Paired Interacting Orbitals Research Articles

A Practical Method for Determining the Cluster Model Size and the Estimation of the Energy Barrier on Heterogeneous Catalysts

A practical method for determining the cluster model size and the estimation of the energy barrier on heterogeneous catalyst, In2O3(111) cluster, on which acetaldehyde is successively converted acetic acid and acetoacetic acid, has been proposed. The cluster size determination is executed as follows; 1) we determine the optimized adsorption structure of reactant, 2) we divide the structure into the reactant fragment and the cluster fragment, and calculate "Paired interacting orbitals (PIO)," 3) we determine the cluster size by using the atomic composition of the PIO of cluster fragment. The estimation of energy barrier is executed as follows; 1) we suppose a transition state, and assume a reactant side intermediate state (RTint), and a product side intermediate state (PDint), 2) we calculate PIOs of the RTint and those of the PDint and compare them, 3) we change the structures of RTint and PDint until the similarities of the PIOs are almost equal, 4) we calculate the total energies of both final intermediates, then, 5) we define an estimated barrier (Ebarriar) according to the following equation;Ebarrier = (ERTint.final +EPDint.final)/2 –Ereactant.

Acid–base concerted mechanism in the dehydration of 1,4-butanediol over bixbyite rare earth oxide catalysts

Catalytic activity of rare earth oxides (REOs) in the vapor-phase dehydration of 1,4-butanediol to produce 3-buten-1-ol varies with lattice parameters of REOs. In order to clarify the adsorption structure and the reaction mechanism, adsorption energy of 1,4-butanediol on bixbyite REO, such as Sc2O3, Y2O3, Dy2O3, Ho2O3, and Er2O3, {222} surface was calculated with density functional theory (DFT), and paired interacting orbitals (PIO) calculation of the adsorption state between 1,4-butanediol and Er2O3 was executed. The DFT study elucidates that the catalytic activity is correlated with adsorption energy. The PIO study clarifies the interactions between the reactive atoms of 1,4-butanediol and Er2O3 surface: tridentate interactions between a position-2 hydrogen atom of diol and an oxygen anion on Er2O3 and between each OH group of diol and erbium cations on Er2O3, and an intramolecular repulsive interaction between the position-1 carbon atom and the oxygen atom of OH group are observed. These results suggest that the position-2 hydrogen atom is firstly abstracted by a basic oxygen anion and that the position-1 hydroxyl group is subsequently abstracted by an acidic erbium cation. Another OH group on position 4 plays an important role of anchoring the diol to the Er2O3 surface. Therefore, it is proved that the dehydration of 1,4-butanediol over REOs proceeds via acid–base concerted mechanism.

Paired interacting orbitals (PIO) study of Mo/SiO2 and Mo/HZSM-5 catalysts for olefin metathesis

Mo/SiO2 and Mo/HZSM-5 olefin metathesis catalysts are theoretically studied using cluster models and the paired interacting orbitals (PIO) method based on the extended Hückel theory. The first step of the catalytic cycle of ethene metathesis, i.e., ethene cycloaddition to the surface Mo methylidene species leading to the molybdacyclobutane formation is analysed. The electronic structure of the Mo ethene–methylidene complexes and the transition states is discussed for different models of the catalysts. It is shown that new MoC and CC bonds are formed in an asynchronous manner. The results support the proposal that the total overlap population between the orbitals of the Mo methylidene and C2H4 fragments of the surface ethene complexes can be an approximate reactivity index for supported Mo catalysts of olefin metathesis.

Theoretical study of heterogeneous Ziegler-Natta catalysts: A comparison between TiCl3 catalysts and MgCl2 supported catalysts by using paired interacting orbitals (PIO) analysis

Two types of industrial heterogeneous Ziegler-Natta propylene polymerization catalysts, TiCl3 type and MgCl2 supported TiCl4 type, were examined by paired interacting orbitals (PIO) analysis proposed by Fujimoto et al.. The ligand located trans to propylene was found to obstruct the insertion but was essential for the stereospecific insertion of propylene. The ligand located trans to the alkyl group of the active site was found to be necessary for activating the alkyl-Ti bond. The regioselectivity and stereoselectivity of the iso-specific polymerization site on the heterogeneous Ziegler-Natta catalyst system were confirmed. The polymerization activity of MgCl2 supported TiCl4 catalyst was larger than that of the TiCl3 catalyst.

DFT and PIO study of the influences of Mo valance state and surface hydroxyl on supported-MoOx catalysts for ethylene polymerization

Phillips catalyst (CrOx/SiO2) is an important industrial catalyst for ethylene polymerization, but the highly toxic chromium may contaminate the environment and do harm to human's health. Supported-MoOx catalyst with low toxicity has potential to replace Phillips catalyst if its catalytic performance could be improved. In this work, models of molybdenum active sites with different valence states (5+, 4+, 3+, 2+) supported on Al2O3 and SiO2, respectively, were established to investigate the effects of the valence states and surface hydroxyl on the catalyst activity using the combination of density functional theory (DFT) and paired interacting orbitals (PIO) methods. DFT results showed that supported Mo2+ center had the lowest energy barrier of ethylene insertion and thus possessed the highest activity for ethylene polymerization. PIO method additionally elucidated the orbital interaction and electron transformation between Mo2+ center and ethylene monomer. Hydroxyl on the support surface could poison the active center by coordination with Mo center. It had been demonstrated that pre-reduction of hexa-valent molybdenum into lower valence state +2 and elimination of surface hydroxyl groups of support were the key factors to obtain highly efficient ethylene polymerization catalyst. The molecular modeling results of this work provided theoretical basis for further experimental developments of green and highly efficient supported Mo-based polyethylene catalysts.

Paired interacting orbitals (PIO) analysis of electronic structures of copper(I) complexes with hydrotris(pyrazolyl)borate ligands

The influence of the hydrotris(pyrazolyl)borate (tpzb) ligand substituents on the electron delocalization of the copper(I) center with carbon monoxide and triphenylphosphine, i.e. the copper(I) carbonyl complexes, [Cu{HB(3,5- iPr 2pz) 3}(CO)] ( 1), [Cu{HB(3,5-Me 2pz) 3}(CO)] ( 2), [Cu{HB(pz) 3}(CO)] ( 3), [Cu{HB(3-CF 3-5-Mepz) 3}(CO)] ( 4), and the copper(I) triphenylphosphine complexes [Cu{HB(3,5- iPr 2pz) 3}(PPh 3)] ( 5), [Cu{HB(3-CF 3-5-Mepz) 3}(PPh 3)] ( 6), are studied by using paired interacting orbitals (PIO) analysis. Carbon monoxide is a weak σ-donor and strong π-acceptor ligand. The Cu(I)–CO interaction is entirely dominated by π back bonding between the two degenerate π∗ orbitals of carbon monoxide and two t 2 type d orbitals of copper(I) ion. This interaction is clearly expressed by two PIOs, PIO-2 and PIO-3, and the strength of this π-acceptability is estimated by the summation of their overlap populations (ΣOP). On the other hand, triphenylphosphine is a strong σ-donor ligand. The Cu(I)–P interaction is dominated by donation from the P atom to the copper(I) ion and expressed by PIO-1, and the strength of this donative interaction ( σ-donationability) is estimated by the overlap population of PIO-1. These results are consistent with experimental data of the C O stretching frequencies by IR spectroscopy and of 13C chemical shifts of 13C O and the broadness of the 31P signals of PPh 3 by 13C and 31P NMR spectroscopy.

Theoretical study on selective oxidation of olefin and alcohol with Mo–peroxo amine complex using “paired interacting orbitals (PIOs)” analysis

Oxidation mechanisms of ethylene and methanol on Mo(O)(OO) 2 (an amine) and the influence of amines on the oxidation are studied by using paired interacting orbital (PIO) analysis of the transition states (TS) of the ethylene epoxidation and of the methanol coordinated complexes, the methoxy complexes, and the hydrogen abstraction of the methanol oxidation. In the case of ethylene oxidation, we are able to recognize three important PIOs: PIO-1 which expresses the electron delocalization from the π-orbital of the ethylene to the σ*-orbital(O 3–O 4) of the Mo peroxo; PIO-2 which expresses the electron delocalization from the p-orbital of O 4 of the Mo peroxo to the π*-orbital of the ethylene; and PIO-3 which expresses the overlap repulsion between the ethylene and occupied (Mo–O 3–O 4) orbitals of the Mo peroxo. This overlap repulsion is enhanced by the coordination of higher alkyl amines to the Mo peroxo, because of the destabilization of these occupied (Mo–O 3–O 4) orbitals. In the case of methanol oxidation, the main components of the PIO-1 of the methanol coordinated complexes and the methoxy complexes are Mo-4d orbitals of the unoccupied orbitals of the Mo peroxo part, and the main components of the PIO-1 of the hydrogen abstraction are O 5-2p, O 6-2p and C-2p orbitals of SOMO of the Mo peroxo part. The energy of the MOs, that contain these Mo-4d, O-2p or C-2p orbitals are not influenced by the amine coordination on the Mo atom. Results suggest that ethylene epoxidation is hindered by the coordination of higher alkyl amines on Mo peroxo complexes, whereas methanol oxidation is not hindered by the coordination of the amines.

Paired interacting orbitals (PIO) study of molybdena-alumina system active in alkene metathesis

The first step of the catalytic cycle of ethene metathesis on molybdena-alumina catalyst, i.e., ethene addition to surface Mo methylidene centres, is investigated by using the paired interacting orbitals (PIO) analysis based on the extended Hückel method. It is shown that the PIO theory is an adequate approach for both qualitative and quantitative description of the process. The total overlap population between the Mo methylidene part and the C 2H 4 fragment of the Mo ethene–methylidene complex decreases with a decrease of the reactivity of the Mo methylidene centre towards ethene. Therefore, the total overlap population is expected to be a useful reactivity index for the molybdenum-based olefin metathesis catalysts.

SIMULATION MODELING ON THE COORDINATION MECHANISM OF ETHYLENE MONOMER ON VARIOUS PREREDUCED Cr(II)Ox/SiO2 PHILLIPS POLYETHYLENE MODEL CATALYSTS

As one of the most important catalysts in polyethylene industry, Phillips catalyst (CrOx/SiO2) was quite unique for its activation by ethylene monomer without using any activator like alkyl-aluminium or MAO. In this work, the density functional theory (DFT) calculation combined with paired interacting orbitals (PIO) method was applied for the theoretical studies on coordination reaction mechanism between ethylene monomer and two model catalysts namely Cr(II)(OH)2 (M1) and silsesquioxane-supported Cr(II) (M2) as surface Cr(II) active site precursors on Phillips catalyst at the early stage of ethylene polymerization. Unexpected multiplicity of the coordination states of ethylene monomer on both M1 and M2 model catalysts had been first reported on a molecular level. In general, increasing the coordination numbers of ethylene, the corresponding binding energy per ethylene for all the complexes was decreased. The supporting effect of chromium oxide onto silica gel surface was found to be destabilizing the corr...

Theoretical study on reaction between bis(μ-peroxo)dicopper(2)complex and phenol by “paired interacting orbitals (PIO)” analysis

The reaction between μ-η 2:η 2-dibridging peroxodicopper complex of bis[3-(2-hydroxybenzylideneamino)phenyl]sulfone and phenol are investigated by performing paired interacting orbitals (PIO) analysis proposed by Fujimoto et al. Perusal of the contour maps and overlap populations of main PIOs suggested that formation of mono-phenoxo complex is easy; however di-phenoxo complex formation is difficult because of the non-flexibility of the SO 2-bridged ligand. This result supplies the reason why poly (phenylene oxide) was not formed in our system.

Simulation of pathways for CO oxidation over Au nano-clusters by paired interacting orbitals (PIO) analysis

CO oxidation pathways over Au nano-clusters, such as face centered cubic Au 14, icosahedron Au 13, cubo-octahedron Au 13 and tetrahedron Au 20 were investigated by using PIO analysis proposed by Fujimoto et al. Four types of oxidation pathways were examined: reaction between (a) adsorbed CO and gaseous O 2 (E–R mechanism), (b) adsorbed O 2 and gaseous CO (E–R mechanism), (c) adsorbed CO and vicinally adsorbed O 2 (L–H mechanism), and (d) adsorbed CO and geminally adsorbed O 2 (L–H mechanism). Only in the case of L–H mechanism, when CO molecule and O 2 molecule are adsorbed on adjacent two surface Au atoms in vicinal manner (c), CO oxidation occurs favorably. In comparison of the vicinal CO and O 2 co-adsorption on icosahedron Au 13 with that on cubo-octahedron Au 13, CO oxidation is favorable in the former, whereas it is unfavorable in the latter because of considerably longer distance between CO and O 2. It is also predicted that CO oxidation is also favorable in the case of vicinally co-adsorbed state of CO and O 2 on tetrahedron Au 20. The present work has shown that the PIO analysis based on extended Hückel MO is an effective way to investigate reaction pathways, especially in large catalytic systems.

Theoretical Study of the Monomer Reaction Mechanism on Phillips CrOx/SiO2 Catalyst Using Density Functional Theory (DFT) and Paired Interacting Orbitals (PIO) Methods

The importance of Phillips CrOx/SiO2 catalyst could still be reflected from its 7 million tons of world HDPE production. However, the activation mechanism of this catalyst by ethylene in terms of active sites formation still remains unclear. In this work, the paired interacting orbital (PIO) method combined with density functional theory (DFT) calculation was applied for the theoretical studies on the intermolecular orbital interactions between ethylene monomer and a molecular model of surface monochromate species under six typical intermolecular geometric orientations (GO-1∼GO-6). Both DFT and PIO results indicated that GO-2 orientation should be the most preferential orientation for the reaction between ethylene monomer and monochromate species. Within GO-2 orientation the ethylene monomer preferentially approaches to the surface monochromate Cr(VI) species in a symmetric orientation relative to the two carbon atoms of ethylene from the upper site between the two double-bonded oxo-atoms of surface monochromate species. The electronic and orbital origin of the GO-2 orientation was elucidated in terms of its low energy increase due to lower repulsive interaction and in-phase overlap of molecular orbital interaction in-between the intermolecular frontier region.

New reaction simulator "LUMMOX" and its application for prediction of catalytic activities.

We developed a new reaction simulator, "LUMMOX." It is an intermolecular interaction analyzer based on the theories of paired interacting orbitals (PIOs) and localized frontier orbitals (LFOs) that have been developed by Fujimoto et al. (Fukui, K.; Koga, N.; Fujimoto, H. J Am Chem Soc 1981, 103, 196; Fujimoto, H.; Koga, N.; Fukui, K. J Am Chem Soc 1981, 103, 7452; Fujimoto, H.; Satoh, S. J Phys Chem 1994, 98, 1436). LUMMOX runs on a Windows PC and displays graphic representation of orbital interactions. Prediction of activities, selectivities, and molecular weight of olefin polymerization catalysts are presented using PIO analysis and LFO calculation. Not only computational chemists but also experimental chemists can easily use this new system for catalyst design or molecular design from the point of view of orbital interaction.

A proposal on an index of activities and selectivities of olefin polymerization catalysts by using 'paired interacting orbitals (PIO)' analysis

Olefin insertion and chain transfer to monomer on [Oh-CH3MCl4]m(M = Ti, Zr, m = –1, –2: models of Ziegler-Natta catalysts) and [Td-CH3ML2]m(M = Ti, Zr; L = Cl, Cp m = 0, +1: models of metallocene catalysts) were analyzed by paired interacting orbitals (PIO) proposed by Fujimoto et al. Polymerization activities, molecular weights and regioselectivities of propylene insertion were easily predicted by using the total overlap population of all PIOs as an index of catalytic reactions.

Theoretical study of ethylene polymerization on Ziegler–Natta catalysts and on metallocene catalysts

Ethylene insertion on O h-d 1CH 3Ti n chloride clusters/C 2H 4 (model systems of Ziegler–Natta catalysts) and T d-d 0,-d 1CH 3TiCp 2/C 2H 4 (model systems of metallocene catalysts) were studied by using paired interacting orbitals (PIO) analysis and LFO calculation. Electron delocalization from catalytic site to ethylene and that from ethylene to catalytic site played a crucial role in ethylene insertion. The former depends on the nucleophilicity of the active site and the latter on the electrophilicity of the active site. They were quantitatively estimated by LFO calculation. In the case of O h systems, the electrophilicity of the catalyst decreased because of the Cl anion located trans to the ethylene in the reaction plane. In the case of Td systems, since the electrophilicity was not weakened because of the absence of the trans ligand to the ethylene, the nucleophilicity and electrophilicity were well balanced. The O h-d 1Ti 4 cluster could be a suitable model of the active site on the TiCl 3 crystalline surface.

A theoretical study of Ziegler-Natta olefin polymerization on the TiCl 3 crystalline surface

Olefin insertion into Ti-methyl bonds of methyltitanium chloride clusters on a TiCl 3 crystalline surface has been studied by using paired interacting orbitals (PIO). We have shown that at least two types of active sites can be assumed on the edge of the basal face of violet TiCl 3 crystallites: a edge type active site which possesses a dangling bonded Cl atom, four bridged Cl atoms and a Cl vacancy and a corner type active site which possesses a dangling bonded Cl atom, three bridged Cl atoms and two Cl vacancies. The size effects on these active site have been estimated by changing the number of titanium atoms of the TiCl 3 model clusters. The most important interaction in the olefin coordinated state is electron delocalization from the occupied Ti d xz orbital of the methyltitanium chlorides to the π∗ orbital of the coordinated olefin molecule, while in the transition state is electron delocalization from the occupied C p and Ti d xz orbital of the methyltitanium chloride species to the π∗ orbital of the olefin. These interactions are compactly shown in PIO's. The coordination energy and the activation energy for the olefin insertion are almost independent of the cluster size when the number of Ti atoms is greater than three. The corner type active site has been shown to be stabilized at the transition state by the absence of one of the Cl atoms which are located orthogonal to the insertion plane. Thus, the olefin insertion on the corner type active site is more favorable than that on the edge type active site. In the case of propylene insertion, syn-orientation of propylene is favored. This regioselectivity should be lower on the corner type active site than that on the edge type active site.

Paired interacting orbitals (PIO) study on the formation of the active site in Ziegler catalyst system

The mechanism of the active site formation in Ziegler polymerization catalysts has been studied using the paired interacting orbitals (PIO) based on an extended Hückel calculation. Deformation of a titanium chloride molecule and partial dissociation of an organoaluminum dimer have been shown to take place by approaching the two species together. A six membered cyclic intermediate complex has been formed in a reaction plane, involving the Ti atom, the C atom, two Al atoms, the Cl* atom and the bridging atom. An elongation of the CAl bond, Al-bridging atom bond, the TiCl* bond and shrinkage of the CTi bond take place in concert in the transition state region. Key factors of activating the complex are the valency of the titanium and the structure whether or not it has a Cl anion trans to the Cl* atom. An active site is the complex between the alkylated titanium chloride and the aluminum chloride produced in the course of the reaction. The formation of the active site in titanium chlorides/organoaluminum systems has been clarified to become easier in the following order: d 1-Sq (square) pyramidal ⪢ d 0-Sq pyramidal ⩾ d 0-hinge like titanium chloride. d 0-triangle pyramidal titanium chloride is unfavorable for the formation of active sites.

Theoretical study of regioselectivity in propylene insertion into Ti-methyl bonds of methyltitanium chlorides: Analysis by paired interacting orbitals

The effect of structural changes of methyltitanium complexes on regioselectivity in propylene insertion into the methyl-Ti bond was studied by applying paired interacting orbitals (PIO). Electron delocalization from the HOMO of the methyltitanium fragment to the π* orbital of the olefin and electron delocalization from the π orbital of the olefin to the unoccupied orbitals of the methyltitanium fragment are important for the olefin insertion. At the same time a repulsive orbital interaction arises between the HOMO of the methyltitanium fragment and the π orbital of the olefin. The HOMO of the d 0-methyltitanium compounds is composed mainly of C m p orbitals and subsidiarily of Ti d z 2 orbital, the composition being not sensitive to the structure of the methyltitanium compounds. The π orbital of propylene has a slightly larger amplitude on C β, while the π* orbital has a slightly larger amplitude on C α. When the methyltitanium compounds and the propylene interact in the syn-form, electron delocalization is facilitated and overlap repulsion is reduced. The primary insertion of propylene is thus favored. The repulsion appears between the methyl group of titanium compounds and the methyl group of propylene. In the anti-form, the repulsion exists between the ligand of the titanium compound and the methyl group of propylene. The repulsion in the anti-form is sensitive to the structure of the methyltitanium compounds. Therefore, the more the space around the titanium is filled with ligands, the stronger the repulsion becomes. As a result, the selectivity of primary insertion of propylene on the d 0-methyltitanium complexes becomes higher in the following order: octahedral > square pyramidal > trigonal bipyramidal > tetrahedral. In d 1 complexes electron delocalization occurs strongly from the SOMO of the methyltitanium compound to the π* orbital of propylene both in the syn-form and in the anti-form, however this electron delocalization is not conducive to insertion. The primary insertion is favoured, but it is not so easy as in the d 0 complexes. By using PIO analyses, we have clarified the importance of the delocalization interaction in determining the regioselectivity in the insertion of propylene into methyl-Ti bonds.

Prediction of reactivities of titanium—methyl bonds in O hd 0 methyltitanium chlorides for insertion of ethylene: Analysis by ‘paired interacting orbitals’ (PIO)

The effect of ligands on ethylene insertion into the methyl-titanium bond of O h -d 0 methyltitanium complexes was studied using paired interacting orbitals (PIO). The most important contribution to this insertion was shown to be electron delocalization from the methyltitanium compound to ethylene, and the next is electron delocalization from ethylene to the methyltitanium compound. The ligand effect, which favors ethylene insertion into the O h d 0 methyltitanum complexes, is summarized as follows: an electron-attracting ligand should be placed at a position trans to ethylene, and an electron-donating one trans to the methyl group; ligands placed at the positions perpendicular to the reaction plane play the role of introducing the ethylene monomer into the reaction site. The effect of coordination of MgCl 2 on the insertion is also discussed.

A theoretical study of the insertion of olefins into Timethyl bonds by “Paired Interacting Orbitals”

Insertion of ethylene into the methylTi bond of ([CH 3Ti(Cl) 4/C 2H 4] n , n = −1 or −3) Oh- d 0, - d 2 and [CH 3Ti(Cl) 2/C 2H 4] n , n = +1 or −1) Td- d 0, - d 2 methyltitanium complexes has been studied by applying Paired Interacting Orbitals (PIO). It is important for the insertion that the PIOs are in-phase between the methyl carbon and the ethylene C β, and between the Ti and the ethylene C α. This condition is satisfied in d 0 complexes. In d 2 complexes the orbital pair in the PIO-1, which consists mainly of the occupied Ti d xz and the unoccupied ethylene π * MO, overlap out-of-phase between the methyl carbon and the ethylene C β. We can predict that, whereas the ethylene insertion is facile in d 0 methyltitanium complexes, it is not in d 2 complexes.