Protonated species, formation

This study demonstrated the power of NMR spectroscopy whereby the use of carbon-13, nitrogen-15, and, more specifically, oxygen-17 in these oxo systems proved that the different species in the protonation, complex formation, ligand exchange, and condensation of the Re(V) system can be exceptionally well characterized in solution. The re-... 

The most intense peak, the base peak represents the M+ and represents the molecular weight as 16. Another small peak representing about 1.1% appears at m/e = 17. Since a molecule of methane can not take up a proton, the formation of this species shows that carbon was in the form of 13C. So it is an isotopic peak and is present in about 1.1%. 

The use of ammonia for the protonation of nitroarenes leads frequently to formation of aduct ions, e.g. [M + NH4]+, but not to the protonated species (MH+)112,113. The ammonia chemical ionization spectrum of nitrobenzene shows, in addition to a series of adduct ions, a dominant signal corresponding to the anilinium ion (m/z 94)112114115. Evidence for the isomerization of the [M + NR ]"1" adduct followed by successive loss of NO and OH or NH3 to give ions corresponding to the substitution products, e.g. the anilinium ion, has been given115 see Scheme 41. 

One may consider the relaxation process to proceed in a similar manner to other reactions in electronic excited states (proton transfer, formation of exciplexes), and it may be described as a reaction between two discrete species initial and relaxed.1-7 90 1 In this case two processes proceeding simultaneously should be considered fluorescence emission with the rate constant kF= l/xF, and transition into the relaxed state with the rate constant kR=l/xR (Figure 2.5). The spectrum of the unrelaxed form can be recorded from solid solutions using steady-state methods, but it may be also observed in the presence of the relaxed form if time-resolved spectra are recorded at very short times. The spectrum of the relaxed form can be recorded using steady-state methods in liquid media (where the relaxation is complete) or using time-resolved methods at very long observation times, even as the relaxation proceeds. 

The metal-peroxo species are considered to have a side-on structure (bidentate coordination of the peroxide ligand) and to be very unstable in protic medium (8). Under physiological conditions, after the first protonation and formation of a hydroperoxo intermediate (Scheme 2), the second protonation of this intermediate can proceed in two distinctly different pathways. In one case the second protonation results in the release of hydrogen peroxide from the metal center, leaving the metal oxidation state unchanged (Scheme 2). This is a crucial step in the catalytic cycles of SODs and SORs, especially in the catalytic mechanism of manganese SODs, which exist in the hydrophobic mitochondrial matrix. If protonation is not efficient, the... 

The formation of 2-(indolin-2-yl)indole dimers from indole-3-acetic acid and its propyl ester in trifluoroacetic acid and phosphoric acid has been studied." The reaction involves electrophilic attack of the protonated species (24) on the free substituted indole to give the trans stereochemistry at the C(2)-C(3) bond. 

Crown ethers are not chromogenic unless they contain a pendant chromogen able to dissociate a proton in a basic medium. The resulting anion interacts strongly with the crown-complexed cation compensating the electric charge. The formation of a zwitterion leads to a hydrophobic extractable species with a considerably shifted absorption maximum compared with the protonated species. This allows the same spectrophotometric determination to be used for a large number of metal ions, provided the appropriate crown compound is used in each case. Another method involves... 

The spectra obtained from the chemisorption of methanol onto catalyst above 100°C indicated the progressive oxidation of methoxy species to formate via dioxymethylene/HCHO and finally to CO, CO2, and H2. Phenol adsorbed on the surface Lewis acid-base pair site and dissociated to phenolate anion and proton. The formation of phenolate anion and proton were discerned from the strong intense C-0 stretching vibration and the disappearence of phenolic 0-H stretching vibration, respectively. Importantly, there were series of definite low intensity bands between 2050 and 1780 cm" that were identified as the out-of-plane aromatic C-H bending vibrations [79, 84-85]. These bending vibrations are possible only if the phenyl ring of phenol is perpendicular to the catalyst surface. 

The emission from [Ru(bpz)3] is quenched by carboxylic acids the observed rate constants for the process can be rationalized in terms of the protonation of the non-coordinated N atoms on the bpz ligands. The effects of concentration of carboxylate ion on the absorption and emission intensity of [Ru(bpz)3] have been examined. The absorption spectrum of [Ru(bpz)(bpy)2] " shows a strong dependence on [H+] because of protonation of the free N sites the protonated species exhibits no emission. Phosphorescence is partly quenched by HsO" " even in solutions where [H+] is so low that protonation is not evidenced from the absorption spectrum. The lifetime of the excited state of the nonemissive [Ru(Hbpz)(bpy)2] " is 1.1ns, much shorter than that of [Ru(bpz)(bpy)2] (88 nm). The effects of complex formation between [Ru(bpz)(bpy)2] and Ag on electronic spectroscopic properties have also been studied. Like bpz, coordinated 2,2 -bipyrimidine and 2-(2 -pyridyl)pyrimidine also have the... 

The first step is a bimolecular reaction leading to the formation of a hydrogen bond the second step is the breaking of the hydrogen bond such that the protonated species H B+ is formed the third step is the dissociation reaction to form the products. In aqueous solutions, the bimolecular reaction proceeds much faster than would be predicted from gas phase kinetic studies, and this underscores the complexity of proton transfer in solvents with extensive hydrogen-bonding networks capable of creating parallel pathways for the first step. In their au-... 

Gas phase measurements of the formation of protonated species can provide similar thermodynamic data. 

Protonation of pyrrole, furan and thiophene derivatives generates reactive electrophilic intermediates which participate in polymerization, rearrangement and ring-opening reactions. Pyrrole itself gives a mixture of polymers (pyrrole red) on treatment with mineral acid and a trimer (146) under carefully controlled conditions. Trimer formation involves attack on the neutral pyrrole molecule by the less thermodynamically favored, but more reactive, (3-protonated pyrrole (145). The trimer (147) formed on treatment of thiophene with phosphoric acid also involves the generation of an a-protonated species. 

Although the quantitative theory of reactions in moderately concentrated solutions of strong acids is unsatisfactory, we do have a good qualitative idea of the processes involved in the acid-catalyzed hydrolysis and formation of esters. Under conditions where the degree of protonation of the substrate is small it is not possible to separate with confidence the factors which affect the solvolytic process and those which affect the preliminary protonation equilibrium. But there have been a number of recent studies of t ie behaviour of carboxylic acids and esters in very strongly acidic media, in. which they are essentially completely protonated. Under these conditions it is possible to observe the behaviour of the protonated species directly. It is appropriate to summarize the results of this research before discussing the reactions under more normal solvolytic conditions. 

---