Carbonate-like species

The room-temperature chemistry of high-surface-area CoO-MgO solid solutions is dominated by the adsorption of CO on edges and steps. Co-ordinatively unsaturated Co2+ and O2 ions react primarily as 02 Co2+02 triplets with formation of [(CCh CoCO]2- species. In samples with high Co contents, the large amounts of clustered cobalt guest species are easily reduced by CO, even at room temperature, with formation of Co(CO)4 and carbonate-like species (377). The formation of polymeric radical anions of CO on high-surface-area CoO-MgO solid solution has also been reported (378). 

CO2 is a poor donor but a good electron acceptor. Owing to its acidic character, it is frequently used to probe the basic properties of solid surfaces. IR evidence concerning the formation of carbonate-like species of different configurations has been reported for metal oxides [31], which accounts for the heterogeneity of the surface revealed by micro-calorimetric measurements. The possibility that CO2 could behave as a base and interact with Lewis acid sites should also be considered. However, these sites would have to be very strong Lewis acid sites and this particular adsorption mode of the CO2 molecule should be very weak and can usually be neglected [32]. 

The components at 1730, 1666, 1622 and 1440 cm are due to various carbonate-like species [25], which are produced by reaction of CO2 with basic centers. On the Mg/MCM-41 (15M) sample, the bands due to carbonate-like groups appeared more intense than Mg/MCM-41(15W) and the distribution of adsorbed carbonate species was similar but not identical. 

It should be noticed that no MgO particles are present in the Mg/MCM-41(15W), as indicated by the DR UV-Vis spectra (Fig. 2, b). Consequently, it can be proposed that basic centers able to react with CO2 to produce carbonate-like species are the oxygen atoms bridging the supported Mg ions with the silica walls of the support. 

It should be noted that a part of the carbonate-like species may desorb as... 

These reactions lead to the formation (transformation) of surface carboxylate and carbonate-like species and to the two electron reduction of the (electrons that can reduce) transition metal ions located in nearest-neighbor positions. On oxidic surfaces that do not contain transition metal ions, redox reactions accompanied by electron transfer from the surface to the adsorbed molecule (or vice versa) are much less probable. 

These spectroscopic studies still need to be complemented by theoretical calculations and modeling to understand in deeper detail why the presence of copper is able to control the outcome of the photocatalytic process and, more specifically, the product distribution. It is very likely that reactive CO2 adsorbs preferentially near copper atoms, probably forming some type of carbonate-like species, and that this adsorption is the key feature controlling the photocatalytic process. It is worth to comment that noble metals and particularly Pt and Au do not interact with CO2. The presence of copper should introduce some affinity for CO2 adsorption on the metal co-catalyst, this being the origin of the change in selectivity. 

Gold is generally considered a poor electro-catalyst for oxidation of small alcohols, particularly in acid media. In alkaline media, however, the reactivity increases, which is related to that fact that no poisoning CO-hke species can be formed or adsorbed on the surface [Nishimura et al., 1989 Tremihosi-Filho et al., 1998]. Similar to Pt electrodes, the oxidation of ethanol starts at potentials corresponding to the onset of surface oxidation, emphasizing the key role of surface oxides and hydroxides in the oxidation process. The only product observed upon the electrooxidation of ethanol on Au in an alkaline electrolyte is acetate, the deprotonated form of acetic acid. The lack of carbon dioxide as a reaction product again suggests that adsorbed CO-like species are an essential intermediate in CO2 formation. 

C-doped Ti02 was employed for the photocatalytic removal of NOx. The presence of carbon, in the form of coke-like species, was responsible for visible light absorption and thus for the good activity of the photocatalyst under green light irradiation [133]. 

The above SKIE data were taken as evidence against an ionic transition state. Allylic cation-like species would result in much larger normal SKIE at C(4) in a polar medium than in non-polar media by approaching the maximum possible value for conversion of an sp3 C(4) of the ether to an sp2 carbon of an allyl cation. 

We have previously seen examples of the carbon-like formulas of mononuclear and dinuclear osmium compounds, namely the methane-like tetrahydride (4.50c), ethylene-like H20s=CH2 (4.51c) and H2Os = OsH2 (Table 4.15), acetylenelike HOs = CH (4.54c) and HOs = OsH (Table 4.15), allene-like H2C = Os = CH2 (4.55a), and so forth. While the coordination numbers and Lewis-like formulas are formally analogous, the actual structures of Os and C species may be quite similar (e.g., the Td structures of OsfL and CH4) or dissimilar (e.g the strongly bent Cs structure of H20s = CH2 [Fig. 4.13(c)] versus the planar D2h structure of H2C = CH2). 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

The mechanism of alkyl hydrogen exchange was not clarified, but a possible mechanism was postulated. Partial hydride abstraction by a Lewis acid site may have occured forming a carbocation-like species followed by exchange of a proton at a R-carbon. Such a mechanism predicts exchange to occur preferentially at methyl groups adjacent to the most stable carbocations (benzylic > 3° > 2° > 1°). This is consistent with the observed relative rates of epimerization of steranes during thermal maturation of sediments (83). 

Trisubstituted carbon-centred radicals chemically appear planar as depicted in the TT-type structure 1. However, spectroscopic studies have shown that planarity holds only for methyl, which has a very shallow well for inversion with a planar energy minimum, and for delocalized radical centres like allyl or benzyl. Ethyl, isopropyl, tert-butyl and all the like have double minima for inversion but the barrier is only about 300-500 cal, so that inversion is very fast even at low temperatures. Moreover, carbon-centred radicals with electronegative substituents like alkoxyl or fluorine reinforce the non-planarity, the effect being accumulative for multi-substitutions. This is ascribed to no bonds between n electrons on the heteroatom and the bond to another substituent. The degree of bending is also increased by ring strain like in cyclopropyl and oxiranyl radicals, whereas the disubstituted carbon-centred species like vinyl or acyl are bent a radicals [21]. 

Fig. 6) can activate hydrogen under mild conditions [219]. In contrast to transition metals, that act as electrophiles towards hydrogen, the (alkyl)(amino)carbenes mainly behave as nucleophiles initially creating a hydride like species, which then attacks the positively polarized carbene carbon atom. 

Since this review has focused on photoelectrochemical conversions of organic compounds, it has neglected the redox reactions of simple inorganic materials like nitrogen, water, and carbon dioxide, species which have a rich photoelectrochemical history. Recent progress made with photoelectrochemical CO2 reduction signals the possibility that in the future organic feedstocks may derive from aldehydes and alcohols produced by photoelectrochemical reductions. 

In marked contrast to this behaviour, monomer 11, l-benzyl-2-methyl-aziridine, gives a relatively slow reaction with Et30+BF4. However, polymerisation is quantitative and all the indications are that the system is living . Why this would be so is difficult to explain. Electronic factors would seem to be elimi-minated since this monomer is intermediate as far as basicity is concerned. Presumably the explanation lies in the stereochemistry of the system since this particular monomer is the only carbon substituted species. Even more strangely, however, the closely related molecule l-benzyl-2-ethyl-aziridine is reported (141) to behave like 8,9, and 10 when treated with boron trifluoride. 

---