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Bond-length Alternation (bond-length + alternation)
Selected AbstractsOligo- and Polyselenophenes: A Theoretical StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 34 2009Sanjio Abstract Recently, a family of conducting polyselenophenes was synthesized, and they were shown to have a number of interesting properties. Here we have studied oligoselenophenes, their cation radicals and dications up to the 50-mer (50,Se), as well as polyselenophene at the B3LYP/6-31G(d) level of theory, and compared them with the corresponding oligothiophenes. Although the calculations reveal many similarities between oligo- and polyselenophenes and their thiophene-based counterparts, they also show the important differences between those two types of conjugated systems. Oligo- and polyselenophenes have a more quinoid character, lower band gap, and importantly, they are more difficult to twist. The theoretical results suggest that the HOMO,LUMO gap (band gap), bond-length alternation (BLA), and charge distribution in oligo- and polyselenophenes are strongly dependent on inter-ring twisting, yet twisting costs little energy. The inter-ring distances in oligo- and polyselenophenes are shorter than the related distances in oligothiophenes, whereas the bond lengths within the selenophene rings are comparable to those of the corresponding oligothiophenes. [source] Tuning of Electronic Properties in Thienyl-Phosphole ,-Conjugated Systems through P-Functionalization Monitored by Raman SpectroscopyCHEMISTRY - A EUROPEAN JOURNAL, Issue 14 2006Juan Casado Dr. Abstract Herein, a Raman spectroscopic study of a new family of 2,5-di(2-thienyl)phospholes and thienyl-capped 1,1,-diphospholes is presented. The Raman spectra have been carefully assigned with the help of density functional calculations. For di(2-thienyl)phospholes, two well-differentiated groups of Raman bands exist that arise either from the central phosphole ring or from the outer thiophene substituents. These data reveal a segmentation of the electronic structure. This paper reports interesting relationships between geometrical data such as the BLA (bond-length alternation) parameter and Raman band wavenumbers. These correlations are unprecedented in the chemistry of phospholes and have been used to interpret the evolution of the electronic structure (aromaticity,,-conjugation) upon 1) substitution of the central sulfur atom of terthiophene by phosphorus and 2) P-functionalization. Increasing the coordination number of the phosphole ring results in intramolecular charge transfer. The best scenario for phosphole aromaticity is found for 1,1,-diphospholes. [source] Solvent and Protein Effects on the Structure and Dynamics of the Rhodopsin ChromophoreCHEMPHYSCHEM, Issue 9 2005Ute F. Röhrig Dr. Abstract The structure and dynamics of the retinal chromophore of rhodopsin are investigated systematically in different environments (vacuum, methanol solution, and protein binding pocket) and with different computational approaches (classical, quantum, and hybrid quantum mechanics/molecular mechanics (QM/MM) descriptions). Finite temperature effects are taken into account by molecular dynamics simulations. The different components that determine the structure and dynamics of the chromophore in the protein are dissected, both in the dark state and in the early photointermediates. In vacuum and in solution the chromophore displays a very high flexibility, which is significantly reduced by the protein environment. In the 11- cis chromophore, the bond-length alternation, which is correlated with the dipole moment, is found to be similar in solution and in the protein, while it differs greatly with respect to minimum-energy vacuum structures. In the model of the earliest protein photointermediate, the highly twisted chromophore shows a very reduced bond-length alternation. [source] | ||||||||||