Chemically activated methylcyclobutane exothermicity of singlet methylene reactions and the heat of formation of singlet methylene

The specific decomposition rates of chemically activated methylcyclobutane produced from CH₂(¹A₁) reaction with cyclobutane have been determined. CH₂(¹A₁)was produced from ketene photolyses at 3340 and 3130 Å and from diazomethane photolyses at 4358 and 3660 Å. Comparisons of the excitation energies of the methylcyclobutane, determined by RRKM theory calculations, and the experimental results for the ketene systems, with thermochemically predicted maximum excitation energies, favor an Arrhenius A factor in the range of 5 × 10¹⁵ to 1 × 10¹⁶ sec^?1 for methylcyclobutane. This result is consistent with (1) the comparison of RRKM theory calculations and the experimental unimolecular falloff for methylcyclobutane, (2) the comparison of experimental A factors for cyclobutane and other alkylcyclobutane decompositions, and (3) two out of three reported experimental A factors for methylcyclobutane. An analysis of these and previous results leads to a value of the CH₂(¹A₁) ? CH₂(³B₁) energy splitting of 9±3 kcal/mole.     

Calculated photoelectron spectra of singlet and triplet methylene

Vertical ionization potentials for singlet and triplet methylene are calculated by a CI perturbation method based on ab initio SCF molecular orbitals (6–31 G^** basis). The shape and vibrational fine structure of the first photoelectron band are investigated using the MINDO/3 method. The computed singlet-triplet splitting for methylene is 16.4 kcal/mol, in reasonable agreement with the experimental value of 19.5 kcal/mol.     

Interaction of O+ ion with water molecules: A quantum-chemical study

The interaction of O⁺ ion with several (from one to four) water molecules was studied by theab initio (UMP4/4-31G^*) and semiempirical (AM1) quantum-chemical methods. It was found that the energy of binding the O⁺ ion to the first water molecule is appreciably higher than those of binding to the subsequent water molecules. In the complex with a water molecule, whose structure corresponds to that of water oxide, the O⁺ ion retains high reactivity. The barrier to the transfer of O⁺ ion to another water molecule is much lower than the barrier to analogous transfer of O atom from the molecule of water oxide, despite the lower dissociation energy of the H₂O−O bond. Consideration of subsequent interactions with water molecules leads to an increase in the barrier to the transfer of O⁺ ion. The doublet and quadruplet excited states of the O⁺+2 H₂O system were also studied. In these cases, the formation energies are well described by the ion-dipole model. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 981–988, June, 2000.     

A quantum-chemical study of 1,2,3,4,5,6,7,8-octaazanaphthalene and itsN-oxides

Calculations of hypothetical molecules of octaazanaphthalene and itsN-oxides were performed by the MNDO method with full geometry optimization. Probable decomposition reactions of these compounds were considered. Compounds with more pronounced alternation of charges on adjacent atoms were shown to be more thermodynamically favorable and thermally stable. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 879–881, May, 1999.     

Cubyl cations: A quantum-chemical study

By the quantum-chemical method PBE0/6-311G (3d5f7,p) equilibrium structural parameters of cubyl cation C₈H ₇ ⁺ , adduct C₈H ₇ ⁺ ·MeOH and methoxycubane molecule C₈H₇OMe were determined. In the free cation the bond lengths are as follows: C¹C^α 1.489, C^αC^β 1,569, C^βC^γ 1.556, and CH 1.085–1.087 Å; angles at C¹ are 98.6°; charge on the C¹ atom is +0.39 au (NPA). In the methoxycubane the cubyl-O bond is shorter, while in the cationic adduct is longer than the O-Me bond: 1.368 < 1.405 Å and 1.494 > 1.469 Å. A relatively low electrophilicity of the cubyl cation compared with CH ₃ ⁺ is concluded and the possibility of its obtaining by radiochemical methods is assumed.     

Quantum chemical study of mechanism for insertion of singlet methylene into C-C1 bond

Conclusions Quantum chemical calculation by the expanded Huckel method revealed that the mechanism for the insertion of singlet methylene into the C-C1 bond of methyl chloride consists in the electrophilic attack by methylene on the C-Cl bond, with a subsequent synchronous transfer of the chlorine atom to the methyl and the formation of ethyl chloride; here the vacant p-orbital of methylene does not react with the unshared pairs of the chlorine atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2622–2625, November, 1973.     

A quantum-chemical study on the Mannich reaction with polynitromethanes

According to the results of SCF-CNDO/2 computation, we propose employing the “ladder weighting summation” to define a G_e index which is used to measure the relative magnitude of the nucleophilicity of the relevant amine and pseudo acid components in a Mannich reaction. We also put forward an A_e index to measure the relative magnitude of the electrophilicity of methyleneamine cationic species in the same reaction. A combined use of both G_e and A_e indices is suggested to evaluate the feasibility of a Mannich reaction for the polynitromethanes and the stability of the products. These results are consistent with those conclusions drawn from “soft-hard acid-base theory”.     

叠氮根电负性的量子化学研究

用ab initio, MINDO/3, MNDO和DV-Xa等量子化学方法计算研究了一些分子型和离子型(碱金属)叠氮化物及相应氯化物的平衡构型和电子结构。结构表明: 在分子型叠氮化物中叠氮根的电负性较氯小、与氮相当; 而在离子型金属叠氮化物中,叠氮根的电负性和氯相当或较氯稍大。将计算所得正则离域分子轨道进行定域化处理, 发现产生这种电负性差异的主要原因是上述两类叠氮化物中N2的成键状况不同, 本文对此进行了较为细致的分析。     

Photosensitization and phototherapy with furocoumarins: A quantum-chemical study

The effect of electromagnetic radiation on biological objects extends from heating to complex photochemistry, and includes DNA alteration, that properly modified in damaged cells may entail beneficial effects. In this regard, psoralen + UV-A (PUVA) therapy, in which furocoumarins, psoralen-like chromophores, are used as photosensitizers and photoreactants with DNA bases, is one of the most promising strategies against a plethora of diseases. Understanding the underlying photochemical mechanisms is crucial to design effective drugs without undesired side effects. We have undertaken a quantum-mechanical study on the photophysics and photochemistry of furocoumarins, analyzing firstly the most efficient way in which the lowest excited triplet state, as protagonist of the photosensitizing action, is populated from the initially promoted singlet states, and secondly the basics of the formation of furocoumarin–DNA photoadducts.     

Comparison of singlet methylene removal rates by group IV hydrides

The technique of laser flash photolysis/laser absorption has been used to obtain an absolute removal rate constant of (3.85 ± 0.18) × 10^?10 cm³ molecule^?1 s^?1 for singlet methylene, ¹CH₂ (ã¹A₁), with germane (GeH₄) at ambient temperature. The removal rate constant is compared with the values for methane (CH₄) and silane (SiH₄) which have been determined previously. © 1995 John Wiley & Sons, Inc.     

Spectral-chemometric and quantum-chemical study of the water-acetonitrile system

Association of molecules in the solution of acetonitrile and its binary mixtures with water has been studied using spectroscopy in the near-IR region (800–1100 nm) and the chemometric method of independent components implemented as a MILCA algorithm. The decomposition of the spectral curves of solutions has aimed to determine the number, structure, size, and stability of the associates in the wide concentration range (0–100%). The well-known MCR-ALS algorithm has been used to confirm the accuracy of the independent component analysis. Quantum-chemical computation has been performed for particles occurring in the solution. The results explain some abnormalities observed for water-acetonitrile solutions in reversephase chromatography.     

A quantum-chemical study on adamantan-2-spirodiazirine protonation

The semiempirical MNDO method has been applied to the protonation of adamantan-2-spirodiazirine to distinguish some hypothetical intermediates in this reaction and to suggest likely mechanisms for various conditions. The framework pan of the substrate can form complexes with protons, which may influence the rate.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 4, pp. 319–325, July–August, 1993.     

A quantum-chemical study on the discharge reaction mechanism of lithium-sulfur batteries

Lithium-sulfur batteries have attracted a great interest in electrochemical energy conversion and storage, but their discharge mechanism remains not well understood up to now. Here, we report density functional theory (DFT) calculation study of the discharge mechanism for lithium-sulfur batteries which are based on the structure of S8 and Li2S x (1≤x≤8) clusters. The results show that for Li2S x (1≤x≤8) clusters, the most stable geometry is chainlike when x = 1 and 6, while the minimal-energy structure is found to be cyclic when x = 2-5, 7,8. The stability of Li2S x (1≤x≤8) clusters increases with the decreasing x value, indicating a favorable thermodynamic tendency of transition from S8 to Li2S. A three-step reaction route has been proposed during the discharge process, that is, S8 →Li2S4 at about 2.30 V, Li2S4 →Li2S2 at around 2.22 V, and Li2S2 → Li2S at 2.18 V. Furthermore, the effect of the electrolyte on the potential platform has been also investigated. The discharge potential is found to increase with the decrease of dielectric constant of the electrolyte. The computational results could provide insights into further understanding the discharge mechanism of lithium-sulfur batteries.     

Characteristics of the complexing of chitosan with sodium dodecyl sulfate,according to IR spectroscopy data and quantum-chemical calculations

The complexing of protonated chitosan with dodecyl sulfate ions in water solutions is studied using IR spectroscopy data and quantum-chemical calculations. It is established that the electrostatic interaction between the protonated amino groups of chitosan and dodecyl sulfate ions is apparent in the IR spectrum as a band at 833 cm^?1. The need to consider the effect the solvent has on the formation of hydrogen-bound ion pairs [CTS⁺ ? C₁₂H₂₅O₃^-] is shown via a quantum-chemical simulation of the equilibrium geometry and the energy characteristics of complexing and hydration.