The vibrational Raman spectra of a decathiophene are provided in three relevant oxidations states: for the radical cation, its class III mixed valence system and its "frozen, -170 degrees C" class II MV analogue; for the dication, its singlet biradical pi-dimer and its "hot, +70 degrees C" magnetically active triplet excited state. Everything is compatible with interpentathiophene charge delocalization occurring at distances of 4-5 A similar to those found in the crystals of pi-stacked oligothiophenes. This stresses the interest of this spectroscopic tool for the analyses of electronic processes in crystals or in thin films of conjugated organic molecules.

The distinct role of the Cu(I) in the Huisgen dipolar cycloaddition of azides to alkynes (denoted as CuAAC) is disclosed by following the evolution of the topology of the Laplacian of the electronic charge density, ∇(2)ρ(r), and its gradient vector field, ∇∇(2)ρ(r), along the reaction coordinate with several density functionals (wB97XD, LCwPBE, M06-2X, M06-L, B3LYP) and the 6-311++G(d,p) basis set. Remarkably, in view of the topology of ∇(2)ρ(r) and ∇∇(2)ρ(r), the mechanism appears to be diverse (asynchronous concerted or stepwise) depending on the reaction conditions. Overall, the catalyst orchestrates first the formation of the external N-C and subsequently the internal one by following alternatively a pericyclic-like or a pseudopericyclic-like mechanism. The role of the catalyst is envisaged as transforming the type of the mechanism from pericyclic to pseudopericyclic, and thence eventually facilitating the process. The mononuclear process (CuAAC) is concerted (with L = CH3CN) with all the functionals tested (i.e., wB97XD, LCwPBE, M06-2X, M06-L), except for the B3LYP who rendered a stepwise mechanism. Nevertheless, with L = H2O and CH3OH attached to the copper, the process becomes asynchronous concerted. Interestingly, upon introduction of the second Cu (Cu2AAC) at our best theory level (i.e., LCwPBE/6-311++G(d,p)), all the processes considered turned out to be concerted except for the 1,4-Cu2AAC which is predicted to be stepwise, with an extremely low enthalpy for the ring-contraction process (0.18 kcal/mol). This fact is explicated by the stability of the intermediate, which is in turn rationalized by the hole within the valence shell of the carbon attached to the Cu and the position of the (3, -3)LP, of the internal N, toward it. Furthermore, due to the tiny energy difference between the stepwise dinuclear and concerted mononuclear mechanisms (0.39 kcal/mol), we argue that the concurrence of both processes (CuAAC and Cu2AAC) is feasible.

The pseudopericyclic character of a group of cycloaddition reactions is theoretically investigated with the quantum theory of atoms in molecules of Bader at B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory. The analysis of the topology of the Laplacian of the charge density, inverted Delta2rho(r), along the reaction coordinate sets out a clear cut between pericyclic and pseudopericyclic processes and also allows one to account for intermediate cases.

In this work, the interactions between heteroatoms (S, Se, and Te) and conjugated skeletons are analyzed. The study is carried out by using electronic absorption and fluorescence spectroscopies, electrochemistry, vibrational Raman spectroscopy, and theoretical calculations in the framework of DFT and TD-DFT theories. Optical spectra are described in terms of one-electron promotions between orbitals around the energy gap. Electrochemistry, in the framework of the Koopman's approach, is also interpreted. The vibrational Raman spectra are assigned to molecular modes and the evolution changing the heteroatom is addressed and an effective tuning of these properties is found. Part of this modulation is associated with local electronic interactions depending on the relative S, Se, and Te electronegativities. Unconventional long-range heteroatom-heteroatom interactions have been proposed which arise from the existence of effective pi-conjugated channels. The molecular level understanding of structure-property relationships in these organic/inorganic semiconductors are of great interest in the interdisciplinary area of material science.