Search results for " Quantum chemistry"
showing 10 items of 549 documents
Dielectric friction effects on rotational reorientation of three cyanine dyes in n-alcohol solutions
1997
We have estimated the effect of dielectric friction on the rotational correlation times of three cationic cyanine dyes. Dielectric corrections were evaluated by using the Stokes–Einstein-Debye hydrodynamic continuum model including the dielectric friction for DiIC2, DiIC6, and DiIC14 in different n-alcohol solutions at room temperature. The dielectric corrections were done to cis and trans conformations of the cyanine dyes. For the trans conformations, which were found more stable than cis conformations, the dielectric model seemed to be more properly suited. The ground and excited state dipole moments for the calculations were evaluated from ab initio molecular orbital calculations and for…
BacSr1−cTiO3 perovskite solid solutions: Thermodynamics from ab initio electronic structure calculations
2005
We suggest theoretical prediction for Ba"cSr"1"-"cTiO"3 perovskite solid solutions (BST) combining ab initio DFT/B3PW calculations and alloy thermodynamics. This approach is based on calculations of a series of ordered super-structures in Ba-Sr simple cubic sublattice immersed in the rest TiO"3 matrix. Although these structures are unstable with respect to the decomposition, the results of total energy calculations allow us to extract the necessary energy parameters and to calculate the phase diagram for the solid solutions (alloys). A novel approach applied to the BST system enables to predict that at T>400 K Ba and Sr atom distribution is random. But below this temperature at small c Ba a…
Experimental Investigations and Ab Initio Studies of Tellurium(II) Dithiolates, Te(SR)2
1999
The reaction between Te(O(i)Pr)(4) and HSR offers a new and effective route to tellurium dithiolates, Te(SR)(2). Te(S(i)Pr)(2) (1) and Te(S(t)Bu)(2) (2) are stable compounds whereas Te(SPh)(2) (3) slowly decomposes at room temperature to give Te and Ph(2)S(2). IR spectra of 1-3 and ab initio calculations (HF/3-21G(d) and MP2 with double-zeta polarization effective core potential basis set) show nu(as)(Te-S) and nu(s)(Te-S) to be around 340 and 380 cm(-)(1), respectively. UV spectra exhibit similar lambda(max) (346-348 nm) for all three compounds, with the greater extinction coefficient of 3 accounting for its different and more intense color. Analysis of the molecular orbitals of the model …
Vibrational spectral studies of methyl 3-(4-methoxyphenyl)prop-2-enoate, a new organic non-linear optic crystal
2005
Single crystals of methyl 3-(4-methoxyphenyl)prop-2-enoate were grown by the slow evaporation technique and vibrational spectral analysis was carried out using near-IR Fourier transform Raman and Fourier transform IR spectroscopy. Ab initio quantum computations were also performed at the HF/6–311G (d,p) level to derive the equilibrium geometry, vibrational wavenumbers and intensities and first hyperpolarizability. The large NLO efficiency predicted for the first time in this new class of compounds was confirmed by powder efficiency experiments. Hartree–Fock calculations reveal that the endocyclic angle at the junction of the propeonate group and the phenyl ring is decreased from 120° by 2.5…
Ab initio study of the electronic and atomic structure of the wolframite-type ZnWO4
2009
Abstract Ab initio quantum chemistry calculations of the structural and electronic properties of monoclinic wolframite-type ZnWO 4 crystal have been performed within the periodic linear combination of atomic orbitals (LCAO) method using six different Hamiltonians, based on density functional theory (DFT) and hybrid Hartree-Fock-DFT theory. The obtained results for optimized structural parameters, band gap and partial density of states are compared with available experimental data, and the best agreement is observed for hybrid Hamiltonians. The calculations show that zinc tungstate is a wide band gap material, with the direct gap about 4.6 eV, whose valence band has largely O 2p character, w…
Ab initio modeling of copper adhesion on regular BaTiO3(001) surfaces
2005
Ab initio calculations have been performed for copper adsorption on a regular, defect-free TiO"2- and BaO-terminated (001) surfaces of a cubic BaTiO"3, using a posteriori HF-CC method as implemented into the CRYSTAL-03 computer code. To clarify the nature of the interfacial bonding, we use slab models of the Cu/BaTiO"3(001) interfaces with different one-side substrate coverages, varied from 1/8 monolayer (ML) up to 1/2 ML, over both TiO"2- and BaO-terminated surfaces. TiO"2 termination has been found to be energetically more favorable for the adsorption of copper atoms. In agreement with previous experimental and theoretical data, our calculations indicate essential contribution of atomic p…
A MRCI PS and CASSCF study of the ground state MgO dissociation energy
1991
Abstract Ab initio calculations at CASSCF and MRCI PS levels are used to determine the dissociation energy for the X 1 Σ + state of MgO, which adiabatically dissociates to the ground state 1 S g of magnesium and to the excited 1 D g state of oxygen, as well as other spectroscopic parameters. Emphasis is placed upon the problem of properly selecting an adequate active space in CASSCF calculations and upon the improvements obtained in MRCI by selecting perturbatively the most important contributions to the total wavefunction and evaluating the remaining ones only by perturbational method. Through a procedure based on stabilizing the computed dissociation energy, values of 3.87 eV (MRCI PS) an…
Theoretical Study on the Structures and Electronic Spectra of TCNE2−
2006
Investigations into the charge-separated states and electron-transfer transitions in tetracyanoethylene (TCNE) compiles have recently generated much interest. In this work we present theoretical calculations showing that the most stable structure of the dianion TCNE 2- has D 2d symmetry in vacuum as well as in the solvents dichloromethane and cicetonitrile. By means of the coupled cluster linear response, we compute the vertical electronic spectrum in both the gas phase and solution. The theoreitcal results, are compared to the experimental data and good agreement is achieved.
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…
Adsorbed CO on group 10 metal fragments: A DFT study
2009
DFT calculations on the helicopter and cartwheel rotations of one CO molecule adsorbed at the bridge site on metal-surface fragments, characterized by two (M(8)) or three (M(14)) metal-atom layers (M = Ni, Pd, Pt) were performed by the B3LYP[LANL2DZ+6-31 g(d,p)] method, to rationalize the adsorption energetics and the steric hindrance characteristics of surface CO molecules. Potential Energy Surfaces were obtained, either fixing the C-O bond-length or allowing it to change. The behavior of the three metals, as obtained from the study of the configurational space characterizing the CO adsorption on the fragments was explained on the basis of the interaction energies involved in the different…