Ionization potentials correlation with proton affinities

The acidity and basicity of 1,2,4-triazines have been calculated by the MOSP method. The pKa values correlate satisfactorily with the total (n + n) atomic charges of the proton accepting atom and the ionization potential.46 The proton affinities of 1,2,4-triazines have been calculated by the 3-21G, 6-31G, MINDO and AMI methods,38,47 the lithium-triazine binding energies by the 6-31 method,48 and the tautomeric equilibrium by the AM1 and MINDO-PM3 method.49... 

In this equation, fH is the ionization potential for H (1312kJ mol-1), fB is the ionization potential for the base B, and EB+ is the energy of the B+-H bond. The term IB is subtracted from fH (the last term is of lesser importance), which leads to the conclusion that the smaller the value for IB, the greater the proton affinity. Because H+ reacts by removing electron density from B, the easier this process is to accomplish, the smaller the value of IB. For the molecules CH4, NH3, H20, and HF, the proton affinities are 527, 841, 686, and 469 kj mol-1, respectively. These values correlate well with the ionization potentials of the molecules, which are in the order NH3 < H20 < CH4 < HF. 

In an attempt to determine the factors which control the rate of protonation, we excunined the correlation of these rates with some basic properties of the molecules, such as electron affinity, ionization potential, and singlet energy, which are known to be interrelated for alternant aromatic hydrocarbons. 

Fig. 1 shows that a plot of log k2 vs Eg g gives a reasonable linear dependence. DeviatioAs from linearity appear to be relatively larger when the rates of protonation are low, probably because the lower rates are more susceptible to experimental complications, particularly impurity effects. It should be noted that the protonation rates determined by Fry and Schuettenberg (5) also give a reasonable straight line when we plot them against AEg g. As expected for alternant hydrocarbons, linearity is also obtained when log k2 is plotted versus the ionization potential I (plot not shown, see values in Table I). In principle, k2 can be correlated also with electron affinity ( ) or with the polarographic reduction half-wave potential, but the literature values for these parauneters are widely scattered. The molecular dimensions seem to have little effect on k2 ... 

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