Neutral versus Ion Pairs

As the acidity of the proton donor AH increases, and there is a concomitant enhancement in the basicity of the acceptor B, the H-bond will become stronger. But it also stands to reason that for a strong enough pair of acid and base, the proton can simply transfer across from A to B, converting the system into an ion pair  

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This question was explored in some detail by ab initio calculations pairing formic acid with methyleneimine, in which the N atom participates in a double bond. As illustrated in Fig. 6.21, the two configurations examined place the N atom either syn or anti with respect to the carboxyl group. The calculations were carried out with a 4-31G basis set at the SCF level. (These results were later confirmed at the correlated MP2 level with a 6-3IG basis set .) The protonation energies of HCOO and NHCHj at the SCF/4-31G level of theory are 360 and 230 kcal/mol, respectively. When adjusted by vibrational and other terms, the calculated values of AH(300 K) are 352.6 and 221.9 kcal/mol, only a little bit larger than the experimental quantities of 345.2 and 214.3 kcal/mol, respectively. Most important for our purposes, the theoretical overestimation is equal to 7 kcal/mol in both cases consequently, the calculated protonation enthalpy difference of 130.7 kcal/mol is very close indeed to the experimental value of 130.9 kcal/mol. And it is this proton affinity difference which is the key element in considering the question of neutral versus ion pairs. 

The Marcus Inverted Region (MIR) is that part of the function of rate constant versus free energy where a chemical reaction becomes slower as it becomes more exothermic. It has been observed in many thermal electron transfer processes such as neutralization of ion pairs, but not for photoinduced charge separation between neutral molecules. The reasons for this discrepancy have been the object of much controversy in recent years, and the present article gives a critical summary of the theoretical basis of the MIR as well as of the explanations proposed for its absence in photoinduced electron transfer. The role of the solvent receives special attention, notably in view of the possible effects of dielectric saturation in the field of ions. The relationship between the MIR and the theories of radiationless transitions is a topic of current development, although in the Marcus-Hush Model electron transfer is treated as a thermally activated process. 

Figure 1. Schematic diagram of the energies of the anion ground state (M S), the lowest covalent state (M-S ), ion-pair state (M S ) and cation ground state (M+S ) versus /j M = alkali metal atom S = solvent molecules). The ion-pair state is expected to correlate with the ground state of the solvated electron plus the solvated M+ ion in bulk fluids. The ground-state electronic character of the neutral cluster changes from covalent to ion-pair type at a certain critical size.

There is need for caution in accepting the above conclusion. The guidelines for the interpretation of data produced from the probes used by Grob et al. in arriving at their conclusion are essentially based on traditional Sj l and S 2 mechanisms and not on the existance of hidden ion-pair return. If hidden ion-pair formation is as common as envisioned by some," the interpretations of m and p values may be based, at least in part, on the reaction of ion pairs and not neutral substrates. The direction of the curvature of the log k versus plot for (83a)-(83d) (if the curvature is real) could be taken as evidence for the ion-pair mechanism. Hence all that can be said until the question of the general involvement of ion pairs is resolved is that nitrogen participation is demonstrated. 

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