Search results for " quantum chemistry"
showing 10 items of 549 documents
Dimers of the Higher-Energy Conformer of Formic Acid: Experimental Observation
2012
We report on the first experimental observation of formic acid dimers composed of two molecules of the higher-energy cis conformer. The cis–cis formic acid dimers are prepared in an argon matrix by selective vibrational excitation of the ground state trans conformer (deuterated form HCOOD) combined with thermal annealing of the matrix at about 30 K. Five cis–cis formic acid dimers are predicted by ab initio calculations (interaction energies from −16.9 to −27.2 kJ molˉ¹), and these structures are used for the assignment of the experimental spectra. Selective vibrational excitation of the obtained cis–cis dimers leads to the formation of several trans–cis dimers, which supports the proposed …
Photolysis of HCOOH monomer and dimer in solid argon: Raman characterization of in situ formed molecular complexes
2010
Raman spectroscopy combined with the matrix isolation technique was employed to study the 193-nm photodecomposition products of formic acid in an argon matrix. The Raman-active fundamentals belonging to the CO(2) + H(2) and CO + H(2)O photoproducts were assigned. Also, bands due to Fermi resonance between the stretching vibration (nu(1)) and the overtone of the bending mode (2nu(2)) of CO(2) were identified. Both ortho- and para-H(2) molecules were identified from their rotational lines S(0)(1) and S(0)(0), respectively. These bands appeared upon matrix annealing as well as after prolonged photolysis. Additionally, photolysis of FA dimers produces oxalic acid and its secondary photoproducts…
An ab initio and DFT study of the fragmentation and isomerisation of MeP(O)(OMe)+
2004
The fragmentation behaviour of the ion MeP(O)OMe+ has been investigated using quantum mechanical calculations at the B3LYP and MP2 levels to support experiments made with an Ion Trap Mass Spectrometer. Two mechanisms for the loss of CH2O are found, one involving a 1,3-H migration to phosphorus and the other a 1,2-methyl migration to give P(OMe)2+ followed by a 1,3-H migration. In each case an ion-dipole complex is formed that rapidly dissociates to yield CH2O. The relative importance of each route has been previously determined experimentally via isotopic labelling experiments, and the theoretical results are found to be consistent with these experimental results. The mechanisms suggested i…
Core hole screening and decay rates of double core ionized first row hydrides.
2013
Because of the high intensity, X-ray free electron lasers allow one to create and probe double core ionized states in molecules. The decay of these multiple core ionized states crucially determines the evolution of radiation damage in single molecule diffractive imaging experiments. Here we have studied the Auger decay in hydrides of first row elements after single and double core ionization by quantum mechanical ab initio calculations. In our approach the continuum wave function of the emitted Auger electron is expanded into spherical harmonics on a radial grid. The obtained decay rates of double K-shell vacancies were found to be systematically larger than those for the respective single …
Ab initio simulations on AgCl(111) surface and AgCl(111)/α-Al2O3(0001) interface
2005
The defect chemistry and ionic transport properties of the AgCl(111)/α-Al 2 O 3 (0001) interface were consid by using ab initio slab calculations. These calculations were performed in the framework of plane-wave basis combined with the density functional theory (DFT), as implemented into the VASP computer code, and Gaus basis set combined with the Hartree-Fock method (CRYSTAL-98 code). We analyze the electron density distribu on the interface and the electrostatic potential distribution near the AgCl surface. The size of the silver ion is great to enter the corundum surface layer and to create excess silver ions in this way. This is in agreement the experiments on heterogeneous doping of Ag…
Ab initio calculations of the hydroxyl impurities in BaF2
2011
Abstract OH − impurities in BaF 2 crystal have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Three different configurations of OH − impurities were investigated and the (1 1 1)-oriented OH − configuration is the most stable one. Our calculations show that OH − as an atomic group has a steady geometrical structure instead of electronic properties in different materials. The studies on band structures and density of states (DOS) of the OH − -impurity systems indicate that there are two defect levels induced by OH − impurities. The two superposed occupied OH − -bands located 1.95 eV above the valance bands (VB) at Γ point mainly consist…
55-Atom clusters of silver and gold: Symmetry breaking by relativistic effects
2006
Abstract Anionic 55-atom clusters of gold and silver are studied using density functional theory, scalar relativistic ab initio pseudopotentials and self-consistent generalized gradient corrections. An almost perfect icosahedron is found to be the clear ground state of Ag 55 - , and its electronic density of states agrees almost perfectly with recently measured high-resolution photoelectron spectra, up to the magnitude of the splitting of the highest free-electron shells by the Ih crystal field. A comparison between theory and a recent experiment allows one to assign icosahedral-based structures also for the Ag 57 - cluster. On the other hand, the Au 55 - cluster has several close-lying low…
Large-scale ab initio modelling of defects in perovskites: Fe impurity in SrTiO3
2002
Abstract Using the unrestricted Hartree–Fock method and supercells containing up to 160 atoms, we calculated the energy level positions in the gap and atomic geometry for the Fe4+ impurity substituting for a host Ti atom in SrTiO3. In agreement with experiment, the high-spin (S=2) state is much lower in energy than the zero-spin state. The energy level positions strongly depend on the asymmetric displacement mode of the six nearest O ions which is a combination of the Jahn–Teller and breathing modes. A considerable covalent bonding between the Fe ion and the four nearest O ions takes place.
Atomic and electronic structure of perfect and defective PbZrO3 perovskite: Hybrid DFT calculations of cubic and orthorhombic phases
2007
Abstract The structural and electronic properties of pure cubic and low-temperature orthorhombic PbZrO3 (antiferroelectric phase), as well as cubic PbZrO3 containing single F-centers (neutral oxygen vacancies) have been simulated by means of ab initio hybrid density functional calculations. We observed a substantial increase of the Pb–O bond covalency in ideal orthorhombic PbZrO3 with respect to its cubic phase. Relatively large displacement of four Pb atoms nearest to the F-center (0.25 A towards the defect) could affect the PbZrO3 ferroelectric properties. An O vacancy in the bulk PbZrO3 attracts ≈0.7 e, and the remaining electron density from the missing O2− is localized mostly on four n…
Excited states of the water molecule: Analysis of the valence and Rydberg character
2008
The excited states of the water molecule have been analyzed by using the extended quantum-chemical multistate CASPT2 method, namely, MS-CASPT2, in conjunction with large one-electron basis sets of atomic natural orbital type. The study includes 13 singlet and triplet excited states, both valence and 3s-, 3p-, and 3d-members of the Rydberg series converging to the lowest ionization potential and the 3s- and 3p-Rydberg members converging to the second low-lying state of the cation, 1 math. The research has been focused on the analysis of the valence or Rydberg character of the low-lying states. The computation of the 1 math state of water at different geometries indicates that it has a predom…