Ab Initio LCAO MO SCF Research Articles

Catalytic activity of the first solvation shell in the CO2 hydration reaction

The catalytic activity of the first solvation shell for the CO2 hydration reaction has been studied within the ab initio LCAO MO SCF supermolecular approach to determine the significance of the specific solvation sites in lowering the activation barrier. The role of OH- ion replacing some solvent molecules has also been investigated suggesting alternative mechanism of CO2 hydration in alkaline solutions. In addition, the basis set dependence of the activation barrier and basis set superposition effect on barrier lowerings were examined.

Ab Initio Structure Investigation of the Enol Forms of β-Diketones RCOCH2COR (R = H, CH3, CF3)

The geometrical parameters, force field parameters, and vibrational frequencies of the enol forms of β-diketones RCOCH2COR (R = H, CH3, CF3) were calculated using the ab initio MO LCAO SCF method involving broad bases of Cartesian Gaussian functions. For acetylacetone (R = CH3), the potential surface of rotation of the two methyl groups was investigated in detail, and the minimal energy route of the transfer of the hydroxyl proton inside the chelate ring was calculated. For hexafluoroacetylacetone (R = CF3) and malonic dialdehyde (R = H), calculations were carried out only for stationary points on the potential energy surface. For all the geometrical configurations under study, the relative energies were refined with inclusion of electron correlation in terms of second-, third-, and fourth-order Moller–Plesset perturbation theory; for molecules with R = H, CH3, the configuration interaction procedure taking account of all single and double excitations was additionally employed. The experimental and theoretical data are compared with those available in the literature. The substituent effect on the geometrical and electronic structure of the chelate ring is studied.

Endohedral analogs of ferrocene: ab initio theoretical predictions

The problem of the existence of endohedral analogs of ferrocene is discussed. It is shown that radical CorH 5 , where Cor is corannulene C 20H 10 and five atoms H are added to the α-positions relative to the five-membered cycle ( pent*) of Cor, can form stable polyhedral dimer D 2 h -C 40H 30 (D). The ground state of D is triplet. The energy of dimerization of CorH 5 is equal to 175 kcal mol −1. The geometric and electronic structure of hypothetical endohedral complex D 5 d -2η 5-Fe@D ( 1) and its isomer D 5 d -2η 5-Fe(CorH 5) 2 ( 1b), which is a sandwich complex, are simulated by the ab initio MO LCAO SCF method (HF/3-21G). It is found that the energy of 1 is less than the energy of 1b by 171 kcal mol −1. The energies of the Fe pent* bond in complexes 1 (115 kcal mol −1) and 1b (111 kcal mol −1) are close to those of the FeCp bond in bis(cyclopentadienyl)iron FeCp 2 (110 kcal mol −1). Complex 1 and also complex D 5 d -2η 5-Fe@C 40H 10 ( 2), carbon skeletons of which have an analogous structure to 1 and with ten H added to the α-positions relative to polar pentagons ( pent*), can be considered as endohedral analogs of ferrocene. Atom Fe in these complexes interacts with atoms of both polar pentagons. The binding energy of the atom Fe with the polyhedral fragment C 40H 10 of 2 is a bit less than in 1 (on 6 kcal mol −1).

Evidence of the molecular orientation of Pt- and Pd-containing organometallic polymers in thick layers: a NEXAFS study

The molecular orientation of rod-like organometallic polymers [–Me(Pn–Bu3)2–CC–C6H4–C6H4–CC–]n, Me=Pt, Pd, in the form of thick films, has been determined from the study of their near-edge X-ray absorption spectra (NEXAFS) recorded in the photon-energy region 280–320 eV. The C K-edge resonances, recorded at grazing (θ=25°) and normal (θ=90°) X-ray beam incidence angles, show a strong polarization dependence of the majority of the peaks. Quantitative evaluation of the intensity ratios suggests that the organic moiety of the polymer, that is –CC–C6H4–C6H4–CC–, is oriented at about 40° with respect to the surface normal, the two benzene rings are nearly coplanar, and the plane containing the metal and the phosphines (Pn–Bu3) is probably aligned with the plane of the rings, since the butyl groups of the Pn–Bu3 ligands also show a non-isotropic character. Although the repeat unit of the polymer is quite complex, the assignment of the π* and σ* resonances has been developed on the basis of (i) the chemical structure provided by traditional spectroscopic investigations, (ii) ab initio LCAO–MO–SCF calculations performed on a simple model molecule (phenylacetylene), and (iii) curve fitting of the spectra. The organometallic polymers containing two different transition metals, Pt and Pd, investigated here, show the same molecular structure and orientation. © 1997 Elsevier Science B.V.

Fractional composition of fly ash from the first unit of the Berezovskaya GRES-1 State regional power plant

The problem of the existence of endohedral analogs of ferrocene is discussed. It is shown that radical CorH5, where Cor is corannulene C20H10 and five atoms H are added to the α-positions relative to the five-membered cycle (pent*) of Cor, can form stable polyhedral dimer D2h-C40H30 (D). The ground state of D is triplet. The energy of dimerization of CorH5 is equal to 175 kcal mol−1. The geometric and electronic structure of hypothetical endohedral complex D5d-2η5[email protected] (1) and its isomer D5d-2η5-Fe(CorH5)2 (1b), which is a sandwich complex, are simulated by the ab initio MO LCAO SCF method (HF/3-21G). It is found that the energy of 1 is less than the energy of 1b by 171 kcal mol−1. The energies of the Fepent* bond in complexes 1 (115 kcal mol−1) and 1b (111 kcal mol−1) are close to those of the FeCp bond in bis(cyclopentadienyl)iron FeCp2 (110 kcal mol−1). Complex 1 and also complex D5d-2η5[email protected]40H10 (2), carbon skeletons of which have an analogous structure to 1 and with ten H added to the α-positions relative to polar pentagons (pent*), can be considered as endohedral analogs of ferrocene. Atom Fe in these complexes interacts with atoms of both polar pentagons. The binding energy of the atom Fe with the polyhedral fragment C40H10 of 2 is a bit less than in 1 (on 6 kcal mol−1).

An ab initio mo study on the structure dependence of Mösssbauer spectra in iron porphyrin complexes

For bis-pyridine(porphinato)iron (FeP(py) 2), the Mössbauer spectral parameters, the quadrupole splitting (QS) and isomer shift (IS), were calculated for various FeN py (N py: pyridine nitrogen) bond distances by using ab initio LCAO SCF MO wave-functions. At the equilibrium FeN py bond distances, QS and IS were in agreement with the experimental values for ferric and ferrous low-spin states. It was found that QS and IS are increasing functions of the FeN py distance for the main configurations of the ground states. From these results, the pressure dependence of the Mössbauer parameters for molecular crystals is qualitatively explained.

An ab initio study of anharmonic potential energy surfaces for bound water molecules

One-dimensional potential energy expansions in stretching, bending and rigid-body translational and rotational coordinates up to fourth order have been calculated for bound H2O molecules by abinitio MO LCAO SCF methods. Water in crystalline ice and LiOH·H2O have been simulated by calculations on (H2O)4·H2O and (Li+)2(OH−)2H2O clusters, each surrounded by point charges. The force constants ƒ 2, ƒ 3 and ƒ 4 in the potential energy expression V=ƒ 2Δr3+ƒ 3Δr3+ƒ 4Δr4 were calculated, where Δr is a simultaneous stretching of the OH bonds in the water molecule. The force constants were found to decrease linearly with the equilibrium OH distance. Anharmonicity constants X were calculated for rigid-body vibrations of H2O in LiOH·H2O and found to be 14 cm−1 for the rotational vibration and less than 1 cm−1 for the translational vibration.

Ab initio LCAO-MO calculations on α-D-glucopyranose, β-D-fructopyranose, and their thiopyranoid-ring analogues. Application to a theory of sweetness

Ab initio SCF LCAO-MO calculations at the STO-3G level have been performed on the title compounds and the results are discussed in terms of net atomic charges and Mulliken population analysis, with a view to providing an explanation for the greater sweetness observed in the case of the thio analogues.

Examination of the molecular properties of D-fructose and L-sorbose by abinitio LCAO-MO calculations

Abinitio SCF LCAO-MO calculations at the STO-3G level have been performed on β-D-fructopyranose (1) and α-L-sorbopyranose (2) using crystallographic data as the geometrical input. Molecular properties of 1 and 2 are discussed in terms of orbital energies, total energy, ionization potentials, Mulliken population analysis, and electrostatic potentials, with a particular emphasis on the possible consequences of these features as regards the sweet taste of these two ketoses. No correlation was found, for example, with the electrostatic, point-charge distribution since the calculated hydrogen-bonding abilities would lead to the prediction of 2 being sweeter than 1. On the other hand, non-bonded overlap populations between oxygen and hydroxyl-hydrogen atoms reveal the presence of intramolecular interactions, which may have a determinant influence on the taste of these molecules and which could explain why D-fructose is much sweeter than its epimer at C-5, namely, L-sorbose.

Computer experiments on aqueous solutions. III. Monte Carlo calculation on the hydration of tertiary butyl alcohol in an infinitely dilute aqueous solution with a new water–butanol pair potential

Monte Carlo calculations have been carried out for an infinitely dilute aqueous solution of tertiary butylalcohol (TBA) at 298.15 K and ordinary density by the Metropolis scheme in an NVT ensemble. The total number of molecules is 216, one of which is TBA. For water–water interaction, the MCY (Matsuoka–Clementi–Yoshimine) potential is used, whereas a new pair potential is determined for water–TBA interaction from ab initio LCAO SCF MO calculations for more than 380 different dimeric configurations with an STO-3G basis set and subsequent multiparameter fitting of MO data thus obtained to an appropriate potential function with a nonlinear optimization method. In the Monte Carlo run, 3 600 000 configurations have been generated and final 2 000 000 configurations have been used to obtain energetic and structural information on the hydration structure of TBA. As in the case of the methanol solution studied previously, the potential energy and structure of water tend to be stabilized by the introduction of one TBA molecule, and two strong hydrogen bonds between TBA and the surrounding water molecules favor the formation of a bulky and stable hydration shell which includes cooperatively connected hydrophobic hydration structures. This is clearly shown in density diagrams for water around TBA which visualize remarkable structuration of water. The iceberglike structure around TBA is more pronounced than that of methanol.

Ab initio calculation of equilibrium geometry and hyperfine coupling constants of the cyclopentane cation

The equilibrium geometry and hyperfine coupling constants of the cyclopentane cation have been calculated by the UHF method, within the ab initio MO LCAO SCF approximation. A non-planar carbon framework of C 5 symmetry was found with one carbon atom out of the plane of the others. The geometry and the calculated isotropic constants give support to the ESR identification of the ion.

Theoretical studies on low-lying electronic states of the CCl+, SiCl+, and GeCl+ ions

Potential energy curves for the ground state X 1Σ+ and low-lying A 1Π and a 3Π states of the CCl+, SiCl+, and GeCl+ ions have been calculated by ab initio LCAO–SCF–MO configuration interaction method. Calculated molecular constants Te, ωe, and ωexe agree well with available experimental data. The assignments of A 1Π–X 1Σ+ and a 3Π–X 1Σ+ bands for CCl+ and a 3Π–X 1Σ+ band for SiCl+ have been confirmed.

Three‐body nonadditive effects for hydrogen bonded water molecules: SCF study of hydrated species including H2S and various metal ions

AbstractThe relative importance of purely nonadditive three‐body terms in solvated systems is assessed through ab initio LCAO–MO–SCF studies of hydrogen bonded structures containing two or more water molecules plus a solvate species. Among the latter the cases of Li+, Ca2+, and Mg2+ ions (of biological interest for aminoacid release processes in membrane transport) and of HDS molecules (of industrial importance in processes for the production of heavy water) are studied.

Computer experiments on aqueous solution. I. Monte Carlo calculation on the hydration of methanol in an infinitely dilute aqueous solution with a new water–methanol pair potential

Monte Carlo calculations have been carried out both for pure water and an infinitely dilute aqueous solution of methanol at 298.15 K and ordinary density by the Metropolis scheme in NVT ensemble. The total number of molecules is 216, one of which is methanol in the case of aqueous solution. For water–water interaction, the MCY (Matsuoka–Clementi–Yoshimine) potential is used, whereas a new pair potential is determined for water–methanol interaction from ab initio LCAO SCF MO calculations for more than 450 different dimeric configurations with a STO-3G basis set and subsequent multiparameter fitting of MO data thus obtained to an appropriate potential function with a nonlinear optimization method. For the aqueous methanol solution, 4 500 000 configurations have been generated and the final 1 500 000 are used to obtain average quantities for energy and various distribution functions. It is found that, with the introduction of one methanol molecule, the potential energy and structure of water tend to stabilize as a whole. Such stabilizations in energy and structure are found to be angular dependent, owing to anisotropic nature of methanol–water interactions. Calculations for several regions classified according to the direction from the methanol molecule have been attempted to examine the contribution of hydrophilic and hydrophobic regions to the total hydration structure. The hydrophilic region as a whole does not show a large energetic stabilization, though pair interaction distribution function clearly indicate a larger hydrogen bonding interaction for some water molecules in this region. In contrast to this, the hydrophobic region is characterized by larger overall energy stabilization as a result of the sum of a large number of smaller interactions. Strong hydrogen bondings in the hydrophilic region seem to act cooperatively with the structural stabilization in the hydrophobic region to form a fairly stable hydration structure around methanol. This is clearly shown by density distribution diagrams for water around methanol which visualize an iceberglike structure of water, remarkably in comparison with the structure in pure water.

Improved procedure for analysis of electron density redistribution in molecular complexes

An improved method for theoretical determination of the charge redistribution, i.e., the intramolecular charge shifts, intermolecular charge transfer, and dipole moment enhancement upon complexation, is proposed. This approach consists of counterpoise correction of the basis set superposition error on calculated electron density redistribution in molecular complexes. Ab initio LCAO MO SCF calculations for HF⋅⋅⋅HNH2, H2O⋅⋅⋅HOH, H2O⋅⋅⋅HF, H3N⋅⋅⋅HF, H3N⋅⋅⋅LiF, CO⋅⋅⋅HCN, H2S⋅⋅⋅OCHNH2, H2NHCO⋅⋅⋅HSH, [Mg⋅⋅⋅OH]+ [HO⋅⋅⋅HOH]−, [Be(OH)(NH3)2]+ molecular complexes in different basis set have been performed leading to more consistent results in contrast to sometimes largely overestimated uncorrected values reported previously. In addition, the charge redistribution upon complex formation has been analyzed for some dimers in terms of atomic multipole expansion of electron density and difference molecular electrostatic potentials.

Ground electronic state and geometry of SF 3

Ab initio LCAO MO SCF calculations with DZ + 3d(S) basis functions show that the sulphur trifluoride radical is a planar π-radical having a 2B 1 ground state. Like ClF 3, it has an umbrella-structure. However, it becomes Y-shaped in its first 2A 1 excited state which has been calculated to lie only ≈ 2.4 eV above the 2B 1 ground state.

Ab initio molecular orbital study on the effects of zinc ion, its ligands and ionic amino acid residues for proton transfer energetics between glu 270 and Zn co-ordinated water molecule in carboxypeptidase A

Thezinc-water-Glu 270 system was reported from the X-ray crystallographic study of native carboxypeptidase A(CPA) (Lipscomb et al., 1968). General base catalysis by the γ-carboxylate of Glu 270 was proposed for peptidase activity of CPA. The effects of zinc ion and its ligands (Glu 72, His 69-Asp 142, His 196) for proton transfer between Glu 270 and Zn co-ordinated water molecule in CPA were studied by the ab initio SCFLCAO-MO method. The results show that the proton transfer from the Zn co-ordinated water molecule to the γ-carboxylate of Glu 270 is greatly promoted by the Zn ion and, conversely, is greatly inhibited by its ligands. The facilitation effect of Zn ion and the inhibition effect of its ligands for the proton transfer were analysed by using the energy decomposition analysis. Moreover, calculations including all side chains of ionic amino acid residues and main chain residues in CPA as point fractional charges were performed. The results show that the proton transfer is affected by the ionic amino acid residues and is not affected by the main chain residues.

Localized and delocalized molecular orbitals within the model of single-orbital relaxation energies

Ab initio LCAO MO SCF calculations have been performed on CO and N 2. Single-orbital relaxation energy contributions obtained by means of molecular orbitais with different localization character are compared. Although satisfactory agreement with directly calculated relaxation energy has been found in all cases, different partitioning of the total relaxation energy into orbital contributions appears.

Ab initio molecular orbital calculation of fe‐porphine with a double zeta basis set

AbstractAn ab initio LCAO SCF MO calculation was performed on planar Fe‐porphine with a double zeta basis set consisting of 300 CGTO's. SCF wave functions of several states of Fe‐porphine and its cation were obtained. The net charge of Fe is in the range of 1.39 to 1.53. The highest occupied orbital is ascertained to be a pure porphine π‐MO, 1a1u. The calculated ionization potentials of the two highest occupied orbitals, 1a1u and 5a2u are 5.98 and 6.43 eV, respectively. They are in good agreement with experiments. The role of the porphine macrocycle on the oxidation of Fe is discussed in terms of gross atomic populations and with contour maps of the density difference.

ChemInform Abstract: THE POTASSIUM FLUORIDE‐BORIC ACID SYSTEM. HYDROGEN BONDING IN KF.H3BO3

AbstractAus ab initio‐LCAO‐MO‐SCF‐Berechnungen zu den wahrscheinlichsten Wechselwirkungen zwischen F‐ und H3BO3 geht hervor, daß das Ion [BF(OH)3]‐ das stabilste ist, jedoch relativ zum H‐Bindungssystem F‐...HOB(OH)2 nur um 33 kJ mol‐1.