Ab initio studies of

Ab initio calculations of the [1,5]-H shift in (3Z)-penta-1,3-diene and other substituted pentadienes and heteroanalogues using the hybrid density functional Becke3LYP with the 6-31G basis set are presented. Electron-donating substituents, such as methoxy in (3Z)-3-methoxypenta-1,3-diene 1, or heteroatoms such as a nitrogen atom in (Z)-ethylidenevinylamine 2, (1Z)-buta-1,3-dienylamine 3, (2Z)-but-2-enylideneamine 4, (Z)-allylidenemethylamine 5, and methylene-(Z)-propenylamine 6 are introduced. The electron-withdrawing fluoride is substituted for the hydrogen atoms in (3Z)-3-fluoropenta-1,3-diene 7, (3Z)-2,4-difluoropenta-1,3-diene 8, (3Z)-1,1',2,3,4,5,5'-heptafluoropenta- 1,3-diene 10, (1E,3E)-1,3,5-trifluoropenta-1,3-diene 11, and (1Z,3E)-1,3,5- trifluoropenta-1,3-diene 13. A detailed analysis of the geometries, energies, and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted case provides insights into substituent effects of this prototype of pericyclic reaction. The inductive and mesomeric effects of heteroatoms or heterosubstituents are of a great importance and in a continuous balance in the energetics of the transformation. Sterics can also play an important role due to the geometrical constraints of the reaction. As a general trend, decreasing the electron density of the phi system destabilizes the aromatic transition structure and increases the activation energy, and vice versa.     

Ab initio studies on the thermolysis of azetidine

The reaction mechanism of the thermolysis of azetidine to form ethylene and methylen-imine has been studied by ab initio SCF MO method at STO--3G and 3-21G levels. Two possible step-wise pathways are explored. One is the breaking of C--C bond as the first step, while the other is thebreaking of C--N bond. All the stationary points on the potential energy surface (PES) are fully optimiz-ed. MP2 / 3-21G single point calculations on all stationary points and MCSCF / STO-3G computationsfor some stationary points are also carried out. The calculations indicate that azetidine decomposesvia biradicaloid intermediates and the cleavage of C--N bond is preferable to that of C--C bond.     

Ab initio effective core potential studies on polymers

Ab initio crystal orbital calculation with the effective core potential (ECP) approximation is performed on infinite poly-yne, all-trans-polyethylene, and all-trans-polysilane. The optimized bond lengths of poly-yne are predicted to be 1.130 Å and 1.321 Å with the split valence LP-31G basis set and agree fairly well with 4-31G results, 1.166 Å and 1.339 Å. The energy band structures of poly-yne and all-trans-polyethylene obtained from ECP calculations are in reasonable agreement with those from the all electron calculations. The fully optimized geometries of all-trans-polysilane are also predicted with the LP-31G basis set asr _SiSi = 2.264 Å,r _SiH = 1.493 Å, \(\sphericalangle\) SiSiSi = 118.97 °, and \(\sphericalangle\) HSiH = 100.35 °. The computational time for calculations of polysilane is found roughly to be comparative to that of polyethylene under ECP approximations.     

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 studies of peroxynitrite anion-water complexes

Quantum mechanical methods have been applied to thecis-ONOO^–-H₂O,cis-ONOO^–-(H₂O)₂ andtrans- ONOO^–-H₂O complexes. Equilibrium geometries, binding energies, net atomic charges and vibrational frequencies are presented for several different arrangements. The MØller-Plessett second-order perturbation (MP2) method predicted shorter hydrogen bonds than the SCF method, but the computed Hartree-Fock (HF) binding energies are similar to counterpoise corrected MP2 values. The geometry changes of ONOO^– and water after solvation are examined. The ONOO^– and H₂O bond length changes follow typical hydrogen bond structural trends, whereas bond angles in ONOO^– are unaffected when the hydrogen bond is formed, similar to the conclusions from NO ₂ ^– -(H₂O) _n HF/6-31G studies and Monte Carlo simulations. Thecis-ONOO^–-(H₂O) _n frequencies are compared with the solution Raman spectrum and with calculations on isolated ONOO^–.     

Ab initio studies of arginine and deaminoarginine

L-Arginine and Deaminoarginine were studied via quantum chemical ab initio calculations using the STO -3G basis set for arginine and the STO -3G and 3–21G basis sets for deaminoarginine. It was found that the most stable conformations are the ones featuring the carboxyl group slightly twisted versus the rest of the molecule, which adopts an extended conformation. It was also found that the cyclic zwitterion is less stable than the neutral cyclic conformation and that there is a barrier to the proton transfer from the carboxyl's oxygen to one of guanidine moiety's nitrogens.     

Ab initio studies of benzocyclopropenone, benzocyclopropenone-containing

Ab initio calculations were carried out on cyclopropenone, 1, benzocyclopropenone, 2, the benzocyclopropenone-containing [2.2]paracyclophane derivative tetracyclo[8.3.2.(4,7)O(11,13)]heptadeca-1(13),4,6,10,14,16-hexaen-12-one, 3, its decarbonylation product tricyclo[8.2.2.2(4,7)]hexadeca-1(12), 4,6,10,13-pentaen-15-yne, 5, a benzyne intermediate, and the bridged benzobarrelene derivative, pentacyclo[5.5.2.2.(1,4)O(4,14)O(10,13)]hexadeca-2,7,9,13,15-pentaene, 6. These calculations suggest that benzocyclopropenone-containing [2.2]paracyclophane, 3, and highly strained bridged benzobarrelene, 6, could exist as stable species. Both aryl rings of the benzocyclopropenone derivative 3 are predicted to be distorted from planarity. This distortion relieves some angle strain present in planar benzocyclopropenone due to the presence of the annulated three-membered ring. Calculations on benzobarrelene, 8, and [2.2]paracyclophane, 4, were performed for comparison to gain a better understanding of the strain borne in bridged benzobarrelene 6. The activation barrier for the intramolecular [4 + 2] cycloaddition of 5 to give 6 was estimated at 18.8 kcal/mol while that for the corresponding [2 + 2] cycloaddition, giving the less stable 9, was 54.5 kcal/mol. The [2 + 2] cycloaddition's transition state was twisted in a manner reminiscent of the conservation of orbital symmetry prediction for an unstrained system.     

Ab initio studies on infinite linear hydrogen fluoride chains

Ab initio crystal orbital calculations on linear infinite chains of hydrogen fluoride and MO calculations on HF and the linear dimer have been performed. Equilibrium geometries, force constants, band structures, densities of states and longitudinal phonon dispersions are presented, and compared with the available data. In agreement with experiment the most common features of hydrogen bonding, elongation of HX bond length (ΔR_HX and decrease in HF stretching force constants, are much more pronounced in the solid state than in the isolated dimer.     

Ab initio studies on polymers. VII. Polyoxymethylene

The structure of an isolated, infinite polyoxymethylene chain has been investigated with the aid of the ab initio crystal orbital method applying a basis set of double-zeta quality. Restricting the primitive unit cell to a single CH₂O group, conformational potential curves as a function of the torsional angle have been evaluated. Only a single minimum closely corresponding to an all-gauche structure was detected. The all-trans conformation is a maximum on the energy curve for simultaneous rotation around C? O single bonds. Detailed geometry optimization in the vicinity of the all-gauche conformation led to the following structure: r_CO = r_OC = 1.425 Å, r_CH = 1.072 Å, ∠HCH = 111.7°, ∠OCO = 110.9°, ∠COC = 115.1°, and τ_OCOC = 70.75°. The computed torsional angle τ_OCOC lies midway between the hexagonal (78.2°) and the orthorhombic (63.5°) modification of solid polyoxymethylene.     

Ab initio molecular orbital studies of polyethylene deformation

A molecular LCAO Hartree-Fock procedure was used to calculate total energies of axially stretched normal paraffins containing up to nine carbon atoms. The results are used to model the mechanical properties of polyethylene.     

Ab initio studies of methylenecarbene and isoelectronic species

Optimum equilibrium geometries, energetics, harmonic vibrational frequencies, and infared intensities within the double harmonic approximation are computed for methylenecarbene, CCH₂, and isoelectronic species involving silicon and germanium at both the SCF level of theory and the level of second-order perturbation theory using a 6-311G(2df, 2p) basis set or its equivalent. Optimum equilibrium geometries and energetics are also computed at both levels of theory using a smaller 6-311G(d, p) basis set or its equivalent. This investigation of these species is the first to include all of the systems with germanium. In addition, this present work is the first study to includef-type polarization functions in a systematic investigation of the molecular structure and properties of all the molecules in the series. Acetylenic structures are also computed for energy comparisons. Of all the linear isomers, only acetylene is found to be a minimum on the potential energy surface. However, all of the C_2v structures are found to be local minima. Both the C_2v and linear structures will serve as a basis for future work involving mapping the entire hyperenergy surfaces of all of the molecular systems in the series.     

Ab initio and DFT studies on hydrolyses of phosphorus halides

Hydrolyses of phosphorus halides, (RO)(2)POX where R = H or Me and X = F or Cl, in the gas phase and in the reaction field have been investigated theoretically with ab initio and the density functional theory (DFT). The free energy of activation in the reaction field was also estimated using the Onsager method with a correction of entropy change and basis set superposition error (BSSE). The reaction of (MeO)(2)POF proceeds through a path with bifunctional catalysis regardless of the medium, but the reaction of (MeO)(2)POCl proceeds through bifunctional and general base catalysis in the gas phase and in water, respectively. The estimated free energy barrier of 23 kcal/mol for the hydrolysis of (MeO)(2)POF is in good agreement with the experimental values of 24 kcal/mol, and relative barrier of 3 kcal/mol to the (MeO)(2)POCl is also in good agreement with the experimental values of 5 kcal/mol of diisopropyl phosphorus halides ((Pr(i)O)(2)POX, X = F and Cl).     

二元低价铟溴化合物结构不稳定的从头算研究

应用较大基组的从头算法,以八配位立方构型多面体为例,研究了二元低价铟溴化合物不稳定性的微观结构原因,分析了构型的势能曲线状况和分子轨道性质．结果表明, 高对称的八配位立方构型多面体势能曲线异常平缓且分布各向异性,中心力场很弱,导致构型极易在外界因素的影响下发生变化．二元低价铟溴化合物具有无机固态化合物中少见的二阶Ｊａｈｎ－ Ｔｅｌｌｅｒ 效应     

Ab initio studies of properties of small potassium clusters

We have studied the properties of various isomers of potassium clusters containing even number of atoms ranging from 2 to 20 at the ab initio level. The geometry optimization calculations of the isomers of each cluster are performed by using all-electron density functional theory with gradient corrected exchange-correlation functional. Using the optimized geometries of different isomers we investigate the evolution of binding energy, ionization potential, and static polarizability with the increasing size of the clusters. The polarizabilities are calculated by employing M?ller-Plesset perturbation theory and time-dependent density functional theory. The polarizabilities of dimer and tetramer are also calculated by employing large basis set coupled cluster theory with single and double excitations and perturbative triple excitations. The time-dependent density functional theory calculations of polarizabilities are carried out with two different exchange-correlation potentials: (i) an asymptotically correct model potential and (ii) within the local density approximation. A systematic comparison with the other available theoretical and experimental data for various properties of small potassium clusters mentioned above has been performed. These comparisons reveal that both the binding energy and the ionization potential obtained with gradient-corrected potential match quite well with the already published data. Similarly, the polarizabilities obtained with M?ller-Plesset perturbation theory and with model potential are quite close to each other and also close to experimental data.     

Ab initio crystal orbital studies on linear chains of H atoms

Anab initio crystal orbital method is used to calculate the energies of an infinite chain of H atoms and of linear arrangements of H₂ molecules with different interatomic distances. The H₂ arrangements are not stable in respect to isolated molecules. The cohesive energy of an optimized arrangement of H atoms chain is 0.0354 a.u.     

Ab initio studies for the photodissociation mechanism of hydroxyacetone

The reaction pathways for CH(3)COCH(2)OH (hydroxyacetone) photodissociation on the low-lying electronic states have been studied with use of the CASSCF energy gradient techniques. The S(0)/S(1) and S(1)/T(1) intersection points were determined by the state-average CASSCF method. Two main reaction pathways, which are possible to the photodissociation, have been studied. It has been found that the mechanism is stepwise, and belongs to Norrish type-I reaction. The n --> pi* excitation leads to the first excited singlet state, followed by the intersystem crossing from S(1) to T(1). On the T(1) potential energy surface, the system can dissociate adiabatically to CH(3)(x) +COCH(2)OH( x) and CH(3)CO(x)+CH(2)OH(x). The COCH(2)OH(x) and CH(3)CO(x) radicals can further dissociate into CO, OH, and other fragments. Our calculated results are in good agreement with recent experimental results.     

Ab initio molecular orbital studies for compounds of magnesium

Using a 6-311G** basis set with estimation of correlation energy at the MP2 level, structural and energetic data for 40 molecular species containing magnesium have been calculated. For about half the species studied, further energetic data were obtained using Pople's G2 method. Enthalpy changes at 298.15 K were obtained for isogyric reactions and standard enthalpies of formation were derived from these. Comparison of the standard enthalpies of formation with the sparse literature data suggests the MP2/6-311G** standard enthalpies of formation are accurate to ± 15 kJ mol^?1 and the corresponding G2 enthalpies accurate to ± 10 kJ mol^?1. The calculated ΔH [MgN, g] revealed a gross error in the currently accepted value for this function. It is intended that these results will be used to parameterize the semiempirical molecular orbital package, MOPAC, for the element magnesium. © John Wiley & Sons, Inc.