Computational studies on complex reaction mechanics

Molecular modeling with density functional and higher level methods was used to study mechanisms for the reactions of carbonyl compounds with ozone, the Doering-Moore-Skattebol Rearrangement, bis-lactam cyclizations, and thermal rearrangements of ortho-ethynyltoluene. Ozonation of both formaldehyde and acetone can proceed by one of two slow pathways: stepwise addition across the carbonyl group or hydrogen atom abstraction. The Doering-Moore-Skattebol Rearrangement proceeds as a triangular lithium-halogen carbenoid, opening stereospecifically to an allene, with lithium-halogen dissociation occurring after the transition state. Bis-lactam cyclization is rapid, reversible, and thermodynamically controlled. The experimentally observed major product is confirmed by computations as thermodynamically most stable. Thermal rearrangements of o-ethynyltoluene proceed through competitive [1,2] and [1,5] H-shifts. Chrysene is formed as a minor product by dimerization of a novel intermediate, orthoxylallene

On the chemistry of non-aromatic thiophen derivatives

Imperial Users onl

Investigations of the structures and reactions of neutral free radicals in solution

Before this work, no direct evidence for cyclisation of the 2-(isocyanatocarbonyl)ethyl radical to give the succinimidyl radical existed, because the latter radical had never been generated from acyclic reagents. Using a combination of e.s.r. spectroscopic techniques and product analysis, the cyclisation of ω-(isocyanatocarbonyl)alkyl radicals derived from the co-bromoalkanoyl isocyanates has been studied. Imidyl radicals, generated by photolysis of, or halogen-atom abstraction from, N-halogenoimides, are efficiently trapped by But2C=CH2 to give relatively persistent adducts which have been characterised by e.s.r. spectroscopy. Bromine-atom abstraction from BrCH2CH2C(O)NCO yields H2CCH2C(O)NCO which undergoes rapid 1,5-endo-cyclisation to give the succinimidyl radical. This cyclisation has been investigated using e.s.r. spectroscopy in conjunction with spin-trapping by But2C=CH2 and ButN=O. The rate coefficient for cyclisation of H2CCH2C(O)NCO has been estimated to be 3.7 x 10 6 s-1 at 328 K in cyclohexane from analysis of the products fromthe radical-chain reaction between BrCH2CH2C(O)NCO and triethylgermane. E.s.r. and product-analysis studies show that H2CCMe2C(O)NCO cyclises, more rapidly than H2CCH2C(O)NCO, to give the 2,2-dimethylsuccinimidyl radical, which subsequently undergoes ring opening to yield Me2CCH2C(O)NCO. The overall rearrangement of H2CCMe2C(O)NCO to Me2CCH2C(O)NCO represents a 1,2-shift of the -C(O)NCO group via an intermediate imidyl radical. The glutarimidyl radical is formed by 1,6-endo-cyclisation of H2CCH2CH2C(O)NCO. It is proposed that the rapid cyclisation of ω-isocyanatoalkyl radicals provides strong evidence that the unpaired electron occupies a σ-orbital in the product imidyl radicals. Section B Polarity reversal catalysis (PRC) of hydrogen-atom abstraction reactions, in particular when the slow abstraction of electron deficient hydrogen by electrophilic t-butoxyl radicals was studied. A number of amine-alkyIboryl radicals have been generated in fluid solution, by hydrogen-atom abstraction from the corresponding amine-alkylborane complexes, using photochemically-produced t-butoxyl radicals, and studied by e.s.r. spectroscopy. The air-stabilities of the parent complexes have been investigated. Amine-alkylboryl radicals abstract halogen rapidly from both alkyl bromides and chlorides. These radicals are highly nucleophilic and rapidly abstract an electron deficient α-hydrogen atom from esters, ketones, lactones, anhydrides, and imides. Because of this property of the derived boron-centred radicals, amine-alkylboranes act as polarity reversal catalysts for the net abstraction of electron deficient hydrogen atoms by the electrophilic t-butoxyl radical. Relative reactivities of alkyl bromides and of esters have been measured by e.s.r. spectroscopy using competition techniques

Reactions of fluorine and bromine atoms

Hydrocarbons (chiefly the lower alkanes) have been fluorinated by a competitive method in a high vacuum, all glass, static system. Monofluorides formed by hydrogen abstraction from pairs of reactants have been measured by gas chromatography and ratios of A factors and activation energy differences found for reactions of the type,F• + RH —» HF + R•Absolute A factors for (2) have been obtained by assuming that the A factor for ethane calculated from transition -state theory was the same as the experimental A factor. Absolute activation energies for reaction (2) have also been obtained by assuming the activation energies for the higher of the alkanes investigated were zero.Bromine hydrogen abstraction reactions analogous to (2) have been investigated by a similar method. Absolute A factors and activation energies have been derived by relating the results to the previously found parameters for bromine atom hydrogen abstraction from methyl bromide.Previous work, both quantitative and qualitative, of bromine and fluorine atom hydrogen abstraction and related reactions has been described and the kinetic data summarised. The activation energies and A factors obtained here for bromine and fluorine atoms have been compared with previously found figures for chlorine. Good agreement has been found between experimental A factors and values calculated from transition -state theory.From the bromine work carbon -hydrogen bond strengths have been derived which compare well with the generally accepted values. In order to obtain more accurate values a method has been developed for obtaining the activation energies of reactions of the type,R• + HBr —» RH + Br•Preliminary results are reported. Arrhenius parameters for other reactions in the bromine hydrocarbon systems have been considered. Future work has been suggested

Spectroscopy And Reactivity Of Silicon Surfaces

Thesis (Ph.D.) - Indiana University, Chemistry, 2010Functionalized silicon surfaces constitute an area of intense scientific interest. Our theoretical work in this exciting area can be broadly divided into two categories; the first is calculating the spectra of functionalized silicon surfaces to understand surface structure and the second is calculating the chemistry of funtionalization of the silicon surface with the surface as a reagent. Using electronic structure theory we have investigated the relationship between the molecular vibrations of cluster and periodic models for the functionalized Si(111) surface. From this work, we have developed a technique for calculating periodic vibrational frequencies using small cluster models. In addition, we have used the method to explain the coupling of near-surface phonon modes with adsorbate vibrational frequencies seen in experimental spectra. The method has been shown to be robust for uniform and mixed coverage (halide/hydrogen) surfaces. A careful study of the silicon hydrogen modes for many reconstructions at the same level of theory has been carried out to understand the spectra of hydrogen-passivated silicon surfaces. For the Si(100) surface we have examined the radical-initiated chain reactions of allylic mercaptan that have the unique property of growing exclusively across dimer rows on the Si(100) surface. We discuss the mechanisms of Lewis acids degrading on the silicon surface and the hydrogen induced insertion of oxygen or nitrogen into the silicon surface

Synthesis and study of novel silicon-based unsaturated polymers

The novel, unsaturated polymers (1-5*) have been synthesized and studied as precursors to silicon carbide (1 and 2) and third order nonlinear optical (NLO) materials (3-5). In the process of synthesizing polymers 1 and 2, corresponding cyclic forms, bisallene 1C and dimethylenedisilacyclobutane 2C were also obtained. X-ray structures of the highly strained 1C and its disilanyl analog were obtained. Extensive kinetic and mechanistic studies of the unique thermal isomerization of 2C to a carbene were also conducted. ftn*Please refer to dissertation for diagram

Diels-alder reactions of perfluorocyclohexa-1,3-diene with compounds containing triple bonds

Not availabl