Proton donor strength

TABLE 6.5. Effects of Proton-Donor Strength on Dihydrogen Bonding in BeH2 Complexes... 

The niobinm trihydride (1 to 7) illustrates clearly the influence of proton-donor strength The dihydrogen bonding energy increases from the weak proton donor CH3OH (—6.7 kcal/mol), to the relatively strong acid CF3OH (—12.3 kcal/mol). 

The data in Table 7.2 show that the dihydrogen-bonding energy increases with the proton-donor strength of proton donors. In this context, systems 28 to 31 are most representative where the A.E value changes from —4.1 kcal/mol for CH3OH to —6.6 kcal/mol for (CF3)2CHOH. In accordance with the theoretical data, the... 

The great proton donor strength of superacids is due to the stabilization of the pro-tonated forms of the Br0nsted acid in an ion pair H F SbF for example. 

The highest proton-donor strengths are exhibited by zeolites with the lowest concentrations of AlOY tetrahedra such as H-ZSM-5 and the ultrastable zeolite HY. These are superacids, which at high temperatures (ca. 500 °C) can even protonate alkanes. It was foimd that the acid strength depends on the number of A1 atoms that are adjacent to a silanol group. Since the A1 distribution is nonuniform, a wide range of acid strengths results. 

Abraham and coworkers [36] have estabhshed a five-parameter equation, r = 0.995, in gas-water partitioning, one term of which is the solute proton donor quantity. Among these are compounds (51, X=0, NMe, and S) whose Sal values appear in Table 12.8. We postulate that the values of 52a and 53a will each be enhanced and proportional to those of 51 that the still more enhanced values for 52b and 53b will continue this proportionahty and that solvation for each of these species will quantitatively reflect this trend, so that, for each series, there will exist a linear relation between the proton donor strength of XH and A(XMe) for the corresponding substituted pyridinium cation. Since the A(XMe) values for X=0 and NMe are known, it should then be possible to extrapolate this two-point correlation to X=S. 

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