Migrations of H Atoms

Fig.4.12. The sample construction 1 - polished quartz plate 2 — semiconductor sensor (ZnO) i - a strip of marblyte glass 4 - a layer of titanium (palladium) X = 0.027 cm ( the length of surface migration of H atoms h = 0.0025 cm ( the air gap between the quartz plate and the glass strip).

The biradical reacts unimolecularly in several ways (a) it is converted to hot epoxide by ring closure, (b) it rearranges by internal migration of H atoms or alkyl groups into hot carbonyl compounds, (c) it decomposes ( pressure-independent fragmentation occurs—the biradical is too short-lived to undergo collisional stabilization). 

Unlike [l,3]-sigmatropic migrations of H-atom, which cannot occur under thermal conditions, the [ 1,3] -sigmatropic migrations of carbon can occur under thermal conditions. This is because carbon has two-lobed p-orbital. Therefore, carbon can simultaneously interact with the migrating source and the migrating terminus using either one of its lobes or both of its lobes, as shown in Fig. 8.58. 

Migration of H atoms into subsurface sites, with a (coverage-dependent-full and dashed lines) sorption energy Ejs-... 

During the past three years, we have studied the reactions of thermally generated silicon atoms with low molecular weight reaetants. Reaction products were isolated in argon matrices and identified by means of IR spectroscopy, aided by calculated vibrational spectra. The method turned out to be very versatile and successful [1-4]. When the reactions of silicon atoms with unsaturated substrates are compared, a general sequence of products can be outlined as follows if a heteroatom is participating in the multiple bond, the silicon atom is bound to a lone pair in the primary product. This primary product can be isomerized photochemically to a cyclic silylene, the formal addition product of the Si atom to the Jt bond, which is the first product in case of substrates with C-C multiple bonds. Consecutive photoisomerizations lead to the formal insertion product of the Si atom into a C-H bond and, finally, to the product(s) of (successive) migration of H atoms to the Si atom. 

K (4). Stars show the beginning of sharp rise of the sensor signal under the influence of H-atoms migrating toward it. 

In some, though not necessarily all, cases loss of H and Cl is thought to be concerted, leading directly to the carbene (cf. p. 50) intermediate (75) formation of the product alkene from (75) then requires migration of H, with its electron pair, from the /J-carbon atom. A 1,1-elimination (Ea) will be indistinguishable kinetically from 1,2-(E2), and evidence for its occurrence rests on isotopic labelling, and on inferential evidence for the formation of carbenes, e.g. (75). 

Ab initio molecular orbital calculations have been carried out by Ignacio and Schlegel on the thermal decomposition of disilane and the fluorinated disilanes Si2H F6 17. Both 1,1-elimination of H2 or HF and silylene extrusion by migration of H and F atoms concerted with Si—Si bond cleavage were considered. The transition states for the extrusion reactions all involved movement of the migrating atom toward the empty p-orbital of the extruded silylene in the insertion which is the retro-extrusion (equation 5). 

In the last expression, Vo is a pre-exponential factor, kB—Boltzmann constant, T— annealing temperature, Emi is the migration energy of H atoms over the z -th scenario . It corresponds to their activation energy, Eai, in a case of spatial redistribution of H atoms between the (tetrahedral) interstices (Ea Emi). Therefore, the temperature dependence of x follows the so-called Arrhenius law ... 

Similar QC studies have been made on Ni(l 0 0) [42,44]. One of these allowed for lattice motion and H penetration into the bulk [44], and we focus on those issues here. To accurately simulate the evolution of a large slab (1197 atoms in 7 layers) of Ni atoms, a sizable number of surface adsorbates, H migration into the bulk, and reactions between pairs of H atoms on the surface, we require a model PES were the H-metal and the H-H interactions in the presence of the metal are functions of the instantaneous positions of the Ni atoms. We used a many-body PES of the form [44] ... 

Ab initio study of H-atom migration in HjCOj. Con- 707 elude migration channel is not accessible in room-temperature studies of O + HjCO... 

The dissolution of hydrogen in palladium to form a- and (i-phase palladium hydrides has been assumed to present problems in determining the surface area of palladium catalysts by hydrogen chemisorption, but under normal chemisorption conditions this is probably not a factor. The a-phase Pd-H forms initially by the migration of hydrogen atoms from the surface into the interstitial volume of the palladium crystals. Hydrogen pressures near atmospheric are... 

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