Pyridinium ions, structure

However, an evaluation of the observed (overall) rate constants as a function of the water concentration (5 to 25 % in acetonitrile) does not yield constant values for ki and k2/k i. This result can be tentatively explained as due to changes in the water structure. Arnett et al. (1977) have found that bulk water has an H-bond acceptor capacity towards pyridinium ions about twice that of monomeric water and twice as strong an H-bond donor property towards pyridines. In the present case this should lead to an increase in the N — H stretching frequency in the o-complex (H-acceptor effect) and possibly to increased stabilization of the incipient triazene compound (H-donor effect). Water reduces the ion pairing of the diazonium salt and therefore increases its reactivity (Penton and Zollinger, 1971 Hashida et al., 1974 Juri and Bartsch, 1980), resulting in an increase in the rate of formation of the o-complex (ik ). 

Species with the structure XGy in which the active site is part of the skeletal group. An example is a set of 3-substituted pyridinium ions in which the pyridine ring is the skeletal group and the N atom in the ring is the active site. 

Aromatic six-membered heterocycles, isoelectronic with benzene, are widely distributed in nature, and in the world of synthetic chemistry. Since N+ and C are isoelectronic, the simplest and most direct hetero-analogue of benzene (1) is the pyridinium ion (2). Further azonia substitution of this kind is theoretically possible, but knowledge of this type of structure does not extend beyond the disubstituted species (3)-(5). 

Jacobs and Uytterhoeven (199, 200) observed a band in the 3700 to 3675 cm-1 region in addition to the bands reported by Ward. The intensities of the acidic bands at 3650 and 3550 cm-1 were greater than those observed by Ward, which probably resulted from a lesser degree of aluminum removal. The new bands at 3700 and 3600 cm-1 arose from hydroxyls that were nonacidic to ammonia (199, 200) and pyridine (198, 199), although bands from pyridinium ions were observed in the IR spectrum. The latter bands were attributed to interaction of pyridine with the 3650 cm-1 hydroxyls (200). Jacobs and Uytterhoeven (199) and Scherzer and Bass (198) attributed the 3700 and 3600 cm-1 bands to structural hydroxyl groups associated with removal of aluminum from the zeolite framework. The 3600 cm-1 band arose from weakly acidic hydroxyls (200) since the band was removed by treatment with 0.1 W NaOH solution. The 3700 cm-1 band was unaffected by a similar treatment. 

A C2 transition structure [50] for hydride transfer from 1,4-dihydropyridine to pyridinium ion has been calculated using the semi-empirical MNDO/MO... 

When R is primary alkyl, the second-order rate constant k2 is obtained by taking the slope of kobs vs. concentration of the nucleophile. The plot passes through the origin, indicating a pure SN2 mechanism without SN1 participation. The reference pyridinium ion is the 2,4,6-triphenyl derivative (because pyrylium precursors with phenyl substituents are more easily prepared) (82AHC(Suppl 2)1) but numerous other substituents have been introduced into the ring. Rate constant values reported in Table XIX, where release of steric strain has a major influence, are in agreement with the role of structural factors discussed in Section IV,A. 

The silicate anion with a double five-membered ring structure is mainly formed as a crystalline solid from the tetra-n-butylammonium (N+(n-C Ho) ) silicate solutions whose N/Si ratios range from 0.78 to 1.0 (20,21). Pyridinium ions are also effective in forming silicate anions with cage-like structures (27). 

Fig. 9. Chemical structure of the synthetic neurotoxin l-methyl-4-phenyl-tetrahydro-pyridine (MPTP) and its metabolism, with monoamine oxidase B as substrate, via MPDP+ to the methyl-phenyl-pyridinium ion (MPP+), which is the active toxin. (For further details see Feldman (1997).)...

A parallel reaction is also observed for 2 and 2". Detailed analysis suggests that the disproportionation proceeds via dimerization, i.e., that the anion adds to the cation to form a covalent bond. The dimer subsequently dissociates to form two free radicals, rapidly in the case of the 4 radical and less rapidly (1 sec ) in the case of the 2 pyridinyl (for dimer structure, cf. Sect. 4.1). Pyridinyl radicals disproportionate in water to pyridinium ions and dihydropyridines (Sect. 4.4). 

For the samples studied, no pyridine adsorbed on Lewis acid sites was detected, indicating that the zeolite had not been partially dehy-droxylated to form such sites. The band reflecting pyridine-cation interaction was detected only after about 16 alkaline earth ions had been introduced into the unit cell. It grew steadily in intensity as more divalent ions were introduced into the structure. In Figure 1, the intensity of the pyridinium ion band, expressed as absorbance/sample mass, is plotted as a function of the per cent exchange by divalent ion. The concentration of acid sites is a function of the degree of exchange. 

Chemisorption of pyridine results in the disappearance of these groups and the formation of pyridinium ions. The concentration of pyridinium ions, and hence accessible Bronsted acid sites, follows a similar relationship to that of the hydroxyl group concentrations. Thus, the acid site concentration remains constant until about 16-18 calcium ions have been introduced. The acidity concentration then decreases rapidly as the calcium ions are exchanged for ammonium ions in accessible positions. At the same degree of exchange, the cation-pyridine band near 1444 cm" is observed first, confirming the appearance of calcium ions in accessible positions in the structure. 

The term synchronous is reserved to denote a mechanism where formation and fission occur to the same extent in the transition structure of a concerted mechanism the transition structure is anywhere on the diagonal dashed line (Figure 1). Bond order balance can be deduced from the changes in effective charge in the transition structure provided the charge is defined by the same standard dissociation equilibrium. This is illustrated by the displacement reaction of pyridines on 7V-phos-phopyridinium species (Scheme 4) the standard equilibrium for both bond formation and fission is the dissociation of substituted pyridinium ions (XpyH ) and hence the effective charges can be directly compared for both processes. 

The effective charge map for the sulfation of phenols with pyridine-A/ -sulfonate esters is illustrated in Schemes 13 and 14 for N-S bond fission and S-O bond formation. Effective charges are defined respectively by the dissociation reactions of substituted pyridinium ions and phenols. The pyridine nitrogen and the phenol oxygen suffer respectively changes in effective charge of - 1.00 and 4-0.23 units from reactant to transition state structure. 

There is no change in effective charge on the -SO3- group from reactant to transition state structure as measured by variation in substituent on the pyridinium ion. The location of the residual effective charge of -1.25 is spread over the SO3-O atoms. 

The increase in positive charge of 0.23 on the attacking aryl oxygen atom leads to an effective charge of -0.77 on this oxygen in the transition structure. This value cannot be utilised to decide the location of the -1.25 units determined from variation of substituent in the pyridinium ion because of the differing reference reaction for the effective charges. 

In vitro metabolic studies with rodent and human liver microsomal prepara- tions have established that MPTP undergoes both oxidative N-demethylation and C-6 (allylic) oxidation in reactions that are -nicotinamide adenine dinucleotide phosphate (NADPH) dependent and therefore likely to be cytochrome P-450 catalyzed (Weissman et al. 1985 Ottoboni et al. 1990). Although the latter transformation can lead to the toxic pyridinium metabolite MPP, the cytochrome P450-catalyzed pathway is unlikely to contribute significantly to the neurotoxicity of MPTP. As mentioned above, liver aldehyde oxidase diverts the inter-mediate dihydropyridinium metabolite away from pyridinium ion formation by catalyzing the conversion of structure 40 to the nontoxic lactim structure 41. Further-more, even if formed in the periphery, the polar pyridinium metabolite would have limited access to the central nervous system (CNS). The low... 

Fig 1 gives IR spectra for pyridine adsorbed on previously dehydrated samples. The spectrum of starting zirconium dioxide obtained through an alcogel step lacks the band eharacteristic for Brdnsted acid sites. The addition of Cr203 into zirconium dioxide leads to acidic B-sites characteristic for pyridinium ions with a band at 1540 cm". This may be related to formation of structure such as [3] ... 

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