A NMe for Compounds with Contiguous Nitrogen Atoms

The C tautomer of s-triazoles contains a normal amidine N H for which A (NMe) = 0.3 is expected. If this value is assumed, it is possible to use the data of Table 12.6 to establish A (NMe) for A/B by trial and error. The fit obtained for R = Me, and for one of the R = H pairs, is very close if A(NMe) 0.55 is used. It is stated [2d] that Kj for 37 lies in the range 4-10, and our own survey [3] suggests a value close to 4 that for R = Me is unknown. The imides 33-36, also triazoles, can be used a second 

These compounds dilFer from their oxygen analogs in that in no case has the thioiminol tautomer been detected. Hence the parent compound s pK values may in aU cases be compared directly with those of their NMe derivatives, as hsted below 

Since experiment cannot provide a value for A(SMe), we have to rely on an indirect approach. That which follows postulates a proportionahty between the free energies of hydrogen bonding for neutral species and the solvation of a related cation. 

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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