Structure and Mechanism in Carbene Chemistry

Formation of cyclopropane rings via Zr—X (X = Cl, Br) γ-elimination

M. V. Frash, V. N. Solkan and V. B. Kazansky, J. Chem. Soc., Faraday Trans., 1997, 93, 515 DOI: 10.1039/A606280F

Since the postulation of carbenes by Buchner (1903) and Staudinger (1912) as electron-deficient transient species carrying a divalent carbon atom, carbenes have emerged as key reactive intermediates in organic synthesis and in molecular mass growth processes leading eventually to carbonaceous nanostructures in the interstellar medium and in combustion systems. Contemplating the short lifetimes of these transient molecules and their tendency for dimerization, free carbenes represent one of the foremost obscured classes of organic reactive intermediates. Here, we afford an exceptional glance into the fundamentally unknown gas-phase chemistry of preparing two prototype carbenes with distinct multiplicities-triplet pentadiynylidene (HCCCCCH) and singlet ethynylcyclopropenylidene (c-C5H2) carbene-via the elementary reaction of the simplest organic radical-methylidyne (CH)-with diacetylene (HCCCCH) under single-collision conditions. Our combination of crossed molecular beam data with electronic structure calculations and quasi-classical trajectory simulations reveals fundamental reaction mechanisms and facilitates an intimate understanding of bond-breaking processes and isomerization processes of highly reactive hydrocarbon intermediates. The agreement between experimental chemical dynamics studies under single-collision conditions and the outcome of trajectory simulations discloses that molecular beam studies merged with dynamics simulations have advanced to such a level that polyatomic reactions with relevance to extreme astrochemical and combustion chemistry conditions can be elucidated at the molecular level and expanded to higher-order homolog carbenes such as butadiynylcyclopropenylidene and triplet heptatriynylidene, thus offering a versatile strategy to explore the exotic chemistry of novel higher-order carbenes in the gas phase.

The molecular geometries of the 1-chloro-, 1-fluoro-, 1-methyl-, and 1-hydrogenosilatranes were fully optimized by the restricted Hartree-Fock (HF) method supplemented with 3-21G, 3-21G(d), 6-31G(d), and CEP-31G(d) basis sets; by MP2 calculations using 6-31G(d) and CEP-31G(d) basis sets; and by GGA-DFT calculations using 6-31G(d5) basis set with the aim of locating the positions of the local minima on the energy hypersurface. The HF/6-31G(d) calculations predict long (>254 pm) and the MP2/CEP calculations predicted short (∼225 pm) equilibrium Si(SINGLE BOND)N distances. The present GGA-DFT calculations reproduce the available gas phase experimental Si(SINGLE BOND)N distances correctly. The solid phase experimental results predict that the Si(SINGLE BOND)N distance is shorter in 1-chlorosilatrane than in 1-fluorosilatrane. In this respect the HF results show a strong basis set dependence, the MP2/CEP results contradict the experiment, and the GGA-DFT results in electrolytic medium agree with the experiment. The latter calculations predict that 1-chlorosilatrane is more polarizable than 1-fluorosilatrane and also support a general Si(SINGLE BOND)N distance shortening trend for silatranes during the transition from gas phase to polar liquid or solid phase. The calculations predict that the ethoxy links of the silatrane skeleton are flexible. Consequently, it is difficult to measure experimentally the related bond lengths and bond and torsion angles. This is the probable origin of the surprisingly large differences for the experimental structural parameters. On the basis of experimental analogies, ab initio calculations, and density functional theory (DFT) calculations, a gas phase equilibrium (re) geometry is predicted for 1-chlorosilatrane. The semiempirical methods predict a so-called exo minimum (at above 310 pm Si(SINGLE BOND)N distance); however, the ab initio and GGA-DFT calculations suggest that this form is nonexistent. The GGA-DFT geometry optima were characterized by frequency analysis. © 1996 by John Wiley & Sons, Inc.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMetal carbene chemistry. Formation and reactions of (.eta.5-C5H5)Fe(CO)n(CH2)+ (n = 1,2) in the gas phase by ion cyclotron resonance spectroscopyAmy E. Stevens and J. L. BeauchampCite this: J. Am. Chem. Soc. 1978, 100, 8, 2584–2585Publication Date (Print):April 1, 1978Publication History Published online1 May 2002Published inissue 1 April 1978https://doi.org/10.1021/ja00476a072RIGHTS & PERMISSIONSArticle Views80Altmetric-Citations28LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (300 KB) Get e-Alerts Get e-Alerts

Manganese oxide-doped zirconium oxide samples, prepared by heating mixtures of coprecipitated hydroxides at 1073 K in a hydrogen stream (water content 0.2% by volume), were analysed to obtain information on the solid solution formation. The state and the thermal stability of the incorporated species were also investigated. The samples (manganese content up to 14.74 mass%), were studied ‘as-prepared’ and after subsequent thermal treatments in oxygen up to 753 K. The results of several techniques [X-ray diffraction (XRD), Raman spectroscopy, thermogravimetry (TG) and magnetic susceptibility measurements] show that in the ‘as-prepared’ samples (1073 K, H2) a high fraction of manganese is incorporated in the zirconia structure, only a small fraction being present as an MnO separate phase. Most of the manganese in solid solution is present in the +2 oxidation state, the remainder as + 3 and +4. TG experiments and magnetic susceptibility measurements reveal that the Mn3+ and/or Mn4+ are formed both during the cooling in hydrogen by reaction with water present as an impurity in the gas phase, and during the exposure to the atmosphere. As the amount of manganese in solid solution increases, the volume of the zirconia unit cell slightly decreases. The solid-solution formation favours the tetragonal and the cubic modifications at the expense of the thermodynamically stable monoclinic phase. When the samples are heated up to 753 K in oxygen, the Mn2+ in solid solution is partially oxidized to Mn3+ and/or Mn4+. TG and XRD experiments show that the oxidation starts at low temperature and takes place in solid solution without appreciable manganese oxide segregation.

Calculations of the bond orders, net charge densities and energy levels for the π-electrons in a number of conjugated hydrocarbons by the valence-electron SCF MO method and by the π-electron SCF MO method are compared. The distinctions between alternant and non-alternant hydrocarbons, and in bond orders among single, aromatic, and double bonds, established by π-electron methods is maintained to a high degree of approximation in the full valence-electron calculations. An excellent correlation exists between the π-electron energy levels calculated by the two methods. The valence-electron calculations indicate that the hyperconjugative release of electrons in neutral molecules such as propene and toluene is negligible, but that this release can be substantial in carbonium ions.

Es werden der H‐D‐Austausch in Propen/DzO‐ und Isobuten/D2O‐ Gemischen sowie der Anteil an 1°C O‐markiertem Acrolein als Produkt der Oxidation von Propen mit I′ O2 oder 1°C O2 an markiertem Metalloxid untersucht.

CNDO/2 Calculations on the acidities of alkanes and saturated alcohols and the basicities of methylamines

A quantitative analysis is made on the correlation ship of thermodynamic property, i.e., standard enthalpy of formation (ΔH f0) with Kier’s molecular connectivity index(1Xv),vander waal’s volume (Vw) electrotopological state index (E) and refractotopological state index (R) in gaseous state of alkanes. The regression analysis reveals a significant linear correlation of standard enthalpy of formation (ΔH f0) with 1Xv, Vw, E and R. The equations obtained by regression analysis may be used to estimate standard enthalpy of formation (ΔH f0) of alkanes in gaseous state.

The data of the experimental and theoretical investigations into the electronic, optical, and recombination properties of AlP, InP, GaP, AlAs, GaAs, and their solid solutions exposed to hard radiation are discussed. The influence of hard radiation on the properties of laser and light-emitting diode (LED) structures based on these materials is analyzed. Light emitting diodes (LEDs) based on AlP, InP, GaP, AlAs, GaAs, and their solid solutions find wide use in alarm, display, and other systems operating in the visible region of the spectrum. Currently, to create an active region in these structures, the producers mainly use multiple quantum wells based on solid solutions of these semiconductors grown on the lattice matched substrates using epitaxy of organometallic compounds from the gas phase (MOCVPE, MOCVD). These LEDs are used in the board installation in the near space region and in nuclear power industry. In these cases, the radiation defects (RD) are formed in the semiconductor lattice, which leads to the changes in the electronic, optical, and recombination properties of the material. All this affects the parameters of LEDs and, first and foremost, their radiative efficiency, thereby reducing their operation time under the action of hard radiation. Maintaining working capacity of LEDs in hard radiation fields is a complex task that requires understanding the physics of the process of radiation degradation of both the semiconductor material and the structure as a whole. An analysis of the scientific, technical, and patent literature shows that the studies of radiation degradation of LEDs, at best, reduce to an analysis of the corresponding empirical relationships. In this paper, we analyze the effect of hard radiation on the electronic, optical, and recombination properties of AlP, InP, GaP, GaAs, and AlAs semiconductors and their solid solutions used for producing visible-range LEDs.

Organometallic Benzheterocycles

A nomenclature system for labeling cyclic peptide fragments

Computational investigations by an ab initio molecular orbital method (HF and MP2) with the 6-311+G(d,p) and 6-311++G(2df, 2pd) basis sets on the tautomerism of three monochalcogenosilanoic acids CH3Si(O)XH (X = S, Se, and Te) in the gas phase and a polar and aprotic solution tetrahydrofuran (THF) was undertaken. Calculated results show that the silanol forms CH3Si(X)OH are much more stable than the silanone forms CH3Si(O)XH in the gas-phase, which is different from the monochalcogenocarboxylic acids, where the keto forms CH3C(O)XH are dominant. This situation may be attributed to the fact that the SiO and OH single bonds in the silanol forms are stronger than the SiX and XH single bonds in the silanone forms, respectively, even though the SiX (X = S, Se, and Te) double bonds are much weaker than the SiO double bond. These results indicate that the stability of the monochalcogenosilanoic acid tautomers is not determined by the double bond energies, contrary to the earlier explanation based on the incorrect assumption that the SiS double bond is stronger than the SO double bond for the tautomeric equilibrium of RSi(O)SH (RH, F, Cl, CH3, OH, NH2) to shift towards the thione forms [RSi(S)OH]. The binding with CH3OCH3 enhances the preference of the silanol form in the tautomeric equilibrium, and meanwhile significantly lowers the tautomeric barriers by more than 34 kJ/mol in THF solution. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Pure fluoroformylchlorodifluoroacetyl disulfide, FC(O)SSC(O)CF2Cl, has been prepared by the reaction of FC(O)SCl and CF2ClC(O)SH in quantitative yield. The conformational properties of the novel molecule have been studied by vibrational spectroscopy (IR – gas phase, Raman – liquid phase) and quantum chemical calculations (B3LYP and MP2 methods). The gaseous compound exhibits a conformational equilibrium at room temperature where the most stable form adopts a C1 symmetry and a syn‐periplanar (sp) orientation of both carbonyl groups with respect to the disulfide bond. A second form, observed in the IR spectrum of the vapor, corresponds to a conformer in which the carbonyl bond of the FC(O) moiety adopts an anti‐periplanar (ap) position with respect to the S–S single bond, and gauche with respect to the ClC–C=O moiety in the chlorodifluoroacetyl group. The experimental free energy difference value ΔG0 = G0(ap–sp) – G0(sp–sp) = 1.4(3) kcal/mol (IR) is reproduced well by the B3YLP/6‐311+G(3df) (1.1 kcal/mol) and the MP2/6‐31G* (1.8 kcal/mol) methods. In addition, the structure of a single‐crystal, grown in situ, was determined by X‐ray diffraction analysis at low temperature. The crystalline solid [monoclinic, P21/n, a = 5.579(3) Å, b = 16.615(7), c = 8.455(4) Å, β = 106.876(8)°] consists exclusively of molecules with the (sp–sp) conformation and the usual gauche orientation around the disulfide bond [φ(CS–SC) = 84.2°]. Conformational transferability is thus demonstrated once again for species that contain the –C(O)SSC(O)– group as part of a systematic program designed to analyze the behavior of this class of molecules. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)