Hydrogen, spectroscopic detection

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

Kinetic investigation of the reaction of cotarnine and a few aromatic aldehydes (iV-methylcotarnine, m-nitrobenzaldehyde) with hydrogen eyanide in anhydrous tetrahydrofuran showed such differences in the kinetic and thermodynamic parameters for cotarnine compared to those for the aldehydes, and also in the effect of catalysts, so that the possibility that cotarnine was reacting in the hypothetical amino-aldehyde form could be completely eliminated. Even if the amino-aldehyde form is present in concentrations under the limit of spectroscopic detection, then it still certainly plays no pfi,rt in the chemical reactions. This is also expected by Kabachnik s conclusions for the reactions of tautomeric systems where the equilibrium is very predominantly on one side. 

Su Z, Wen Q, Xu Y (2006) Conformational stability of the propylene oxide-water adduct direct spectroscopic detection of O-H... 0 hydrogen bonded conformers. J Am Chem Soc 128 6755-6760... 

In the first step the spectroscopically detectable ketyl radical 36 is formed, which then recombines to form benzopinacol (37) (Weiner, 1971). The same pinacol is obtained by reacting benzophenone with 2-propanol, since the dimethylketyl radical (38) produced in the hydrogen abstraction step is a strong reductant and transfers a hydrogen atom to the excess benzophenone to form another molecule of the diphenylketyl radical (36). 

Mechanistic studies of the formation of the zirconium alkylidene complexes of type 462 have been carried out.326 Neither the independently prepared monobenzyl complex nor the tribenzyl derivative is thermally sensitive or reacts with light as a separate species, but the equimolar mixture is thermolyzed to give the benzylidene species. The reaction of the trichloride 461 with 2 equiv. of KCH2Ph generates an equilibrium mixture of benzyl complexes consisting of monobenzyl 464, tribenzyl 463, and the bis(benzyl) derivative. The dibenzyl complex is spectroscopically detectable, but not isolable and co-exists with the mono- and tribenzyl species however, it is the bis(benzyl) species that is photochemically and thermally labile for a facile cr-hydrogen abstraction process, leading... 

The transient nature of spectroscopically detected reaction intermediates in the reaction cycle of nonheme di-iron enzymes makes it difficult to obtain detailed structural information. Characterization of biomimetic 62-adducts derived from synthetic systems facilitates our understanding of the electronic and geometric structures of their biological counterparts. Within synthetic platforms, (peroxo)di-iron(III) species can be accessed either by oxygenation of mono- or di-iron(II) complexes or by reaction of di-iron(III) complexes with hydrogen peroxide. Formation of high-valent iron(III)iron(IV) species proceeds either via biomimetic one-electron reduction of the (peroxo)di-iron(III) core or by one-electron chemical oxidation of a di-iron(III) precursor. Decomposition of an alkylperoxoiron(III) adduct can afford a di-iron(IV) species. 

Halogen Compounds. Weakly hydrogen-bound van der Waals complexes of PH3 with hydrogen halides were generated from the diluted gases. The reactions were carried out either by pulsed-nozzle supersonic expansion with pulse-coupled microwave spectroscopic detection (MW) or by isolation in matrices at low temperature with identification by IR spectroscopy (IR). Experimentally observed complexes are H3P---HF (MW [2], IR [8]), H3P---HCI (MW [4], IR [3, 5]), and H3P---HBr (MW [6], see also [4]). 

Rhodium. In addition to the studies on the alkene hydrogenation catalyst [RhCl(PPhs)3] (see Vol. 3), some other very detailed studies on this system have appeared. It is now apparent that the complexes [RhClLJ [L = PPh, or P(p-tolyl)3] do not dissociate to a spectroscopically detectable extent. However, despite its low concentration a species of composition [RhClLJ must be postulated, as previously reported, in order to explain the kinetics of H2 reactions with solutions of [RhOLs]. The complexes [RhClLa], howev, are in equilibrium with the dimers, [RhClLJa. The dimer reacts with H2 to form (47) and is also readily cleaved with ethylene or L, but not cyclohexene, to form [RhCl(C2H4)L2] or [RhClLJ. ... 

Unexpectedly strong intermolecular hydrogen bonding has been reported by IR spectroscopic studies for tetrahydro-4,7-phenanthroline-l,10-dione-3,8-dicarboxylic acids, which exist in the oxo-hydroxy form 165 in both solid state and in solution [78JCS(CC)369].Tlie conclusion was based on comparison of B-, C-, and D-type bands for 165 and their dimethyl esters (detection of hydrogen bonding) and on analysis of IR spectra in the 6 /xm region (pyridine- and pyridone-like bands). 

As far as oxidation of the polymer with oxygen of the air is concerned, the /3-hydrogen atom in the neighborhood of the C=C double bond is the most likely one to be attacked by oxygen with the formation of hydroperoxide which undergoes further decomposition [19]. OH and CO groups have been detected spectroscopically in the polymer [67,83]. 

Nuclear magnetic resonance, NMR (Chapter 13 introduction) A spectroscopic technique that provides information about the carbon-hydrogen framework of a molecule. NMR works by detecting the energy absorptions accompanying the transitions between nuclear spin states that occur when a molecule is placed in a strong magnetic field and irradiated with radiofrequency waves. 

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