Ab initio thermal rate coefficients for H + NH3 ⇌ H2 + NH2

The reversible reaction NH₃ + H ⇌ H₂ + NH₂, which plays an important role in NH₃ fuel combustion, is studied with a theoretical approach that combines the high-accuracy extrapolated ab initio thermochemistry (HEAT) protocol with semiclassical transition state theory (SCTST). The calculated forward reaction is endothermic by 11.8 ± 1 kJ/mol, in nearly perfect agreement with the active thermochemical tables (ATcT) value of 11.5 ± 0.2 kJ/mol. Using this improved thermochemistry yields better rate constants, especially at low temperatures. Experimental rate constants available from 400 to 2000 K for the forward and reverse reaction pathways can be reproduced (within 20%) by the calculations from first principles.     

Ab initio study on the dynamical properties of the hydrogen abstraction reaction NH2 + OH → NH + H2O

 The geometry of the transition state of the title reaction was optimized at the unrestricted Hartree–Fock, the spin-unrestricted second-order M?ller–Plesset, and the spin-unrestricted quadratic configuration interaction with all single and double substitutions levels of theory. The changes in the geometry, the bound vibrational modes, and the potential energy along the minimum energy path are discussed. Variational transition-state theory rate constants calculated with the tunneling and curvature effect correction agree very well with the experimental values. Received: 23 April 1999 / Accepted: 9 June 1999 / Published online: 15 December 1999     

Ab initio study of the complexes of first-row transition-metal ions with CH, CH_2, and CH_3

The geometries and bonding characteristics of the complexes of the first-row transition-metal ions with CH, CH_2 and CH_3 were investigated by ab initio molecular orbital theory. MCH~ and MCH_2~ are linear and coplanar, re spectively. Both of them are with obvious treble or double bond characteristics, but these multiple bonds are mostly "im perfect". The calculated bond dissociation energies of C--M~ , C=M~ and C≡M~ are mostly close to the experi mental values, and appear in similar periodic trends from Sc to Zn.     

Study of the structure and stability of aqua complexes Y(H2O) n 3+ (n = 1–10) by Ab initio methods

The structural and energetic characteristics of aqua complexes Y(H₂O) _n ³⁺ (n = 1–10) have been calculated by the ab initio RHF and MP2 methods.     

Ab initio study of the HCO 3 − /H2O exchange in the (NH3)3 ZnII(HCO 3 − ) complex

Summary The displacement of bicarbonate anion in the (NH₃)₃Zn^II(HCO ₃ ^– ) complex with water has been studied throughab initio calculations. It has been found that H₂O binds to the (NH₃)₃Zn^II(HCO ₃ ^– ) species yielding a stable pentacoordinate (NH₃)₃Zn^II(HCO ₃ ^– )(H₂O) complex. The results also indicate that deprotonation of water in the pentacoordinate species facilitates the release of HCO ₃ ^– , although, the presence of HCO ₃ ^– in the coordination sphere of Zn^II makes such deprotonation more difficult. Environmental effects have been considered in the study of HCO ₃ ^– /H₂O exchange.A contribution from the Grup de Química Quàntica de l'Institut d'Estudis Catalans     

Ab. initio calculations of Cl2SO ·nAICl3 complexes (n = 1, 2)

Fragments' of the potential energy surfaces (PES) for the SOCl₂ ·nAICl₃ (n = 1, 2) complexes were calculated by theab initio MO LCAO method using the RHF approximation for the STO-3G basis set and adding a 3d-AO for the S atom, as well as by the semiempirical MNDO method. Two local minima, assigned to the donor-acceptor complex Cl₂SO+AlCl₃ (la) and to Cl₃SOAlCl₂ (1b) were located on the PES atn = 1. Two local minima corresponding to two donor-acceptor complexes Cl₂SOAl₂Cl₆ and, were also located on the PES atn = 2. An analysis of the enthalpies of cation formation in the SOC1₂ +nAICl₃ (n = 1-3) systems calculated by theab initio method shows that the enthalpy of formation of the SOCl₊ cation atn = 2 is 17 kcal mol^–1 less than that atn = 1; the structure of the Cl₂SOAlCl₂ ⁺ cation with two strong electrophilic centers at the Al and S atoms becomes more favorable energetically atn = 3. The results of calculations for Cl₂SO ·nAICl₃ complexes by the MNDO method are in agreement with those obtained by theab initio method except for the geometry of complexes containing the Cl₃SO fragment and the charge values on the S atoms.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1116–1120, May, 1996.     

二维配位聚合物(NH3+CH2CH2NH3+)(NH2CH2CH2NH3+)2[(VO)4(PO4)4(H2O)2]·2H2O的水热合成及晶体结构

无机-有机杂化钒氧酸盐由于其结构的多样性以及在催化、医药、光、电、磁等材料领域中的应用前景而受到人们的广泛关注。近年来这一研究领域的重大进步是将有机氮配体或者过渡金属配合物直接连接到矾氧骨架上以获得各种新奇结构。合成出许多属于L/V/O、MXLY/V/O、L/P/V/O和MXLY     

Intramolecular Rearrangements of >Si(O–C·=O)(CH2–CH3) and >Si(CH2–CH·–CH3)(CH2–CH3) Radicals

Using ESR and IR spectroscopy, the structures of >Si(O–C^·=O)(CH₂–CH₃) (1) and >Si(CH₂–CH^·–CH₃)(CH₂–CH₃) (2) radicals were deciphered. The directions and kinetic parameters of reactions of intramolecular rearrangements in these radicals were determined. The reactions of hydrogen atom abstraction in radical (1) from the CH₂ and CH₃ groups were studied. It was found that the endothermic reaction of hydrogen atom abstraction from the methyl group occurs at a higher rate than the exothermic reaction with the methylene group. The differences are determined by changes in the size of a cyclic transition state. Based on the experimental data, the strengths of separate C–H bonds in surface fragments are compared. The rearrangement >Si(CH₂–CH^·–CH₃)(CH₂–CH₃) >Si(C^·(CH₃)₂)(CH₂–CH₃) was discovered and its mechanism was determined. One of its steps is the skeletal isomerization Si- (2)- _. (1)Si- (1)- _. (2). Experimental data are analyzed using the results of quantum-chemical calculations of model systems.     

Ab initio electron correlated studies on the intracluster reaction of NO+ (H2O)(n) → H3O+ (H2O)(n-2) (HONO) (n = 4 and 5)

Hydrated nitrosonium ion clusters NO(+)(H(2)O)(n) (n = 4 and 5) were investigated by using MP2/aug-cc-pVTZ level of theory to clarify isomeric reaction pathways for formation of HONO and fully hydrated hydride ions. We found some new isomers and transition state structures in each hydration number, whose lowest activation energies of the intracluster reactions were found to be 4.1 and 3.4 kcal mol(-1) for n = 4 and n = 5, respectively. These thermodynamic properties and full quantum mechanical molecular dynamics simulation suggest that product isomers with HONO and fully hydrated hydride ions can be obtained at n = 4 and n = 5 in terms of excess hydration binding energies which can overcome these activation barriers.     

Structure of [NH3(CH2)4NH3]2P4O12·2H2O

After a short survey of what is the present state of the cyclophosphates associated with the organic molecule NH₂(CH₂)₄NH₂, we report chemical preparation and crystal structure for a new example of such compounds. [NH₃(CH₂)₄NH₃]₂P₄O₁₂.2H₂O is monoclinic (S.G. : P2₁/n), with Z = 2 and the following unit-cell parameters : a = 7.6728(8) Å, b = 18.962(3) Å, c = 7.9789(9) Å β = 111.751(9)°. Bidimensional layer arrangement of P₄O₁₂ rings connected to the water molecules thanks to weak H-bonds run parallel to the ab plane. The organic groupements located between these inorganic planes perform the three-dimensional cohesion by NH····O hydrogen bonds.     

Coordination and ligands’ effects in trinuclear [Pd3(COT)2(L)]2+ (L = H2O,CO, N2, HCN,HNC, NH3, PH3, PCl3, PF3, CS,CH2) sandwich complexes of cyclooctatetraene: theoretical investigation

Structural Chemistry - This paper reports the molecular structure, the electronic structure, and the decomposition energies of the [M3(COT)2(L)]2+ (M = Cr, Fe, Pd, and L = H2O, CO, N2, HCN, HNC,...     

Ab initio potential energy surface and quantum dynamics for the H + CH4 → H2 + CH3 reaction

A new full-dimensional potential energy surface for the title reaction has been constructed using the modified Shepard interpolation scheme. Energies and derivatives were calculated using the UCCSD(T) method with aug-cc-pVTZ and 6-311++G(3df,2pd) basis sets, respectively. A total number of 30,000 data points were selected from a huge number of molecular configurations sampled by trajectory method. Quantum dynamical calculations showed that the potential energy surface is well converged for the number of data points for collision energy up to 2.5 eV. Total reaction probabilities and integral cross sections were calculated on the present surface, as well as on the ZBB3 and EG-2008 surfaces for the title reaction. Satisfactory agreements were achieved between the present and the ZBB3 potential energy surfaces, indicating we are approaching the final stage to obtain a global potential energy surface of quantitative accuracy for this benchmark polyatomic system. Our calculations also showed that the EG-2008 surface is less accurate than the present and ZBB3 surfaces, particularly in high energy region.     

Ab initio Study of H2O–HCl (1:1, 1:2, 2:1) Complexes

This paper reports on an ab initio (6-31G**) study of 1:1, 1:2, and 2:1 (H₂O) _n (HCl) _m complexes. Stable configurations of the 1:2 and 2:1 (H₂O) _n (HCl) _m complexes and their geometrical and energy characteristics were determined. The vibrational analysis of the complexes was carried out. The effect of hydrogen bonding due to S₁S₀ and T₀T₁ electronic excitations is considered.     

XPS study of the electronic structure of heterometallic complexes [Fe2MO(O2CCH3)6(H2O)3] · 3H2O (M = Co,Ni)

Heterometallic compounds of general formula [Fe ₂ ^III M^IIO(O₂CR)₆(H₂O)₃] · 3H₂O (R = CH₃, M = Co, Ni; R = CCl₃, M = Co, Ni) have been studied by XPS. The compounds have been identified as high-spin complexes with metal atoms in oxidation states M(II) and M(III). Analysis of the XPS data revealed the tendency of the XPS pattern and magnetic parameters of molecules to change with a change in the electronic nature of metal atoms. The assignment is based on the degree of covalence of the M-O bond. In chloro-substituted heterocomplexes, electron density delocalization on the metal atoms with metal-to-ligand charge transfer through three bonds (M-O-C-C) is observed. The substitution in terminal groups leads to the change in the electron density distribution between the carboxylate and terminal groups.     

三核过渡金属配合物[M3ⅢO(OOCR)6L3]+(M=Cr,Fe,Mn;R=CH3,C2H5,CH2NH2;L=C5H5N,H2O)的1H NMR性质

采用1H NMR谱研究了通式为[M3ⅢO(OOCR)6L3]+(M=Cr,Fe,Mn;R=CH3,C2H5,CH2NH2;L=C5H5N,H2O)的一系列氧心三核过渡金属配合物,主要考察其1H化学位移随金属、配体、温度、溶剂等因素变化而变化的规律.结果表明,骨架金属对化学位移的影响最大,M3O中的3个金属离子间存在反铁磁交换相互作用.对Mn配合物中顺磁中心对化学位移和线宽的影响机制的研究表明,其1H各向同性位移主要由接触作用贡献.     

Ab initio and kinetic study of the reaction of ketones with OH for T = 500-2000 K. Part I: hydrogen-abstraction from H3CC(O)CH(3-x)(CH3)x, x = 0 ↦ 2

A theoretical study is presented of the mechanism and kinetics of the reactions of the hydroxyl radical with three ketones: dimethyl (DMK), ethylmethyl (EMK) and iso-propylmethyl (iPMK) ketones. CCSD(T) values extrapolated to the basis set limit are used to benchmark the computationally less expensive methods G3 and G3MP2BH&H, for the DMK + OH reaction system. These latter methods are then used in computations involving the reactions of the larger ketones. All possible abstraction channels have been modeled. A stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel has been recognized in part of the abstracting processes. High-pressure limit rate constants of the title reactions have been calculated in the temperature range of 500-2000 K using the Variflex code including Eckart tunneling corrections. Variable reaction coordinate transition state theory (VRC-TST) has been used for the rate constants of the barrier-less entrance channel. Calculated total rate constants (cm(3) mol(-1) s(-1)) are reported as follows: k(DMK) = 1.32 × 10(2)×T(3.30)exp(503/T), k(EMK) = 3.84 × 10(1)×T(3.51)exp(1515/T), k(iPMK) = 2.08 × 10(1)×T(3.58)exp(2161/T). Group rate constants (on a per H atom basis) for different carbon sites in title reactions have also been provided.     

Ab initio study of ternary radical–molecule complexes between HCN(HNC) and HO(HS) species

MP2 calculations with the cc-pVTZ basis set were used to analyze the intermolecular interactions in ternary radical–molecule complexes between HCN(HNC) and HO(HS) species, in gas phase and in water media. Particular attention was given to parameters such as the cooperative energies and many-body interaction energies. The results indicate that hydrogen bonding between two HCN(HNC) molecules gives more stability to triads than hydrogen bonding between HCN(HNC) and OH(SH) species. The electronic properties of the complexes were analyzed using the parameters derived from the atoms in molecules methodology. The water media has an enhancing influence on the stabilities of studied clusters versus the ones obtained in gas phase.     

The spin-forbidden reaction CH(2∏) + N2→ HCN + N(4S) revisited I. Ab initio study of the potential energy surfaces

High-level ab initio electronic structure theories have been applied to investigate the detailed reaction mechanism of the spin-forbidden reaction CH(²∏) + N₂ → HCN + N(⁴S). The G2M(RCC) calculations provide accurate energies for the intermediates and transition states involved in the reaction, whereas the B3LYP/6-311G(d,p) method overestimates the stability of some intermediates by as much as about 10 kcal/mol. A few new structures have been found for both the doublet and quartet electronic states, which are mainly involved in the dative pathways. However, due to the higher energies of these structures, the dominant mechanism remains the one involving the C ₂ intersystem-crossing step. The C ₂ minima on the seam of crossing (MSX) structures and the spin-orbit coupling between the doublet and quartet electronic states are rather close to those found in previous studies. Vibrational frequencies orthogonal to the normal of the seam which have been applied in a separate publication to calculate the rate of the CH(²∏) + N₂ → HCN + N(⁴S) reaction with a newly proposed nonadiabatic transition-state theory for spin-forbidden reactions have been calculated at the MSX from first principles. Received: 23 June 1998 / Accepted: 21 September 1998 / Published online: 8 February 1999     

Ab initio MO study of ethylene insertion into the Sm–C bond of H2SiCp2SmCH3

Ethylene insertion into the Sm–C bond of H₂SiCp₂SmCH₃, a model reaction of an olefin polymerization propagation step, has been studied by ab initio molecular orbital methods. The small electronegativity of the Sm atom makes the Sm–C bond ionic, the methyl group being negatively charged by −0.75. The reaction passes through a loose ethylene complex with a binding energy of 15 kcal/mol and then a tight four-centered transition state with an agostic interaction between the Sm atom and one of the methyl CH bonds. A small activation energy of 14 kcal/mol is required to pass through this transition state, indicating that this is an easy reaction. Compared with the reactions with group 4 cationic silylene-bridged metallocenes the activation energy is higher and the reaction is less exothermic. The origin of these differences is discussed. The results of molecular mechanics calculations on regio- and stereoselectivities in the insertion reaction of propylene are also reported. Received: 13 July 1998 / Accepted: 28 August 1998 / Published online: 2 November 1998