Ab initio study on the adsorption and oxidation of HCHO with Ag2 cluster

We present an ab initio study on the adsorption and oxidation of HCHO on the Ag₂ dimer. HCHO is found to weakly adsorb on the neutral Ag₂ cluster and the adsorption energy increases when the Ag₂ cluster is positively charged. The adsorption of HCHO on Ag₂ is promoted by pre-adsorbed OH species while coadsorbed H has no effect. The formation of η²-methylendioxy is inhibited if the Ag₂ cluster is positively and properly charged, which enhance the selectivity of the partial oxidation of methanol. These results provide insight into the reaction mechanism of HCHO on silver surfaces and may guide the design of novel catalysts.     

Kinetic study of the reaction H2O2+H→H2O+OH by ab initio and density functional theory calculations

Theoretical investigations on the kinetics of the elementary reaction H₂O₂+H→H₂O+OH were performed using the transition state theory (TST). Ab initio (MP2//CASSCF) and density functional theory (B3LYP) methods were used with large basis set to predict the kinetic parameters; the classical barrier height and the pre-exponential factor. The ZPE and BSSE corrected value of the classical barrier height was predicted to be 4.1 kcal mol⁻¹ for MP2//CASSCF and 4.3 kcal mol⁻¹ for B3LYP calculations. The experimental value fitted from Arrhenius expressions ranges from 3.6 to 3.9 kcal mol⁻¹. Thermal rate constants of the title reaction, based on the ab initio and DFT calculations, was evaluated for temperature ranging from 200 to 2500 K assuming a direct reaction mechanism. The modeled ab initio-TST and DFT–TST rate constants calculated without tunneling were found to be in reasonable agreement with the observed ones indicating that the contribution of the tunneling effect to the reaction was predicted to be unimportant at ambient temperature.     

Molecular structure of the azidoalane [Me2N(CH2)3]AltBu(N3). X-ray structural determination and ab initio calculations

A single crystal of the azidoalane [Me₂N(CH₂)₃]AltBu(N₃) (1a), grown in a capillary using a miniature zone melting procedure, was investigated by X-ray analysis. Compound 1a (C₉H₂₁AlN₄) is a monomeric species in the solid state, which crystallizes in the monoclinic space group P2₁ with a=6.8560(14) Å, b=12.251(3) Å, c=7.786(2) Å, β=108.51(3)° and Z=2. The results of the X-ray structural determination are compared with the calculated structure of 1a (HF/6-31G(d) and B3LYP/6-31G(d) level of theory). Whereas the overall agreement between the measured and calculated structure is good, the Al–N donor-bond length differs by 11 and 12 pm at the HF and B3LYP level, respectively. To evaluate the effects of a polar environment on the molecular structure of 1a self-consistent reaction field (SCRF) calculations at the HF and B3LYP level with the 6-31G(d) basis set were performed.     

An ab initio study of the Cl(P)+C2H6→C2H5+HCl abstraction reaction

Electronic energies, geometries, and harmonic vibration frequencies for the reactants, products, and transition state for the Cl(³P)+C₂H₆→C₂H₅+HCl abstraction reaction were evaluated at the HF and MP2 levels using several correlation consistent polarized-valence basis sets. Single-point calculations at PMP2, MP4, QCISD(T), and CCSD(T) levels were also carried out. The values of the forward activation energies obtained at the MP4/cc-pVTZ, QCISD(T)/cc-pVTZ, and CCSD(T)/cc-pVTZ levels using the MP2/cc-pVTZ structures are equal to −0.1, −0.4, and −0.3 kcal/mol, respectively. The experimental value is equal to 0.3±0.2 kcal/mol. We found that the MP2/aug-cc-pVTZ adiabatic vibration energy for the reaction (−2.4 kcal/mol) agrees well with the experimental value −(2.2–2.6) kcal/mol. Rate constants calculated with the zeroth-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment. In general, the theoretical rate constants differ from experiment by, at most, a factor of 2.6.     

An ab initio potential energy surface for the υ4 vibrational mode of hydrogen peroxide

In this paper, the levels and the torsional microstates of hydrogen peroxide are determined from fully optimized ab initio calculations using a nuclear model in one dimension. Calculations have been performed at the MP2 level with the 6-311 G(2df,2pd), 6-31 1+G(2df,2pd), cc-pVTZ and AUG-cc-pVTZ basis sets including polarization orbitals and diffuse functions. The most stable conformation, calculated with the MP2/AUG-cc-pVTZ approach, is a trans–gauche conformer lying at 67.5° from the trans structure. By using the same level of calculations, the heights of the trans and cis barriers have been determined to be 386.5 and 2643.8 cm⁻¹ in a good agreement with the experimental data. The variational torsional levels split into four components by the tunnelling effect of the barriers. The splitting of the fundamental level caused by the trans barrier has been found to be 11.8683 cm⁻¹, whereas the splitting caused by the cis barrier is insignificant under n=2. Current ab initio energies confirm the experimental assignments and verify the separability of the torsion from the rest of the vibrations. However, the experimental relation of dependence on the torsion of the rotational constants cannot be reproduced in one-dimension and depends on several additional vibrational effects.     

An ab initio and matrix isolation infrared study of the 1:1 C2H2–CHCl3 adduct

The details of weak C–Hπ interactions that control several inter and intramolecular structures have been studied experimentally and theoretically for the 1:1 C₂H₂–CHCl₃ adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C₂H₂ and CHCl₃ species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels. Both the computational and experimental data indicate that the C₂H₂–CHCl₃ complex has a weak hydrogen bond involving a C–Hπ interaction, where the C₂H₂ acts as a proton acceptor and the CHCl₃ as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl₃ and a hydrogen in C₂H₂. The combination of the C–Hπ interaction and the secondary ClH interaction determines the structure and the energetics of the C₂H₂–CHCl₃ complex. In addition to the vibrational assignments for the C₂H₂–CHCl₃ complex we have also observed and assigned features owing to the proton accepting C₂H₂ submolecule in the acetylene dimer.     

Chemisorption of HSi(OH)3 on silica surfaces: an ab initio periodic study

The study of the grafting of trialkoxysilane R′Si(OR)₃ molecules on amorphous silica has been undertaken at the Hartree–Fock level using a biperiodic model for the surface. Different types of slab cut out from the model system Edingtonite (a tetragonal silica structure with five SiO₂ groups per unit cell) have been used to simulate isolated and interacting silanol sites at the amorphous silica surface, while only the simple case of HSi(OH)₃ has been considered for the interacting molecule. In a first step, for each type of surface the geometrical parameters have been optimised and the surface formation energy determined. The geometrical structure of the grafted molecule is compared with that of the isolated one. The geometrical strains of the surfaces with either isolated or interacting silanols are also compared before and after the grafting reactions. The calculated values of the chemisorption energies show that the grafting process is favored on isolated silanols only if correlation effects are included.     

Direct dynamics studies on the hydrogen abstraction reactions CF3O+CH4(CD4)→CF3OH(CF3OD)+CH3(CD3)

The dynamics properties of the hydrogen abstraction reaction CF₃O+CH₄→CF₃OH+CH₃ are studied by dual-level direct dynamics method. Optimization calculations are preformed by B3LYP and MP2 with the 6-311G(d,p) basis set, and the single-point calculations are done at the multi-coefficient correction method based on quadratic configuration interaction with single and double excitations (MC-QCISD) method. The rate constants are evaluated by canonical variational transition-state theory with a small-curvature tunneling correction over a wide range of temperature 200–2000 K. The agreement between theoretical and experimental rate constants is good in the measured temperature range. The calculated results show that the variational effect is small and almost neglected over the whole temperature range, whereas, the tunneling correction plays a role in the lower temperature range. The kinetic isotope effect for the reaction is ‘normal’. The value of k_H/k_D is 2.38 at room temperature and it decreases with the temperature increasing.     

Conformation of N,N′-bis (2,6-dimethylmorpholino), N″-dichloroacetyl phosphoric triamide: CHCl2C(O)NHP(O)[2,6(CH3)2(NC4H6O)]2. NMR and ab initio studies

Using phosphorus pentachloride as a substrate, a new carbacyclamidophosphate, N,N″-bis (2,6-dimethylmorpholino), N″-dichloroacetyl phosphoric triamide (1) has been synthesized and characterized by ¹H, ³¹P and ¹³C NMR, IR spectroscopy and elemental analysis. Due to the presence of methyl disubstituted morpholine rings and the dichloroacetamide group, several conformers can be considered for this molecule. The ³¹P{¹H} NMR spectra for the isomeric mixture of synthesized compound showed four signals with the ratio 67.1; 19.0; 12.2; 1.7, which indicates four independent conformers. The ¹H NMR spectra confirmed these results. The conformational space and the molecular geometry of the molecule in the gaseous phase have been studied using the B3LYP method of approximation, with 6-31G and 6-311++G^** basis sets.     

An ab initio potential energy surface and spectroscopic constants for the XΣg state of NO2

A three-dimensional potential energy function has been calculated for the X¹Σ⁺_g state of NO⁺₂ from ab initio MRD-CI data. With this PE function, converged vibrational calculations have also been performed for ten vibrational states, with the aid of a computer program developed in the present work for this purpose. The calculated harmonic frequencies, vibrational term values and rotational constants are in good agreement with experimental data.     

Ab initio molecular orbital study on the gas phase SN2 reaction F + CH3Cl → CH3F + Cl

Ab-initio molecular orbital (MO) and direct ab initio dynamics calculations have been applied to the gas phase S_N2 reaction F⁻ + CH₃Cl → CH₃F + Cl⁻. Several basis sets were examined in order to select the most convenient and best fitted basis set to that of high-quality calculations. The Hartree–Fock (HF) 3−21+G(d) calculation reasonably represents a potential energy surface calculated at the MP2/6−311++G(2df,2pd) level. A direct ab initio dynamics calculation at the HF/3−21+G(d) level was carried out for the S_N2 reaction. A full dimensional ab initio potential energy surface including all degrees of freedom was used in the dynamics calculation. Total energies and gradients were calculated at each time step. Two initial configurations at time zero were examined in the direct dynamics calculations: one is a near collinear collision, and the other is a side-attack collision. It was found that in the near collinear collision almost all total available energy is partitioned into two modes: the relative translational mode between the products (40%) and the C − F stretching mode (60%). The other internal modes of CH₃F were still in the ground state. The lifetimes of the early- and late-complexes F⁻ … CH₃Cl and FCH₃ … Cl⁻ are significantly short enough to dissociate directly to the products. On the other hand, in the side-attack collision, the relative translation energy was about 20% of total available energy.     

Ab initio and density function theory computational studies of the CH4 + H → CH3 + H2 reaction

The activation barrier for the CH₄ + H → CH₃ + H₂ reaction was evaluated with traditional ab initio and Density Functional Theory (DFT) methods. None of the applied ab initio and DFT methods was able to reproduce the experimental activation barrier of 11.0-12.0 kcal/mol. All ab initio methods (HF, MP2, MP3, MP4, QCISD, QCISD(T), G1, G2, and G2MP2) overestimated the activation energy. The best results were obtained with the G2 and G2MP2 ab initio computational approaches. The zero-point corrected energy was 14.4 kcal mol⁻¹. Some of the exchange DFT methods (HFB) computed energies which were similar to the highly accurate ab initio methods, while the B3LYP hybrid DFT methods underestimated the activation barrier by 3 kcal mol⁻¹. Gradient-corrected DFT methods underestimated the barrier even more. The gradient-corrected DFT method that incorporated the PW91 correlational functional even generated a negative reaction barrier. The suitability of some computational methods for accurately predicting the potential energy surface for this hydrogen radical abstraction reaction was discussed.     

精密从头算与ABEEM/MM模型对水团簇(H2O)11 9种低能结构的计算

用精密从头算方法研究了(H2O)11的9种低能异构体的性质, 包括优化的几何结构、结合能、偶极矩和氢键个数等, 并且得出了515-a是(H2O)11的全局最低能结构. 同时, 也用ABEEM/MM(atom bond electronegativity equalization method/molecular mechanics) 模型研究了这些性质, 与从头算的结果进行了比较, 得到了相符合的结果. 这显示了ABEEM/MM模型在描述中等大小的水分子团簇结构上是成功的.     

Co(H2O)62+/3+体系电子转移反应动力学的理论研究

A theoretical scheme is presented which is based on an activation model for calculating the rate of the electron- exchange reaction between transition metal complexes in aqueous solution and applies to Co(H2O)6 2+/3+ electron transfer system. The activation parameter and activation energy of the system are obtained via the activation model. The slopes of the potential energy surfaces (curves) of the reacting system at the separated reactants are calculated from the fitted potential energy curves. The coupling matrix element is determined by using the perturbation theory and numerical integral method. Theoretical rate constants are obtained for the system at both UHF/6-311G and UMP2/6-311G levels. The agreement of the theoretical results with experimetal values is excellent. This fact indicates the scheme proposed is feasible and accurate in studying the self- exchange eletron transfer reaction.     

Structures and conformations of trifluoromethanesulfonic anhydride, (CF3SO2)2O, and bis(trifluoromethylsulfonyl)difluoromethane, (CF3SO2)2CF2

The geometric structures and conformational properties of trifluoromethanesulfonic anhydride, (CF₃SO₂)₂O, and bis(trifluoromethylsulfonyl)difluoromethane, (CF₃SO₂)₂CF₂ have been studied by gas electron diffraction (GED) and ab initio calculations (HF/3–21G*). The calculations predict for both systems two stable conformers with C₂ symmetry and one with C₁ symmetry. In both compounds structures with C₂ symmetry and dihedral angles SOSC ≈ 100° ((CF₃SO₂)₂O) and SCSC≈ 150° ((CF₃SO₂)₂CF₂ are lowest in energy. According to the GED analyses the dominant conformer of (CF₃SO₂)₂O₂ possesses C₂ symmetry with SOSC dihedral angles of 99.1(14)°. The presence of up to 30% of the two other conformers cannot be excluded; for (CF₃SO₂)₂CF₂ only one conformer with C₂ symmetry and SCSC dihedral angles of 143(2)° is observed. A complete set of geometric parameters is given.     

Structures and conformations of trifluoromethanesulfonic anhydride, (CF3SO2)2O, and bis(trifluoromethylsulfonyl)difluoromethane, (CF3SO2)2CF2

The geometric structures and conformational properties of trifluoromethanesulfonic anhydride, (CF₃SO₂)₂O, and bis(trifluoromethylsulfonyl)difluoromethane, (CF₃SO₂)₂CF₂ have been studied by gas electron diffraction (GED) and ab initio calculations (HF/3–21G*). The calculations predict for both systems two stable conformers with C₂ symmetry and one with C₁ symmetry. In both compounds structures with C₂ symmetry and dihedral angles SOSC ≈ 100° ((CF₃SO₂)₂O) and SCSC ≈ 150° ((CF₃SO₂)₂CF₂) are lowest in energy. According to the GED analyses the dominant conformer of (CF₃SO₂)₂O possesses C₂ symmetry with SOSC dihedral angles of 99.1(14)°. The presence of up to 30% of the two other conformers cannot be excluded; for (CF₃SO₂)₂CF₂ only one conformer with C₂ symmetry and SCSC dihedral angles of 143(2)° is observed. A complete set of geometric parameters is given.     

Theoretical study on the reaction pathways of HFCO + H2O

For the reaction of methanoyl fluoride with water, both optimized structures and vibrational wavenumbers of reaction intermediates, transition structures and product complexes were calculated and characterized with theory at the MP2/6-311++G(d,p) level. Including a catalytic path and concerted and stepwise hydrolysis paths, possible reaction mechanisms were also investigated. The catalytic reaction of HFCO yielding HF and CO has the smallest activation barrier, 29.6 kcal/mol, whereas for the concerted hydrolysis 33.0 kcal/mol is required to overcome the barrier to form transoid HCOOH + HF, which is less than for the stepwise counterpart, 42.0 kcal/mol.     

Applicability of the H NMR chemical shifts computed by the ab initio/GIAO-HF methodology to the study of geometrical features of Zn-porphyrin dimers

Several porphyrin dimers have been newly designed and synthesized to construct assemblies with 1,4-diazabicyclo[2.2.2]octane (DABCO) as a bidentate binding ligand. Semi empirical (AM1) and ab initio calculations have been used to study the assemblies generated by the organization of dimers and DABCO, including the computation of ¹H NMR complexation-induced chemical shifts using the ab initio/GIAO methodology. The diagnostic capacity of the theoretical method has been applied to explain experimental results and geometrical features of the complexes.