Complex, ion-dipole

Schematic representation of some of the lower frequencies in the ion-dipole complex for the Cl + MeCl m and the imaginary frequency of the transition structure, calculated using a 6-31G basis set. 

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

The 3-21+G reaction path to 13 is illustrated in Figure 7 and involves the intermediacy of the ion-dipole complex 12. As shown in Figure 5, the well-depth for 12 is reduced to 14.3 kcal/mol since the chloride ion is kept by the methyl group about 1.5 A farther from the carbonyl carbon than in 9. This interaction energy for Ci" CH3C0Ci compares favorably with the value of 11 kcal/mol estimated by Asubiojo and Brauman from their ICR experiments (4). 

Taking all of these results together, some general patterns emerge. Foremost, the tetrahedral adducts 1 are found to be energy minima when the substituents X and Y are both first-row elements. However, when X and Y are both second-row elements, the tetrahedral species is a transition state and the only minima are ion-dipole complexes, 1. Clearly, two key factors in the formation of the tetrahedral adduct 1 are the difference in gas-phase basicities for the two anions (X" and 1) and the difference... 

When X and Y are identical halogen atoms, the potential surface for this reaction is represented by a well for the frontside ion-dipole complex [X CHaY] , a TS for the [X CH3 Y] stmcture in which the methyl moiety is planar, and a backside ion-dipole complex [XCH3 Y] that leads to products. These... 

If trapping in the ion-dipole complexes is important, the SN2 reaction mechanism may be written as ... 

Another important question deals with the intramolecular and unimolecular dynamics of the X-—RY and XR -Y- complexes. The interaction between the ion and molecule in these complexes is weak, similar to the intermolecular interactions for van der Waals molecules with hydrogen-bonding interactions like the hydrogen fluoride and water dimers.16 There are only small changes in the structure and vibrational frequencies of the RY and RX molecules when they form the ion-dipole complexes. In the complex, the vibrational frequencies of the intramolecular modes of the molecule are much higher than are the vibrational frequencies of the intermolecular modes, which are formed when the ion and molecule associate. This is illustrated in Table 1, where the vibrational frequencies for CH3C1 and the Cr-CHjCl complex are compared. Because of the disparity between the frequencies for the intermolecular and intramolecular modes, intramolecular vibrational energy redistribution (IVR) between these two types of modes may be slow in the ion-dipole complex.16... 

If crossing the central barrier is not rate-controlling in TST, then trapping in the ion-dipole complex must be incorporated into the statistical model and it is more difficult to represent the effect of central barrier recrossings correcting TST with the K factor is not sufficient. The recrossings and presence of both intermolecular and intramolecular complexes are expected to affect the k, kisom, and k rate constants in equation 6. The value for k should be smaller than that of a capture model, and kisom and k 8 should disagree with the predictions of RRKM theory. 

In LiF (,1 1) ion-dipole complexes, donor-acceptor charge leakage from F to Li+ is apparent in each member of the series, but the incremental CT, expressed as a percentage of a full electronic charge, is progressively weakened with each added F ion,... 

Figure 5.8 Optimized structures of CT-deleted ion-dipole complexes HO HA (A = F, OH, NH2, CH3).

Supercritical water (SCW) presents a unique combination of aqueous and non-aqueous character, thus being able to replace an organic solvent in certain kinds of chemical synthesis. In order to allow for a better understanding of the particular properties of SCW and of its influence on the rate of chemical reactions, molecular dynamics computer simulations were used to determine the free energy of the SN2 substitution reaction of Cl- and CH3C1 in SCW as a function of the reaction coordinate [216]. The free energy surface of this reaction was compared with that for the gas-phase and ambient water (AW) [248], In the gas phase, an ion-dipole complex and a symmetric transition... 

Table 11 The translational, rotational and vibrational contributions to the secondary a-deuterium KIEs for the three halide ion-methyl halide identity SN2 reactions calculated from the separated reactants and from the ion-dipole complexes."...

The anti fluorine in this carbanion is almost a free fluorine ion, i.e. the carbanion is effectively an F CFH=CH2 ion-dipole complex. 

This volume seeks in a small way to bridge the wide gap between organic chemistry in the gas and condensed phases. The same types of chiral ion-dipole complexes that form as intermediates of solvolysis may be generated in the gas phase by allowing neutral molecules to cluster with chiral cations. The reactions of these chiral clusters have been characterized in exquisite detail by mass spectrometry. The results of this work are summarized by Maurizio Speranza in a chapter that is notable for its breadth and thoroughness of coverage. This presentation leaves the distinct impression that further breakthroughs on the problems discussed await us in the near future. 

Ion-dipole complexes in dense gases the radiolytic approach 178... 

Enantioselective self-assembling of amino acids 209 Host-guest inclusion complexes 213 Reactivity of chiral ion-dipole complexes 233... 

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