Stability protonic sites

Tautomerism Kinetic vs. Thermodynamic Stability Protonation Sites in Conjugated Molecules N/N Alternatives. ... 

In conjugated molecules one or other of the possible protonation sites may be more or less favoured by solvation effects and for this reason sites of protonation are often solvent dependent. In some instances, similar stability of two possible cations results in tautomeric equilibria and these too may be solvent dependent. Just as solute-solvent interactions have an effect on the relative stability of two possible cations formed from a conjugated molecule, so in solid salts stability relationships depend on the mode of packing of ions, which determines interactions with the nearest neighbours. Therefore the types of cation observed in solid salts are not necessarily the most stable ones in solution. 

Both steric and electronic effects of substituents upon experimental pXa-Vcdues provide information about protonation sites. In view of the small steric requirement of the proton, steric effects in protonation arise most often from steric inhibition of solvation of cations, which results in a reduction of their stability (pA a smaller). 

In any kind of conjugated system with two protonation sites [I] there is the possibility of both cations [3] and [4] being formed in varying amounts, depending on conditions. As has been pointed out in the preceding section, solvation effects may play a dominant role here in tipping the balance from resonance-stabilized forms to solvation-stabilized forms. Thus under a given set of conditions we may have protonation at both sites as shown in equations (21) and (22). 

Replacement of the proton at N1 of G by a metal ion has qualitatively similar consequences as substitution of the imino proton of T and U. The Pt(II) binding to this site increases the basicity of the purine ring, but it is not clear how the residual electron density is distributed. The N1 platinated guanine bases titrate with a pA a of 5 (176, 177), and from the high pD dependence of the H8 resonances in the H NMR spectra it is concluded that the most likely protonation site is N7 (cf. Fig. 16). In its protonated state, the nucleobase represents again a metal-stabilized rare tautomer. 

Zeolite polarity and reaction rate The competition between sulfolane, PA and product molecules for the adsorption on the active protonic sites is sufficient enough to explain the differences in reaction orders and catalyst stability and selectivity between PA transformation in sulfolane and in dodecane. However, the competition for the occupancy of the zeolite micropores plays a significant role as well. This was demonstrated by studying a related reaction the transformation of an equimolar mixture of PA with phenol in sulfolane solvent on a series of H-BEA samples with different framework Si/Al ratios (from 15 to 90).[49] According to the largely accepted next nearest neighbour model,[50,51] the protonic sites of these zeolites should not differ by their acid strength, as furthermore confirmed by the... 

In the case of HSAPO-34, the empirical force field results are in good agreement with experimentally established protonation sites at 02 and 04, while in ab initio calculations a slightly different, 01 < 02 < 04, order of stability was fotmd, similar to that in the HSSZ-13. 

Compound I must be activated by the strongly acidic medium. One possible protonation site is the basic pyrrolidinium nitrogen atom (structure X), which would favor the formation of carbenium ion XI (see Scheme 16.3). This intermediate would gain additional stabilization from the polar solvent. Conditions for an SnI substitution would prevail to yield XII, whose fragmentation would resemble, formally speaking, the retro cycloaddition process used in the synthesis of I (i.e., VIII - I). The resulting ketone (XIII) would contain the elements required for the construction of a five-membered carbocycle with the desired incorporation of the functional carbon of one of the nitrile groups, as... 

This process occurs at either the amine nitrogen or the ring carbon atoms. In solution, the nitrogen atom is the preferred ANI protonation site, due to a higher stabilization by solvent of the A-protonated form86. 

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