Cation chemistry

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

The Group 13 metals differ sharply from the non-metallic element boron both in their greater chemical reactivity at moderate temperatures and in their well-defined cationic chemistry for aqueous solutions. The absence of a range of... 

Niobium and tantalum provide no counterpart to the cationic chemistry of vanadium in the -t-3 and -t-2 oxidation states. Instead, they form a series of cluster compounds based... 

When produced by such dry methods it is frequently unreactive but, if precipitated as the hydrous oxide (or hydroxide ) from aqueous chromium(III) solutions it is amphoteric. It dissolves readily in aqueous acids to give an extensive cationic chemistry based on the [Cr(H20)6] ion, and in alkalis to produce complicated, extensively hydrolysed chromate(III) species ( chromites ). 

Another milestone discovery in the held of silyl cations chemistry was achieved by Reed and co-workers in the same year, 1993, when Lambert published his Et3Si study. Reed synthesized his t-PrjSi (CBuHsBrg), (2+ (CBnH5Br5), by the hydride transfer reaction of /-PrjSiH and Ph3C (CBnH6Br6) in toluene,taking advantage of the very low nucleophilicity of the carborane anion" (Scheme 2.9). 

Although cycloaddition reactions have yet to be observed for alkene radical cations generated by the fragmentation method, there is a very substantial literature covering this aspect of alkene radical cation chemistry when obtained by one-electron oxidation of alkenes [2-16,18-26,28-31]. Rate constants have been measured for cycloadditions of alkene and diene radical cations, generated oxidatively, in both the intra- and intermolecular modes and some examples are given in Table 4 [91,92]. 

Technetium and rhenium differ markedly from manganese, but they are very similar to each other. They have little cationic chemistry, few compounds in the oxidation state II, more extensive chemistry in the IV and V states. The metals resemble Pt in their appearance (usually, however, they are in the form of a grey powder) they tarnish slowly in moist air, do not react with water. Metal dust is a fire and explosion hazard. 

Al, Ga, In and T1 differ sharply from boron. They have greater chemical reactivity at lower temperatures, well-defined cationic chemistry in aqueous solutions they do not form numerous volatile hydrides and cluster compounds as boron. Aluminium readily oxidizes in air, but bulk samples of the metal form a coherent protective oxide film preventing appreciable reaction aluminium dissolves in dilute mineral acids, but it is passivated by concentrated HN03. It reacts with aqueous NaOH, while gallium, indium and thallium dissolve in most acids. 

Due to the fact that the removal of a bonding electron from the HOMO of the substrate RH leads to a radical cation with enhanced reactivity with respect to fragmentation reactions, the pathway often employed in radical cation chemistry results in the separation of charge and spin by dissociative processes, such as deprotonation, desilylation or cleavage of a stable cationic leaving group... 

Among the 1,3-oxazinium derivatives, the 3-azapyrylium salts are the most interesting [they are also called 1 -1,3-oxazinium (72S333), 1,3-oxazin-1-ium (83MI1), 1,3-oxazinium (78AHC1 84MI1), and 1,3-oxazinylium salts (81BCJ2387)]. The absence of a universally accepted name for these compounds indicates the novelty of this field of heterocyclic cation chemistry. 

Ultraviolet-curable laminating adhesives use cationic chemistry and visible initiating systems to solve laminating problems with polyester films, which absorb most UV radiation.15... 

Long-lived cyclopropylcarbinyl cation chemistry, including spiro cations and dications, has been reviewed,7 and some of the more interesting newer carbocations, such as (1), are the subject of a short survey.8 The use of secondary deuterium isotope effects in the study of carbocation-forming reactions has been revisited,9 and the... 

Radical cation structure types can be classified according to the nature of the donor molecules, viz., it-, n-, or cr-donors, from which they are generated. Radical cations derived from typical it-donors may be closely related to the structure of their precursors, whereas substantial differences may be observed between the structures of radical cation and precursor for cr-donors. The potential surfaces of radical cations and their parents may differ in three features reaction barriers may be reduced, free energy differences between isomers may be reduced or reversed and energy minima on the radical cation surface may have geometries corresponding to transition structures on the parent potential surface. The pursuit of such novel structure types has given new direction to radical cation chemistry. Representative radical cation structures are discussed to document their rich variety and to illustrate the molecular features that determine their structures. 

Compared to the analogous reactions of the parent molecules, many radical cation reactions show a dramatic decrease in activation barriers, one of the most striking aspects of radical cation chemistry. Intuitively, this observation can be ascribed to the fact that the highest occupied molecular orbital (HOMO) of a radical cation is occupied by a single electron. As a result, the bond strength of one or more key bonds must be reduced and the bonds more easily decoupled. However, the barriers to some radical cation rearrangements appear to lie even lower than might be expected on the basis of this simple model. 

Both we and others have established various radical cation structure types, which deviate in important features from the structures of their neutral diamagnetic precursors. The pursuit of these novel structure types has given new direction to radical cation chemistry. We have noted that some of these species resemble plausible transition structures for the thermal rearrangement of the parent molecules, i.e. saddle points on the corresponding potential surfaces. From a different point of view, they can be envisaged as one-electron oxidation products of biradicals or zwitterions. However, this relationship rarely serves as a practical approach to their generation, since the potential bifunctional precursors are often unstable and not readily accessible. These radical cations are usually generated from related hydrocarbons or cyclic azo compounds. 

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