Ab initio prediction of MnOH2/MnOH2 electron transfer reactivity at electron correlation level

An ab initio calculation of electron transfer reactivity of Mn²⁺OH₂/Mn³⁺OH₂ redox system, in gas and in solution, has been made in this paper. The detailed geometry optimization and the scanning of the potential energy surfaces have been carried out at UMP2 (full)/6-311+G^* level. The relevant energy quantities (such as the activation energy and the binding energy) are calculated at different levels of theory (HF, MP2, MP3, MP4 and QCISD and corresponding spin-projection PUHF, PMP2 and PMP3) with the same basis set (6-311+G^*). Both all-electrons and valence electrons have been correlated in energy calculations. The electronic transmission coefficient is calculated using the ab initio potential energy surface slopes and the coupling matrix element determined from the two-state model and the Slater-type d-electron wave functions. The pair distribution function is obtained using both a classical sphere approximation scheme and a novel scheme. The relevant kinetic parameters are obtained at different ab initio levels in terms of the new model. The contact-distance dependence of these parameters and the applicability of the presented models are also discussed.     

O2+O2 electron transfer reactivity in the quartet state from ab initio calculation including electron correlation

The structures and properties of the O₂+O₂⁻ electron transfer system in the quartet state, both in the gaseous phase and in solution, were studied at the UMP2(full)/6-311+G* basis set level for the five selected coupling structures: two T-type, collinear, parallel, and crossing. The stabilities of these encounter complexes were compared. The activation barriers, coupling matrix elements, and the electron transfer rate at two theoretical levels (semiclassical and quantum mechanical) were also calculated for the quartet state, and the effect of the solvent medium evaluated at the self-consistent reaction field level. Results indicate that the structures and properties of the encounter complexes directly affect the mechanism and rate of the electron transfer reaction, the contact distances for this O₂…O₂⁻ were generally large (3 Å), the interaction between the donor and the acceptor was weak, and the structures are floppy. The electronic transmission factor for the reacting system, O₂+O₂⁻, was less than unity (ca. 001–0.6), thus the electron transfer reaction was non-adiabatic in nature. Analysis of the dependence of relevant kinetic parameters on various influencing factors showed that the effect of the solvent medium on the coupling matrix element was small, but that on the electron transfer rate was very large, and the gaseous phase results for the molecular geometrical parameters and their contributions can directly transfer to solution. Among the five selected transition state structures, the electron transfer was more likely to take place via the T-type and the P-type structures, the rate values from two theoretical levels were in good agreement with each other and were also very close to the experimental findings. If the various anharmonic vibrational contributions, the effect of the solvent molecular electronic structures and the interaction between the reacting species and the solvent medium are taken into account, the results can be improved.     

Theoretical study on the Co~(2+)OH_2/Co~(3+)OH_2 electron transfer reactivity

This paper presents a contact distance dependence analysis scheme and an ab initio calculation application for the electron transfer (ET) reactivity of Co2+OH2/Co3+OH2 reacting pair. The applicability of these schemes and the corresponding models has been discussed. The contact distance (Rcoco) dependence of the relevant quantities has been analyzed. The results indicate that the activation energy from the accurate PES method agrees well with that from the anharmonic potential method, and they are obviously better than that from the harmonic potential method. The pair distribution function varies from 10~(-2) to 10~(-5) along with Rcoco changing from 1.20 to 0.35 nm. The coupling matrix element exponentially decays along with the increase of Rcoco, and the effective electronic coupling requires Rcoco smaller than 0.75 nm. In the range from 0.50 to 0.75 nm for Rcoco, the corresponding electronic transmission coefficient falls within 1.0-10~(-6). The local ET rate also exponentially decays along with the incre     

Theoretical sstudy on the Co2+OH2+/Co3+OH2+ electron transfer reactivity

This paper presents a contact distance dependence analysis scheme and an abinitio calculation application for the electron transfer (ET) reactivity of Co²⁺OH₂/Co³⁺OH² reacting pair. The applicability of these schemes and the corresponding models has been discussed. The contact distance (R _CoCo) dependence of the relevant quantities has been analyzed. The results indicate that the activation energy from the accurate PES method agrees well with that from the anharmonic potential method, and they are obviously better than that from the harmonic potential method. The pair distribution function varies from 10⁻² to 10⁻⁵ along withR _CoCo changing from 1.20 to 0.35 nm. The coupling matrix element exponentially decays along with the increase ofR _CoCo, and the effective electronic coupling requiresR _CoCo smaller than 0.75 nm. In the range from 0.50 to 0.75 nm forR _CoCo, the corresponding electronic transmission coefficient falls within 1.0—10⁻⁶. The local ET rate also exponentially decays along with the increase ofR _CoCo due to the electronic factor. Since the contribution from the pair distribution function to the total ET rate is an inverse measure of that from the electronic factor, the variation of the spherically averaged local ET rate along withR _CoCo exhibits a parabola with a maximum at 0.50 nm ofR _CoCo. This maximum is close to the overall observed ET rate value. For this mono-hydrated transition metal ionic system, the ET rate generally is about 10⁶ L·mol⁻¹·s⁻¹ in gaseous process. Further, since it is impossible to experimentally determine the structures and their PESs of these hydrated systems, especially for the unstable intermediate species,ab initio calculations can play an effective auxiliary role in discussing the ET reactivities of these kinds of reacting systems.     

Ab initio molecular orbital study of Fe2Cl6 and FeAlCl6

Ab initio molecular orbital theory was used to determine the equilibrium structure and vibrational frequencies of Fe₂Cl₆ and FeAlCl₆. The equilibrium structure the Fe₂Cl₆ dimer has D_2h symmetry with a planar arrangement of the four membered {FeCl_brFeCl_br} ring, similar to the Al₂Cl₆ dimer. The calculated bond distances and vibrational frequencies are in good agreement with experiment. The potential energy surface for the puckering of the {FeCl_brFeCl_br} ring is extremely flat. This prevents an unambiguous assignment of either D_2h or C_2v symmetry to the Fe₂Cl₆ structure in electron diffraction measurements. The FeAlCl₆ molecule is found to have a C_2v structure similar to Fe₂Cl₆ with vibrational frequencies in good agreement with experiment.     

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.     

Theoretical Study of the Inner/sphere Reorganization Energy and Activation Energy of Self/exchange Electron Transfer Reaction for M(H2O)26+/3+ (M=V, Cr, Mn, Fe and Co) Systems

通过建立电子转移过程的活化模型和重组模型, 提出了用量子化学从头算方法研究电子转移过程内层重组能和活化能的新方法. 在UMP26/311G水平上获得了5对过渡金属水合离子体系M(H     

Electron transfer reactivity of O2+O system in low-spin coupling: Ab Initio study at electron correlation level

The electron transfer reactivity of the O₂+O system in low-spin coupling is studied at the second-order unrestricted Møller–Plesset (full)/6-311+G* basis set level by using different transition state structures. The properties and stabilities of the encounter complexes are compared for the five selected coupling structures: two T type, collinear, parallel, and crossing. The activation barriers and the coupling matrix elements are also calculated. The results indicate that the structures of the encounter complexes directly affect the electron transfer mechanism and rate. These encounter complexes are structurally unstable, the contact distances between the acceptor O₂ and the donor O are generally large, the interaction is weak, and the structures are floppy. The electronic transmission factor for the reacting system, O₂+O, is less than unity; thus, the electron transfer reaction is nonadiabatic in nature. Analysis of the dependence of relevant kinetic parameters on various influencing factors has shown that the effect of the solvent medium on the coupling matrix element is small but that on the electron transfer rate is very large. Among the five selected transition state structures, the electron transfer is more likely to take place via T₁-type and P-type structures. In the low-spin coupling the favorable electronic states for two reacting species are ¹∑(O₂) and X²Π_g(O) instead of X³∑(O₂) and X²π_g(O), which are favorable for the high-spin (quartet state) coupling mechanism. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 989–998, 1999     

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.     

Complete basis set, the MP2 ab initio, and hybrid density functional theory evaluation of ionization potential and electron affinity for PH, PH2, PHF, PF, and PF2

A systematic study was performed on the small molecular systems built from phosphor, hydrogen and fluorine with the target being to evaluate accurately their ionization potentials and electron affinities, as well as influence fluorine on the ionization potential of phosphor as a central atom. To determine the accuracy of hybrid density functional methods for computing those energies, ionization energies for hydrogen, fluorine and phosphor were calculated and compared with the experimental and CBSQ values. To demonstrate the accuracy of this method, both the ionization potential and the electron affinity for phosphorus and fluorine atoms were calculated and compared with the experimental data. For both PF and PF₂, an identical electron affinity of 0.72 eV and for PH and PHF 1.0 eV were suggested.     

CCl2自由基与H2O分子反应动力学研究

用213 nm激光光解CCl4产生CCl2自由基，用LP LIF技术测定了室温下基态CCl2自由基与H2O分子的反应速率常数为(5.45±0.95)×10－14 cm3•molecule－1•s－1.在G2MP2理论水平上计算了CCl2＋H2O反应的最低单重态势能面，揭示了插入与加成 消除两种反应机理，得到了三个可能的产物通道:HCl＋HClCO、HCl＋trans ClCOH以及HCl＋cis ClCOH.并用RRKM TST和传统过渡态理论计算了这三个通道的分支比及其温度效应.结果说明在低温下（273 K），插入机理的产物通道的分支比远大于加成 消除机理的产物通道， HCl＋HClCO是主要产物，分支比为77.4％，其次是HCl＋cis ClCOH，分支比为22.6％.而在高温下（3000 K），加成 消除机理的反应通道大于插入机理， HCl＋trans ClCOH分支比为82.3％.     

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.     

Ab initio MO study on the global minimum structure of C3H3 anion: propynl-1-yl versus allenyl anions

The geometrical structures of the C₃H⁻₃ anion are surveyed at the coupled-cluster doubles (CCD) level of theory with the aug-cc-pVDZ basis set. To clarify the CCD geometries, the stable two isomers -- propynl-l-yl 1 and allenyl 2 anions -- are further optimized at the coupled-cluster singles, doubles (triples) (CCSD(T)) level of theory both with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. The final energies are calculated at the CCSD(T) and the complete active space self-consistent field (CASSCF) multi-reference internally contracted CI (MRCI) levels of theory with the aug-cc-pVTZ basis set. At the MRCI level of theory including both the corrections due to the cluster energies (MRCI+Q) and the zero-point vibrational energies, the allenyl anion 2 is about 1.3 kcal mol⁻¹ lower in energy than the propynl-l-yl anion 1. These results contrast with the previous theoretical estimates, where the propynl-l-yl anion 1 is 2-3 kcal mol⁻¹ lower in energy than the allenyl anion 2. The activation energies of the intramolecular hydrogen transfer in the 1 → 2 conversion reactions are 63.5 kcal mol⁻¹ at the MRCI+Q level of theory with the aug-cc-pVTZ basis set including the zero-point energy corrections. The adiabatic electron affinity of the planer propargyl (H₂CCCH) radical, which is the global minimum of the C₃H₃ radical, is calculated to be 0.976 eV (after correction for the zero-point energy changes) at the CCSD(T) level of theory with the aug-cc-pVTZ basis set. The present electron affinity is in fairly good agreement with the experimental one (0.893 eV) observed by Oakes and Ellison.     

Ab initio molecular orbital calculations of nuclear spin-spin coupling constants in PH2, PH3, PH4 and P2H4

Ab initio molecular orbital calculations of nuclear spin-spin coupling constants in PH⁻₂, PH₃, PH⁺₄ and P₂H₄ have been carried out employing SCF perturbation theory. Basis set dependence of all the four contributing terms has been studied in order to find the criterion for the selection of basis sets to be employed for computing this property. The dependence of the coupling constants of PH⁻₂ on its geometry has also been found. This study also discusses the requirement for satisfactory computation of couplings in cases where none of the coupling nuclei is a proton. It is found that bond-centred functions along with at least double zeta basis sets reproduce coupling constants quite satisfactorily. In all the cases studied, uncontracted core basis functions yield couplings which are in better agreement with experimental couplings than those obtained with contracted core functions.     

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.     

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 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.     

Gas phase and water solution conformational analysis of the herbicide diuron (DCMU): an ab initio study

In the present work, the conformational equilibrium for the herbicide diuron (DCMU) has been investigated using high level ab initio calculations. The solvent effect was included through two different continuum models: (1) the real cavity IPCM method and (2) the standard dipole Onsager model SCRF. The effect due to solute-solvent hydrogen-bond interactions was analyzed considering a hybrid discreet-continuum model. At the Hartree-Fock level, the gas phase results showed that only the trans forms (A and B) are present in the equilibrium mixture, with the relative concentrations found to be 33% (A) and 67% (B) (HF/6-311+G**//6-31G**). When the electronic correlation effect is included (MP2/6-31G*//HF/6-31G*), a relative stabilization of the cis forms was observed, with the conformational distribution calculated as 38% (A), 50% (B), 6% (C) and 6% (D). The trans conformations were found to be completely planar, which has been considered to be a prerequisite for the herbicide binding. In water solution, the trans conformation A should be the most abundant conformer, the IPCM and SCRF values being ca. 100% and ca. 85% respectively. The IPCM calculations with the isodensity level set to 0.0005 present a conformational distribution close to that obtained from the hybrid model [92% (A) and 8% (B)], which has been considered our best solvent approach. Regarding the biological action of urea-type herbicides, the results presented here are important, because some QSAR studies have suggested that the partition coefficient is related to the herbicide activity, so the conformational equilibrium may play a role in the biological action. Received: 23 February 1998 / Accepted: 28 May 1998 / Published online: 19 August 1998