Search results for " Quantum chemistry"
showing 10 items of 549 documents
Hartree–Fock study of adhesion and charge redistribution on the Ag/MgO(0 0 1) interface
2001
Abstract Ab initio study of the Ag/MgO(0 0 1) interfaces based on a quantitative analysis of the bonding in the interfacial region is provided in the framework of Hartree–Fock approach. We are describing the way interfacial electronic and other properties evolve as a function of metal coverage. General conclusion that could be drawn from our calculations is that chemical bond formation is not important for the Ag/MgO(0 0 1) perfect interface. Physisorption of Ag atoms over surface O 2− ions associated with atomic polarization and charge redistribution in the metal planes are the dominant effects. The adhesion energy is enhanced by the interaction of the substrate Mg 2+ ions with the surplus…
An ab Initio MO Study of Silver Triflate Complexation in [2.2.1]Cyclophane π-Prismands
2002
Ab initio Hartree-Fock and DFT MO calculations have been used to study the conformations of six [2.2.1]cyclophane π-prismands and the formation of their π-complexes with silver triflate (AgSO 3 CF 3 ). The lowest energy cyclophane conformations and their silver triflate π-complexes have been calculated with HF/3-21G* and B3LYP/3-21G* levels of theory. The nature of bonding in silver triflate π-complexes has been studied with natural bond orbital analysis (NBO). Energies of the calculated cyclophanes and complexes, together with the formation energies of those complexes, have also been discussed. The results have been compared to available X-ray crystal structures and also to results of the …
Structural and electronic contributions to hyperpolarizability in methyl p-hydroxy benzoate
2006
Abstract The single crystals of methyl p-hydroxy benzoate (MPHB) are grown from methanol solution by a low temperature solution growth technique. The SHG efficiency is tested using Q-switched Nd:YAG laser of wavelength λ at 1064 nm, which is approximately 1.2 times that of urea. Vibrational spectral analysis using NIR-FT Raman and FT-IR spectra is carried out to understand the structural and electronic contributions to hyperpolarizability in MPHB. The DFT computations are also performed at B3LYP/6-311G(d,p) level to derive equilibrium geometry, vibrational wavenumbers and intensities. The results of ab initio calculations at HF/6-311G(d,p) level show that the vibrational contribution for th…
Raman spectroscopy of acetic acid monomer and dimers isolated in solid argon
2011
Acetic acid (AA) monomer and its dimers were studied by means of Raman spectroscopy combined with the matrix isolation technique. All fundamental bands of CH3COOH monomer were identified, including the CH3 torsional mode. Additionally, three overtone or combination modes were observed as a result of their enhanced intensities by Fermi resonance (FR). Twenty bands of the cyclic dimer (C2h) were identified and assigned, among which appear all intermolecular modes. Bands due to two different higher energy forms of the dimer were also identified. The experimental assignments are supported by ab initio calculations. Copyright © 2011 John Wiley & Sons, Ltd.
First principles slab calculations of the regular Cu/MgO(001) interface
2004
Ab initio slab calculations are performed for the copper adhesion over magnesium ions on the perfect MgO(0 0 1) surface with 1/4 monolayer (ML), two types of 1/2 ML and 1 ML substrate coverages. Results of our calculations are compared with various experimental and theoretical data. Both small atomic polarization and charge redistribution give the dominant contributions to the physisorption bonding on a regular Cu/MgO(0 0 1) interface.
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.
Treatment of scalar-relativistic effects on nuclear magnetic shieldings using a spin-free exact-two-component approach.
2013
A cost-effective treatment of scalar-relativistic effects on nuclear magnetic shieldings based on the spin-free exact-two-component theory in its one-electron variant (SFX2C-1e) is presented. The SFX2C-1e scheme gains its computational efficiency, in comparison to the four-component approach, from a focus on spin-free contributions and from the elimination of the small component. For the calculation of nuclear magnetic shieldings, the separation of spin-free and spin-dependent terms in the parent four-component theory is carried out here for the matrix representation of the Dirac equation in terms of a restricted-magnetically balanced gauge-including atomic orbital basis. The resulting spin…
Atomic and electronic structure of both perfect and nanostructured Ni(111) surfaces: First-principles calculations
2011
article i nfo In this study, we perform first principles simulations on both atomically smooth and nanostructured Ni(111) slabs. The latter contains periodically distributed nickel nanoclusters atop a thin metal film gradually growing from adatoms and serving as a promising catalyst. Applying the generalized gradient approximation within the formalism of the density functional theory we compare the atomic and electronic structures of Ni bulk, as well as both perfect and nanostructured (111) surfaces obtained using two different ab initio approaches: (i) the linear combination of atomic orbitals and (ii) the projector augmented plane waves. The most essential inter-atomic forces between the …
Molecular structure of p -methylaniline and its van der Waals complexes with CF 3 H, CH 4 and CF 4 studied by laser induced fluorescence spectroscopy…
2002
The UV fluorescence excitation and dispersed fluorescence spectra of jet-cooled p-methylaniline have been obtained for the S1←S0 transition. The main spectral bands have been assigned by comparison with those of other relevant substituted benzenes and with the help of computed vibrational frequencies. The structure of p-methylaniline in both the S0 and S1 states has been studied at ab initio quantum chemical calculations at MP2 and CIS levels of theory using the 6-31+G∗ basis sets. Both low- and high-resolution laser induced fluorescence spectra of p-methylaniline van der Waals complexes with CF3H, CH4 and CF4 have been characterised following jet-expansion investigations. The equilibrium g…
Ab initioanalysis on metal ion catalysis in the enolization reactions of some acetylheterocycles: kinetics of the enolization reactions of 3-acetyl-5…
2002
Kinetic data on the enolization reaction of 3-acetyl-5-methylisoxazole, 5-acetyl-3-methylisoxazole, 3(5)-acetylpyrazole and some previously studied acetylheterocycles have been the object of a comprehensive ab initio analysis. Enolization rate constants were measured spectrophotometrically by the halogen trapping technique at 25 °C and ionic strength of 0.3 mol dm−3 in water, in acetate buffers, in dilute hydrochloric acid, in dilute sodium hydroxide and in the presence of some metal ion salts. In the spontaneous (water) and base (acetate) catalysed reactions the ketones investigated are generally more reactive than acetophenone, according to the electron-withdrawing effect of the heterocyc…