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A Theoretical Study on the Low-Lying Excited States of 2,2′:5′,2′′-Terthiophene and 2,2′:5′,2′′:5′′,2′′′-Quaterthiophene
2005
The nature and properties of the low-lying singlet and triplet valence excited states of 2,2':5',2-terthiophene (terthiophene) and 2,2':5',2:5,2'-quictterthiophene (tetrdthiophene) are discussed on the basis of high-level ab initio. computations. The spectrascopic features determined experimentally for short α-oligothiophenes are rationalised on theoretical grounds. Special attention is devoted to the nonradiative decay process through intersystem crossing (ISC) from the singlet to the triplet manifold, which is known to be relatively less efficient in tetrathiophene. Along the geometry relaxation of the S 1 state of terthiophene, the S' 1 and T 2 states become degenerate, which leads. to a…
A theoretical determination of the electronic spectrum of formaldehyde
1995
The electronically excited states of formaldehyde are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis sets. The calculations comprised five valence excited states plus all singlet 3s, 3p, and 3d members of the Rydberg series converging on the first ionization. The computed vertical excitation energies were found to be within 0.2 eV of the available experimental energies. Full geometry optimization has been performed for five valence excited states. Assuming a planar geometry, the “0-0” transition for the valence1A1(π → π*) state is calculated to appear near 7.9 eV, close to the (n y → 3p) region. This state is, however, not pl…
Theoretical study of the electronic spectrum of p-benzoquinone
1999
The electronic excited states of p-benzoquinone have been studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The calculation of the singlet–singlet and singlet–triplet transition energies comprises 19 valence singlet excited states, 4 valence triplet states, and the singlet 3s,3p, and 3d members of the Rydberg series converging to the first four ionization limits. The computed vertical excitation energies are found to be in agreement with the available experimental data. Conclusive assignments to both valence and Rydberg states have been performed. The main features of the electronic spectrum correspond to the π…
Theoretical Study of the Electronic Spectrum of Imidazole
1996
The complete active space (CAS) self-consistent field (SCF) method and multireference second-order perturbation theory (CASPT2) have been used to study the electronic spectrum of imidazole and the imidazolium ion. The calculations comprise a large number of, both singlet and triplet, valence and Rydberg excited states. A newly developed continuum model has been used to compute solvatochromic shifts. In the gas phase the first and second π → π* excited singlet valence states of imidazole are computed at 6.72 and 7.15 eV, and they shift to 6.32 and 6.53 eV upon solvation. The gas-phase values are somewhat too large (≈0.3 eV) due to an erroneous valence−Rydberg mixing in the CASSCF wave functi…
Structure and Properties of YbZnSn, YbAgSn, and Yb2Pt2Pb
1999
Abstract YbZnSn, YbAgSn, and Yb2Pt2Pb were synthesized by reacting the elements in sealed tantalum tubes in a high-frequency furnace. The structures of YbAgSn and Yb2Pt2Pb were refined from single crystal X-ray data: YbAgPb type, P 6 m2, a=479.2(2) pm, c=1087.3(3) pm, wR2=0.050, BASF=0.34(8), 509 F2 values, 18 variables for YbAgSn and Er2Au2Sn type, a=776.0(1) pm, c=701.8(2) pm, wR2=0.072, 426 F2 values, 18 variables for Yb2Pt2Pb. The lattice constants of YbZnSn are confirmed: NdPtSb type, P63mc, a=464.7(1) pm, c=747.7(2) pm. The stannides YbZnSn and YbAgSn crystallize with superstructures of the AlB2 type. The zinc (silver) and tin atoms form ordered Zn3Sn3 and Ag3Sn3 hexagons, respectivel…
New alkoxo-bridged mixed-valence cobalt clusters: Synthesis, crystal structures and magnetic properties
2008
Two new Co II /Co III complexes, [{Co II Co III (mea) 3 } 2 (bpe) 3 ](ClO 4 ) 4 · 1.5CH 3 OH · 1.5H 2 O ( 1 ) and [ Co 4 II Co 3 III ( dea ) 6 ( CH 3 COO ) 3 ] ( ClO 4 ) 0.75 ( CH 3 COO ) 1.25 · 0.5 H 2 O ( 2 ) [Hmea = monoethanolamine H 2 dea = diethanolamine and bpe = 1,2-bis(4-pyridyl)ethane], have been obtained by reacting cobalt(II) perchlorate ( 1 and 2 ), Hmea ( 1 )/H 2 dea ( 2 ), bpe ( 1 ) and sodium acetate ( 2 ). The crystal structures of 1 and 2 have been solved by single crystal X-ray diffraction. Crystal 1 contains “Chinese lantern”-like shaped cations, resulting by connecting two {Co II Co III (mea) 3 } moieties with three flexible bpe ligands. The coordination sphere of the …
A theoretical study of the electronic spectrum of bithiophene
1995
The electronic spectrum of bithiophene in the energy range up to 6.0 eV has been studied using multiconfigurational second order perturbation theory (CASPT2) and a basis set of ANO type, with split valence quality and including polarization functions on all heavy atoms. Calculations were performed at a planar (trans) and twisted geometry. The calculated ordering of the excited singlet states is 1Bu, 1Bu, 1Ag, 1Ag, and 1Bu with 0–0 transition energies: 3.88, 4.15, 4.40, 4.71, and 5.53 eV, respectively. The first Rydberg transition (3s) has been found at 5.27 eV. The results have been used in aiding the interpretation of the experimental spectra, and in cases where a direct comparison is poss…
Theoretical study of the electronic spectra ofcis-1,3,5-hexatriene andcis-1,3-butadiene
1994
The electronic spectra forcis-1,3-butadiene andcis-1,3,5-hexatriene have been studied using multiconfiguration second-order perturbation theory (CASPT2) and extended ANO basis sets. The calculations comprise all singlet valence excited states below 8.0 eV, the first 3s, 3p, 3d Rydberg states, and the second 3s state. The four lowest triplet states were also studied. The resulting excitation energies forcis-hexatriene have been used in an assignment of the experimental spectrum, leading to a maximum deviation of 0.13 eV for the vertical transition energies. The calculations place the 11 B 2 state 0.04 eV below the 21 A 1 state. 16 excited states were studied incis-butadiene, using a CASPT2 o…
Electronic Transitions in Tetrathiafulvalene and Its Radical Cation: A Theoretical Contribution
2001
The low-lying electronic states of tetrathiafulvalene (TTF) and its radical cation (TTF+) have been studied using the multistate extension of a multiconfigurational second-order perturbation method (MS−CASPT2). The minimum-energy equilibrium geometries optimized at the CASSCF level have a boatlike conformation for the neutral molecule, with no significant barrier toward planarity. A more aromatic planar structure is, however, found for the ionic system. For TTF, the calculations of the vertical excitation energies comprise valence singlet and triplet states as well as the lowest members of the Rydberg series converging to the first ionization limit. Valence doublet states have been consider…
Comparative Mössbauer spectroscopic study of iron impurities in ZnSe
2001
Abstract An anomalous spectral doublet attributed to Fe 2+ impurity pairs is observed in Mossbauer absorption spectra of 57 Fe 0.01 Zn 0.99 Se below room temperature. In Mossbauer emission experiments, ZnSe: 57 Co polycrystal and powder sources exhibit below 45 K the nucleogenic Fe 1+ valence state born as an after effect of the electron-capture decay of 57 Co. Above 8 K, the proportion of the nucleogenic Fe 1+ decreases under the effect of the thermally activated Fe 1+ →Fe 2+ relaxation. The activation energy is estimated to 0.004 eV. While nucleogenic Fe 1+ is absent in ZnSe: 57 Co single crystal spectra, increasing proportions are detected in samples with an enhanced specific surface. Th…