Electronic structure calculations, thiophene

The first attempt to give a description of the electronic structure of thiophene including all valence electrons was made by Clark <1968T2663>. He used the complete neglect of differential overlap CNDO/2 method of Pople, Santry, and Segal, modified in order to include 3d-, 4s-, and 4p-AOs for sulfur in the basis set. The results showed a rather large participation of the 3d-orbitals in the sulfur-carbon bond. The calculated population was 0.24 and 0.14 for 3d and 3d,t. respectively. Calculation of the dipole moment made it clear that the effect of the 3d-orbitals was overestimated by the CNDO/2 method. 

The 3dxz- and 3dyz-AOs on sulfur are included in the linear combination of atomic orbitals in a semi-empirical SCF MO study of thiophene. The extension of the SCF MO method to include more than one atomic orbital per atomic site is accomplished by a point-charge model for the evaluation of two-center repulsion integrals. A comparison of the SCF molecular orbitals with and without the inclusion of these higher atomic orbitals shows that the d orbitals participate in the 7t-electronic structure of thiophene to only a small extent, but that their participation affects the calculated electronic properties to a great extent <1966JA4804>. 

The NICS of each ring, as a criterion of aromaticity, has been used to explain the stability order of benzo[/)]thio-phene and its isomer. The results indicate that the benzene ring is aromatic in all the systems. The five-membered ring of benzo[. ]thiophene is also aromatic, whereas in benzo[r]thiophene it is nonaromatic. This could be an explanation of the stability of the former molecule. The MOS and the condensed Fukui functions derived from the electronic-structure calculations explain the expected electrophilic substitution of these compounds. The theoretical structure, ionization energies, order of aromaticity, stability, and reactivity are in good agreement with the experimental results <2003T6415>. 

All calculations on the electronic structures of thiophene are performed using the GAMESS program [122]. The calculations are based on the... 

The results of geometry optimization and electronic structure calculations for a class of thiophene copolymers, which were synthesized [175,176], with mixed aromatic/quinoid character are reported [177], The orbital charges on their aromatic/quinoid segments are calculated and a relation is offered between them and the energy dispersion of the orbital. Acceptable localization properties are predicted for the HOMO and LUMO in the copolymers with longer quinoid sequences. 

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

Theoretical calculations using the semiempirical CNDO/2 method including d- orbital participation in (Id X = O, NH, S Y = S) predicted singlet ground states for these systems as well as substantial n- bond character between the S—C bonds arising from dir-pTr overlap, and very little double bond character in the central C—C bond (74JA1817). There is also considerable contribution to the electronic structure from dipolar canonical forms in (Id X = O, NH Y = S) compared to thieno[3,4-c]thiophene (9) as evidenced by the respective calculated dipole moments of 0.15 and 3.21 D for the former and 0.00 D for the latter. 

Several calculations have been made of the electronic structure of benzo[c]thiophene.11-15 The earliest calculation11 based on the... 

Although the molecular structure of thiophene has been actively investigated, there have not been many reports on those of the methyl derivatives. The molecular structure of 2,5-dimethylthiophene has been determined by gas electron diffraction (GED) and the results compared with ab initio calculations carried out at the 3-21G" level. 

Poly(thieno[3,4-h]thiophene), like the monomers, was the last of the thienothiophenes to be synthesized. Hong and Marynick [28] had obtained the electronic structures of poly(thieno[3,4-h]thiophene) using modified extended-Huckel band calculations and predicted that PT34bT connected through 4-and 6-positions will have a bandgap of 1.5-1.6 eV (tt-tt, comparable to that of polyacetylene. In... 

This chapter has summarized some of the recent trends in thiophene polymers. The key factors determining the bandgap are connectivity, bond alternation and the effect of conjugation over the sulfur n-centers. Copolymers frequently have electronic structures that are not intermediate between those of the component repeat units. Calculations can help to select systems for synthetic targets whether the aim is a small bandgap or a particular specific bandgap value. Naturally, one has to bear in mind that the theoretical calculations refer to periodic, defect-free and very long polymers and thus the actual polymers may differ in essential ways from this ideal. Further uncertainty results from the usually unknown interchain overlap, which tends... 

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