An ab initio benchmark study of the H + CO --> HCO reaction

The H + CO --> HCO reaction has been characterized with correlation consistent basis sets at five levels of theory in order to benchmark the sensitivities of the barrier height and reaction ergicity to the one-electron and n-electron expansions of the electronic wave function. Single and multireference methods are compared and contrasted. The coupled cluster method RCCSD(T) was found to be in very good agreement with Davidson-corrected internally-contracted multireference configuration interaction (MRCI+Q). Second-order Moller-Plesset perturbation theory (MP2) was also employed. The estimated complete basis set (CBS) limits for the barrier height (in kcal/mol) for the five methods, including harmonic zero-point energy corrections, are MP2, 4.66; RCCSD, 4.78; RCCSD(T), 4.15; MRCI, 5.10; and MRCI+Q, 4.07. Similarly, the estimated CBS limits for the ergicity of the reaction are: MP2, -17.99; RCCSD, -13.34; RCCSD(T), -13.79; MRCI, -11.46; and MRCI+Q, -13.70. Additional basis set explorations for the RCCSD(T) method demonstrate that aug-cc-pVTZ sets, even with some functions removed, are sufficient to reproduce the CBS limits to within 0.1-0.3 kcal/mol.     

MgH^+,Mg2H^+量子化学从头计算的研究

用量子化学自洽场分子轨道从头计算方法,采用STO-3G基组计算了MgH~+,Mg_2H~+离子的位能曲线与位能面;给出电子波函数与电子集居数;得到MgH~+的平衡构型键长γ_(Mg-H)1.60A.Mg_2H~+的平衡构型有C_(∞v)与D_(∞h)两类,前者为[Mg—Mg—H]~+,键长γ_(Mg-Mg)2.41,γ_(Mg-H)1.63A;后者为[Mg—H—Mg]~+,γ_(Mg-H)1.73A.前者的总能量比后者低50kcal/mol.还讨论了它们的稳定性.MgH~+的键长计算结果和实测值较吻合,Mg_2H~+的平衡构型计算结果支持了Porter从热力学研究提出的假设.     

The ab initio quantum chemical calculation of MgH+ and Mg2H+

The potential energy curve of MgH+ and the potential energy surface of Mg₂H+ are calculated by quantum chemical ab initio SCF MO method with STO-3G basis set. The electronic wave functions and populations are obtained. The eqilibrium internuclear distance of MgH+ is 1.60 Å. There are two possible configurations of Mg₂H+: C and D . The former is (Mg-Mg-H)+, with bond length rM_g-M_g = 2.41Å and rM_g-H =1.63Å. The latter is (Mg-H-Mg)+, with bond length rM_g-H = 1.73Å. The cause of the stability of these species is discussed. The result of calculation about the bond length of MgH+ agrees reasonably well with experimental value. The conclusion about the stability of (Mg-Mg-H)+ ion supports the hypothesis proposed by Porter, based on thermodynamical calculation.     

Highlevel ab initio and hybrid density functional theory study of the energy profile for the CO+CO reaction

Two complete basis set and three hybrid density functional computational studies were applied in the exploration of the ¹CO+²CO⁺ reaction potential energy surface. One molecular carbon monoxide–carbon monoxide cation molecular associate was elucidated as the structure with the lowest energy on the potential energy surface. Ionization energies, bond dissociation energies, and enthalpies of formation for every di and tri-atomic molecule on the potential energy surface were estimated with the two complete basis sets and the three hybrid density functional theory methods. Six different endothermic channels for the ¹CO+²CO⁺ reaction were evaluated with ab initio and DFT methods. The computed energies and structural parameters are compared with experimental values where available. Some new energies for this reaction system were suggested.     

Modeling the global potential energy surface of the N + N2 reaction from ab initio data

A new global potential energy surface for the N + N2 exchange reaction has been built from ab initio data. To overcome the difficulty of carrying out ab initio calculations for a large set of geometries the alternative strategy of fitting the minimum energy paths of the surface and their angular dependence using a modified LAGROBO functional form has been adopted. In this way we have been able to reproduce all the main features of the potential using a fairly small set of ab initio values.     

Direct ab initio dynamics study of the OH + HOCO reaction

The reaction between OH and HOCO has been examined using the coupled-cluster method to locate and optimize the critical points on the ground-state potential energy surface. The energetics are refined using the coupled-cluster method with basis set extrapolation to the complete basis set (CBS) limit. Results show that the OH + HOCO reaction produces H2O + CO2 as final products and the reaction passes through an HOC(O)OH intermediate. In addition, the OH + HOCO reaction has been studied using a direct dynamics method with a dual-level ab initio theory. Dynamics calculations show that hydrogen bonding plays an important role during the initial stages of the reaction. The thermal rate constant is estimated over the temperature range 250-800 K. The OH + HOCO reaction is found to be nearly temperature-independent at lower temperatures, and at 300 K, the thermal rate constant is predicted to be 1.03 x 10(-11) cm3 molecule(-1) s(-1). In addition, there may be an indication of a small peak in the rate constant at a temperature between 300 and 400 K.     

High level ab initio and density functional theory calculations of the dissociation energies,ionization energies,geometrical variations,and vibrational modes of ground and excited CO2, CO2+, and CO22+

The potential energy surface of CO₂²⁺ ( ³Σ_g^?) is investigated with HF, MP2, MP4, CBS‐Q, G1, G2MP2, G2, G3B3, and B3LYP/6‐311++G(3df,3pd) methods. Density functional theory shows the lowest dissociation channel of this compound to be the formation of CO⁺ ( ²Σ⁺)+O⁺ ( ⁴S_u) and to have a barrier of around 2 eV as well as a dissociation energy of around ?3.2 eV. Thus we propose that with enough correlation it is possible to accurately predict the energies of dissociation and barrier widths and heights to test for the stability of a particular molecular species. We also propose a refinement of current understanding by observing HOMO–LUMO gaps, Lowdin and Mulliken bond orders (to test for bond orbital overlap and hence qualitatively describe bonding and fragmentation in these complexes) and predicted spectrum for such studies as ZEKE spectroscopy (to study cationic states) and REMPI (to study the first excited states) of these class of molecules and, we hope, provide future insight into larger and more interesting systems. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

DFT and ab initio study on the reaction mechanism of CH2SH + O2

The mechanism for the CH₂SH + O₂ reaction was investigated by DFT and ab initio chemistry methods. The geometries of all possible stationary points were optimized at the B3LYP/6-311+G(d,p) level, and the single point energy was calculated at the CCSD(T)/cc-pVXZ(X = D and T), G3MP2 and BMC-CCSD levels. The results indicate that the oxidation of CH₂SH by O₂ to form HSCH₂OO is a barrierless process. The most favorable channel is the rearrangement of the initial adduct HSCH₂OO (IM1) to form another intermediate H₂C(S)OOH (IM3) via a five-center transition state, and then the C–O bond fission in IM3 leads to a complex CH₂S. . .HO₂ (MC1), which finally gives out to the major product CH₂S + HO₂. Due to high barriers, other products including cis- and trans-HC(O)SH + HO could be negligible. The direct abstraction channel was also determined to yield CH₂S + HO₂, with the barrier height of 22.3, 18.1 and 15.0 kcal/mol at G3MP2, CCSD(T)/cc-pVTZ and BMC-CCSD levels, respectively, it is not competitive with the addition channel, in which all stationary points are lower than reactant energetically. The other channels to produce cis- and trans-CHSH + HO₂ are also of no importance.     

Accurate ab initio binding energies of the benzene dimer

Accurate binding energies of the benzene dimer at the T and parallel displaced (PD) configurations were determined using the single- and double-coupled cluster method with perturbative triple correction (CCSD(T)) with correlation-consistent basis sets and an effective basis set extrapolation scheme recently devised. The difference between the estimated CCSD(T) basis set limit electronic binding energies for the T and PD shapes appears to amount to more than 0.3 kcal/mol, indicating the PD shape is a more stable configuration than the T shape for this dimer in the gas phase. This conclusion is further strengthened when a vibrational zero-point correction to the electronic binding energies of this dimer is made, which increases the difference between the two configurations to 0.4-0.5 kcal/mol. The binding energies of 2.4 and 2.8 kcal/mol for the T and PD configurations are in good accord with the previous experimental result from ionization potential measurement.     

Gas-phase reaction of the isobutenyl anion with N2O from ab initio calculations

Calculations using conventional ab initio theory are performed to investigate the reaction mechanism associated with the gas-phase ion/molecule reaction of isobutenyl anion with N₂O. As a result, our theoretical findings strongly suggest that the main pathway is the reaction pattern of end-N attack and that the corresponding reaction mechanism basically relates to hydrogen migration, which may yield products cis-CH₂(CH₃)CCN ₂ ^? , trans-CH₂(CH₃)CCN ₂ ^? , and H₂O. Those are in good agreement with the experimental observations. Moreover, based on the NBO, Activation Strain model and methyl group effect analysis, we also explored the characters of rate-determining step of the main pathway.     

An ab initio molecular orbital study on the gas-phase rapid reaction of the silicon cation Si with ammonia

The gas-phase rapid ion-molecule reaction Si⁺ (²P) + NH₃→ SiNH₂⁺ + H is theoretically investigated by the ab initio molecular orbital methods. Several possible pathways (A, B, C) on its potential energy surface have been examined, discussed and compared. Theoretical calculations indicate that pathway A is favourable in energy and that the reaction begins by forming a collision complex of the ion-dipole molecule Si-NH⁺₃, which forms with no barrier into the first energy well of the reaction coordinate. Migration of an H atom from an N atom to a Si atom forms the intermediate HSi-NH⁺₂, which corresponds to the second energy well and can fragment to the observed product SiNH⁺₂ by losing an H atom from the Si atom. The barriers for migration and fragmentation are 52.5 and 38.6 kcal mol⁻¹ respectively. Pathway A has a negative activation energy of −42.1 kcal mol⁻¹.     

Theoretical ab initio study of NO and CO depollution reaction catalyzed by copper

The reaction between NO and CO leading to N₂ and CO₂ is the most studied depollution process of the former molecules. An ab initio study of a multistage mechanism of this reaction catalyzed by copper was performed at SCF level. Many intermediates intervene in the proposed mechanism, such as CuCO, CuNO, CuO, and NCO. Geometrical parameters, atomic charge, dipole moment, vibrational normal mode wave number, and dissociation energy of intervening molecules were calculated. Thermochemistry parameters (ΔH, ΔG, ΔS) were also obtained. Transition state has also been determined and has allowed us to discuss the reaction mechanism. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

An ab initio study on the allene-isocyanic acid and ketene-vinylimine [2 + 2] cycloaddition reaction paths

The reaction paths of [2 + 2] cycloadditions of allene (H2C=C=CH2) to isocyanic acid (HN=C=O) and ketene (H2C=C=O) to vinylimine (H2C=C=NH), leading to all the possible 14 four-membered ring molecules, were investigated by the MP2/aug-cc-pVDZ method. In the two considered reactions, the 2-azetidinone (beta-lactam) ring compounds were predicted to be the most stable thermodynamically in the absence of an environment. Although 4-methylene-2-azetidinone is the most stable product of the ketene-vinylimine cycloaddition, its activation barrier is higher than that for 4-methylene-2-iminooxetane by ca. 6 kcal/mol. Therefore, the latter product can be obtained owing to kinetic control. The activation barriers in the allene-isocyanic acid reactions are quite high, 50-70 kcal/mol, whereas in the course of the ketene-vinylimine cycloaddition they are equal to ca. 30-55 kcal/mol. All the reactions studied were found to be concerted and mostly asynchronous. Simulation of the solvent environment (toluene, tetrahydrofuran, acetonitrile, and water) by using Tomasi's polarized continuum model with the integral equation formalism (IEF-PCM) method showed the allene-isocyanic reactions remained concerted, yet the activation barriers were somewhat higher than those in the gas phase, whereas the ketene-vinylimine reactions became stepwise. The larger the solvent dielectric constant, the lower the activation barriers found. The lowest-energy pathways in the gas phase and in solvent were confirmed by intrinsic reaction coordinate (IRC) calculations. The atoms in molecules (AIM) analysis of the electron density distribution in the transition-state (TS) structures allowed us to distinguish pericyclic from pseudopericyclic from nonplanar-pseudopericyclic types of reactions.     

High-level ab initio potential energy surfaces and vibrational energies of H2CS

Six-dimensional (6D) potential energy surfaces (PESs) of H(2)CS have been generated ab initio using the recently proposed explicitly correlated (F12) singles and doubles coupled cluster method including a perturbational estimate of connected triple excitations, CCSD(T)-F12b [T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys. 127, 221106 (2007)] in conjunction with F12-optimized correlation consistent basis sets. Core-electron correlation, high-order correlation, scalar relativistic, and diagonal Born-Oppenheimer terms were included as additive high-level (HL) corrections. The resulting 6D PESs were represented by analytical functions which were used in variational calculations of the vibrational term values below 5000 cm(-1). The best PESs obtained with and without the HL corrections, VQZ-F12(*HL) and VQZ-F12?, reproduce the fundamental vibrational wavenumbers with mean absolute deviations of 1.13 and 1.22 cm(-1), respectively. A detailed analysis of the effects of the HL corrections shows how the VQZ-F12 results benefit from error cancellation. The present purely ab initio PESs will be useful as starting points for empirical refinements towards an accurate "spectroscopic" PES of H(2)CS.     

Interaction of Ca2+ and Mg2+ with Ionophores Studied by Using a Pair-Potential Model Based on ab initio Calculations

Atom pair potentials are obtained from ab initio SCF-LCAO-MO calculations for model complexes of Mg²⁺ and Ca²⁺ with N, N-dimethylacetamide, and malonamide. The SCF-LCAO-MO interaction energies for 271 complexes of Mg²⁺ and 271 complexes of Ca²⁺ with these amides were fitted with a simple analytical potential by a least-square procedure. Interaction energies and optimal ion locations obtained by pair-potential calculations are compared with values obtained by ab initio calculations for some related amides. The application of the atom pair potentials to the structure of the Mg²⁺-complex [MgCl² (C³H⁷ON)⁶] of N-ethylacetamide is discussed.     

Accurate ab initio binding energies of alkaline earth metal clusters

The effects of basis set superposition error (BSSE) and core-correlation on the electronic binding energies of alkaline earth metal clusters Y(n) (Y = Be, Mg, Ca; n = 2-4) at the Moller-Plesset second-order perturbation theory (MP2) and the single and double coupled cluster method with perturbative triples correction (CCSD(T)) levels are examined using the correlation consistent basis sets cc-pVXZ and cc-pCVXZ (X = D, T, Q, 5). It is found that, while BSSE has a negligible effect for valence-electron-only-correlated calculations for most basis sets, its magnitude becomes more pronounced for all-electron-correlated calculations, including core electrons. By utilizing the negligible effect of BSSE on the binding energies for valence-electron-only-correlated calculations, in combination with the negligible core-correlation effect at the CCSD(T) level, accurate binding energies of these clusters up to pentamers (octamers in the case of the Be clusters) are estimated via the basis set extrapolation of ab initio CCSD(T) correlation energies of the monomer and cluster with only the cc-pVDZ and cc-pVTZ sets, using the basis set and correlation-dependent extrapolation formula recently devised. A comparison between the CCSD(T) and density functional theory (DFT) binding energies is made to identify the most appropriate DFT method for the study of these clusters.     

Systematic ab initio studies of the conformers and conformational distribution of gas-phase tyrosine

A full structural assignment of the conformers of gaseous tyrosine is presented. A total of 1296 unique trial structures were generated by allowing for all combinations of internal single-bond rotamers and optimized at the B3LYP6-311G* level of theory and then subjected to further optimization at the B3LYP6-311++G** level. A total of 76 conformers are found and their dipole moments, rotational constants, and harmonic frequencies are determined. Accurate relative energies are given at the MP26-311G(2df,p)B3LYP6-311++G** level of theory. Characteristic H-bonding types are classified and listed for all the conformers. The four most stable conformers display an intramolecular H bond, COOH...NH(2), and an additional H-bonding interaction between the amino group and pi electron of the aromatic ring. The results further confirm that the global minimum conformations of the aromatic amino acids have the same H-bonding type. Combined with statistical mechanics principles, conformational distributions at various temperatures are computed and the temperatures with which the theoretical results match that of experiments are indicated.     

Ab initio investigation on the reaction path and rate for the gas-phase reaction of HO + H2O <--> H2O + OH

This article describes an ab initio investigation on the potential surfaces for one of the simplest hydrogen atom abstraction reactions, that is, HO + H2O <--> H2O + OH. In accord with the findings in the previously reported theoretical investigations, two types of the hydrogen-bonding complexes [HOH--OH] and [H2O--HO] were located on the potential energy surface. The water molecule acts as a hydrogen donor in the [HOH--OH] complex, while the OH radical acts as a hydrogen donor in the [H2O--HO] complex. The energy evaluations at the MP2(FC) basis set limit, as well as those through the CBS-APNO procedure, have provided estimates for enthalpies of association for these complexes at 298 K as -2.1 approximately -2.3 and -4.1 approximately -4.3 kcal/mol, respectively. The IRC calculations have suggested that the [H2O--HO] complex should be located along the reaction coordinate for the hydrogen abstraction. Our best estimate for the classical barrier height for the hydrogen abstraction is 7.8 kcal/mol, which was obtained from the CBS-APNO energy evaluations. After fitting the CBS-APNO potential energy curve to a symmetrical Eckart function, the rate constants were calculated by using the transition state theory including the tunneling correction. Our estimates for the Arrhenius parameters in the temperature region from 300 to 420 K show quite reasonable agreement with the experimentally derived values.