Search results for " orbita"
showing 10 items of 447 documents
Prediction of Magnetic Properties in Oxovanadium(IV) Phosphates: The Role of the Bridging PO4 Anions
1998
Oxovanadium phosphates constitute a crystallochemically very rich family that, in turn, results in a seemingly intricate magnetochemistry including from isolated dimers to 3-D systems. This magnetic diversity is due, in part, to the possible participation of phosphate groups in the spin transfer between VIV centers. This way, 31P solid-state NMR becomes a key tool in determining the exchange paths involving phosphorus orbitals. The magnetic behavior of several layered oxovanadium phosphates M(VOPO4)2·4H2O (M = Na+, Ca2+, Ba2+, and Pb2+) has been investigated. Like it occurs in the case of other previously studied lamellar derivatives, the best fit of the temperature-dependent magnetic susce…
Carbazole-based green and blue-BODIPY dyads and triads as donors for bulk heterojunction organic solar cells.
2020
Two BODIPY derivatives with one (B2) and two (B3) carbazole moieties were synthesized and applied as electron-donor materials in organic photovoltaic cells (OPV). Their optical and electrochemical properties were systematically investigated. These BODIPY dyes exhibit excellent solubility in organic solvents and present high molar extinction coefficients (1.37–1.48 × 105 M−1 cm−1) in solutions with absorption maxima at 586 nm for mono-styryl groups and at 672 nm for di-styryl groups. The introduction of the styryl moieties results in a large bathochromic shift and a significant decrease in the HOMO–LUMO energy-gaps. The BODIPY dyes show relatively low HOMO energies ranging from −4.99 to −5.1…
Wannier90 as a community code: new features and applications
2019
Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, s…
Tuning of the electronic and photophysical properties of ladder-type quaterphenyl by selective methylene-bridge fluorination
2016
The photophysics (spectral positions, band shapes, fluorescence quantum yields and lifetimes) of a series of fluorinated ladder type quaterphenyls L4P and L4P-Fn (n = 2, 4, 6) depend strongly on the degree and position of fluorine, despite the fact that substitution is not performed in the rings but only in methylene-bridges. This is driven by subtle differences in the molecular orbitals (MOs) participating in the electronic transitions, and in the vibronic pattern of the S0 and S1 electronic states as revealed by (TD)DFT calculations. Solid state spectra for n = 0, 2, 4 are similar to those of solution due to small intermolecular interactions as revealed by combined X-ray and (TD)DFT analy…
Theoretical rotational constants of MeCnN species
1990
Abstract By means of SCF HF “ab initio” calculations with STO-3G and 6-31G basis sets, the geometric parameters of methylcyanopolyynes (MeCnN n=3, 5, 7 and 9) have been obtained. B0=0.3748 GHz for MeC7N and B0=0.2708 GHz for MeC9N, with a STO-3G basis set, were obtained. Both species, unstable in the laboratory, are still undetected in the interstellar medium, although their existence is very probable.
The Low-Lying Excited States of 2,2′-Bithiophene: A Theoretical Analysis
2004
The low-energy region of the singlet →singlet, singlet →triplet, and triplet→triplet electronic spectra of 2,2'-bithiophene are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1 1 B u and 2'B u , are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2 3 A g and 2 3 B u ,…
Size-intensive decomposition of orbital energy denominators
2000
We introduce an alternative to Almlöf and Häser’s Laplace transform decomposition of orbital energy denominators used in obtaining reduced scaling algorithms in perturbation theory based methods. The new decomposition is based on the Cholesky decomposition of positive semidefinite matrices. We show that orbital denominators have a particular short and size-intensive Cholesky decomposition. The main advantage in using the Cholesky decomposition, besides the shorter expansion, is the systematic improvement of the results without the penalties encountered in the Laplace transform decomposition when changing the number of integration points in order to control the convergence. Applications will…
EXTENDED HÜCKEL MOLECULAR ORBITAL CALCULATION OF THE TEMPERATURE DEPENDENCE OF THE QUADRUPOLE SPLITTING OF [Fe(H2O)6] SiF6 AND KFeCl3
1976
A number of papers have appeared in which the temperature dependence of the quadrupole splitting has been treated, whereby covalency effects have been considered by introducing the orbital reduction factor k, with k ranging from 1.-0.7. There are, however, two problems : (1) k is unisotropic if the symmetry is lower than cubic, (2) the spin orbit coupling of the ligand electrons are usually neglected. These two problems have been treated here by using SCCEHMO [1] calculations on two examples, [Fe(H2O)6]SiF6 and KFeCl3, which have been previously delt with by other authors using ligand field theory [2, 3]. For both compounds the differences between the measured and the calculated quadrupole …
Comparative G2(MP2) Molecular Orbital Study of B3H7XH3 and H3BXH3 Donor−Acceptor Complexes (X = N, P, and As)
2002
B3H7XH3 and H3BXH3 (X = N, P, and As) have been studied as donor−acceptor complex type at the G2(MP2) level of theory. Both single- and double-bridged structures of B3H7 Lewis acid are taken into account. Although the double-bridged structure is energetically favored in the isolated state, the coordination prefers the single-bridged one. The monoborane adducts adopt the staggered arrangement with C3v symmetry. The energetic analysis by natural bond orbital theory shows that the decrease of hyperconjugative contribution upon complexation in the B3H7 moiety has, as consequence, a loss of symmetry for B3H7XH3 (X= P and As) complexes. In the two series, the coordination is the result of two opp…
Heterocyclic systems containing tin(IV)—XIII [1]. Possible ceasing or inversion of the structuraltrans influence during the course of a bimolecular n…
1997
Abstract In the series of the heterocycles X[(CH2)3]2SnR2 (16 newly synthesized compounds and two taken from the literature) and in the germanium analog BuiN[(CH2)3]2GeCl2, the donor group X (= NMe, NBz, NBui, NPri, O, S) intramolecularly attacks the Lewis acidic atoms Sn or Ge. Eight structure determinations (and, in addition, 2 taken from the literature) are compared. The nucleophilic attack at Sn is stronger for the ligands R2 =Cl2 than for the more electronegative R2 = (OSiPh3)2. Overall the familiartrans influence holds: simultaneous approach of X and weakening of the ligand R (trans). In three cases of subtly graded donor strength a slight inversion or ceasing is observed: shorter dis…