Nitrogenous protonic species

Neither the existence nor the geometry of the ammonium ion NH4 or its solvates, or the hydrazinium ions N2H5 and have ever been 

NH4 and H2O were identified in the former, and NH4 and HjO (from electrolysis of water of crystallization) in the latter. 

It may be expected that because of its large ionic radius NH4 should have a coordination number similar to those of K , Rb or Cs, namely between 6 and 12. Its appreciable tendency for hydrogen bond formation, however, can act to reduce its coordination to hydrogen bond acceptors to 4 or 5 .  

The ammonium-ammonia system might be expected to show much similarity to the oxonium-water series, both in the extent of solvates of the [NH4(NH3) ] formed in the solid state, and in the richness of structural arrangements of the resulting hydrogen bonded ions. Disappointingly, the literature does not bear witness to this. 

In summary, although it is tempting to draw parallels between the ammonium amines and the isoelectronic oxonium-water series, this is simply not possible the structural variety displayed by this latter family as hydration is increased finds no comparison, nor does the broad range of anions accompanying [H(H20) ] in the solid state find any counterpart. 

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The slope of the straight lines in Fig. 1 is pH-dependent. This has been explained on the ground of an equilibrium between the free enamine and the nitrogen-protonated species. This acid-base equilibrium is built up very rapidly [Eq. (3)], and causes a decrease in concentration of the reactive... 

The double bond of the nitrogen-protonated species is stable with respect to electrophilic attack under the reaction circumstances, since the free... 

Thorium compounds of anionic nitrogen-donating species such as [Th(NR2)4], where R = alkyl or sdyl, are weU-known. The nuclearity is highly dependent on the steric requirements of R. Amides are extremely reactive, readily undergoing protonation to form amines or insertion reactions with CO2, COS, CS2, and CSe2 to form carbamates. Tetravalent thorium thiocyanates have been isolated as hydrated species, eg, Th(NCS)4(H20)4 [17837-16-0] or as complex salts, eg, M4 Th(NCS)g] vvH20, where M = NH, Rb, or Cs. 

The reason for the slow hydrolysis compared to that of structurally similar compounds like nitrones or 0,lV-acetals might be the following (b-67MI50800) in the protonated species (77) assistance of the lone pair of electrons at nitrogen is sterically hindered due to the large angle of its orbital to the plane of the ring. 

Quantum calculations on metal-assisted tautomerization indicate a substantial stabilization to protonation of the endocyclic nitrogen atoms (99). In the case of HgCHg, adducts are formed by binding at either A-N6 or C-N4, which increases the stability of the respective N1H+ or N3H+ protonated species by 10-14 kcal/mol. For Pt11 binding at C-N4 the effect is much greater at 30-34 kcal/mol. 

Conversely, Gourec et al. suggested a mechanism involving a non-metallic active site.85 In the proposed mechanism, based on XPS, SIMS, and CV analysis, dioxygen reacts with protonated nitrogen surface species to form oxidized nitrogen surface species and water. After further reaction with protons and electrons, the oxidized nitrogen is reduced, and water is produced. 

As mentioned earlier, products of disproportionation often accompany the rearrangement products. This reaction is also acid-catalysed and it is a reasonable assumption that reaction proceeds via the protonated species. Experiments with the 4,4 -diiodohydrazobenzene (19) showed that there were significant nitrogen and para-carbon kinetic isotope effects23. This implies that disproportionation must take place after C—C bonding has occurred, i.e. that the intermediate must be the quinonoid form 20 (and cannot, for example, be a jr-complex), which is then believed to react with another reactant molecule to give the disproportionation products (Scheme 4). 

It is a well established fact that the preferred site of protonation of aniline in solution is the nitrogen atom, due to the higher stabilization upon solvatation of the N-protonated species with respect to the ring protonated ion (/). In the gas phase however, the situation is less clear, and the preferred site of protonation has been the subject of many experimental and theoretical studies. Based on proton transfer equilibria at 600 K and upon correlation of the proton affinities... 

On the other hand, the PAs at the para and ortho positions are manifestly more sensitive to the electron correlation treatment than the PA at the nitrogen. From the point of view of the % diagnostic (a measure for the importance of nondynamical correlation), aniline (7 =0.0113) and N-anilinium (7 =0.0096) are very similar (and basically purely single-reference), while the p- and o-protonated species exhibit very mild multireference character (0.0149 and 0.0157, respectively). Since protonation at these sites thus involves a noticeable change in 7i, the PA is expected to be more sensitive to the correlation method. 

Protonation to the conjugate acid (iminium cation) increases the potential of the itnine to act as an electrophile (compare carbonyl see Section 7.1), and this is followed by nucleophilic attack of water. The protonated product is in equilibrium with the other mono-protonated species in which the nitrogen carries the charge. We shall meet this mechanistic feature from time to time, and it is usually represented in a mechanism simply by putting H+, +H+ over the equilibrium arrows. Do not interpret this as an internal transfer of a proton such transfer would not be possible, and it is necessary to have solvent to supply and remove protons. 

In the case of the nitrogen compounds, the bond angles in the unprotonated species are rather close to those of the protonated species 97). Since there are only rather small geometric changes, the proton affinity of pyridine is close to that of trimethylamine. 

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