilbene dimers for an interchain distance ranging from 30 to 3.5 A. 

Table 5. Core level binding energies, and transition energies and intensities for low energy shake up structures in poly para-substituted styrenes...

A number of correlations have been made, with moderate success, between the transition energies and intensities in the ultraviolet (UV)... 

If one or more spin-coupling terms also occur (e.g. hyperfine or nuclear spin-spin), then each such spin-Hamiltonian parameter matrix Y occurs in the transition energy and intensity expressions within combinations of type... 

F nmr spectra have been reported for tetrafluoro-l,3-dithietane in solution with nematic liquid crystals D2h symmetry is imputed. " Theoretical calculations of the transition energies and intensities of three different tetraalkyl-1,3-dithietanes indicate a need for the inclusion of d orbitals. Theoretical calculations also have been made on a dimer of 1,3-dithietane. The UV spectrum of 2,2,4,4-tetrachloro-1,3-dithietane has been interpreted with the aid of molecular orbital calculations, interaction between the two sulfur atoms is suggested. The photoelectron spectra of tetrafluoro and tetrachloro-l,3-dithietane and 1,3-dithietane itself have been reported. The negative-ion mass spectrum of tetra-fluoro-1,3-dithietane has a base peak corresponding to fluoride ion the base peak of tetra(trifluoromethyl)-l,3-dithietane corresponds to the anion of hexafluoro-thioacetone. The oxidation potential of 1,3-dithietane does not show any unusual transannular interactions in the cation radical. ... 

The Pariser-Parr-Pople method, self-consistent molecular orbital calculations, and the Huckel approach have been applied to the theoretical calculation of transition energies and intensities in the ultraviolet spectra of pyrido[2,3-ft]pyrazine and other azanaphthalenes. Good agreement with experimental data was obtained. The infrared spectra of the parent heterocycle" and its 8-hydroxy derivative have been discussed the 0X0 form is preferred to the hydroxy form both in chloroform solution and in the solid state. As with other a-hydroxyazines, the oxo form is also favored in the 2-, 3-, and 6-hydroxy compounds. Ultraviolet spectra have been measured for the parent heterocycle and various substituted compounds. The spectrum of the parent is substantially similar to other azanaphthalenes. In ethanol the principal bands are at ca. 260,300, and 350 nm. Both the infrared and ultraviolet spectra of about 60 derivatives of the ring system have been assembled. ... 

Molecular orbital calculations have been performed on pyrido[3,4-fojpyrazine using several approaches. Along with pyrido[2,3- )]pyrazine and other azanaphthalenes, these calculations provide good estimates of the transition energies and intensities of the ultraviolet spectrum of the parent heterocycle. The infrared spectra of the parent and other azanaphthalenes have been determined and discussed. Infrared data for several a- and y-hydroxy compounds confirm their existence, in solution and in the solid state, as oxo compounds. The visible and ultraviolet spectrum of the parent has been analyzed. In 95% ethanol the principal peaks are at 232 and 309 nm. The spectrum is substantially the same as that of quinoxaline, and analogous derivatives of the two heterocycles also possess similar spectra. Comparative ultraviolet data have been used to differentiate between amino derivatives of pyrido[3,4-b]pyrazine and pyrido[2,3-fc]pyrazine and between isomeric dihydro and tetrahydro derivatives of the [3,4-b]system. PMR data are available for certain... 

The accurate prediction of electronic transition energies and intensities remains a challenging area of quantum chemistry. Many of the early successes of MO theory involved qualitative interpretations of electronic spectra. The development of the Huckel model made semiquantitative calculations for conjugated organic molecules possible for the first time. The crystal field model (see Chapter 2.35) was successfully applied to calculate d-d transitions in TM complexes. 

Another important method to calculate transition energies and intensities is time-dependent density functional theory (TDDFT) (see Chapter 2.40). 

In brief, then, the evidence indicates that reasonably accurate atomic ionization energies can be obtained by high-quality SCF calculations on the neutral and ionized species (ASCF, not —6), but that transition energies and intensities require Cl sufficient to account for much of the valence electron correlation. 

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