Ion pair configurations

Almost certainly the configurations A + MandA + M, where A denotes an alkali atom and M a molecule, do not interact directly the coupling between them is made by the ion-pair configuration A+ + M . The excitation functions can be divided in two groups according to whether a rapid onset of the excitation cross section near threshold and a maximum at low energies is found or not.2... 

K + S02. A sharp maximum, followed by a marked decrease towards higher energies occurs in the emission cross section, when the channel for collisional ionization opens. When 1 - (M) < I, but still I - (M) > , high yields for collisional ionization are found,30,46 while in cases where I - (M) S /, they are very low this indicates that when the ion-pair configuration cuts across all Rydberg states, inelastic collisions mostly lead to excitation and not to ionization. 

Further evidence for the importance of the ion-pair configuration comes from A + N02, where I — E E for K, but 1 — (M) > E for Na. Consequently the cross section for excitation of potassium is much lower than that for sodium.76... 

When I - (M) < , the ion-pair configuration does not couple the initial and final channel, and the threshold will be at much higher energies. When the bond lengths are too different, the Franck-Condon factors are unfavourable for a transition A+ + M- -> A + M in the interaction region.2,6,15 Consequently the excitation cross section will be small. 

For all diatomic molecules is of the order of 8 eV for emission in the visible range. Since / - (M) < the ion-pair configuration A+ + M will not couple the configurations A + M and A + M at low energies. Therefore in this picture no excitation of diatomic molecules by alkali atoms is expected at low collision energies. For NOz and S02 / — (M) > , so that molecular excitation could be possible even at low collision energies. 

Figure 7. Atomic structures of the three neutral-pair and two ion-pair configurations of HAN molecule. The neutral-pair structures in order of stability are labelled as HAN-NO, HAN-N, and HAN-O. The ion-pair structures are labelled as ts-HAN-NO and ts-HAN-O.

Figure 11. Schematic representation of sequential events of an SNj ionization reaction in a polar liquid. Elementary events involve contact ion pairs (CIP) and solvent-separated ion pairs (SSIP). In ionic aqueous solutions, the influence of different ion-pair configurations on early electron-transfer trajectories can be considered through the investigation of ultrafast electronic dynamics and radical ion-...

Figure 11 Potential of mean force for a model ion pair in a dipolar solvent. Also shown are the configuration of the nearest-neighbor solvent molecules for several of the important ion pair configurations contact ion pair, transition state ion pair, and solvent-separated ion pair. Adapted from ref. 130.

In the second part we study the ion speciation in infinitely dilute NaCI aqueous solutions by determining the constant of as.sociation by constraint molecular dynamics via mean-force potential calculations. We determine the temperature and density dependence of the extent of the ion association. In addition we analyze the kinetics of the interconversion between two ion pair configurations, the contact ion pair and the solvent-shared ion pair, by determining the transition state theory (TST) kinetic rates. 

Several studies have been reported on the determination of the mean-force potential between aqueous ion pairs at ambient conditions, " yet little is known about the speciation in aqueous solutions at near-critical and supercritical conditions " which are typically encountered in technological processes where supercritical water is either the reaction medium or the energy carrier. In this section we analyze the association, equilibrium, and the kinetic (interconversion) rate constants for an infinitely dilute aqueous Na /CI" solution as described by a water-electrolyte model at several supercritical state conditions. In Section 3.3.1 we briefly describe the statistical mechanical formalism for the determination of the thermodynamic constants and the molecular dynamic determination via constraint dynamics. In Section 3.3.2 we discuss the actual kinetics of the inteicon-version between two ion pair configurations leading to the definition of the corresponding equilibrium constant. Finally, in Section 3.3.3 we discuss the outcome of the comparison between the association constants from simulation and... 

Figure 4.16 Computed most stable ion-pair configuration for (a) [(PPh3)Au(4-Me-styrene)][BFj and (b) [(IPr)Au(4-Me-styrene)][BF4]. Reproduced with permission from D. Zuccacia et Am. Chem. Soc., 2009,131, 3170-3171. Copyright (2009) American Chemical Society. "...

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

Attack by a nucleophile or the solvent can occur at either of the ion pairs. Nucleophilic attack on the intimate ion pair would be expected to occur with inversion of configuration, since the leaving group would still shield the fiont side of the caibocation. At the solvent-separated ion pair stage, the nucleophile might approach fiom either fece, particularly in the case where solvent is the nucleophile. Reactions through dissociated carbocations should occur with complete lacemization. According to this interpretation, the identity and stereochemistry of the reaction products will be determined by the extent to which reaction occurs on the un-ionized reactant, the intimate ion pair, the solvent-separated ion pair, or the dissociated caibocation. 

If it is assumed that ionization would result in complete randomization of the 0 label in the caihoxylate ion, is a measure of the rate of ionization with ion-pair return, and is a measure of the extent of racemization associated with ionization. The fact that the rate of isotope exchange exceeds that of racemization indicates that ion-pair collapse occurs with predominant retention of configuration. When a nucleophile is added to the system (0.14 Af NaN3), k y, is found to be imchanged, but no racemization of reactant is observed. Instead, the intermediate that would return with racemization is captured by azide ion and converted to substitution product with inversion of configuration. This must mean that the intimate ion pair returns to reactant more rapidly than it is captured by azide ion, whereas the solvent-separated ion pair is captured by azide ion faster than it returns to racemic reactant. 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

Entry 4 shows that reaction of a secondary 2-octyl system with the moderately good nucleophile acetate ion occurs wifii complete inversion. The results cited in entry 5 serve to illustrate the importance of solvation of ion-pair intermediates in reactions of secondary substrates. The data show fiiat partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and in the alcohol resulting from reaction with water. The alcohol of retained configuration is attributed to an intermediate oxonium ion resulting from reaction of the ion pair with the dioxane solvent. This would react until water to give product of retained configuratioiL When azide ion is present, dioxane does not efiTectively conqiete for tiie ion-p intermediate, and all of the alcohol arises from tiie inversion mechanism. ... 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

The stereochemistry of hydrogen-deuterium exchange at the chiral carbon in 2-phenylbutane shows a similar trend. When potassium t-butoxide is used as the base, the exchange occurs with retention of configuration in r-butanol, but racemization occurs in DMSO. The retention of configuration is visualized as occurring through an ion pair in which a solvent molecule coordinated to the metal ion acts as the proton donor... 

Figure 11.11 Ion pairs in an S l reaction. The leaving group shields one side of the carbocation intermediate from reaction with the nucleophile, thereby leading to some inversion ol configuration rather than complete racemization.

Short-lived chiral ion pairs are intermediates in the Haller-Bauer cleavage 14 15 of enantiomer-ically enriched 2,2-disubstituted 1,2-diphenylethanones, which give optically active phenylalka-nes on in situ protonation with partial retention of the configuration. 

Alkenyllithium derivatives, carrying carbanion-stabilizing substituents, which facilitate the formation of solvent-separated ion pairs, can also exhibit preparatively useful configurational stability in respect to the double bond of the precursor. 

Like the kinetic evidence, the stereochemical evidence for the SnI mechanism is less clear-cut than it is for the Sn2 mechanism. If there is a free carbocation, it is planar (p. 224), and the nucleophile should attack with equal facility from either side of the plane, resulting in complete racemization. Although many first-order substitutions do give complete racemization, many others do not. Typically there is 5-20% inversion, though in a few cases, a small amount of retention of configuration has been found. These and other results have led to the conclusion that in many SnI reactions at least some of the products are not formed from free carbocations but rather from ion pairs. According to this concept," SnI reactions proceed in this manner ... 

In a few cases, SnI reactions have been found to proceed with partial retention (20-50%) of configuration. Ion pairs have been invoked to explain some of these. For example, it has been proposed that the phenolysis of optically active a-phenyl-ethyl chloride, in which the ether of net retained configuration is obtained, involves a four-center mechanism ... 

No matter how produced, RN2 are usually too unstable to be isolable, reacting presumably by the SnI or Sn2 mechanism. Actually, the exact mechanisms are in doubt because the rate laws, stereochemistry, and products have proved difficult to interpret. If there are free carbocations, they should give the same ratio of substitution to elimination to rearrangements, and so on, as carbocations generated in other SnI reactions, but they often do not. Hot carbocations (unsolvated and/or chemically activated) that can hold their configuration have been postulated, as have ion pairs, in which OH (or OAc , etc., depending on how the diazonium ion is generated) is the coun-... 

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