The UV-visible absorption spectra of [Ru(E)(E')(CO)(2)(iPr-DAB)] (E = E' = SnPh(3) or Cl; E = SnPh(3) or Cl, E' = CH(3); iPr-DAB = N,N'-di-isopropyl-1,4-diaza-1,3-butadiene) are investigated using CASSCF/CASPT2 and TD-DFT calculations on model complexes [Ru(E)(E')(CO)(2)(Me-DAB)] (E = E' = SnH(3) or Cl; E = SnH(3) or Cl, E' = CH(3); Me-DAB = N,N'-dimethyl-1,4-diaza-1,3-butadiene). The calculated transition energies and oscillator strengths allow an unambiguous assignment of the spectra of the nonhalide complexes [Ru(SnPh(3))(2)(CO)(2)(iPr-DAB)] and [Ru(SnPh(3))(Me)(CO)(2)(iPr-DAB)]. The agreement between the CASSCF/CASPT2 and TD-DFT approaches is remarkably good in the case of these nonhalide complexes. The lowest-energy part of the spectrum (visible absorption) originates in electronic transitions that correspond to excitations from the axial E-Ru-E' sigma(2) orbital into the low-lying pi(DAB) orbital (sigma-bond-to-ligand charge transfer, SBLCT, transitions), while the absorption between 25 000 and 35 000 cm(-1) is due to metal-to-ligand charge transfer (MLCT) excitations from the 4d(Ru) orbitals to pi(DAB) (MLCT). Above 35 000 cm(-1), the transitions mostly correspond to MLCT and SBLCT excitations into pi(CO) orbitals. Analysis of the occupied sigma orbitals involved in electronic transitions of the nonhalide complexes shows that the Kohn-Sham orbitals are generally more delocalized than their CASSCF/CASPT2 counterparts. The CASSCF/CASPT2 and TD-DFT approaches lead to different descriptions of electronic transitions of the halide complexes [Ru(Cl)(2)(CO)(2)(Me-DAB)] and [Ru(Cl)(Me)(CO)(2)(Me-DAB)]. CASSCF/CASPT2 reproduces well the observed blue-shift of the lowest absorption band on going from the nonhalide to halide complexes. TD-DFT systematically underestimates the transition energies of these complexes, although it reproduces the general spectral features. The CASSCF/CASPT2 and TD-DFT techniques differ significantly in their assessment of the chloride contribution. Thus, CASSCF/CASPT2 assigns the lowest-energy absorption to predominantly Ru --> DAB MLCT transitions, while TD-DFT predicts a mixed XLCT/MLCT character, with the XLCT component being predominant. (XLCT stands for halide (X)-to-ligand-charge transfer.) Analysis of Kohn-Sham orbitals shows a very important 3p(Cl) admixture into the high-lying occupied orbitals, in contrast to the CASSCF/CASSPT2 molecular orbitals which are nearly pure 4d(Ru) with the usual contribution of the back-donation to pi(CO) orbitals. Further dramatic differences were found between characters of the occupied sigma orbitals, as calculated by CASSCF/CASPT2 and DFT. They differ even in their bonding character with respect to the axial E-Ru and Cl-Ru bonds. These differences are attributed to a drawback of the DFT technique with respect to the dynamical correlation effects which become very important in complexes with a polar Ru-Cl bond. Similar differences in the CASSCF/CASPT2 and TD-DFT descriptions of the lowest allowed transition of [Ru(Cl)(2)(CO)(2)(Me-DAB)] and [Ru(Cl)...

The second-order nonlinear optical properties of helicenes and phenylenes have been theoretically investigated at the time-dependent Hartree-Fock level using the Austin model 1 semiempirical Hamiltonian. Both the antisymmetric isotropic component of the first hyperpolarizability (beta) and its projection on the dipole moment (beta(parallel)) have been determined for increasingly large helical systems as well as for their analogs substituted by donor/acceptor pairs. It is found that (i) in nonsubstituted helicenes and phenylenes, beta increases monotonically with the size of the system and slightly depends on the nature of the helix; (ii) the corresponding beta(parallel) is mostly determined by the radial component of the first hyperpolarizability vector; (iii) in helicenes, beta(parallel) is positive and presents quasiperiodic oscillations with the helix; (iv) in phenylenes, beta(parallel) depends upon the size of the helix and it can be either positive or negative as a result of the differences in evolution with N of the radial components of the dipole moment and first hyperpolarizability. Substituting the helicenes and phenylenes by the prototypical NH2/NO2 donor/acceptor pair provides a diversity of effects on beta and beta(parallel) that encompasses decrease, increase, and change in sign.

SF5CF3, recently found in the Earth's atmosphere, is an extremely potent greenhouse gas. Its behaviour under irradiation, as in the upper atmosphere, is of great importance for the possible impact on the global climate. The vertical absorption spectrum of the title compound is studied by various single- and multi-reference ab initio methods and the results are compared to experiments. The best results for valence states are obtained by the multi-state complete active space second order perturbation theory (MS-CASPT2) approach. In contrast, the popular time-dependent density functional theory (TDDFT) gives very poor results. Multi-reference configuration interaction (MRCI) does not yield very accurate energies because only a limited number of valence electrons can be included in the correlation treatment. The Rydberg states are calculated very accurately and efficiently by a frozen-core multi-configuration self-consistent field (FC-MCSCF) method. The accuracy is assessed by quantum defect theory and the experimental and ab initio calculated first ionization potential. The photoelectron spectrum is investigated by the outer-valence Green's functions (OVGF) method. The simulated spectrum is in excellent agreement with a recent experiment.