Ab Initio SCF Calculations Research Articles

Bis(η 5-dicarbollide)aluminum: Ab initio investigation of structure and NMR spectra

We address the question of the similarity between the title compound and the newly discovered aluminocene sandwich complex bis(cyclopentadienyle)aluminum ([AlCp 2 *] +). Ab initio SCF calculations of the structural parameters of the bis (η 5-dicarbollide)aluminum anion are in good agreement with X-ray data. The calculated 11B NMR spectrum allows a direct assignment of three measured signals and the theoretical signal range is consistent with experiment. The aluminum atom is strongly deshielded compared to the isolobal Cp * compound by about 170 ppm showing that the carbollide ligand exhibits a much weaker aromatic ring current effect than the Cp * ligand.

Ab initio SCF investigation of γ-hydroxybutyric acid

The potential energy surface of γ-hydroxybutyric acid was investigated by means of ab initio 4-31G SCF calculations. Sixty-six symmetry unique local minima are contained in this potential energy surface. Intramolecular interactions and all reactions, which involve the intramolecular hydrogen bond CO−O−H … O−H are discussed. Combination with the results for γ-aminobutyric acid yields a comparison of hydrogen bonds involving the groups −OH and −NH 2.

Application of potential constants: Electronic chemical potentials of polyatomic molecules—VIII

Abstract A new approach for the calculation of electronic chemical potentials of polyatomic systems is developed by applying the quadratic potential (force) constants which are available from normal coordinate analyses using spectroscopic data. The approach is constructed within the framework of density-functional theory into which the simple bond-charge model is incorporated. To evaluate the utility of such an approach, we have calculated electronic chemical potentials for various kinds of polyatomic molecules, and the calculated results have been compared favorably with experimental values as well as those obtained from ab initio SCF calculations. It seems that this approach offers the possibility of chemical potential calculations for polyatomic molecules whose quadratic stretching force constants are obtained by normal coordinate analyses.

Interactions in Bimetallic Bonding: Electronic and Chemical Properties of PdZn Surfaces

The properties of PdZn surfaces have been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, and CO chemisorption. The formation of Pd-Zn bonds increases the binding energy of the core and valence levels of Pd and reduces the binding energy of the core and valence levels of Zn. An equiatomic PdZn alloy exhibits a large depletion of Pd(4d) states near the Fermi level and a shift of [approximately] +0.7 eV in the binding energy of the Pd(3d) levels. Ab initio SCF calculations for the PdZn molecule and Pd[sub n]Zn[sub n] (n = 2-4) clusters show a decrease in the electron population of the Pd(4d) orbitals as a consequence of Pd(4d) [yields] Zn(4p) charge transfer and Pd(4d) [yields] Pd(5s,5p) rehybridization. These electron transfers reduce electron-electron repulsion within a Pd atom, shifting its core and valence levels toward higher binding energy. The electronic perturbations induced by Zn on Pd reduce the CO-chemisorption ability of Pd by weakening the Pd(4d)-CO(2[pi]) bonding interactions. PdZn surfaces that are rich in Zn show Pd-CO bonds that are 12-16 kcal/mol weaker than those seen on pure Pd surfaces. 57 refs., 12 figs., 1 tab.

Spectroscopic and theoretical studies of 4-nitropyridine N-oxide and of its related charge transfer compounds in their excited state T 1

Lowest triplet state T 1 Raman spectra of 4-nitropyridine Noxide NPO, its deuterated derivative NPO- d 4 and its related compound 3-methyl 4-nitropyridine Noxide (POM) obtained by time-resolved resonance Raman spectroscopy (TR 3) are reported. NPO and NPO- d 4 keep the C 2v symmetry in their T 1 state but the structure of the cycle is modified towards a pronounced quinonoid conformation. An important charge transfer from the N-oxide to the NO 2 group is observed. The TR 3 activity of T 1-POM vibrations is very similar to T 1-NPO, the T 1 → T n transitions of these molecules are of the same nature. Ab initio SCF calculations have been performed and the results obtained for both geometry and electron deformation density are in good agreement with spectroscopic data.

Theoretical study of the H 2O2CO hydrogen-bonded ternary complexes

Ab initio SCF and MP2 calculations have been performed on the structural and energetic properties of H 2O2CO. Three possible energy minima were found, all forming almost linear hydrogen bonds. The most stable species was observed when both carbon monoxide monomers were attached to water via the carbons. An energy decomposition analysis was performed on various complexes in order to study the nature of the intermolecular interactions. The co-operative effects in the different species are found to be negligible.

Ab initio molecular orbital calculations including solvent effects by generalized Born formula. Conformation of zwitterionic forms of glycine, alanine and serine in water

The continuum model, in which the solvation energy is expressed by the generalized Born formula and the atomic fractional charges are evaluated by Löwdin population analysis, has been incorporated into ab initio SCF calculations. The effective atomic radius was expressed as a function of the atomic charge and optimized in the variational procedure to obtain the Fock matrix elements. The method has been applied to conformational analysis of the zwitterionic forms of glycine, alanine and serine in water. The solvent effects on the molecular and electronic structures of these species were also examined.

A theoretical study of conformations and vibrational frequencies in (NH 2) 2C=X compounds (X=O, S, and Se)

Results of ab initio SCF and MP2 calculations of urea (NH 2) 2C=O, thiourea ((NH 2) 2C=S) and selenourea ((NH 2) 2C=Se) have been reported. The non-planar transoid structure turned out to be more stable than the cissoid form, and the planar form is characterized as the transition structure throughout. Harmonic force field and vibrational frequencies have been calculated for the normal species as well as the corresponding deuterated ones. Experimental IR spectra and assignments of previous empirical and semiempirical studies have been critically examined, and the electronic structures of three species are compared by calculating some molecular properties.

Effect of basis sets on ab initio SCF calculations of molecular hardness

Effect of basis sets on ab initio SCF calculations of molecular hardness

Metal-metal bonding on surfaces: molecular orbital study of Pd/Ti(001) and Pd/Ru(001)

The interaction of a Pd atom with Ti(001), Ru(001) and Pd(111) surfaces has been studied using semiempirical MO-SCF calculations (INDO/1) and cluster models. In addition, the electronic properties of the diatomic PdTi, PdRu and Pd 2 molecules have been examined using ab initio SCF calculations. The results of the calculations indicate that the charge transfer in the Pd-Ti and Pd-Ru bonds is small. For supported Pd monolayers, the Pd-substrate bonds can be described as mainly metallic, with a small degree of ionic character. Adsorption of Pd on an early transition metal induces a reduction in the electron population of the Pd(4d) orbitals by: (1) charge transfer from Pd to the metal substrate, and (2) rehybridization of the Pd(4d,5s,5p) levels. The magnitude of both phenomena increases when the fraction of empty orbitals in the valence band of the metal substrate rises. The Pd(4d) → Pd(5s,5p) electron transfer plays an important role in the strength of the bimetallic bonds: the larger this rehybridization, the stronger the Pd-substrate bond. Electronic perturbations induced by Ti or Ru on Pd reduce the CO-chemisorption ability of Pd by weakening simultaneously the Pd(4d)-CO(2π ∗) and Pd(5s,5p)-CO(5σ) bonding interactions. For adsorption of CO on supported Pd, the π back-donation and strength of the Pd-CO bond decrease when the fraction of empty states in the valence band of the support increases.

Ab initio SCF and force-field calculations on low-energy conformers of 2-acetylamino-2,N-dimethylpropanamide

The understanding of the conformational properties of α,α-disubstituted amino acids is related to the design of structural mimics for proteins. More specifically, 2-amino-2-methylpropanoic acid has been widely used in constructing helical peptide modules. In this paper, we report the results of ab initio SCF calculations with 3-21G, 6-31G* and 6-31G** basis sets on four low-energy conformations of 2-acetylamino-2,N-dimethylpropanamide. The results are used to assess the reliability of empirical potentials used in force-field methodologies. For this purpose, averaged multiple conformations' atomic point charges were determined with the 6-31 G* basis set. The results point to some important deficiencies in current force-fields. Thus, the relative energies of dipeptide conformations with excessive repulsive or excessive attractive interactions are not well described by some empirical potentials.

Complexes of 4-aminopyrimidine and 4-hydroxypyrimidine with water: computed relative thermodynamic stabilities

Relative thermodynamic stabilities in two series of isomeric complexes of substituted pyrimidine (4-aminopyrimidine; 4-hydroxypyrimidine, 4-oxypyrimidine) with water have been computed. Description of the isomers is based on ab initio SCF calculation, with energetics corrected by the second order perturbation treatment. The relative stabilities are evaluated in terms of partition functions constructed from the calculated dimeric parameters. In each of the sets several relative stability interchanges are found. The interchanges are manifested in a considerable heat capacity term enhancement.

Calibration of angular functions of the?-fluorine isotropic hyperfine interaction based on theab initio LCAO-MO UHF SCF calculations of?-fluorinated ethyl radicals

Ab initio LCAO-MO UHF SCF calculations have been carried out for the s-fluorinated ethyl radicals R-CH2, where R=CH3-nFn (n=0,1,2,3). Parameteres of the s-fluorine angular functions of the isotropic hyperfine interaction have been calculated for the ⇏-radical and radical anion fragments\(R - \mathop C\limits^. \begin{array}{*{20}c} / \\ \backslash \\ \end{array} \) and\(R - \overline {\mathop C\limits^. } \begin{array}{*{20}c} / \\ \backslash \\ \end{array} \)

Electron diffraction study of the molecular structure of 2,5-dimethylthiophene

The molecular structure of 2,5-dimethylthiophene has been determined by gas electron diffraction. Ab initio SCF calculations at the 3-21G* level were carried out and used as an aid to the analysis. The torsional vibrations of two methyl groups were treated as large-amplitude motions. A CH Me bond was found to be cis with respect to the SC bond in the equilibrium structure and the three-fold potential barrier was determined to be 1.0(7) kcal mol −1. The structural parameter values with estimated error limits (3σ) in parentheses are as follows: r g(SC) = 1.736(2) Å, r g(CC) = 1.372(4) Å, r g(CC Me) = 1.510(5) Å, r g(CH ring) = 1.098(6) Å, r g(CH Me) = 1.112 Å, ∠ αCSC = 92.6(5)°, ∠ αSCC ring = 110.7(5)°, ∠ αSCC Me = 121.6(4)°, ∠ αCCH = 123.4° and 〈∠ αHCH〉 = 108.1°, where 〈〉 denotes the average value. The values of r(CH Me) − r(CH ring), ∠CCH and 〈∠HCH〉 were taken from the 3-21G* calculations.

Use of molecular dynamics simulations with ab initio SCF calculations for the determination of the deuterium quadrupole coupling constant in liquid water and bond lengths in ice

AbstractThe deuterium quadrupole coupling constant and asymmetry parameter in heavy water were determined using ab initio SCF calculations. Snapshots from a molecular dynamics simulation were used to give liquid water cluster configurations and the influence of simulation parameters on the quadrupole coupling constant was investigated. The electronic potential model and the number of molecules in the molecular dynamics simulation and the pressure of the system were found to have only a small influence on the quadrupole coupling constant. The average value of the quadrupole coupling constant at room temperature, corrected for the known deficiency of the ab initio calculation in the gas phase, yields a quadrupole coupling constant of 253 kHz, in perfect agreement with the most recent experiments. The oxygen—deuterium bond lengths in ice Ih, ice II, and ice IX were determined using experimental quadrupole coupling constants and a model equation. An averaged bond length of 98.9 pm was obtained for the Ih form, which is approximately 2 pm shorter than that determined by neutron diffraction studies, whereas the bond lengths for the four deuterium sites in ice II and the three sites in ice IX are in fair agreement with experiment. © John Wiley & Sons, Inc.

Theoretical study of hydrogen-bonded formaldehyde-water complexes

The system H 2 CO+H 2 O has been investigated by means of ab initio SCF and CI calculations. The geometry was SCF-optimized for the complex H 2 CO+H 2 O and for one and two additional H 2 O molecules bound to the optimal H 2 CO...H 2 O nuclear arrangements. The binding energy was evaluated with basis set superposition errors and zero-point vibrations taken into account. The solvent shift for the singlet and triplet n→π * transitions was determined in CI calculations for the entire system. The solvent shift is found to be strongest (around 1140 cm -1 ) for the optimally H 2 O-bonded formaldehyde complexes with one water molecule

Complexes of 2-aminopyrimidine, 2-hydroxypyrimidine and their tautomers with water: Relative thermodynamic stabilities

Relative stabilities in two pairs of complexes of tautomers of substituted pyrimidine (2-aminopyrimidine and 2-iminopyrimidine; 2-hydroxypyrimidine and 2-oxypyrimidine) with water have been computed. Description of the isomers is based on ab initio SCF calculation, with energetics corrected by the second-order perturbation treatment. The relative stabilities are evaluated in terms of partition functions constructed from the calculated dimeric parameters. Complexes of 2-hydroxypyrimidine and 2-oxypyrimidine with water were found to coexist at higher temperatures. However, the overall heat capacity can be enhanced by about 3%, at most, by the isomeric interplay. The relation to non-equilibrium situations is discussed.

Ab initio SCF and MP2 calculations of the structures and relative stabilities of the isomeric hydrocarbons C 7H 8

The molecular structures and relative stabilities of some selected hydrocarbon isomers of the C 7H 8 species are reported at the HF/6–31 G ∗ and MP2/6–31 G ∗ levels of theory. Except for the strained bicyclic species, the relative energies obtained by ab initio calculations are found to be qualitatively in agreement with those of previous semiempirical MO and molecular mechanics studies. The calculated CC and CC bond lengths are, however, considerably different in some cases from the semiempirical MO values.

Electronic Structure of the 1-(Benzoyloxy)ethyl Radical

Abstract In the polymerization of vinyl benzoate, the tacticity of the obtained polymer is changed toward less isotactic by UV irradiation. To explain this phenomenon, it was assumed that the growing terminal of the polymer was excited by UV irradiation and that the coplanarity of the vinyl benzoates was broken. It was thus naturally concluded that the existence of a twisted radical reduced the isotacticity. To examine the above assumption, the electronic structures of the ground and excited states of 1-(benzoyloxy)ethyl radical () were studied as a model of the growing terminal of the polymer by ab initio SCF and CI calculations. The present calculation, however, showed that the planer structure was stable in both the ground and excited states.

Theoretical studies of surface reactions: embedded cluster theory

A theoretical approach to the description of the electronic structure of molecules adsorbed on solid surfaces and surface reactions is described. The objective is a quantitative, molecular level, understanding of surface processes, including chemisorption energetics, adsorbate structure and reaction mechanisms. The electronic structure problem is formulated as an embedded cluster of atoms in which a localization transformation is used to define the electronic subspace that interacts strongly with the adsorbate and surface region. This permits ab initio SCF and CI calculations of molecular quality to be performed on a portion of the lattice—adsorbate system. Assumptions of the theory are discussed and applications to chemisorption on transition metal substrates are referenced.