Protonated methanol, electrostatic

FIGURE 4 2 Electro static potential maps of methanol and chloro methane The electrostatic potential is most negative near oxygen in methanol and near chlorine in chloromethane The most positive region is near the O—H proton in methanol and near the methyl group in chloromethane... 

The S—H bond is less polai than the O—H bond, as is cleaiTy seen in the electrostatic potential maps of Figure 15.7. The decreased polarity of the S—H bond, especially the decreased positive character of the proton, causes hydrogen bonding to be absent in thiols. Thus, methanethiol (CH3SH) is a gas at room temperature (bp 6°C), whereas methanol (CH3OH) is a liquid (bp 65°C). 

For comparison, consider now the same ions in methanol solution. Each ionic field will contain more electrostatic energy than the corresponding ionic field in aqueous solution. Suppose that again we raise a proton from the occupied level of a (NIIi) ion to the vacant level of a (CH3COO)- ion. In this process the amount of electrostatic energy released will be greater than in water. If then the value of, / is roughly the same as before, the total amount of work required to transfer the proton will be smaller than in water. Hence, in the chart of the proton levels in methanol, we expect that the interval between these two proton levels will be narrower than in Fig. 36. 

As expected, this is smaller than the value in aqueous solution. Although in both solvents a positive amount of work is required to transfer the proton, the amount in methanol is smaller, owing to the greater amount of electrostatic energy released in the disappearance of the ionic fields. 

The question of methanol protonation was revisited by Shah et al. (237, 238), who used first-principles calculations to study the adsorption of methanol in chabazite and sodalite. The computational demands of this technique are such that only the most symmetrical zeolite lattices are accessible at present, but this limitation is sure to change in the future. Pseudopotentials were used to model the core electrons, verified by reproduction of the lattice parameter of a-quartz and the gas-phase geometry of methanol. In chabazite, methanol was found to be adsorbed in the 8-ring channel of the structure. The optimized structure corresponds to the ion-paired complex, previously designated as a saddle point on the basis of cluster calculations. No stable minimum was found corresponding to the neutral complex. Shah et al. (237) concluded that any barrier to protonation is more than compensated for by the electrostatic potential within the 8-ring. 

It is a serious drawback that it is not possible to determine the transfer activity coefficient of the proton (or of any other single-ion species) directly by thermodynamic methods, because only the values for both the proton and its counterion are obtained. Therefore, approximation methods are used to separate the medium effect on the proton. One is based on the simple sphere-in-continuum model of Born, calculating the electrostatic contribution of the Gibb s free energy of transfer. This approach is clearly too weak, because it does not consider solvation effects. Different ex-trathermodynamic approximation methods, unfortunately, lead not only to different values of the medium effect but also to different signs in some cases. Some examples are given in the following log yH+ for methanol -1-1.7 (standard deviation 0.4) ethanol -1-2.5 (1.8), n-butanol -t-2.3 (2.0), dimethyl sulfoxide -3.6 (2.0), acetonitrile -1-4.3 (1.5), formic acid -1-7.9 (1.7), NH3 -16. From these data, it can be seen that methanol has about the same basicity as water the other alcohols are less basic, as is acetonitrile. Di-... 

Of particular interest is the very high conductivity of the hydrogen ion. This ion has an unexpectedly high conductivity in a number of hydroxylic solvents such as water, methanol and ethanol, but behaves more normally in nonhydroxylic solvents such as nitrobenzene and liquid ammonia. At first sight the high values might seem to be due to the small size of the proton. However, there is a powerful electrostatic attraction between a water molecule and the proton, which because of its small size can come very close to the water molecule. As a consequence the equilibrium... 

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