Benzooxirene. Ab initio calculations

Ab initio calculations have been performed on benzooxirene, the corresponding oxo carbene (“ketocarbene”), and the transition state linking the two. At the highest level used, QCISD(T)/6-31G^*//MP2(FULL)/6-1G^* with MP2(FULL)/ 6-31G^* zero point energy corrections, the relative energies of the oxirene, the transition state and the carbene are 0, 24.6, and −17.8 kJ mol⁻¹. Correlation energy effects are very important in this system: at the QCISD(T) level the oxirene lies above the carbene, as at the MP4 and HF levels, but at the MP2 level the ordering is reversed. Benzooxirene is probably slightly nonplanar: the HF/6-31G^* geometry is C_2v but the MP2(Fermi contact)/6-31G^* geometry is C_s with a 6-/3-ring coplanarity deviation of about 6.9 °, although in the MP2(FULL)/6-31G^* geometry this is reduced to about 3.1 °.     

Ab initio studies of small AlmFen clusters

The equilibrium geometrical structures of small Al_mFe_n clusters have been determined through ab initio calculation of the cluster total energy at the UB3LYP/Lanl2dz level. For dimers of iron and aluminum, the dissociation energies, the equilibrium atomic distances, and the vibrational frequencies were calculated. The agreement between calculations and experiments is reasonable. The ground stable geometrical structures of Fe₄, FeAl₃, Fe₃Al and Fe₂Al₂ clusters favor three-dimension configurations, but Al₄ tetramers are planar structures. The Al-rich tetramers are more stable than the other two composition tetramers. This is different from that of bulk alloys.     

Ab initio calculations on the molecular structure of fluorocyanopolyynes

The molecular structure of the first three members of the fluorocyanopolyynes was studied by ab initio Hartree-Fock calculations with a polarized double zeta basis set and at MP2 level with the same basis set. Alternating triple and single bonds were found; a theoretical estimate of rotational constants and dipole moments was performed and a comparison with the available experimental data was made. An analysis of the theoretical vibrational frequencies of the title compounds was carried out.     

Cu+和Ag+叠氮盐晶体的周期性ab initio计算

过渡金属叠氮盐为化学结构简单的爆燃爆炸性物质．可作为起爆剂或快速反应研究的模型体系。在炸药化学、固体化学和固体物理等领域受到广泛关注．其晶体结构、物理、化学和爆炸性质已有许多报道．但迄今对该类化合物的理论研究方法仅局限于半经验的DV-Xa方法和EH-CO法．     

Ab initio calculations on Pro-Ala and Pro-Gly dipeptides

The geometries of the dipeptides -Pro- -Ala, -Pro- -Ala and -Pro-Gly were investigated by a grid scan ab initio calculation. The 6-31G basis set was used to estimate the effect of the alanyl side-chain on the conformation of the peptide backbone and to provide a computational basis for the interpretation of known physical-chemical properties of larger peptides that contain these dipeptides. These calculations furnish a direct quantum mechanical assessment of the energetic consequences of a methyl side-chain in the i + 2 position of a turn. The results of the calculation support the current view that the presence of a -Ala residue in the i + 2 position favors a type II β-turn over a type I β-turn conformation, while -Ala has the opposite effect. Total and relative energies for all the optimized conformations identified by the grid search are given and geometric parameters (bond lengths, bond angles and dihedral angles) and net atomic charges have been calculated.     

Ab initio and density functional theory calculations of molecular structure and vibrational spectrum of ethyl azidoacetate

Here we report ab initio and density functional results for molecular properties of ethyl azidoacetate (N₃CH₂COOC₂H₅) and for the corresponding singly ionized structure (N₃CH₂COOC₂H₅⁺). Ab initio ionization energies based on Koopmans’ theorem are in excellent agreement with the experimental data from ultraviolet photoelectron spectroscopy. DFT adiabatic energy differences between neutral and ionized structures are very sensitive to electronic correlation effects and are not in very good agreement with experiment. The results for the structure and vibrational frequencies are compared with the experimental data of related molecular structures.     

硫代甲酰胺双聚体的量子化学计算

在MP2/6 31G(d)和MP2(FC)/6 311 G(d,p)水平上,对硫代甲酰胺(HC-SNH2)及其3种构型双聚体进行几何全优化计算,经振动频率分析,确认为势能超曲面上的稳定驻点.然后在MP2/6 311 G(2df,2p)水平上进行单点能计算和基组重叠误差(BSSE)校正以获得相互作用能.并利用自然键轨道(NBO)理论和分子中的原子(AIM)理论探讨HCSNH2之间相互作用的本质.     

Ab initio prediction of vibrational spectra: A database approach

Efforts to develop a database of quadratic force fields for organic molecules are described. The database is based on systematic ab initio calculations, scaled to reproduce the experimentally observed frequencies. The choice of the theoretical method, the basis sets, geometries, internal coordinates and the scaling procedure are discussed. A key point in the procedure is the automatic generation of the internal valence coordinates. This is also very advantageous for geometry optimization. The database should permit the prediction of vibrational frequencies for most organic molecules to 10–20 cm⁻¹, together with semiquantitative intensities. The accuracy is sufficient to identify unknown compounds from a list of reasonable candidates.     

Ab initio calculations of pKa values of some organic acids in aqueous solution

The acidity of different classes of organic compounds in aqueous solution has been calculated. The calculations are carried out at the SCF level with inclusion of entropic and thermochemical correction to yield free energies of dissociations.

The polarized continuum model is used to describe the solvent. The model furnishes pK_a values in relatively good agreement with experimental data. Scaling different parts of solvation energies provides a significant improvement in results and signifies the importance of balance of individual contributions from electrostatic, cavity, dispersion and repulsion interactions.     

Ab initio Investigation of Decomposition of HCF_2OH

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CnAl-(n=1~11)结构的量子化学从头计算

在RHF／ROHF6－311G*水平上，优化了直链构型CnAl－（n＝1～11）的几何参数，计算了体系的能量以及它们失去C，C2，C3，Al，CA1，C2Al等六种解离方式的解离能，并与CnP－的计算结果加以对比．计算结果证实了我们以往的实验观察，即CnAl的结构稳定性具有奇偶交替的变化规律，其中n为偶数的族离子相对比较稳定．还根据对计算结果的分析，探讨了CnAl－的其它结构特性．     

Ab initio and density functional calculations of mean-square amplitudes of vibration for benzene and cubane

Mean-square amplitudes of vibration were calculated using ab initio and density functional methods for benzene and cubane. Both 6-31G^* and 6-311G^** basis sets were employed. It was found that significant improvements were achieved when electron correlation was introduced, even if only at the local density functional level. The mean-square amplitudes calculated were not effected by the basis set used for benzene and slightly improved for the highly strained cubane molecule when the larger basis set was used. An attempt was also made to improve the calculated mean-square amplitudes by making use of scale factors found in the literature, which were developed to improve the calculated frequencies. It was found that only the SCF mean-square amplitudes were significantly improved.     

Structural transition of NaBH4 under high pressure: Ab initio calculations

The pressure induced structural transition of NaBH₄ from β-NaBH₄ (tetragonal-P42_1c) to γ-NaBH₄ (orthorhombic-Pnma) is investigated by ab initio plane-wave pseudopotential density functional theory method (DFT). The BaSO₄-type structure of orthorhombic high-pressure phase is testified theoretically for the first time. The calculated transition pressure of β-NaBH₄ (tetragonal-P42_1c) to γ-NaBH₄ (orthorhombic-Pnma) is 9.66 GPa and the orthorhombic high-pressure phase is stable up to 30 GPa. Our results agree well with previous experimental results and demonstrate that high-pressure phase transition from β-NaBH₄ to γ-NaBH₄ may occur at low temperature. At last, the pressure effects on the electronic structures of α-, β- and γ-NaBH₄ are discussed.     

Ab initio analysis of the interaction of CO2 with acetylated d-glucopyranose derivatives

Peracetylated d-glucopyranose has a high solubility in CO₂ and can be a promising phase-change physical solvent or absorbent for CO₂, as reported recently. However, peracetylated d-glucopyranose is unstable under acidic atmospheres, especially in sulfur-containing waste gases, and the possibly major decomposition products are 2,3,4,6-tetra-O-acetyl-d-glucopyranose, 1-thiol-d-glucopyranose tetraacetate, and 1-mercaptoethyl-d-glucopyranose tetraacetate. Therefore, it is highly interesting to investigate the interaction between CO₂ and these three compounds using ab initio calculations, including geometry optimizations with HF/3-21G, B3LYP/6-31+G** and single-point energy calibration with MP2/aug-cc-pVDZ. The results indicate that the electrostatic interactions between the substrates and CO₂ are mainly influenced by the interaction distance and the numbers of negative charge donors or the interacting pairs involved in the complex. It is furthermore found that ΔE increases significantly if S and O atoms could interact with CO₂ simultaneously. The binding energy is irrelevant if one considers the chemical environment of the O atom (i.e. O_Ac, O_E or O_S) or the S atom (i.e. SEt or SH), and the electronegativity difference between the S and O atoms. The three substrates studied are still excellent CO₂-philes, although their average ΔE (–20 kJ/mol) is slightly lower than that of peracetylated d-glucose (–22 kJ/mol), which has one more O atom that can interact with CO₂. Therefore, the applications of carbohydrates can be expanded to include adsorbents for CO₂, SO₂ or both, and the functional groups attached to the carbohydrate can vary from those to the acetyl groups.     

Ab initio Study on the Tautomerization Mechanism and the Kinetics of Formamide

The tautomerism of formamide has been investigated by ab initio calculation using RHF/4-31G basis set. The change in some physical properties along the reaction path is presented. In the study of molecular reaction kinetics, we have calculated all the kinetic parameters according to statistical thermodynamics and transition state theory. The combination of kinetics analysis with IRC analysis has indicated that the tautomerism is concerted but the geometric changes are nonsynchronous; the lifetime of the transition state is short and the transition state is tight type; the formamide is more stable than formimidic acid; the kinetic rate constant of backward reaction is larger than that of the forward reaction in the range of temperature studied.     

铍硼四原子簇合物的量子化学从头算

铍和硼簇合物的实验和理论研究表明,它们有特殊的成键方式,BBe2,B2Be2,B6Be和B4Be等簇合物结构已由晶体粉末法测得^[1]。这类化合物可用于带电粒子的活化分析和低能电子的遏止实验,有一定应用前景^[2],对BnBem簇电子结构和成键性质进行研究,不仅对理解大簇和凝聚相的形成机制及微观结构等有重要意义,而且有一定的应用价值^[3-6],B2Be/B2Be^ 的结构^[7]已有论证。本文选用高精度大基组二次组态相互作用QCISD（T）／6－311G^**方法对BBe3,B2Be2和B3Be的电子结构进行了计算,并在HF／6－31G^*水平上作了频率计算。     

Molecular structure of phenylsilane: a study by gas-phase electron diffraction and ab initio molecular orbital calculations

The molecular structure of phenylsilane has been determined accurately by gas-phase electron diffraction and ab initio MO calculations at the MP2(f.c.)/6-31G* level. The calculations indicate that the perpendicular conformation of the molecule, with a Si–H bond in a plane orthogonal to the plane of the benzene ring, is the potential energy minimum. The coplanar conformation, with a Si–H bond in the plane of the ring, corresponds to a rotational transition state. However, the difference in energy is very small, 0.13 kJ mol⁻¹, implying free rotation of the substituent at the temperature of the electron diffraction experiment (301 K). Important bond lengths from electron diffraction are: <r_g(C–C)>=1.403±0.003 Å, r_g(Si–C)=1.870±0.004 Å, and r_g(Si–H)=1.497±0.007 Å. The calculations indicate that the C_ipso–C_ortho bonds are 0.010 Å longer than the other C–C bonds. The internal ring angle at the ipso position is 118.1±0.2° from electron diffraction and 118.0° from calculations. This confirms the more than 40-year old suggestion of a possible angular deformation of the ring in phenylsilane, in an early electron diffraction study by F.A. Keidel, S.H. Bauer, J. Chem. Phys. 25 (1956) 1218.     

Ab initio molecular orbital calculations on large lattice cluster models: Use of translational symmetry

Summary Translational symmetry has been shown to be useful in the calculation of electronic structures of large lattice models. The number of unique integrals has been derived for cases of different dimensionality. For the unique integrals zero screening and approximation methods are described. The method has been applied to arrays of hydrogen atoms and to a zincblende surface model. When the size of the system is increased the translationally unique integrals are shown to become either zero or they can be calculated by simple coulombic approximations.     

Ab initio calculations on the multipole moments and dipole polarisabilities of the PO molecular ion (XΣ)

The potential energy, dipole, quadrupole and octopole moments and dipole polarisabilities have been calculated at CASSCF level for the ground X¹Σ⁺ state of the PO⁺ molecular ion as a function of internuclear distance. Most of the electrical properties have not previously been calculated and show rapid variations around 5 a.u. due to a perturbation. The calculated vibrational frequency of 1410.4 cm⁻¹ and the integrated IR absorption intensity of 984 cm² mol⁻¹ should lead towards the first observation of the vibrational spectrum.     

富勒烯结构Si60的从头算研究

采用量子化学从头算方法, 系统地研究了Si60-Ih及其各种降低对称性后的扭曲构型的稳定性. 找到了5个低能量低对称性(对称性分别为T, Ci, C1, CS和C2) Si60的稳定结构. 分析计算结果表明, 典型的低能量Si60结构对应着一些硅原子凸出球外和一些硅原子凹进球内, 部分Si原子间的成键呈sp3杂化方式.