Ion-dipole potential

The possibility of a barrier which inhibits a reaction in spite of the attractive ion-dipole potential suggests that one should make even crude attempts to guess the properties of the potential hypersurface for ion reactions. Even a simple model for the long range behavior of the potential between neutrals (the harpoon model ) appears promising as a means to understand alkali beam reactions (11). The possibility of resonance interaction either to aid or hinder reactions of ions with neutrals has been suggested (8). The effect of possible resonance interaction on cross-sections of ion-molecule reactions has been calculated (25). The resonance interaction would be relatively unimportant for Reaction 2 because the ionization potential for O (13.61 e.v.) is so different from that for N2 (15.56 e.v.). A case in which this resonance interaction should be strong and attractive is Reaction 3 ... 

A further advance occurred when Chesnavich et al. (1980) applied variational transition state theory (Chesnavich and Bowers 1982 Garrett and Truhlar 1979a,b,c,d Horiuti 1938 Keck 1967 Wigner 1937) to calculate the thermal rate coefficient for capture in a noncentral field. Under the assumptions that a classical mechanical treatment is valid and that the reactants are in equilibrium, this treatment provides an upper bound to the true rate coefficient. The upper bound was then compared to calculations by the classical trajectory method (Bunker 1971 Porter and Raff 1976 Raff and Thompson 1985 Truhlar and Muckerman 1979) of the true thermal rate coefficient for capture on the ion-dipole potential energy surface and to experimental data (Bohme 1979) on thermal ion-polar molecule rate coefficients. The results showed that the variational bound, the trajectory results, and the experimental upper bound were all in excellent agreement. Some time later, Su and Chesnavich (Su 1985 Su and Chesnavich 1982) parameterized the collision rate coefficient by using trajectory calculations. 

Now the transition state, termed loose , is ill-defined and product-like. For ion-molecule reactions, reverse activation barriers are often absent because of the strong long-range ion-induced dipole or ion-dipole potential [1]. Further, for many systems of interest, the cleavage of the AB+ bond is heterolytic (i.e., the pair of electrons forming the bond is removed by one of the fragments). Quan-... 

In this way it is possible to deal with short range functions and the long range behavior of the ion-dipole potential can be treated explicitly. 

A quantitative evaluation of the effect of ion-dipole interactions on the rate constant in an infinitely dilute solution of ions can be obtained by inserting the ion-dipole potential energy of interaction from Eqs. (7-7) and (7-8) into Eq. (7-2) or by calculating the free energy AG" necessary to bring the ion and dipole together and then using transition-state theory. Since... 

The extensive use by Dugan and Magee of trajectory calculations to compute close-collision cross sections for the collision of ions with polar molecules has been reviewed in Section 4.2.2d. For such calculations, a form for only the attractive part of the potential need be assumed and, in this case, a particular value of the ion-molecule separation was used to define a close collision. The form chosen for the potential was the simple, anisotropic, electrostatic ion-dipole potential plus the ion-induced-dipole potential and, for reasons discussed in that section, such a model may only be applied to ion-molecule collisions at thermal energies. 

Su T, Viggiano A A and Paulson J F 1992 The effect of the dipole-induced dipole potential on ion-polar molecule collision rate constants J. Chem. Phys. 96 5550-1... 

Ion-Dipole Forces. Ion-dipole forces bring about solubihty resulting from the interaction of the dye ion with polar water molecules. The ions, in both dye and fiber, are therefore surrounded by bound water molecules that behave differently from the rest of the water molecules. If when the dye and fiber come together some of these bound water molecules are released, there is an increase in the entropy of the system. This lowers the free energy and chemical potential and thus acts as a driving force to dye absorption. 

Decide if ion-dipole interactions are responsible for the observed substituent effects. Obtain the charge on carbon and nitrogen in each cyano group. What evidence is there for a polar CN bond Should the ion (O )-dipole (CN) interaction be stabilizing or destabilizing Can these interactions explain the trends in electrostatic potential (Hint Focus on changes in O—CN distance and in orientation of the cyano group.)... 

Exercise 3.5. Consider the simple case of a positive ion surrounded by 10 dipoles, using for the dipole-dipole interaction the potential of Exercise 3.3 and for the ion-dipole interaction the potential... 

N2 - NO+ + N, compared with a theory (7) based on classical trajectories subject to an ion-induced dipole potential. The assumptions involved in calculating the measured cross-sections are noted in the text... 

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... 

The surface potential of a solution can be calculated, according to Eq. (10.18), from the dilference between the experimental real energy of solvation of one of the ions and the chemical energy of solvation of the same ion calculated from the theory of ion-dipole interaction. Such calculations lead to a value of -1-0.13 V for the surface potential of water. The positive sign indicates that in the surface layer, the water molecules are oriented with their negative ends away from the bulk. 

Results of parameter optimization and MD simulations of small model compounds have been published, including alcohols [63], alkanes [63], aromatic [64] and heteroaromatic [209] compounds and liquid amides [65], Studies of ions in aqueous solution were also performed [61, 88] and results from an MD simulation on a DPPC lipid monolayer have been reported (Harder, MacKerell, Roux, submitted). Notable from the monolayer study was the reproduction of the dipole potential across the monolayer, a value that cannot be reproduced using non-polarizable models. This exciting, unforeseen observation points to the types of results that may be obtained from polarizable macromolecular force fields that are not accessible to the present additive models. 

Because T -> V energy transfer does not lead to complex formation and complexes are only formed by unoriented collisions, the Cl" + CH3C1 -4 Cl"—CH3C1 association rate constant calculated from the trajectories is less than that given by an ion-molecule capture model. This is shown in Table 8, where the trajectory association rate constant is compared with the predictions of various capture models.9 The microcanonical variational transition state theory (pCVTST) rate constants calculated for PES1, with the transitional modes treated as harmonic oscillators (ho) are nearly the same as the statistical adiabatic channel model (SACM),13 pCVTST,40 and trajectory capture14 rate constants based on the ion-di-pole/ion-induced dipole potential,... 

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