Oxygen atoms

The Fuchs group, in preparing acetylenic triflones 25 for an unrelated purpose, noted an unexpected exotherm upon mixing with tetrahydrofuran 26. Investigation of the remnants uncovered coupling of the alkyne to the carbon atom a to the 

Similar to the Fuchs group s acetylenic triflones 25, acetylenic bromides 28 can couple with carbon-centered radicals. The Li group used peroxides to abstract a hydrogen atom a to an oxygen in cyclic ethers 29. Homolytic snbstitution at the sp-hybridized carbon liberates a bromine-centered radical which can abstract another hydrogen atom from the cyclic ether to propagate the chain reaction. 

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Restructuring of a surface may occur as a phase change with a transition temperature as with the Si(OOl) surface [23]. It may occur on chemisorption, as in the case of oxygen atoms on a stepped Cu surface [24]. The reverse effect may occur The surface layer for a Pt(lOO) face is not that of a terminal (100) plane but is reconstructed to hexagonal symmetry. On CO adsorption, the reconstruction is lifted, as shown in Fig. XVI-8. 

Figure XVIII-2 shows how a surface reaction may be followed by STM, in this case the reaction on a Ni(llO) surface O(surface) + H2S(g) = H20(g) + S(surface). Figure XVIII-2a shows the oxygen atom covered surface before any reaction, and Fig. XVIII-2h, the surface after exposure to 3 of H2S during which Ni islands and troughs have formed on which sulfur chemisorbs. The technique is powerful in the wealth of detail provided on the other hand, there is so much detail that it is difficult to relate it to macroscopic observation (such as the kinetics of the reaction).

Fig. XVni-2. Successive STM images of (a) Ni(llO) with a chemisorbed layer of oxygen atoms and (b) after exposure to 3 1 of H2S. The area shown in 85 x 91 A. [From F. Besenbacher, P. T. Sprunger, L. Ruan, L. Olesen, I. Stensgaard, and E. Lcegsgaard, Tap. Catal., 1, 325 (1994).]...

Fig. XVIII-15. Oxygen atom diffusion on a W(IOO) surface (a) variation of the activation energy for diffusion with d and (b) variation of o- (From Ref. 136. Reprinted with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

Some detailed calculations have been made by Tully [209] on the trajectories for Rideal-type processes. Thus the collision of an oxygen atom with a carbon atom bound to Pt results in a CO that departs with essentially all of the reaction energy as vibrational energy (see Ref. 210 for a later discussion). 

Chemisorption occurs when the attractive potential well is large so that upon adsorption a strong chemical bond to a surface is fonued. Chemisorption involves changes to both the molecule and surface electronic states. For example, when oxygen adsorbs onto a metal surface, a partially ionic bond is created as charge transfers from the substrate to the oxygen atom. Other chemisorbed species interact in a more covalent maimer by sharing electrons, but this still involves perturbations to the electronic system. 

The hydration of more inert ions has been studied by O labelling mass spectrometry. 0-emiched water is used, and an equilibrium between the solvent and the hydration around the central ion is first attained, after which the cation is extracted rapidly and analysed. The method essentially reveals the number of oxygen atoms that exchange slowly on the timescale of the extraction, and has been used to establish the existence of the stable [1 10304] cluster in aqueous solution. 

This potential will lead to a single water molecule adsorbing at the PZC on Pt with the dipole pointmg axi ay from the surface and the oxygen atom pointing directly at a Pt-atom site (on-top configuration). 

Classic examples are the spontaneous emission of light or spontaneous radioactive decay. In chemistry, an important class of monomolecular reactions is the predissociation of metastable (excited) species. An example is the fonnation of oxygen atoms in the upper atmosphere by predissociation of electronically excited O2 molecules [12, 13 and 14] ... 

The recombination of oxygen atoms affords an mstnictive example ... 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

Figure Bl.23.9. Scattering intensity of 4 keV Ne versus azimuthal angle 8 for a Ni 110] surface in the clean (1 X 1), (1 X 2)-H missing row, and (2 x l)-0 missing row phases. The hydrogen atoms are not shown. The oxygen atoms are shown as small open circles. 0-Ni and Ni-Ni denote the directions along which O and Ni atoms, respectively, shadow the Ni scattering centre.

Figure Bl.24.12. Elastie eross seetion of helium ions seattered from oxygen atoms. The pronouneed peak in the speetnim around 3.04 MeV represents the resonanee seattering eross seetion that is often used in deteetion...

Thennal dissociation is not suitable for the generation of beams of oxygen atoms, and RF [18] and microwave [19] discharges have been employed in this case. The first excited electronic state, 0( D), has a different spin multiplicity than the ground 0( P) state and is electronically metastable. The collision dynamics of this very reactive state have also been studied in crossed-beam reactions with a RF discharge source which has been... 

Wang L S, Wu FI and Desai S R 1996 Sequential oxygen atom chemisorption on surfaces of small iron clusters Phys. Rev. Lett. 76 4853... 

Figure C2.12.1. Origin of ion exchange capacity in zeolites. Since every oxygen atom contributes one negative charge to the tetrahedron incoriDorated in the framework, the silicon tetrahedron carries no net charge while the aluminium tetrahedron carries a net charge of-1 which is compensated by cations M.

Figure C2.12.3. Secondary building units in zeolites. Each comer represents a T-atom (Si, Al) while tire connecting lines represent oxygen bridges witli tire oxygen atom in tire middle.

The ozone fonnation in the atmosphere is induced by radiation and a result of tliree-body collisions of the oxygen atoms with O2 molecules. This process requires a higher gas density and is, therefore, not efficient in the ionosphere. 

Other compounds containing lone pairs of electrons readily form co-ordinate links and in each case a change in spatial configuration accompanies the bond formation. The oxygen atom in dimethyl ether, CHj—O—CHj, has two lone pairs of electrons and is able to donate one pair to, for example, boron trichloride ... 

If the formulae of the acids are written as shown on the right, it becomes apparent that acid strength increases as the number oj oxygen atoms not involved in O—bonding increases. 

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