Positive protonation reaction

As an example of the problem of species in solution, consider the case of a solution made by dissolving some potassium chrome alum, KCrfSO s-12H20, in water. On testing, the solution is distinctly acidic. A currently accepted explanation of the observed acidity is based upon the assumption that, in water solution, chromic ion is associated with six H20 molecules in the complex ion, Cr(H20) a. This complex ion can act as a weak acid, dissociating to give a proton (or hydronium ion). Schematically, the dissociation can be represented as the transfer of a proton from one water molecule in the Cr(H20) 3 complex to a neighboring H20 to form a hydronium ion, H30+. Note that removal of a proton from an H20 bound to a Cr+3 leaves an OH- group at that position. The reaction is reversible and comes to equilibrium ... 

Equation 4.9 has been extensively applied to study the mechanisms of electrophilic (e.g., protonation) reactions, drug-nucleic acid interactions, receptor-site selectivities of pain blockers as well as various other kinds of biological activities of molecules in relation to their structure. Indeed, the ESP has been hailed as the most significant discovery in quantum biochemistry in the last three decades. The ESP also occurs in density-based theories of electronic structure and dynamics of atoms, molecules, and solids. Note, however, that Equation 4.9 appears to imply that p(r) of the system remains unchanged due to the approach of a unit positive charge in this sense, the interaction energy calculated from V(r) is correct only to first order in perturbation theory. However, this is not a serious limitation since using the correct p(r) in Equation 4.9 will improve the results. 

It has been suggested that an increase in the coordination number of vanadium from 4 to 5 already takes place in the second protonation step, i.e. when [H2V04] is formed (21). For reactions (1) and (2), however, the protonation constants and thermodynamic parameters are comparable with those reported for P04 and As04 , providing firm evidence that reaction (2) is not accompanied by incorporation of water in the vanadate ion (15, 17). Further, the estimated thermodynamic quantities for reaction (6), AH° = -39 kJ/mol and AS0 = —51 J/(mol K), obtained by extrapolation from the experimental values for reactions (1) and (2) and those for the three protonation steps of P04 and As04 , are not typical of a simple protonation reaction (17). For such a reaction the entropy change is normally a positive quantity often amounting to 100 50 J/(mol K) and the enthalpy... 

Further computational studies on the azaBaP system are currently in progress. We are also evaluating the protonation reactions on different positions of the rings in order to establish the most favored protonation site. The effect of methylation/protonation at nitrogen on epoxide reactivity is also being examined. 

The energy Ea is a quantum term associated with the proton reaction coordinate coupling to the Q vibration, Ea = h1 /2m. and Co is the tunneling matrix element for the transfer from the 0th vibrational level in the reactant state to the 0th vibrational level in the product state. The term AQe is the shift in the oscillator equilibrium position and F L(Eq, Ea, Laguerre polynomial. For a thorough discussion of Eq. (8), see [13],... 

Transition metals also catalyze isotopic exchange reactions. Platinum is the most active catalyst for most heterocycles. The mechanism may involve metallation, addition, o--addition and ir-complex formation. a-Hydrogen exchange in pyridine is favored over 3- and 7-positions, particularly by a cobalt catalyst whereas platinum is much less selective. In isoquinoline both the 1- and 3-position protons are exchanged at almost the same rates with very little exchange at any other position. In 3-substituted pyridines exchange is preferred at the 6-position, the more so as the size of the 3-substituent increases (73AHC(15)140). 

In step 2 of the reaction, the electrophilic carbon atom of C02 attacks the ring at the ortho position. The reaction proceeds via the transition states oTSl and TS2 (Figure 5.3), with formation of the intermediates C and D. The final product, sodium salicylate, is obtained from intermediate D through a proton shift from the ring to the phenoxide ion via the transition state TS3. It can be shown from the figure that intermediates C and D can easily undergo the reverse reactions, thus confirming the role of the C02-pressure to shift the equilibrium towards the final products of the reaction. 

The qualitative applications of mass spectrometry are based on the determination of the mass-to-charge m/z) ratio of an analyte. ESI and APCI MS are well known as a soft ionization technique. This means that relatively small amount of energy are needed to ionize the analyte. The efficiency of ion formation depends on a molecule s ability to associate and carry a charge. In a positive ion experiment, this ionization process can be defined by the following simple protonation reaction ... 

It is interesting to note that when the positions of the proton and the activating group are exchanged so that the proton assumes an axial position, no reaction was observed. 

After consideration of the mechanism of the Schmidt reaction of carboxylic acids (cf. section III.A)the prediction was made and verified that the Curtins rearrangement would be subject to acid catalysis The charge and bond distribution in the substrate can almost certainly be best represented by 1 and protonation (reaction 7) was believed to form 2 in which the positive charge on the nitrogen of the... 

In the positive-ion mode, the ion-molecule reactions are determined by the proton affinities of the reactants. The proton affinity (PA) or gas-phase basicity of molecule M is defined as the exothermicity of its protonation reaction ... 

Similarly, nucleophiles do not attack species like H30 at oxygen, even though it is the oxygen atom that carries the positive charge. Reaction occurs at one of the protons, which also neutralizes the positive charge. Or, to put it another way, H30 is an acid (electrophilic at hydrogen) and not electrophilic at oxygen. 

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