Metal catalyzed ethylene epoxidation:A comparative density functional theory study

Ethylene epoxidation on Ag(111), Pt(111), Rh(111) and Mo(100) has been studied by density functional theory (DFT) calculations. The results show that the adsorption energies of possible adsorbed species involved in the ethylene epoxidation increase in the order: Ag相似文献   

Mechanism of olefin epoxidation with transition metal peroxo complexes: DFT study

Using density functional calculations over the last decade led to considerable progress in understanding the mechanism of olefin epoxidation with Ti, V, Mo, W, and Re peroxo complexes. According to calculations, the reaction occurs by direct electrophilic transfer of one of the atoms of the peroxo group to the olefin. The alternative stepwise mechanism, which has been discussed for a long time and suggested the formation of a metallocyclic intermediate, is characterized by higher activation barriers than direct transfer. The electrophilic character of the direct transfer of oxygen was interpreted at the level of molecular orbital analysis as interaction between the HOMO of the olefin π(C-C) and the LUMO of the peroxo group σ*(O-O). The factors determining the activity of various metal complexes in epoxidation were examined in relation to the ligand environment and the structure of the peroxo group.     

Olefin epoxidation by molybdenum peroxo compound: molecular mechanism characterized by the electron localization function and catastrophe theory

The oxygen atom transfer reaction from the Mimoun-type complex MoO(η(2)-O(2))(2)OPH(3) to ethylene C(2)H(4) affording oxirane C(2)H(4)O has been investigated within the framework of the Bonding Evolution Theory in which the corresponding molecular mechanism is characterized by the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT). Topological analysis of ELF and electron density analysis reveals that all Mo-O bonds in MoO(η(2)-O(2))(2)OPH(3) and MoO(2)(η(2)-O(2))OPH(3) belong to closed-shell type interactions though negative values of total energy densities E(e)(r(BCP)) imply some covalent contribution. The peroxo O(i)-O(j) bonds are characterized as charge-shift or protocovalent species in which pairs of monosynaptic basins V(3)(O(i)), V(3)(O(j)) with a small electron population of ~0.25e each, are localized between core basins C(O(i)), C(O(j)). The oxygen transfer reaction from molybdenum diperoxo complex MoO(η(2)-O(2))(2)OPH(3) to C(2)H(4) system can be described by the following consecutive chemical events: (a) protocovalent peroxo O(2)-O(1) bond breaking, (b) reduction of the double C(1)=C(2) bond to single C(1)-C(2) bond in ethylene, (c) displacement of oxygen O(1) with two nonbonding basins, V(i=1,2)(O(1)), (d) increase of a number of the nonbonding basins to three (V(i=1,2,4)(O(1))); (e) reorganization and reduction in the number of nonbonding basis to two basins (V(i=1,4)(O(1))) resembling the ELF-topology of the nonbonding electron density in oxirane, (e) formation of the first O(1)-C(2) bond in oxirane, (f) C(2)-O(1)-C(2) ring closure, (g) formation of singular nonbonding basin V(O(2)) in new Mo=O(2) bond. The oxygen atom is transferred as an anionic moiety carrying a rather small electronic charge ranging from 0.5 to 0.7e.     

Sulfide and sulfoxide oxidations by mono- and diperoxo complexes of molybdenum. A density functional study

The molecular mechanism for the oxidation of sulfides to sulfoxides and subsequent oxidation to sulfones by diperoxo, MoO(O(2))(2)(OPH(3)) (I), and monoperoxo, MoO(2)(O(2))(OPH(3)) (II), complexes of molybdenum was studied using density functional calculations at the b3lyp level and the transition state theory. Complexes I and II were both found to be active species. Sulfide oxidation by I or II shows similar activation free energy values of 18.5 and 20.9 kcal/mol, respectively, whereas sulfoxides are oxidized by I (deltaG = 20.6 kcal/mol) rather than by II (deltaG = 30.3 kcal/mol). Calculated kinetic and thermodynamic parameters account for the spontaneous overoxidation of sulfides to sulfones as has been experimentally observed. The charge decomposition analysis (CDA) of the calculated transition structures of sulfide and sulfoxide oxidations revealed that I and II are stronger electrophilic oxidants toward sulfides than they are toward sulfoxides.     

Octahedral bipyridine and bipyrimidine dioxomolybdenum(VI) complexes: characterization,application in catalytic epoxidation,and density functional mechanistic study

Complexes of the general formula [MoO(2)X(2)L(2)] (X=Cl, Br, Me; L(2)=bipy, bpym) have been prepared and fully characterized, including X-ray crystallographic investigations of all six compounds. Additionally, the highly soluble complex [MoO(2)Cl(2)(4,4'-bis(hexyl)-2,2'-bipyridine)] has been synthesized. The reaction of the complexes with tert-butyl hydroperoxide (TBHP) is an equilibrium reaction, and leads to MoV(I) eta(1)-alkylperoxo complexes that selectively catalyze the epoxidation of olefins. Neither the Mo-X bonds nor the Mo-N bonds are cleaved during this reaction. These experimental results are supported by theoretical calculations, which show that the attack of TBHP at the Mo center through the X-O-N face is energetically favored and the TBHP hydrogen atom is transferred to a terminal oxygen of the Mo=O moiety. After the attack of the olefin on the Mo-bound peroxo oxygen atom, epoxide and tert-butyl alcohol are formed. The latter compound acts as a competitive inhibitor for the TBHP attack, and leads to a significant reduction in the catalytic activity with increasing reaction time.     

Olefin epoxidation with tert-butyl hydroperoxide catalyzed by MoO2X2L complexes: a DFT mechanistic study

DFT calculations suggest that the catalytic epoxidation of olefins by Mo(vi) complexes, modeled by MoO2Br2(MeN=C(H)-C(H)=NMe), in the presence of MeOOH, the model for tert-butyl hydroperoxide, starts with a hydrogen transfer from the peroxide to one of the terminal Mo=O oxygen atoms and the remaining MeOO anion binds as a seventh ligand, forming a five-membered Mo-O(alpha)-O(beta)(Me)...H-O-Mo ring held together by a hydrogen bond. In the second step, a concerted approach of ethylene to the Mo-O(alpha) bond gives rise to an intermediate containing a seven-membered Mo-C-C-O(alpha)-O(beta)(Me)...H-O-Mo ring. In the final step, decomposition of the intermediate leads to the starting complex, alcohol and the epoxide. The activation energy for the addition of the olefin (second step) is the highest one, in agreement with available kinetic studies showing that the catalyst formation is not always a rate-limiting step. DFT calculations also show that the alcohol by-product (MeOH) can react with the starting complex, competing with ROOH and hence leading to the progressive catalyst poisoning, which has been observed experimentally.     

Peroxovanadate imidazole complexes as catalysts for olefin epoxidation: density functional study of dynamics, 51V NMR chemical shifts, and mechanism

A density functional study of [VO(O(2))(2)(Im)](-) (1, Im = imidazole) is presented, calling special attention to effects of dynamics and solvation on the (51)V chemical shift. According to Car-Parrinello molecular dynamics simulations, rotation of the Im ligand can be fast in the gas phase, but is more hindered in aqueous solution. In the latter, bonding between Im and V is reinforced, and dynamic averaging of GIAO-B3LYP magnetic shieldings affords a gas-to-liquid shift of ca. -100 ppm for delta((51)V). A complete catalytic cycle has been characterized for olefin epoxidation mediated by 1, using H(2)O(2) as oxidant. The rate-determining step is indicated to be initial oxygen atom transfer from 1 to the substrate via a spiro-like transition state. Substituent effects on this barrier are examined, and a significant decrease (by 2-6 kcal/mol) is revealed upon removal of the Im proton or upon complexation with a H-bond acceptor. Implications for the mechanism of the oxidative chemistry of vanadium-dependent haloperoxidases and requirements for prospective biomimetic analogues are discussed.     

Metal ion complexes of thioformin: A density functional study

Some metal ion complexes of thioformin have been investigated by the density functional approach in order to understand its coordination chemistry. Due to the presence of the soft sulfur atom in addition to the oxide ion, thiohydroxamates exhibit a different coordination chemistry compared to their hydroxamate counterparts. It is found that the oxide ion forms bonds with metal ions that are predominantly ionic in nature while the sulfur atom is involved in weak covalent bond formation. Most of the complexes are found to be tetrahedrally distorted square planar structures, showing distinct cis and trans isomers. The exception is Zn(II), which forms tetrahedral complexes. The stabilities of the complexes have been linked with their antibiotic activity.     

A study of resveratrol-copper complexes by electrospray ionization mass spectrometry and density functional theory calculations

Resveratrol is a polyphenolic compound found in plants and human foods which has shown biological activities including chemoprevention, acting through a mechanism which involves the reduction of Cu(II) species. By electrospray ionization (ESI) mass spectrometry we have produced and detected the resveratrol-copper complexes [Resv+Cu](+), [Resv+Cu+H(2)O](+) and [2Resv+Cu](+) by using a resveratrol/CuSO(4) solution in CH(3)CN/H(2)O. The most stable structures of the detected complexes have been calculated at the B3LYP/6-311G(d) level of theory. Resveratrol interacts with the copper ion through nucleophilic carbon atoms on the aromatic ring and the alkenyl group. The fact that only singly charged ions were observed implies that Cu(II) is reduced to Cu(I) in the ESI process. For investigating the structure-reactivity correlation, we have carried out a similar study on the synthetic analogue dihydroresveratrol (DHResv). For the latter only the [DHResv+Cu](+) complex has been detected.     

A density functional study of pseudotetrahedral metal–nitrosyl complexes

Local and nonlocal density functional computations have been carried out to study the electronic structure and the equilibrium geometry of the isoelectronic series Cr(NC)₄, Mn(NO)₃(CO), Fe(NO)₂(CO)₂, and Co(NO)(CO)₃ and model compounds Fe(NO)₂L₂ (L = Cl, HCN, NH₃, PH₃, and C₂H₄). The structure of Fe(NO)₂(C₄H₆) is also described. The discussion is focused on structural modifications through a change of ligand, in particular those concerning the metal-nitrosyl conformation (linear vs. bent). Though this is a preliminary study of metal–nitrosyl properties by DFT methods and more computations are required to analyze the mechanism of homogeneous catalysis processes, our results support the hypothesis that structural reorganization from linear to bent metal–nitrosyl plays a key role in some reactions, such as in butadiene dimerization. © 1994 John Wiley & Sons, Inc.     

Allylic alcohol epoxidation by methyltrioxorhenium: a density functional study on the mechanism and the role of hydrogen bonding.

By locating all relevant transition structures with a hybrid density functional method, we explored the three most reasonable mechanisms for H2O2 epoxidation of propenol catalyzed by methyltrioxorhenium (MTO), namely: (i) coordination of propenol as lone pair donor to rhenium mono- and bis-peroxo complexes followed by intramolecular epoxidation, (ii) formation of a metal alcoholate, derived from addition of propenol to the Re complex with the formation of a metal-OR bond, followed by intramolecular epoxidation, (iii) intermolecular oxygen transfer assisted by hydrogen bonding where the rhenium complex acts as hydrogen bond acceptor and HOR as hydrogen bond donor. The computational results demonstrate that the last route is highly favored over the other two and, in particular, they provide the first unambiguous and compelling evidence that alcoholate-metal complexes, mechanism (ii), do not appreciably contribute to product formation. In keeping with experimental findings, theoretical data predict that the monoperoxo Re complex should be considerably less reactive than its bis(peroxo) counterpart and suggest that the hydrated form of the latter complex should be the actual active epoxidant species. All transition structures exhibit a distorted spiro-like structure, while the most stable ones feature hydrogen bonding to the attacking peroxo fragment with the olefinic OH group either in an "outside" (OC1C2C3 approximately 128 degrees ) or "inside" (OC1C2C3 approximately 14 degrees ) conformation. Previous qualitative models for transition structures of Re-catalyzed epoxidation of allylic alcohols are discussed in the light of our computational data.     

A gas- and condensed-phase density functional study of donor-acceptor complexes of sulfur trioxide

A series of donor-acceptor complexes containing sulfur trioxide have been studied in the gas and condensed phases using density functional theory. The condensed phase is represented using the polarizable continuum model. The systems investigated include complexes of nitrogen-containing donor molecules, (CH(3))(n)H(3-n)N (n = 0-3), with SO(3) and complexes of oxygen-containing donor molecules, (CH(3))(m)H(2-m)O (m = 0-2), with SO(3). Significant differences are observed between the gas- and condensed-phase properties of the complexes as a result of the ability of the condensed-phase medium to support higher charge separation between the donor and acceptor. The gas/condensed-phase behavior of two nitrogen-containing complexes, (CH(3))H(2)N-SO(3) and (CH(3))(2)HN-SO(3), has been investigated for the first time. These complexes exhibit properties intermediate to the previously observed H(3)N-SO(3) and (CH(3))(3)N-SO(3) complexes. Systematic trends in the gas- and condensed-phase structure and properties have been observed as methyl groups are added to the donor molecule. In addition, two oxygen-containing complexes, CH(3)OH-SO(3) and (CH(3))(2)O-SO(3), have been characterized for the first time. The differences between the gas- and condensed-phase properties of the oxygen-containing complexes are, in many cases, larger than those of the nitrogen-containing complexes, and therefore they represent an intriguing new class of complexes for potential experimental observation. Finally, a strong correlation between the charge transfer and binding energy has been obtained for both the nitrogen- and oxygen-containing complexes of sulfur trioxide.     

Disproportionation of Gold(II) complexes. A density functional study of ligand and solvent effects

A computational study of gold(II) disproportionation is presented for the atomic ion as well as complexes with chloride and neutral ligands. The Au2+ atomic ion is stable to disproportionation, but the barrier is more than halved to 119 kcal/mol in an aqueous environment vs 283 kcal/mol in the gas phase. For dissociative disproportionation of chloride complexes, the loss of chlorine, either as an atom (Delta G(aq) = +20 kcal/mol) or as an anion (Delta G(aq) = +15 kcal/mol) represents the largest calculated barrier. The calculated transition state for associative disproportionation is only 9 kcal/mol above separated Au(II)Cl3(-) anions. For the disproportionation of Au(II)L3 complexes with neutral ligands, disproportionation is highly endergonic in the gas phase. Calculations imply that for synthesis of a monometallic Au(II) complex, a nonpolar solvent is preferred. With the exception of [Au(CO)3]2+, disproportionation of Au(II)L3 complexes to Au(I)L and Au(III)L3 is exergonic in solution phase for the ligands investigated. The driving force is provided by the very favorable solvation free energy of the trivalent gold complex. The solvation free energy contribution to the reaction (Delta G(solv)) is very large for small and polar ligands such as ammonia and water. Furthermore, calculations imply that choosing ligands that would yield neutral species upon disproportionation may provide an effective route to thwart this decomposition pathway for Au(II) complexes. Likewise, bulkier ligands that yield larger, more weakly solvated complex ions would appear to be desirable.     

Iodo-methyl ligand exchange reaction in platinum complexes: A density functional study

A theoretical investigation at the gradient-corrected density functional (BP86) level of theory on the iodo-methyl ligand exchange reaction in platinum-diphosphine complexes is discussed. The reaction consists of two elementary steps: the oxidative addition of methyl-iodide, and reductive elimination of ethane from the intermediate Pt(bdpp)(CH₃)₃I complex which is the rate determining step with a free energy of activation of 19.5 kcal/mol in acetonitrile phase. The oxidative addition step takes place with S_N2 mechanism via a transition state with a collinear arrangement of the I-CH₃-Pt moiety.     

A comparative study of unrelaxed surfaces on quartz and kaolinite, using the periodic density functional theory

To investigate surface properties of fractured silica particles, which are commonly connected to the etiology of silica toxicity, models of low-index unrelaxed surfaces of quartz and kaolinite were constructed and analyzed using the periodic density functional theory calculations. The models were used to investigate surface sites that emerge in the processes of heterolytic and homolytic cleavage of quartz. It is found that the quartz surface is stabilized by two types of interactions. One, due to a more even charge distribution of sites, was characterized by surface energies of up to 0.025 eV x A(-2) and the other, due to a more even oxygen distribution between complementary surfaces, was up to 0.036 eV x A(-2). The total specific surface energies of unrelaxed surfaces ranged from 0.161 to 0.200 eV x A(-2) for quartz and from 0.017 to 0.158 eV x A(-2) for kaolinite. For the conchoidal fracture of quartz an average specific surface energy of 0.187 eV x A(-2) was obtained. These results provide a foundation for further characterization of the surface properties of mechanically comminuted respirable silica particulate and for reduction of occupational health hazards due to pulverized silica.     

Internal carbon monoxide exchange and CO dissociation in cobalt carbonyl carbene complexes. A density functional study

Structures, intramolecular CO-exchanges, and CO-dissociation of ethoxycarbonylcarbene-bridged dicobalt carbonyl complexes [μ₂-{ethoxycarbonyl(methylene)}-μ₂-(carbonyl)- bis(tricarbonyl-cobalt) (Co-Co)] Co₂(CO)₇(CHCO₂Et) (1) and [di-μ₂-{ethoxycarbonyl(methylene)}-bis(tricarbonyl-cobalt) (Co-Co)] Co₂(CO)₆(CHCO₂Et)₂ (2) were investigated by utilizing the density functional theory at the B3LYP/6-31G(d) level. In the lowest energy isomer of 1 the equatorial carbonyl group cis to the bridging ethoxycarbonylcarbene is prone to dissociate resulting in a coordinative unsaturated Co₂(CO)₆(CHCO₂Et) complex stabilized by an intramolecular cobalt-oxygen orbital interaction. Several mechanisms describing the fluxional behavior of 1 and 2 were found. It was found that the internal transformation designated as ‘tripodal rotation’ is responsible for the temperature-dependent broadening of the ¹³C NMR signals for compound 2. For 1 the tripodal rotation needs to be taken into account as well, however an even faster internal CO-exchange allows for the carbonyls to switch between the terminal and bridging positions. In the coordinative unsaturated complexes Co₂(CO)₆(CHCO₂Et) and Co₂(CO)₅(CHCO₂Et)₂ the CO ligands show also many variations of internal rearrangements. In complex 1 the effect of the rotation of the C_carbene-C_carbonyl bond on the energy of the rotamers was also examined.     

A density functional theory study of uranium(VI) nitrate monoamide complexes

Density functional theory calculations were performed on uranyl complexed with nitrate and monoamide ligands (L) [UO(2)(NO(3))(2)·2L]. The obtained results show that the complex stability is mainly governed by two factors: (i) the maximization of the polarizability of the coordinating ligand and (ii) the minimization of the steric hindrance effects. Furthermore, the electrostatic interaction between ligands and uranium(vi) was found to be a crucial parameter for the complex stability. These results pave the way to the definition of (quantitative) property/structure relationships for the in silico screening of monoamide ligands with improved extraction efficiency of uranium(vi) in nitrate acidic solution.     

五味子素A、B和五味子丙素的密度泛函研究

采用密度泛函B3LYP方法在6-31G基组水平上对五味子素A、B及五味子丙素3种五味子提取物进行了优化计算,并从平衡几何构型、前线分子轨道、净电荷分布等方面对计算结果做了比较.计算结果表明分子中的二氧五环对分子的药物活性具有较大影响.随着分子中二氧五环数目的增加,分子中联苯环扭转角减小,前线轨道能级和能级差都减小,联苯环上正电荷增加,由此可判断3种分子活性顺序应为五味子丙素>五味子素B>五味子素A.     

A screened hybrid density functional study on energetic complexes: Metal carbohydrazide nitrates

The molecular geometry, electronic structure and thermochemistry of a series of metal carbohydrazide nitrates were investigated using the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid density functional. The results show that Ca, Sr, and Ba complexes have additional coordinated oxygen atoms from the nitrate ion, which differ obviously from Cu, Ni, Co, and Mg complexes in terms of the geometric structure. Detailed NBO analyses clearly indicate that the metal–ligand interactions in Cu, Ni, and Co complexes are covalent, whereas those of Mg, Ca, Sr, and Ba complexes are ionic in nature. Furthermore, the donor–acceptor interactions result in a reduction of occupancies of σ_{C? O} and σ_{N? H} orbitals. Consequently, the bond lengths increase and the bond orders decrease. Finally, the calculated heats of formation predict that the ionic alkaline‐earth metal carbohydrazide nitrates are more stable than the covalent transition metal carbohydrazide nitrates. It agrees well with the available experimental thermal stabilities, indicating that the metal–ligand bonding character plays an important role in the stabilities of these energetic complexes. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A comparative post-Hartree–Fock and density functional theory study of monochalcogenide diatomic molecules

HOMO and LUMO (FMOs) play important role in the optical properties of meridianal isomer of tris(8-hydroxyquinolino)aluminum (mer-Alq3) and its derivatives. The frontier molecular orbitals (FMOs) also play a vital role in the process of charge transport. It is urgent to find the reason of FMO distribution pattern among the ligands. The structures of mer-Alq3 and its “CH”/N substituted derivatives have been optimized at the B3LYP/6-31G^* level. Energy decomposition analysis has been performed at the B3LYP/DZP level. The results of energy-partitioning analysis of ground states are discussed. It has been explained that HOMOs are on A-ligands due to weaker electrostatic interaction energy between L_a-AlL_bL_c fragments while LUMOs are on B-ligands due to weaker orbital interaction energy between L_b-AlL_aL_c fragments.