Naked proton

The hydrogen ion (H+) represents a very different situation. When hydrogen is released into the soil solution by ionization, it loses its electron. The naked proton (H+) is naturally attracted to the partially negative oxygen of water and its lone pair of electrons (Figure 5.8, equation 2). The result of this interaction is the species H30+, which is called a hydronium ion. This is the true species in the soil solution even though scientific papers and texts will use the simpler H+ when writing equations. 

H+ Ion. The naked proton H+ exists only in the gas phase. In the condensed state, the proton is always solvated, thus no free proton is capable of existence. It is customary, however, as short hand notation to depict H+ as the solvated proton. 

The method has been used to study the LiH system [13,14,15] for which the main interest was in the first excited state, which governs the dynamical behaviour of the neutral LiH molecule in the presence of a naked proton. Various nuclear configurations have been sampled, both in the subreactive [14] and reactive regions of the configuration space [13]. It turned out that a simple two-reference VB wavefunction was sufficient for the subreactive study, while the stretching of the LiH bond in the reactive regions required the use of an additional reference function. For this system, the ground state SC wavefunction has the form ... 

Up to now, we have considered the resonance of a naked proton in a magnetic field, but real protons in organic compounds are not naked. They are surrounded by electrons that partially shield them from the external magnetic field. The electrons circulate and generate a small induced magnetic field that opposes the externally applied field. 

If all protons were shielded by the same amount, they would all be in resonance at the same combination of frequency and magnetic field. Fortunately, protons in different chemical environments are shielded by different amounts. In methanol, for example, the electronegative oxygen atom withdraws some electron density from around the hydroxyl proton. The hydroxyl proton is not shielded as much as the methyl protons, so the hydroxyl proton absorbs at a lower field than the methyl protons (but still at a higher field than a naked proton). We say that the hydroxyl proton is deshielded somewhat by the presence of the electronegative oxygen atom. 

At temperatures above about 200°C, however, HC1 does begin to dissociate, but not into ions. Instead of the chlorine atom taking both bonding electrons with it, leaving a naked proton, the electron pair forming the H-Cl bond is shared out between the two atoms. AG for this reaction is a much more reasonable +431 kj mol-1 and, at high temperatures (above about 200°C, that is), HC1 gas can be dissociated >200 °C... 

In calculations, the energy of the naked proton is zero, so that the corresponding protonation energy, PE, at 0 K in the gas phase for an isolated molecule is obtained by equation 10. 

A shorthand notation you may see in some texts is to drop a proton off by itself or show a naked proton being picked up by a base. Under the conditions found in almost all... 

Proton transfer can occur from any acidic to any basic groups or to and from the solvent. A common shortcut in writing mechanisms is to draw just the proton rather than the acid or the protonated solvent, but you should remember that a naked proton will never be floating free in solution. Always check the proton transfer (Section 3.3). 

The notation (aq) refers to solutes in aqueous solution, whereas (.) refers to water in the liquid state. It is well established that there are no naked protons (free hydrogen ions) in solution even the hydronium ion (H3O+) is an underestimate of the degree of hydration of hydrogen ion in aqueous solution. All solutes are extensively hydrated in aqueous solution. We will write the short form of equations for acid dissociation in the interest of simplicity, but the role of water should be kept in mind throughout our discussion. 

Protonation can in fact be seen in a more subtle way the protonating reagent is not necessarily a naked proton, but the ion, a species always supposed abundant [3,83,87], but only detected recently in interstellar clouds [121]. The corresponding reaction thus becomes ... 

The chloride anion is the same in both cases the only difference is that a very unstable naked proton would have to be the other product in the gas phase but a much more stable H3O cation would be formed in water. In fact it s even better than that, as other molecules of water cluster round ( solvate ) the HjO cation, stabilizing it with a network of hydrogen... 

Observe that each of these reactions accomplishes a net heterolytic cleavage of an H2 molecule, an incredibly weak acid (pAT = 42), with the P picking up a proton and the B a hydride. How might this occur Neither a naked proton nor a naked hydride anion is plausible as an intermediate. A reaction path in which both the P and the B act in concert to break the H-H bond heterolytically is envisioned below, even though explicit experimental evidence for such a pathway is still lacking ... 

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