Search results for "basis"
showing 10 items of 760 documents
Ground and excited state polarizabilities and dipole transition properties of benzene from coupled cluster response theory
1999
Abstract The electronic properties and transition properties have been investigated for the lowest singlet electronic states of benzene using coupled cluster response theory. The polarizabilities have been calculated for the ground state and the 1 1 B 2u , 1 1 B 1u 1 1 E 1u and 2 1 E 1u excited states. The dipole allowed transitions out of these states have also been calculated and discussed in the context of the calculated polarizabilities. Oscillator strengths and the second electronic moments of the charge distributions have been used to characterize and identify qualitative features of the individual states. The performance of coupled cluster singles (CCS), the recently proposed CC2 mod…
Theoretical study on hydration of symmetrically different diazanaphthalenes
2005
Abstract Quinazoline (symmetrical) and quinoxaline (unsymmetrical), diazanaphthalenes involved in certain biological reactions, have been studied computational with the purpose of comparing their protonation and covalent hydration mechanisms. Geometry optimizations of neutral, mono and diprotonated cations and hydrated products have been carried out at three levels of theory. Geometry optimizations were performed at HF, DFT/B3LYP levels of theory using 6-311G* basis set and single point energies were calculated at the MP2 level of theory using the same basis set. In agreement with experimental results, calculations predict a two steps mechanism resulting in a hydrated cation in which the OH…
Ab initio study of rotational isomerism and electronic structure of isomeric bipyrroles
1985
Abstract Ab initio calculations using STO-3G and 4-31G basis sets have been performed on the internal rotation barriers and conformational stabilities for 2,3′- and 3,3′-bipyrrole. The twofold rotation potential predicted for both isomers at minimal basis level becomes a more involved fourfold potential when the split-valence basis set is employed, because it takes into account more properly the nonbonded interannular interactions. A transoid-gauche minimum is predicted to have the minimal absolute conformational energy in both isomers. The electronic structure of the highest occupied MOs of 2,2′-, 2,3′- and 3,3′-bipyrrole are analyzed in terms of the single pyrrole MO pattern and a similar…
Theoretical study of the electronic spectrum of magnesium-porphyrin
1999
Multiconfigurational self-consistent field (SCF) and second order perturbation methods have been used to study the electronic spectrum of magnesium-porphyrin (MgP). An extended ANO-type basis set including polarization functions on all heavy atoms has been used. Four allowed singlet states of E1u symmetry have been computed and in addition a number of forbidden transitions and a few triplet states. The results lead to a consistent interpretation of the electronic spectrum, where the Q band contains one transition, the B band two, and the N band one. The computed transition energies are consistently between 0.1 and 0.5 too low compared to the measured band maxima. The source of the discrepan…
Vibrational spectra and DFT calculations of PPV-oligomers
2003
The first two members of the p-phenylenevinylene- oligomer family (i. e. 1, 4-distyrylbenzene [DSB] and 4, 4'-distyrylstilbene [DSS]) were synthesized and their infrared and Raman spectra recorded and empirically assigned. Molecular geometries were optimized for the planar point group (C2h) by the density functional theory (DFT) method using the B3LYP functional and 6-31G* basis set. Calculations of vibrational spectra, including intensities, were carried out subsequently using the DFT method with the same basis set and linear scaling was applied. Calculated vibrational wavenumbers are in a fair agreement with our own experimental spectra. In order to explore changes in vibrational dynamics…
MCSCF determination of the KO molecule ground state.
1992
Abstract Ab initio calculations at the MC/CASSCF level are used to determine in an accurate way the nature and position of the ground state of the KO molecule. The characteristic 2 Π and 2 Σ + alkali monoxide ionic states show a theoretical energy separation of about 0.04 eV, which produces opposite results by authors in favour of one or the other symmetry. We test the basis set dependence and active space dependence of the calculated energetical ordering of both states, with an extensive study of the active orbitais selection; the results show a ground state of 2 Σ + symmetry.
Determination of the Relative Stereochemistry of Flexible Organic Compounds by Ab Initio Methods: Conformational Analysis and Boltzmann-Averaged GIAO…
2002
Ab initio calculations at the Hartree-Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) 1 3 C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an openchain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental 1 3 C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8-13 relevant conformers per stereoisomer. Thermochemical calculations …
Anharmonic vibrational frequency calculations for solvated molecules in the B3LYP Kohn–Sham basis set limit
2012
Abstract The solvent dependence of harmonic and anharmonic vibrational wavenumbers of water, formaldehyde and formamide was studied using the B3LYP method. The results obtained with the hierarchy of Jensen's polarization-consistent basis sets were fitted with two-parameter formula toward the B3LYP Kohn–Sham complete basis set (CBS) limit. Anharmonic corrections have been obtained by a second order perturbation treatment (VPT2) and vibrational configuration interaction (VCI) method. The solvent environment was treated according to the self-consistent reaction field polarizable continuum model (SCRF PCM) approach.
ChemInform Abstract: Modular Metal Chalcogenide Chemistry: Secondary Building Blocks as a Basis of the Silicate-Type Framework Structure of CsLiU(PS4…
2012
The new title compound is synthesized from a mixture of U, P2S5, Li2S, Cs2S, and S in the molar ratio 2:2:1:1:4 (sealed silica tube, 700 °C, 3 d).
Ab initio calculations and vibrational spectroscopy on the phenylenediamine isomers
1998
Molecular orbital calculations at HF and MP2 levels have been performed using the 6-3IG** basis set for full geometry optimization of the phenylenediamine isomers. Our results show that only a transoid conformer is found for o-phenylenediamine, whereas cis and trans conformers exist for m- and p-phenylenediamine. Vibrational normal modes have been also analyzed for the gas phase and in chloroform solution, and compared with experimental data we have obtained using FTIR spectroscopy. © 1998 Elsevier Science B.V.