Conformational isomerization of 4-methyl-1,3-dioxane

The whole possible course of conformational isomerization of 4-methyl-1,3-dioxane has been established using empirical (MM+) and nonempirical [STO-3G, 3-21G, 6-31G(d) and 6-31G(d,p)] approximations within the limits of the Hartree-Fock method. It was shown that the potential energy surface of this compound contains a principal (equatorial chair conformer) and local minima corresponding to the axial chair conformer and series flexible forms.     

Conformational analysis of 3-methyltetrahydro-1,3-oxazine

Conformational analysis of 3-methyltetrahydro-1,3-oxazine has been performed using HF/6-31G(d), PBE/3z, and RI-MP2/λ2 simulation approximations. The potential energy surface contains eight minima. Interconversion of the axial and equatorial chair conformers (main minima) occurs via several independent pathways involving six twist forms. The preferred path presumes direct chair-chair transition via pyramidal inversion of the nitrogen atom.     

Conformational analysis of 1,3-oxathiane

Computer simulation of pathways of conformational isomerization of 1,3-oxathiane molecule carried out with the help of HF/6-31G(d), MP2.6-31G(d)/HF/6-31G(d), and PNE/3z quantum-chemical approximations showed that interconversion between the degenerate in energy chair conformers proceeds through seven independent pathways: directly and via six flexible forms. Potential energy surface contains eight minimum points including chair conformers and enantiomeric pairs of twist forms, and also five transition states, among them different modification of semi-chair, symmetric and unmmetrical boat. Molecular dynamics methods show that flexible forms at room temperature convert into one another and into the chair conformers.     

Conformational analysis of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes

Conformational isomerization of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes was studied using quantum-chemical HF/6-31G(d), HF/pVDZ, and PBE/3z approximations. The potential energy surface of molecules of both compounds is shown to contain the main minimum corresponding to the equatorial chair conformer C-5e. The calculated potential barriers of the conformational isomerization were found. Based on the experimental (¹H NMR) and theoretical vicinal coupling constants the ??G ⁰ values were determined of the methyl group at the ring C⁵ atom. The reasons for the distinctions between the values of this parameter from the ??G ⁰, obtained by energy minimizations of the equatorial and axial chair conformers are discussed.     

Modern valence-bond description of chemical reaction mechanisms: the 1,3-dipolar addition of diazomethane to ethene

The electronic mechanism for the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane (CH2N2) to ethene (C2H4) is described through spin-coupled (SC) calculations at a sequence of geometries along the intrinsic reaction coordinate obtained at the MP2/6-31G(d) level of theory. It is shown that the bonding rearrangements occurring during the course of this reaction follow a heterolytic pattern, characterized by the movement of three well-identifiable orbital pairs, which are initially responsible for the pi bond in ethene and the C-N pi bond and one of the N-N pi bonds in diazomethane and are retained throughout the entire reaction path from reactants to product. Taken together with our previous SC study of the electronic mechanism of the 1,3-dipolar cycloaddition of fulminic acid (HCNO) to ethyne (C2H2) (Theor. Chim. Acc. 1998, 100, 222), the results of the present work suggest strongly that most gas-phase concerted 1,3-dipolar cycloaddition reactions can be expected to follow a heterolytic mechanism of this type, which does not involve an aromatic transition state. The more conventional aspects of the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane to ethene, including optimized transition structure geometry, electronic activation energy, activation barrier corrected for zero-point energies, standard enthalpy, entropy and Gibbs free energy of activation, have been calculated at the HF/6-31G(d), B3LYP/6-31G(d), MP2/6-31G(d), MP2/6-31G(d,p), QCISD/6-31G(d) and CCD/6-31G(d) levels of theory. We also report the CCD/6-311++G(2d, 2p)//CCD/6-31G(d), MP4(SDTQ)/6-311++G(2d,2p)//CCD/6-31G(d) and CCSD(T)/6-311++G(2d, 2p)//CCD/6-31G(d) electronic activation energies.     

Quantum-chemical study of 1,3-dioxane complexes with two water molecules

Methods PM3, RHF/6-31G(d), and MP2/6-31G(d)//RHF/6-31G(d) were used in calculation of the energy of formation of five 1,3-dioxane complexes with two water molecules formed through hydrogen bonds. The study of the conformational properties of the most stable associate revealed two routes of the chair-chair conformation isomerization. It was shown that the difference between the minima on the potential energy surface in this gase increased, and the barriersto the interconversion decreased as compared to the calculated values for the isolated molecule of 1,3-dioxane.     

New insights into an old reaction. High-resolution x-ray powder diffraction of Wiberg's aminoalane intermediate

In accordance with the procedure described by E. Wiberg, Me(3)Al-NH(3) was heated as a bulk material in inert atmosphere to give a colorless liquid which slowly loses methane. Close to the end of this elimination reaction, the melt crystallized to give a microcrystalline powder of (Me(2)AlNH(2))(x)(). The structure of this intermediate has been solved by the method of high-resolution X-ray powder diffraction. The compound crystallizes in the monoclinic space group C2/c with the cell parameters of a = 15.0047(6) A, b = 8.7500(2) A, c = 24.4702(8) A, and beta = 107.290(2) degrees, with eight trimers (Me(2)AlNH(2))(3) per unit cell. These trimers crystallize in a boat conformation in contrast to the known trimers of the same composition where a twist-boat conformation had been found by single crystal determination. Different conformers of (Me(2)AlNH(2))(3) have been investigated by theoretical methods (HF/6-31G(d), B3LYP/6-31G(d), B3LYP/6-311G(d,p), MP2(fc)/6-31G(d), and MP2(fc)/6-311G(d,p)). The twist-boat and the chair conformer correspond to minima at the potential energy surface, whereas the boat conformer corresponds to a first-order transition state (relative energies of 0.45-2.56 kJ/mol (boat) and 6.66-11.91 kJ/mol (chair)). Relaxed scans of the potential energy surface at the HF/6-31G(d) and B3LYP/6-31G(d) levels have shown that the boat conformer (C(s)() symmetry) connects two enantiomers of the twist-boat form (C(2) symmetry).     

Semi-empirical (PM3 and AM1) and ab initio molecular orbital calculations of 1,2,4-oxadiazoles, 4,5-dihydro-1,2,4-oxadiazoles and 4,4-di-n-butyl-2-phenylbenzo-1,3-oxazine

Molecular orbital calculations using semi-empirical (PM3 and AM1) and ab initio (HF/6-31G) types have been carried out on several 3,5-disubstituted 1,2,4-oxadiazoles 1a–d, 5-n-butyl-3,5-diaryl-4,5-dihydro-1,2,4-oxadiazoles 2a–d, and 4,4-di-n-butyl-2-phenylbenzo-1,3-oxazine 3. A comparison of the results by the two computational procedures has been made. Transformation of the oxadiazole ring to 4,5-dihydro-1,2,4-oxadiazole having both aryl and n-butyl groups at C-5 exhibited interesting conformational features. Also, examination of 1,3-oxazine 3 gave an idea about the structure of this compound. The rotational barrier of each phenyl group in 1a and 1d has been calculated using the ab initio method HF/6-31G(d).     

Conformational analysis of 5,5-dinitro-1,3-dioxane

Study of conformational transformations of 5,5-dinitro-1,3-dioxane by the method HF/6-31G(d), and also hybrid DFT PBE/3z and RI-MP2/λ2 indicated the only inversion route chair-chair through an intermediate minimum corresponding to the 2,5-twist-form. The potential barrier to this process was estimated and also the features of the intramolecular rotation of the axial and equatorial nitro groups in the chair conformer were investigated.     

Special features of 1,3-dipolar cycloaddition of n-methylazomethinylid to nitrobenzazoles

Synthetic approaches to 1,3-dipolar cycloaddition of N-methylazomethinylid to mononitrobenzazole are described. The geometric and electronic structures have been studied by quantum chemical methods (HF/STO-3 G and B3LYP/6-31 G*) and the reactivity indexes of compounds have been estimated. It was shown that 1,3-dipolar cycloaddition of N-methylazomethinylid to the dipolarophile has a polar character and proceeds in accordance with the normal (noninversion) electronic distribution.     

Structures and stabilities of diacetylene-expanded polyhedranes by quantum mechanics and molecular mechanics

The structures, heats of formation, and strain energies of diacetylene (buta-1,3-diynediyl) expanded molecules have been computed with ab initio and molecular mechanics calculations. Expanded cubane, prismane, tetrahedrane, and expanded monocyclics and bicyclics were optimized at the HF/6-31G(d) and B3LYP/6-31G(d) levels. The heats of formation of these systems were obtained from isodesmic equations at the HF/6-31G(d) level. Heats of formation were also calculated from Benson group equivalents. The strain energies of these expanded molecules were estimated by several independent methods. An adapted MM3 molecular mechanics force field, specifically parametrized to treat conjugated acetylene units, was employed for one measure of strain energy and as an additional method for structural analysis. Expanded dodecahedrane and icosahedrane were calculated by this method. Expanded molecules were considered structurally in the context of their potential material applications.     

Conformational analysis of 2-methyl-5-nitro-1,3,2-dioxaborinane

Quantum-chemical methods HF/6-31G(d), HF/6-31+G(d), MP2/6-31G(d)//HF/6-31G(d), and MP2/6-31+G(d)//HF/6-31+G(d) were used to investigate the conformational isomerization of 2-methyl-5-nitro-1,3,2-dioxaborinane. It has been shown that a potential energy surface of this compound includes two minima: an axial form of semi-chair and equatorial sofa together with a transition state belonging to the conformation of 2,5-twist-form. A comparison between experimental NMR ¹H and theoretical vicinal coupling constants was used to determine the quantitative conformational composition of cyclic boric acid ester and a value of ΔG ⁰ for nitro group at the ring carbon atom C⁵ in CCl₄ and C₆D₅NO₂ solutions.     

Quantum chemical studies on structure, conformation and isomerization of nitroso, nitro substituted benzene and 1,3-cyclopentadiene

The molecular structure, conformational stability and isomerization of nitroso, nitro substituted benzene and 1,3-cyclopentadiene in gas phase have been investigated using ab initio and density functional theory methods. The molecular geometries and energetics of possible conformers were obtained by employing MP2, B3LYP and B3PW91 levels of theory implementing 6-31G* basis set. The relative stabilities of the conformations were evaluated from the energy differences of the structure. Chemical hardness (η) and chemical potential (μ) were calculated at HF/6-31G* level of theory for all the positional and geometrical isomers to study the maximum hardness principle. Each optimized structure has been tested against the imaginary frequencies at MP2/6-31G* level of theory in order to be sure they are located at energy minimum.     

Theoretical studies on electrochemistry of p-aminophenol

Geometric parameters, vibrational frequencies and thermochemical values of p-quinonimine (p-Q) and p-aminophenol (p-AP) were computed by ab initio calculation (HF) and density function theory (DFT) with the 6-31G(d,p) basis set. Cyclic voltammetry with a goldren electrode of p-AP solutions in phosphate buffers at pH 7.30 showed that standard electrode potential of half reaction for (p-Q) and (p-AP) is 0.728V. Standard electrode potential of half reaction for p-Q and p-AP were calculated using the sum of electronic and thermal free energies of p-Q and p-AP with normal hydrogen electrode (NHE) as a reference electrode. The results show that the theoretical standard electrode potential of half reaction for p-Q and p-AP is 0.682V at B3LYP/6-31G(d,p) level and 0.622V at HF/6-31G(d,p) level, respectively, indicating that computed standard electrode potential at B3LYP/6-31G(d,p) level are more reliable than that at HF/6-31G(d,p) level.     

All-trans- and t,T,t,C,t,T,t-deca-1,3,5,7,9-Pentaenes: Ab Initio Structures,Vibrational Analyses,and Some Regularities in the Series of Related Molecules

Total geometry optimization and calculation of the force constants for all-transand t,T,t,C,t,T,tdeca-1,3,5,7,9-pentaene were carried out at the ab initio, HF/6-31G level. The HF/6-31G//HF/ 6-31G force fields were modified using empirical scale factors transferred from trans-buta-1,3-diene augmented by an additional scale factor for the central formal carbon-carbon double bond coordinates (determined previously for all-trans-hexa-1,3,5-triene). The total number of scale factors was seven. The vibrational problems for both decapentaenes were solved using the respective scaled HF/6-31G//HF/6-31G force field. Infrared intensities and Raman activities were calculated from the unscaled HF/6-31G//HF/6-31G force fields. Complete assignment of all the fundamental vibrational frequencies is given. Geometrical parameters, vibrational frequencies and force constants are compared with the corresponding values of buta-1,3-diene, hexa-1,3,5-triene and octa-1,3,5,7-tetraene. Regularities in the properties of this molecular series are discussed. Special attention is given to the possibility of using the vibrational spectra for detection of distortions from the regular trans structure of these oligoenes.     

苯并氧化呋咱稳定性和异构化的DFT和ab initio研究

运用B3LYP/6-31G(d)密度泛函理论（DFT）方法对苯并氧化呋咱、邻二亚硝基苯及其间的异构化反应进行了计算研究。结果表明,苯并氧化呋咱的分子总能量比邻二亚硝基苯的低;由苯并氧化呋咱异构为邻二亚硝基苯的正向反应活化能（Ea+=51.0kJ/mol）,与文献实测值（58.6kJ/mol）较接近,而其逆向反应活化能（Ea-=4.6kJ/mol）很小,从而揭示了苯并氧化呋咱比邻二亚硝基苯更稳定·此外,进行了HF/3-21G、HF/6-31G(d)和MP2/6-31G(d)//6-31G(d)水平下相应的计算,发现B3LYP-DFT的结果较abinitio为优。谐振动频率的B3LYP/6-31G(d)计算还支持了邻二亚硝基苯为苯并氧化呋咱“自-自”互变重排反应的中间体。     

Theoretical calculations of the effects of 2-heavier group 14 element and substituents on the singlet-triplet energy gap in cyclopentane-1,3-diyls and computational prediction of the reactivity of singlet 2-silacyclopentane-1,3-diyls

UDFT and CASSCF calculations with the 6-31G(d) basis set were performed to investigate the heavier group 14 element (M) effect on the ground-state spin multiplicity of cyclopentane-1,3-diyls and their reactivity. The calculations find that 2-metallacyclopentane-1,3-diyls (M = Si, Ge) that possess a variety of substituents (X = H, Me, F, OR, SiH(3)) at M(2) are singlet ground-state molecules. The energies of the 1,3-diphenyl-substituted singlet 2-silacyclopentane-1,3-diyls are calculated to be ca. 5 kcal/mol lower than those of the intramolecular ring-closure products, i.e., 1,4-diphenyl-5-silabicyclo[2.1.0]pentanes, at the B3LYP/6-31G(d) level of theory. The energy barrier for the disrotatory ring closure of singlet 2,2-dimethyl-1,3-diphenyl-2-silacyclopentane-1,3-diyl (lambda(calcd) = 757 nm, f = 1.01 at RCIS/6-31G(d)) to the corresponding 5-silabicyclo[2.1.0]pentane is computed to be 11.6 kcal/mol, which is 13.1 kcal/mol lower in energy than that for the conrotatory ring-opening to a 3-silapenta-1,4-diene. The computational work predicts that singlet 1,3-diaryl-2-silacyclopentane-1,3-diyls are persistent molecules under conditions without trapping agents.     

双自由基CF2与O3的反应机理

主要用作致冷剂和发泡剂的氯氟烃（CFCs）是破坏臭氧层的主要物质之一．对氯氟烃类化合物及其降解产物（包括光解、光氧化、化学反应产物等）在大气中行为问题的研究是大气化学研究的重要内容．前人[1-3]从理论和实验两方面研究了自由基与臭氧的反应机制，但是氯氟烃光解过程中     

Quantum chemical calculations based on ONIOM and the DFT methods in the inclusion complex: doxycycline/2-O-Me-β-cyclodextrin

A computational study of inclusion complexes of 2-methyl-βCD with Doxycycline tautomeric (enol and keto form) has been performed with several combinations of ONIOM hybrid calculations. The reliability of the ONIOM2 calculations at the integrated level, ONIOM2 (M05-2X/6-31G(d): M05-2X/3-21G*), ONIOM2 (M05-2X/6-31G(d):HF/3-21G*), ONIOM2 (B3LYP/6-31G(d):HF/3-21G*), ONIOM2 (B3LYP/6-31G(d):B3LYP/3-21G*) and ONIOM2 (B3PW91/6-31G(d):B3PW91/3-21G*) was examined. Their complexation, binding, deformation and stabilization energies, and geometrical data were compared with those of the target geometry structure optimized at the M05-2X/6-31G(d) level of theory. Mixed combinations ONIOM2 (M05-2X 6-31G(d):HF 3-21G*) and ONIOM2 (B3LYP 6-31G(d):HF 3-21G*) reproduces nearly the target geometry structure and provides realistic energetic results at a relatively low computational cost.