Observations supporting formulation

It has long been postulated that these facile reactions occur via formation of a cationic intermediate (upon acid-catalyzed loss of ROH) which can be formulated either as a a-bonded alkylcobalt carbonium ion or a cobalt(III)-olefin m complex. Recently, firm kinetic evidence has been obtained for the occurrence of an intermediate in the acid-catalyzed decomposition of 2-hydroxy- and 2-alkoxyethyl-cobaloximes [94]. Thus, while 2-hydroxyethylcobaloxime decomposes with strictly first-order kinetics in mildly acidic H2SO4/H2O mixtures, the alkoxy derivatives show a substantial lag followed by a first-order decay which is slower than that for the hydroxyethyl complex. In strongly acidic mixtures (//q < —5) all compounds show a rapid burst of absorbance change, followed by a slower first-order decay which is identical for all compounds whether measured spectrophotometrically or manometrically. These observations support the mechanism shown in Eqn. 56. 

It is appropriate to emphasize again that mechanisms formulated on the basis of kinetic observations should, whenever possible, be supported by independent evidence, including, for example, (where appropriate) X-ray diffraction data (to recognize phases present and any topotactic relationships [1257]), reactivity studies of any possible (or postulated) intermediates, conductivity measurements (to determine the nature and mobilities of surface species and defects which may participate in reaction), influence on reaction rate of gaseous additives including products which may be adsorbed on active surfaces, microscopic examination (directions of interface advance, particle cracking, etc.), surface area determinations and any other relevant measurements. 

Formulation of the detailed mechanisms of decomposition of coordination compounds are likely to remain difficult. Reliable kinetic and supporting observations are not easily obtained where several initiating reactions are possible and subsequent chemical changes may occur, before the first-formed product has left the crystallite of reactant. 

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

Spectroscopic evidence (44,45) has been adduced for the formation of electron-gain centres upon y-irradiation of the binuclear carbonyls Mn2(C0)lo and Re2(C0)lo. A study (45) of a single crystal of irradiated Mn2(CO)10 has shown that the radical anion contains two equivalent 55Mn nuclei whose hyperfine tensors lie 118 apart. This has led to the suggestion that the anion radical contains a bridging CO and that its correct formulation is Mn2(C0)9 . The observation of a bridged Mn2(C0)9 species in u.v.-photolyzed material lends some support to this hypothesis (46). 

In quantum statistical mechanics where a density operator replaces the classical phase density the statistics of the grand canonical ensemble becomes feasible. The problem with the classical formulation is not entirely unexpected in view of the fact that even the classical canonical ensemble that predicts equipartitioning of molecular energies, is not supported by observation. 

D-ribitol (21), and the structure formulated as 2,4-0-benzylidene-D-er-ythrose (22), would be 2,3-O-benzylidene-D-erythrose (23). These reassignments are supported by comparison of the properties of the product described as 22 with data from the literature. Thus, an authentic sample of 22, obtained by a different route (32), had an optical rotation value of — 20 °, which greatly differs from that found for the product formulated (31) as 22 (—65.2° — — 62.6°). The fact that mutarotation is observed, as well as the correspondence with the [a]D value (—62°) for 23, would indicate that the latter is the correct structure for the product described as 22. In any event, hydrolysis of the acetal function of both (22 and 23), leads to D-erythrose. 

Observation (iii) above, taken in the context of the triad annihilation in Scheme 12, indicates that the more or less statistical o/p pattern is diagnostic of the homolytic pathway (66) since it will clearly dominate the competition for TOL+- at the high concentrations of added N02 (Scheme 16). Indeed this conclusion is supported by observation (i), in which essentially the same isomeric product distribution (i.e. ortho meta para 70 2 28%) is achieved when the pyridine competition is thwarted for the sterically hindered 2,6-lutidine, an ineffective nucleophile (Schlesener et al., 1984). According to the formulation in Scheme 16, the isomeric product distribution is established from the sterically hindered Me2PyNOj during the homolytic annihilation of TOL+- by N02, most favourably at the ortho and para... 

DMF, and Nbl jSDMF have been shown to involve O-bonded DMF molecules by i.r. studies. The latter compound may be constituted as [Nb-(DMF)g]l4 since no bands attributable to Nb—I were observed, and this formulation is supported by preliminary conductance data in DMF solution. E.s.r. studies of NbCl4,2L complexes (L = DMF, DMA, THA, dioxan, dimethoxyethane, hexamethylphosphoramide, or AA -diethylformamide) indicate that they adopt a trans-octahedral geometry. 

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