Density functional studies on chromium catalyzed ethylene trimerization

Mechanism of ethylene trimerization using chromium catalyst is investigated using density functional methods. Recent experimental results indicate Cr-based homogeneous catalysts to follow metallacycle pathway in ethylene tri-, teta- and oligomerization reactions. Given the importance of chlorinated Cr-based active catalysts in these reactions, we have used “bare” minimal ligands like Cl^? and considered catalytic cycles with neutral or cationic intermediates starting with [Cr(II)Cl₂(ethylene)₂] and [Cr(II)Cl(ethylene)₂]⁺, respectively. We have compared both ‘Cossee’ and the ‘metallacycle’ mechanisms on these model systems utilizing density functional computations at B3LYP/LANL2DZ(d,p) level. The metallacycle mechanism with cationic Cr(II)–Cr(IV) intermediates is found to be the most favored path, with oxidative coupling of two coordinated ethylene to form the chromacyclopentane being the rate determining step (RDS). We also found that with neutral intermediates the Cossee pathway rather than the metallacycle mechanism is followed. Thus in spite of the simplicity of using just Cl^? as ligand in the model catalytic intermediates, our computational results match remarkably well with many recent and important experimental findings.     

Structural diversity of metallacycle intermediates for ethylene dimerization on heterogeneous NiMCM-41 catalyst: a quantum chemical perspective

Structural Chemistry - Nanocluster models were investigated to explore the diversity of metallacycle intermediates for ethylene dimerization over NiMCM-41 at B3LYP/6-311+G* and M06/Def2-TZVP. The...     

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 eflfect of chromium oxide onto silica gel surface was found to be destabilizing the corresponding complexes and decreasing the multiplicity of the coordination states as well due to both electronic and steric effect.Moreover.tri-and tetra-or higher ethylene coordination states could not be possibly formed on the supported catalyst as on the Cr(II)(OH)2.The optimized complex geometries were adopted for determining the intermolecular orbital interactions.In-phase overlap orbiral interaction for all the molecular complexes indicated favorable coordination between ethylene and Cr(II)sites.The molecular orbital origin of the π-bonded Cr(II),and mono-and di-C2H4 M1 complexes had been elucidated by PIO method showing high possibility of the formation of metallacyclopropane or metallacyclopentane active sites in the subsequent initiation of polymerization stage.     

Theoretical Investigation for Two-state Reactivity of CO_2 Hydrogenation Catalyzed by Ru in Gas Phase

Gas-phase CO_2 catalyzed activation hydrogenation by Ru atoms was studied with density functional theory. Based on the structure optimization of different potential energy surfaces,there are two crossing points between singlet and triplet potential energy surfaces and there is a crossing point between quintet and triplet potential energy surfaces in the whole catalytic cycle. Spin transition probabilities in the vicinity of the intersections have been calculated by the Landau-Zener model theory. There are three minimum energy crossing points(MECPs) with strong spin-orbital coupling effect and higher spin transition probability,and all spin inversion occurred in s orbital and different d orbitals of ruthenium,indicating this is a typical two-state reactivity(TSR) reaction. Finally,the lowest energy reaction path is ensured.     

Theoretical study on the reactions of Nb and Nb+ with CO2 in gas phase

Density functional theory calculations have been performed to explore the potential energy surfaces of C? O bond activation in CO₂ molecule by gas‐phase Nb atom and Nb⁺ cation for better understanding the reaction mechanism of second‐row metal with CO₂. The minimum‐energy reaction path is found to involve the spin inversion in the different reaction steps. This potential energy curve‐crossing dramatically affects the reaction energetic. The present results show that the mechanism is insertion‐elimination mechanism along the C? O bond activation reaction. All theoretical results not only support the existing conclusions inferred from early experiment but also complement the pathway and mechanism for this reaction. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Probes of spin conservation in heavy metal reactions: experimental and theoretical studies of the reactions of Re+ with H2, D2, and HD

A guided ion beam tandem mass spectrometer is used to examine the kinetic energy dependence of reactions of the third-row transition metal cation, Re(+), with molecular hydrogen and its isotopologues. A flow tube ion source produces Re(+) in its (7)S(3) electronic ground state. Reaction with H(2), D(2), and HD forms Re H(+)(Re D(+)) in endothermic processes. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energy of D(0)(Re(+)-H)=2.29+/-0.07 eV (221+/-6 kJ/mol). The experimental thermochemistry is consistent with ab initio calculations, performed here and in the literature. Theory also provides the electronic structures of these species and is used to examine the reactive potential energy surfaces. Results from reactions with HD provide insight into the reaction mechanisms and indicate that the late metal ion, Re(+), reacts largely via a statistical mechanism. This is consistent with the potential energy surfaces which locate a stable Re H(2) (+)((5)B(2)) complex. Results for this third-row transition metal system are compared with the first-row congener (Mn(+)) and found to have much higher reactivity towards dihydrogen and stronger M(+)-H bonds. These differences can be attributed to efficient coupling among surfaces of different spin along with lanthanide contraction and relativistic effects.     

Reaction of bare VO+ and FeO+ with ammonia: a theoretical point of view

The potential energy surfaces corresponding to the dehydration reaction of NH(3) by VO(+) ((3)Sigma, (1)Delta, (5)Sigma) and FeO(+) ((6)Sigma, (4)Delta) metal oxide cations have been investigated within the framework of the density functional theory in its B3LYP formulation and by employing new optimized basis sets for iron and vanadium. The reaction is proposed to occur through two hydrogen shifts from the nitrogen to the oxygen atom giving rise to multicentered transition states. Possible spin crossing between surfaces at different spin multiplicities has been considered. The energy profiles are compared with the corresponding ones for the insertion of bare cations to investigate the influence on reactivity of the presence of the oxygen ligand. The topological analysis of the gradient field of the electron localization function has been used to characterize the nature of the bonds for all the minima and transition states along the paths.     

Proposal for the metallacycle pathway during the cyclopropanation catalyzed by cobalt-Schiff base complexes

[reaction: see text] The density functional study of the cobalt(II) complex-catalyzed cyclopropanation revealed that the cobalt(II)-Schiff base complex without an ethylene bridge would be flexible enough to transform into the cis-beta conformer while approaching the olefin; consequently, the metallacycle pathway would be preferred, while the ethylene bridge would stabilize the planar conformer to allow reaction via the concerted mechanism.     

Insights into the cycloaddition reaction mechanism between ketenimine and unsaturated hydrocarbon: A theoretical study

The cycloaddition reaction mechanisms between interstellar molecule ketenimine and unsaturated hydrocarbon (ethyne and ethylene) have been systematically investigated employing the second-order Møller-Plesset perturbation theory (MP2) method. Geometry optimizations and vibrational analyses have been performed for the stationary points on the potential energy surfaces of the system. The calculated results show that it can be produced the five-membered cyclic carbene intermediates through pericyclic reaction processes between ketenimine and ethyne (or ethylene). For the reaction between ketenimine and ethyne, through the following H-transferred processes, carbene intermediate can be isomerized to the pyrrole compounds. For the reaction between ketenimine and ethylene, carbene intermediate can be isomerized to the pyrroline compounds. The present study is helpful to understand the reactivity of nitrogenous cumulene ketenimine and the formation of prebiotic species in interstellar space.     

Formation and decomposition of C3 metallacycles from ethylene and methylene on MoAl alloy thin films

The reaction between adsorbed ethylene and methylene species has been investigated on a molybdenum-aluminum alloy grown from Mo(CO)(6) on a planar alumina film formed on a Mo(100) single crystal in ultrahigh vacuum. Di-sigma-bonded ethylene reacts with carbene species, formed on the surface from methylene iodide, to form a C(3) metallacycle. This predominantly decomposes to yield propylene, while a smaller portion yields cross-metathesis products since (12)C(13)CH(4) is formed from reaction between (13)C(2)H(4) and (12)CH(2). This work demonstrates for the first time that the reaction proceeds in heterogeneous phase via a C(3) metallacycle as proposed in the Hérisson-Chauvin mechanism.     

The Kinetic Isotope Effect as a Probe of Spin Crossover in the C?H Activation of Methane by the FeO+ Cation

Two‐state reactivity (TSR) is often used to explain the reaction of transition‐metal–oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum‐mechanical calculations for energy profiles with a spin crossover in the rate‐limiting step. To prove the TSR concept, kinetic profiles for C H activation by the FeO⁺ cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H‐transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition‐state theory including multidimensional tunneling. The minimum energy crossing point with very large spin–orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H‐transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model.     

The Kinetic Isotope Effect as a Probe of Spin Crossover in the CH Activation of Methane by the FeO+ Cation

Two‐state reactivity (TSR) is often used to explain the reaction of transition‐metal–oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum‐mechanical calculations for energy profiles with a spin crossover in the rate‐limiting step. To prove the TSR concept, kinetic profiles for C? H activation by the FeO⁺ cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H‐transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition‐state theory including multidimensional tunneling. The minimum energy crossing point with very large spin–orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H‐transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model.     

Experiment and theoretical modeling of the luminescence of silver nanoclusters dispersed in oxyfluoride glass

Density functional theory (DFT) and complete active space perturbation theory (CASPT2) have been applied for modeling the configuration, charge, energy states, and spin of luminescent Ag nanoclusters dispersed within the bulk of oxyfluoride glass host. The excitation spectra of luminescence of the Ag nanoclusters have been measured and simulated by means of the DFT and CASPT2. Electron spin resonance spectra have been recorded and suggest diamagnetic state of Ag nanoclusters. The silver nanoclusters have been argued to consist mostly of pairs of Ag(2) (+) dimers, or Ag(4) (2+) tetramers, with different extent of distortion along the tetramer diagonal. The sites for the Ag nanoclusters have been suggested where the pairs of Ag ions substitute onto metal and hole cation sites and are surrounded by fluorine ions within a fluorite-type lattice.     

气相VO（∑＋）与CH3OH（1^A′）分子自旋禁阻反应C—H，O—H键活化机理的理论研究

采用密度泛函理论（DFT）B3LYP与cCsD方法研究了二重态和四重态势能面自旋禁阻反应VO（∑’）活化cH30H（1^A′）分子c—H,0—H键的微观机理．通过自旋一轨道耦合的计算讨论了势能面交叉点和可能的自旋翻转过程．在MEcP处,四重态和二重态问的旋轨耦合常数为131．14cm^-1．自旋多重度发生改变,从四重态系间穿越到二重态势能面形成中间体2^IM1,导致反应势能面的势垒明显降低．     

Decomposition of CH2O by lanthanum: a theoretical study

This work aims to investigate the reaction mechanism of lanthanum atom with formaldehyde in the gas phase using density functional theory and coupled cluster calculations. The results indicate that the minimum energy pathway, similar to the reactions of its neighboring yttrium with formaldehyde, is the formation of the eta2-formaldehyde-metal complex followed by two C-H insertions which leads to metal dihydrides and carbon monoxide. The competing pathway producing a metal-carbonyl compound and hydrogen molecule favors a high-spin state and thus involves a spin conversion from doublet state to quartet state. The crossing region of the doublet and quartet potential energy surfaces (PES) has been estimated by a simple approach as proposed by Yoshizawa et al. Less favorable pathways leading to metal monoxide and carbene radical by C-O insertion as well as formyllanthanum by single C-H insertion are also studied. Compared with the CCSD(T) method, the BP86 method tends to overestimate the binding energies of the d-rich compounds, though the two methods qualitatively agree well on the reaction mechanism. Finally, the (n - 1)d1ns2 to (n - 1)d2ns1 promotion effect is proposed to account for the difference in the formation mechanism of the metal-carbonyl compounds LaCO and YCO, which may also extend to the reactions of formaldehyde with other "general" group III rare earth elements including Sc, Ce, Gd, and Lu.     

The Dewar benzene radical cation and its ring-opening reaction

The radical cation of Dewar benzene, 1*+, has been generated and observed by optical spectroscopy in cryogenic matrices. 1*+ distinguishes itself by a charge resonance band at 600 nm, very similar in shape and position to that observed for the related radical cation of norbornadiene. This coincidence indicates that in ground-state 1*+ the odd electron is also located in a pi-MO. The energy of the charge resonance transition, which is very sensitive to the dihedral angle between the four-membered rings in 1*+, is predicted consistently too low by TD-DFT and CASPT2. Probably this angle is too large in the B3LYP and CASSCF geometries. As 1*+ can be observed at 77 K, it must be separated by a barrier of at least 7-8 kcal/mol from its very exothermic decay to the radical cation of benzene, 2*+. An analysis shows that the ring-opening of 1*+ is a multistep process involving two avoided crossings between potential surfaces of different symmetry and electronic nature. Owing to the orbital symmetry-forbidden nature of the process, the energy of 1*+ starts by increasing steeply on stretching the central C-C bond, but then the system undergoes a crossing to a 2A1 surface which leads adiabatically to an excited state of 2*+. Therefore, another avoided crossing must be transited before the molecule can decay on the ground-state surface of 2*+. The rearrangement of 1*+ to 2*+ is an example of a "pseudodiabatic" thermal reaction that transits between potential surfaces representing very different electronic structures.     

Re-examining the Mechanisms of Competing Pericyclic Reactions of 1,3,7-Octatriene

DFT (both B3LYP and M06-2X), CASSCF, and CASPT2 calculations were used to investigate competing [3,?3] and [3,?5] sigmatropic shifts and intramolecular [4+2] cycloaddition of 1,3,7-octatriene. In accord with previous results on 1,5-hexadiene, CASSCF calculations found both stepwise and concerted pathways for the [3,?3] rearrangement. For the competing [3,?5] sigmatropic rearrangement, CASSCF and CASPT2 calculations revealed three stepwise pathways with similar barriers. UB3LYP and UM06-2X calculations predicted a different potential energy landscape: no stepwise [3,?3] pathway, only two competing [3,?5] sigmatropic shifts, and an intramolecular Diels-Alder cycloaddition/homolytic ring-opening pathway. Significant lowering of barriers for all rearrangements was predicted for some 1,3,7-octatrienes with substituents at the 4- and 7-positions.     

Re‐examining the Mechanisms of Competing Pericyclic Reactions of 1,3,7‐Octatriene

DFT (both B3LYP and M06‐2X), CASSCF, and CASPT2 calculations were used to investigate competing [3, 3] and [3, 5] sigmatropic shifts and intramolecular [4+2] cycloaddition of 1,3,7‐octatriene. In accord with previous results on 1,5‐hexadiene, CASSCF calculations found both stepwise and concerted pathways for the [3, 3] rearrangement. For the competing [3, 5] sigmatropic rearrangement, CASSCF and CASPT2 calculations revealed three stepwise pathways with similar barriers. UB3LYP and UM06‐2X calculations predicted a different potential energy landscape: no stepwise [3, 3] pathway, only two competing [3, 5] sigmatropic shifts, and an intramolecular Diels–Alder cycloaddition/homolytic ring‐opening pathway. Significant lowering of barriers for all rearrangements was predicted for some 1,3,7‐octatrienes with substituents at the 4‐ and 7‐positions.