Onium salts, structures

Simple aryl onium salts (structures a-c in Fig. 10) absorb in the wavelength range of 200-330 nm, making them well-suited for applications requiring DUV... 

BF4. In large rings the fate of the onium ions depends mostly on the structure and degree of unsaturation of the particular compound, and the onium salts range from completely stable to highly unstable. 

A nonionic, non-volatile photoactive acid generator, 2,6-dinitrobenzyl tosylate has been recently reported and shown to be effective in chemically amplified resist systems (10). This ester is a nonionic compound that has a much wider range of solubility in matrix polymers and does not contain undesirable inorganic elements. While it is known to exhibit a lower sensitivity to irradiation than the onium salt materials, many structural variations can be produced to precisely vary the acid properties of the molecule and to control the diffusion of the AG in the polymer matrix (11). 

Tetraorganometal compounds exist as onium salts P4M 1 X with tetracoordinate tetrahedral metal centers or as covalent R4MX species with trigonal bipyramidal structures. 

Radicals are generated at the anode by oxidation of carbanions (Scheme lb), for example, alkoxides and carboxylates (see Chapter 5, 6), and at the cathode by reduction of protonated carbonyl compounds or onium salts (Scheme Ic) (see Chapter 7). Thereby, a wide choice of different radical structures can be mildly and simply... 

Quaternary onium salts were the first phase-transfer catalysts used subsequently, a number of compounds (linear polyethers, polypodands, crown-ethers, cryptands, cage-compounds, etc.) were found effective for the anion activation in two-phase systems. These structurally different systems must satisfy at least two fundamental conditions in order to behave as phase-transfer catalysts i) solubility in the organic phase ii) steric hindrance around the cationic center leading to a good cation-anion separation within the ion-pair. 

The 0X0 structures of l,3-oxazin-4-onium and benzo-l,3-oxazin-4-onium salts, but not their 4-hydroxy forms, have been confirmed by H-NMR spectra in which the chemical shifts of methyl groups in position 2 were almost identical for both NH- and NMe-substituted salts 106 (75KGS460 77KGS328, 77KGS704, 77ZOR2459). 

The tetra alkyl amm onium salts of [B Hg]- , formed by ion-exchange reactions, have proven to be useful synthetic reagents because of their thermal and air stabilities. The structure of the [B Hg]- ion has been determined by an x-ray study (66) and shown to have the 2013 styx structure, C2j) symmetry. Mechanisms for the formation of this ion have been proposed (67). Tetraborane(lO) can be easily obtained from salts of [B3H8] (eq. 9). 

The mechanism of living copolymerisation in the presence of aluminium porphyrin coupled with quaternary onium salt was proposed to involve activation of the anion of the onium salt as the nucleophile by the metallopor-phyrin the structure of the active species was found to be six-coordinate aluminium prophyrin carrying one reactive axial ligand on both sides of its square planar AIN4 skeleton [188] ... 

Effect of Side Chain Length. The side chain bound to the cation also affects the Tm due to flexibility and excluded volume effects. To discuss the relation between side chain structure and Tm, the onium cation structure was fixed. The relation between the side chain structure and the thermal properties of imidazolium salts has already been reported by Seddon et al. [16, 17]. The effect of alkyl chain length of 1-alkyl-3-methylimidazolium tetrafluoroborate on the phase transition temperatures is... 

Some compounds show meso-phases between the solid and liquid phases. These phases are classified into two kinds, namely liquid crystals in which the molecules have orientational order and disorganized position in one or more dimensions, and plastic crystal in which the molecules have organized positions and orientational disorder. Although the component ions in ILs are largely disordered, the appearance of liquid crystalline or plastic crystal phases could be the function of ion structures, when component ions have a tendency towards orientational or positional ordering by alignment of the ions and/or interaction among ions. Onium salt-type plastic crystals have been reported by MacFarlane [12,39]. 

The increase in (salt) for these onium salts can be understood in terms of mutual structure-forming or long-range hydrophobic interaction. The hydrocarbon side chains occupy voids in the solvent-lattice , which is stabilized by co-solvent but which collapses when x2 > xf. It is interesting therefore to note that the partial molar volume of R4N + ions decreases as x2 increases in ethyl alcohol + water mixtures, reaching a minimum near x2 0-1 which becomes more clearly defined with increase in size of the R-groups (Lee and Hyne, 1970). A similar trend is observed in acetone + water mixtures (Macdonald and Hyne, 1970). By way of contrast, the partial molar volumes, F (ion), for I-, Br , and Cl- ions decrease steadily to minima near x2 = 0-3 in ethyl alcohol + water mixtures where V2 (R4N+) is a maximum (Lee, 1972 1973). 

While these experiments stimulated other research, it is sad to relate that the proposed explanation is not correct. It required almost seventy years, however, before it was realized that the curare-like action of the onium salts was a result of their ionic character. Other examples of the role of spatial arrangement were discovered. For example, quoting Stewart Ishizuka (1897) found that maleic acid was a much stronger poison than its stereoisomer, fumaric acid 1.94 grammes for every kilogramme in a dog s weight was a fatal dose of the former acid, while the same dose of fumaric acid was harmless. Similarly these isomers were found to have differential effects on microorganisms. Stewart also rationalized the mydriatic action of tropine and the inactivity of pseudotropine in terms of three-dimensional formulae as shown below (1 and 2). In many ways, these structural representations are close to the present-day conformational structures (pseudotropine = 3, tropine = 4) ... 

The benzo-derivatives of the salts 86, i.e. benzo-l,3-oxazin-4-onium salts 89, are formed under the same conditions by acylation of salicylonitrile 88, which is a structural analog of the enol form 85 of the / -ketonitrile71 (equation 32). 

In a molecular system which clearly possess a ct LUMO the photoexcitation process may involve promotion of an electron from a n HOMO to the ct LUMO. This process may or may not produce bond cleavage. Direct observation of a 7t — ct electronic transition is often difficult due to the localized nature of the ct molecular orbitals resulting in a low probability for the transition [91]. More likely ait->it electronic transition takes place initially and ET, i.e., a - ct, is required to eventually populate the lower energy ct antibonding molecular orbital. Onium salts are examples of chemical structures that possess a ct LUMO and are expected to behave in this manner (see in Sect. 3.3). 

Where compounds have been studied (and the structural data are less complete), there are close parallels between the chloro and bromo species, with one significant variation there is a noticeable tendency for the bromides to form [MnX4]2- tetrahedra more readily. Thus, Rb2[MnBr4] is tetrahedral, whereas the chloro compound (Table 32) is known only as an octahedral polymer and there is no octahedral polymer form of Cs2MnBr4 as there is for the chloro analogue. Similarly, in the various onium salts, discrete [MnBr4]2 tetrahedra are more common. One intriguing compound in... 

Stable salts can be synthesized that have very low melting points for electrochemical and other applications. The neutralization method introduced in this chapter is an easy way to prepare pure salts and to use them in learning about the structure-property relationships of salts. It has been shown that excellent onium salts can be obtained from the results of neutralized amines. Besides serving as sources of qua-temized onium salts, neutralized amines have excellent properties. Some of these are unique and not retained by ordinary onium salts. More vigorous studies on neutralized amines can be expected to be published in the near future. 

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