Search results for " quantum chemistry"
showing 10 items of 549 documents
Studies of a molecular hourglass: synthesis and magnetic characterisation of a cyclic dodecanuclear {Cr10Cu2} complex.
2006
The synthesis, structure, EPR, and magnetic studies of two dodecanuclear heterometallic cyclic clusters are reported. The compounds have the general formula [R(2)NH(2)](2)[Cr(10)Cu(2)F(14)(O(2)CCMe(3))(22)] (R=Me, 1 or iPr, 2). Both structures contain an array of metal centers which describe an approximate "hourglass", with an ammonium cation in the center of each half of the figure. The chromium sites are all six-coordinate, with the two copper sites five-coordinate. The majority of metal-metal edges are bridged by a single fluoride and two pivalate ligands, while two Cr--Cu edges are bridged by a single fluoride and a single pivalate. Magnetic studies show that 1 and 2 exhibit similar (bu…
Calculations of the atomic structure of the KNbO3 (110) surface
2000
Abstract The O-terminated KNbO 3 (110) surface is modeled using a semi-empirical shell model and two different short-range interatomic potentials. We find this surface to be unstable with respect to a strong reconstruction and K-termination. This conclusion is confirmed by preliminary calculations using the ab initio linear combination of atomic orbitals (LCAO) formalism.
Quantum dynamical study of the O(1D) + CH4→ CH3+ OH atmospheric reaction
2014
Time independent quantum mechanical (TIQM) scattering calculations have been carried out for the O((1)D) + CH4(X(1)A1) → CH3(X(2)A2″) + OH(X(2)Π) atmospheric reaction, using an ab initio ground potential energy surface where the CH3 group is described as a pseudo-atom. Total and state-to-state reaction probabilities for a total angular momentum J = 0 have been determined for collision energies up to 0.5 eV. The vibrational and rotational state OH product distributions show no specific behavior. The rate coefficient has been calculated by means of the J-shifting approach in the 10-500 K temperature range and slightly depends on T at ordinary temperatures (as expected for a barrierless reacti…
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 …
Using theozymes for designing transition-state analogs for the intramolecular aldol reaction of δ-diketones
2001
Two theozymes for the intramolecular aldol reaction of δ-diketones have been studied using ab initio methods. The presence of both acid/base residues favors several steps of the aldol reaction. The appropriate positioning of these residues can accelerate one of two diastereromeric reaction pathways, the catalyzed aldol reaction being highly stereoselective. Analysis of the geometrical parameters, charge distribution, and the shape of molecular electrostatic potential for the corresponding acid/base catalyzed transition structure allows us to design adequate transition-state analogs to favor a reactive channel of this intramolecular aldol reaction. © 2001 John Wiley & Sons, Inc. Int J Quant …
The surface electronic structure of stoichiometric and defective LiF surfaces studied with MIES and UPS in combination with ab-initio calculations
1997
Abstract UPS (He I) and metastable impact electron spectroscopy (MIES) spectra of the LiF(100) single crystal surface and stoichiometric LiF films are presented. The spectra are interpreted on the basis of ab-initio electronic structure calculations. Defective surfaces, produced by electron dosing, were studied in the same manner. The MIES spectra reveal that the electron dosing produces metallic patches on the surface, but no uniform Li adlayer. The calculation show that the F-center contribution to the electron emission is very close in energy to that from the metallic patches; thus, the two contributions cannot be distinguished by the present experimental techniques.
Vibrational analysis of the electronic spectrum of ethylene based onab initio SCF-CI calculations
1972
Ab initio calculations for CH2 twisting and CC stretching vibrational wavefunctions and energy levels are reported for various electronic states of ethylene C2H4. Electronic transition moments between these states are also obtained to allow a calculation of the oscillator strengths for vibrational transitions involved in various electronic band systems; from this study it is concluded that thevertical electronic energy differenceΔE e may differ significantly from the energy of the absorption maximumΔE max with which it is often equated. In particular it is found in the case of theπ→π * singlet-singlet excitation of ethylene that theΔE e value overestimates the most probable vibrational tran…
General occurrence of polar twisted minima in the ionic singlet excited surfaces of polyenes
1982
Except for a few cases, the lowest closed shell SCF solutions for the twisted geometries of polyenes involves delocalized nonpolarized MO’s, and represents a meaningless mixture of radicalar gound state and ionic excited states. Higher (quasi) solutions allow a realistic approach of the polarized ionic states. Orthogonality to the radicalar ground state may be imposed without important loss of energy. Further extensive CI does not modify significantly the situation. These statements are illustrated through ab initio calculations of butadiene and hexatriene. This approach allows us to show that slight distortions stablize polar minima even in ’’homosymmetric’’ molecules such as all‐trans hex…
Semiconducting Clathrates Meet Gas Hydrates: Xe24[Sn136]
2014
Semiconducting Group 14 clathrates are inorganic host–guest materials with a close structural relationship to gas hydrates. Here we utilize this inherent structural relationship to derive a new class of porous semiconductor materials: noble gas filled Group 14 clathrates (Ngx[M136], Ng=Ar, Kr, Xe and M=Si, Ge, Sn). We have carried out high-level quantum chemical studies using periodic Local-MP2 (LMP2) and dispersion-corrected density functional methods (DFT-B3LYP-D3) to properly describe the dispersive host–guest interactions. The adsorption of noble gas atoms within clathrate-II framework turned out to be energetically clearly favorable for several host–guest systems. For the energetically…
Ionization Energies of ClO and Cl2O2
1996
The ionization energies of chlorine oxide (ClO) and its dimer (Cl2O2) have been measured using monochromatic synchrotron radiation in the 10−20 eV energy regime in combination with photoionization mass spectrometry. The threshold energy of ClO+ (m/z = 51) is found at 10.85 ± 0.05 eV, whereas the ionization threshold of Cl2O2+ (m/z = 102) occurs at 11.05 ± 0.05 eV. The experimental values are compared to results from ab initio calculations, where three stable isomers of Cl2O2 are considered: dichlorine peroxide (ClOOCl), chloryl chloride (ClClO2), and chlorine chlorite (ClOClO). The results indicate that the experimental threshold energy of Cl2O2+ is due to adiabatic ionization of ClOOCl. T…