Ab Initio Orbitals Research Articles

Exploring the conformational energy landscape of glassy disaccharides by cross polarization magic angle spinning C13 nuclear magnetic resonance and numerical simulations. II. Enhanced molecular flexibility in amorphous trehalose

This paper uses chemical shift surfaces to simulate experimental C13 cross polarization magic angle spinning spectra for amorphous solid state disaccharides, paying particular attention to the glycosidic linkage atoms in trehalose, sucrose, and lactose. The combination of molecular mechanics with density functional theory/gauge invariant atomic orbital ab initio methods provides reliable structural information on the conformational distribution in the glass. The results are interpreted in terms of an enhanced flexibility that trehalose possesses in the amorphous solid state, at least on the time scale of C13 nuclear magnetic resonance measurements. Implications of these findings for the fragility of trehalose glass and bioprotectant action are discussed.

Molecular orbital ab initio and density functional theoretical study on reaction between PH2 and NO

The theoretical study of reaction between PH 2 and NO on the ground state potential energy surface is reported by using molecular orbital ab initio calculation and density function theory (DFT). Equilibrium structural parameters, harmonic vibrational frequencies, total energies and zero point energies of all species during reaction are computed by HF, MP2 (full) and B3LYP theory levels with the medium basis set 6-31G*. Theoretical results indicate that intermediate IM1(H 2 PNO) is firstly formed by overcoming a small energy barrier TS1, and then two four-mem- bered ring transient states TS2 and TS5, with energy barriers 103.3 and 102.6 kJ/mol respectively, then H-migration and isomerization are completed and the products PN and H 2 O are formed. The reaction is exothermic one with -189.6 kJ/mol released.

Determinação da estrutura molecular do ciclooctano por métodos ab initio e difração de elétrons na fase gasosa

The determination of the molecular structure of molecules is of fundamental importance in chemistry. X-rays and electron diffraction methods constitute in important tools for the elucidation of the molecular structure of systems in the solid state and gas phase, respectively. The use of quantum mechanical molecular orbital ab initio methods offer an alternative for conformational analysis studies. Comparison between theoretical results and those obtained experimentally in the gas phase can make a significant contribution for an unambiguous determination of the geometrical parameters. In this article the determination of the molecular structure of the cyclooctane molecule by electron diffraction in the gas phase and ab initio calculations will be addressed, providing an example of a comparative analysis of theoretical and experimental predictions.

Isomerization and dissociation processes of protonated benzene and protonated fulvene in the gas phase

The protonation energetics of benzene and fulvene are investigated through the use of molecular orbital ab initio G2(MP2) calculations using B3LYP/6-31G∗ optimized geometries. Calculated proton affinities of benzene and fulvene are 752 and 842 kJ mol −1 assuming formation of benzenium ion 1 and Cα-protonated fulvene 3, respectively. Isomerization reactions of protonated benzene and protonated fulvene were investigated at the same level of theory. The isomerization of benzenium ion 1 into protonated fulvene implies as a first step the formation of bicyclo[3,1,0]-hexenyl cation, 5, that may eventually evolve to yield methylene-protonated fulvene 2 and Cα-protonated fulvene 3. The latter yields the C β-protonated species 4 through a relatively low activation barrier. The energy required by the dissociation of benzenium ion via a 1,1-H 2 extrusion process is equal to its endothermicity. The excess energy of the dissociating species 1 having a lifetime of 10 −5 s is sufficiently low to explain the absence of an appreciable kinetic energy release during the separation of the products.

Gas-Phase Basicities of Acid Anhydrides

The gas-phase proton affinities (PA's) of acetic anhydride, 1, and several representative cyclic anhydrides (succinic, 2; methylsuccinic, 3; glutaric, 4; and 3-methylglutaric, 5) were measured through the use of Fourier transform-ion cyclotron resonance and high-pressure chemical ionization techniques: PA(1) = 844 ± 1 kJ/mol, PA(2) = 797 ± 1 kJ/mol, PA(3) = 807 ± 1 kJ/mol, PA(4) = 816 ± 3 kJ/mol, PA(5) = 820 ± 3 kJ/mol. The results were analyzed in the light of molecular orbital ab initio (MP2/6-31G*, G2) and density functional theory (B3LYP/6-31G*) calculations. The enol forms of acetic ahydride and its protonated counterparts were predicted to be significantly less stable than the corresponding diketo conformers. The large proton affinity of acetic anhydride takes its origin from the formation of an intramolecular hydrogen bond in the protonated form. This is supported by the computational results and by the measurement of a sizable entropy loss upon protonation. In contrast, the protonation of cyclic ...

Study of NO and CO dissociation on the (100) Cu surface using density functional theory and the topological analysis of the electronic density and its Laplacian

Molecular orbital ab initio Hartree–Fock, post-Hartree–Fock at the MP2 and QCISD levels, and density functional theory calculations of the dipole moment, the topology of the electronic density, ρ(r), and its Laplacian, [Formula: see text], for CO and NO molecules are reported. The results obtained confirm that density functional methods provide remarkably good electronic properties and a good description of the topology of ρ(r) and [Formula: see text]. The Becke exchange functional with the correlation functional of Lee, Yang, and Parr was used to calculate the electronic density of the (100) Cu surface. Topological analysis of ρ(r) shows that the crystal graph corresponds to square pyramids between the atoms of the top of the surface and the atoms of the second layer The topological analysis of [Formula: see text] shows that the atomic graph of the Cu surface exhibits one (3,−3) local charge concentration surrounded by four (3,+1) local charge depletion points. Additionally, there is a (3,+3) local depletion in the midpoint between each of four contiguous Cu atoms corresponding to the active site for the adsorption of the (3,−3) local charge concentration on the C atom of the CO or the N atom of the NO molecule. The larger value of the [Formula: see text] at the nonbonded charge concentration on the atoms and the geometrical configuration of these critical points favor the interaction of the NO over the CO molecule with the (100) Cu surface. This result is in accord with the known reaction barriers for these molecules. Key words: density functional theory, Laplacian of the electronic density, (100) Cu surface, carbon monoxide, nitrogen monoxide, molecular graph, atomic graph.

Theoretical and experimental analysis of properties in heterocycles containing the aminosulphonylamino moiety

A theoretical and experimental analysis of the geometric and electronic properties of compounds containing the aminosulphonylamino moiety was carried out. The theoretical properties were calculated using molecular orbital ab initio methods at the Hartree-Fock (HF) and second order Möller-Plesset (MP2) levels of theory, local density functional (LDF) ab initio methods and the semi-ab initio method, SAM1, on sulphamide and seven heterocyclic compounds containing the aminosulphonylamino group. The experimental analysis has been performed using x-ray structures of related compounds gathered in the Cambridge Structural Database together with experimental dipole moments and 13CNMR shifts of some of the compounds studied. Comparison of the experimental analysis with the theoretical results indicates that none of the methods studied is the most adequate to describe the geometry and electronic distribution of these molecules. The use of the 6-31G* basis set to compute the geometry of these molecules and methods which include electronic correlation (MP2/61G*//RHF/6-31G*, MP2/6-31G* and LDF) to quantify their electronic distribution are proposed.

Valence Electron Studies with Gaussian-Based Model Potentials and Gaussian Basis Functions. I. General Discussion and Applications to the Lowest s and p States of Li and Na

A new model potential for one-valence-electron atoms is introduced. It consists [Eq. (6)] of a core Coulomb potential modified by the addition of a Gaussian screened Coulomb potential. This potential has the desired features outlined for a model potential, and it has particularly convenient and simple mathematical properties when used with Gaussian basis functions. Since the smooth valence orbitals are sought, Gaussian functions are a good basis set because their deficiencies in nuclear regions do not enter the problem. The potential is calibrated to experimental energies for the 2s and 2p states of Li and 3s and 3p states of Na, using extended basis sets. The model Hamiltonians so defined are used with a variety of more modest basis sets to determine pseudowavefunctions. Based on comparisons with ab initio orbitals, energy analyses, radial density calculations, and overlap and expectation value calculations, the conclusion is that good valence pseudowavefunctions can be obtained by this approach with relatively small basis sets. The approach looks promising for valence-only molecular studies.