Ion-dipole reaction

H20 calculates to a dielectric constant (Dc) of 78.54 at 25°C, the highest of the ordinary solvents. Ethanol has D = 24.3 at 25°C. Given its high polarity, H20 easily engages in dipole-dipole and ion-dipole reactions. 

Racemization of neutral substrates by solvolysis through an ion-pair or ion-dipole reaction intermediate will rarely be observed for reactions in water, because few substrates will meet the following strict requirements for the observation of racemization ... 

The elementary reactions of carbocationic polymerizations can be separated into three types. Deactivation of carbenium ions with anions and transfer to counteranion are ion-ion reactions, propagation and transfer to monomer are ion-dipole reactions, and ionization is a dipole-dipole reaction [274]. Ion-ion and dipole-dipole reactions with polar transition states experience the strongest solvent effects. Carbocationic propagation is an ion-dipole reaction in which a growing carbenium ion adds electro-philically to an alkene it should be weakly accelerated in less polar solvents because the charge is more dispersed in the transition state than in the ground state [276]. However, a model addition reaction of bis(p-methoxyphenyl)carbenium ions to 2-methyl- 1-pentene is two times faster in nitroethane (e = 28) than in methylene chloride (e = 9) at - 30° C [193]. However, this is a minor effect which corresponds to only ddG = 2 kJ morit may also be influenced by specific solvation, polarizability, etc. [276,277]. 

In Eq. (2.1 10), kD= is the rate constant at infinite dielectric constant, ZA, p, and r are ion charge, dipole moment and the shortest ion-dipole distance, respectively, and k is the Boltzmann constant. The term 0 represents the alignment of reactants, and cos 0 is unity in the case of head-on alignment. Thus, as the dielectric constant decreases, the rates of anion-dipole reactions decrease and the rates of cation-dipole reactions increase. As indicated by a linear relationship with a positive slope in log k versus 1/D plots (Fig. 84), the hydrolysis rate constant for chloramphenicol in water-propylene glycol mixtures increases with decreasing dielectric constant, suggesting a hydronium ion-dipole reaction.397... 

Temperature-Dependent Rate Constants For Ion-Dipole Reactions C+(2p) + HCl(X S+)... 

ABSTRACT. Calculation of the rate constant at several temperatures for the reaction +(2p) HCl X are presented. A quantum mechanical dynamical treatment of ion-dipole reactions which combines a rotationally adiabatic capture and centrifugal sudden approximation is used to obtain rotational state-selective cross sections and rate constants. Ah initio SCF (TZ2P) methods are employed to obtain the long- and short-range electronic potential energy surfaces. This study indicates the necessity to incorporate the multi-surface nature of open-shell systems. The spin-orbit interactions are treated within a semiquantitative model. Results fare better than previous calculations which used only classical electrostatic forces, and are in good agreement with CRESU and SIFT measurements at 27, 68, and 300 K. ... 

A large number of approximate theories have been proposed for ion-dipole reactions. Some of these include the average dipole orientation (ADO) approximation and its extension to include conservation of angular momentum (the AADO method ), various transition-state theories involving variational and statistical modifications, the semiclassi-cal perturbed rotational state (PRS) approximation, classical trajectory studies, the adiabatic invariance method, and the statistical adiabatic channel model (SACM). 

In the present study, we employ the ACCSA method which involves a combined rotationally adiabatic capture and centrifugal sudden approximation. The method enables the computation of cross-sections and rate constants which are state-selective in the initial rotational states of the reactant molecule. The ACCSA approach has been successful for ion-dipole reactions which involve proton transfer. Predicted rate constants for the reactions -f HCN H2CN + H2 and HCO + HCN - - CO are in... 

In this study we have investigated the ion-dipole reaction -h HCl X E )... 

TEMPERATURE-DEPENDENT RATE CONSTANTS ION-DIPOLE REACTIONS C+(= P) + HCl X F, )... 

In this section we, first of all, compare various calculations for ion-dipole reaction cross sections and rate coefficients, and then go on to make several comparisons of theory with experimental data. 

Unfortunately, the experimental methods for fast neutral reactions have not yet developed to enable rate coefficients to be obtained at interstellar temperatures (less than 100 K). Thus it has not yet been possible to test theory in the rigorous way that has been done for ion-dipole reactions. For reviews of experiment and theory in this area see references [3] and [8]. 

When the interaction depends on the orientation of the neutral molecule, as is the case, for example, for ion-dipole reactions, the simple treatment outlined above is no longer appropriate. The adiabatic channel method is often used in this context [32]. The rotational energy levels in the separated reactants are coupled with the orbital energy levels to define a set of channels for the collision complex. The number of open states, N(EJ), is the number of channels with an energy maximum below the energy E. Examples of this approach include the adiabatic channel centrifugal sudden approximation (ACCSA) of Clary [33] and the statistical adiabatic channel model (SACM) of Troe and co-workers [34]. 

Swart ER, le Roux LJ. 413. The effect of solvent on a simple ion-dipole reaction. Part... 

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