Ion-dipole association

Using different ligands for L permitted the charge of the complex to be varied from +2 to —2. The constant kw is the second-order rate constant not corrected for ion-dipole association. However, a direct comparison can be made of the —2 and +2 rates as well as the — 1 and +1 because NH3 is neutral and the outer sphere attraction should be approximately the same for the same absolute charge on the complex. 

Figure 2. Ion-dipole association between hydroxyl group and ionic oxygen of phosphate group of sphingomyelin...

The double bond adjacent to the polar group strongly influences surface potentials of plasmalogen and sphingomyelin. In sphingomyelin, an ion-dipole association between the hydroxyl and ionic phosphate groups of the molecule results in a net positive surface charge. 

Na, Zn, or Al. Multivalent cations may form a bridging linkage between the ionizable groups of the two immiscible polymers resulting in interchain copolymer formation by ion-ion association. Monovalent cations such as Na" or may also be used to promote associaticMi through ion-dipole association. With either type of cation, a morphology is formed in which there are concentrated domains of associated ionic species (ion clusters) in a matrix of the immiscible homopolymers. 

In another study of the same type of latex, Ottewill and Vincent have shown that even with a non-ionic adsorbate there may be interactions with the surface ionic groups. They studied ethanol, n-propanol and n-butanol and concluded that the initial adsorption, at low concentrations of alkanol, probably involved ion-dipole association of hydroxyl with surface carboxylate anions. This would result in the hydrophobic tails of the alkanols being oriented towards the aqueous phase and, more importantly, a desorption of counterions from the double layer, thus affecting the surface electrical potential, IPq [33]. 

In contrast, reaction of ligand 72 with 4,4 -biphenyldiboronic acid has been successful and diboronate 73 is obtained in yields of 33%. This complex acts as a receptor for the paraquat dication forming a 1 1 complex with an association constant of 320 in acetone. The intermolecular forces responsible for the complexation are ion-dipole stabilization between the dative N B dipoles and the two cationic centers in paraquat, attractive tz-tz interactions between... 

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... 

Interest within the physical organic community on the mechanism for the formation and reaction of ion-pair and ion-dipole intermediates of solvolysis peaked sometime in the 1970s and has declined in recent years. The concepts developed during the heyday of this work have stood the test of time, but these reactions have not been fuUy characterized, even for relatively simple systems. Richard and coworkers have prepared a short chapter that summarizes their recent determinations of absolute rate constants for the reactions of these weak association complexes in water. This work provides a quantitative basis for the formerly largely qualitative discussions of competing carbocation-nucleophile addition and rearrangement reactions of ion and dipole pairs. 

Racemization of chiral a-methyl benzyl cation/methanol adducts. The rate of exchange between water and the chiral labeled alcohols as a function of racemization has been extensively used as a criterion for discriminating the Sn2 from the SnI solvolytic mechanisms in solution. The expected ratio of exchange vs. racemization rate is 0.5 for the Sn2 mechanism and 1.0 for a pure SnI process. With chiral 0-enriched 1-phenylethanol in aqueous acids, this ratio is found to be equal to 0.84 0.05. This value has been interpreted in terms of the kinetic pattern of Scheme 22 involving the reversible dissociation of the oxonium ion (5 )-40 (XOH = H2 0) to the chiral intimate ion-dipole pair (5 )-41 k-i > In (5 )-41, the leaving H2 0 molecule does not equilibrate immediately with the solvent (i.e., H2 0), but remains closely associated with the ion. This means that A inv is of the same order of magnitude of In contrast, the rate constant ratio of... 

Some obscure facets of this intricate picture have been unveiled by Filippi and Speranza who investigated the stereochemistry and the intimate mechanism of a model solvolytic reaction taking place in an ion-dipole pair in the gaseous phase. ° Adducts (7 )-56 and R)-51 are obtained in the gas phase by association of the relevant chiral alcohols, i.e., (/ )-(- -)-l-phenyl-ethanol ((I )-44) and (/f)-(- -)-l-(pentafiuorophenyl)ethanol K)-55), with the CHs OHJ ion, generated by y-radiolysis of CH3F/H2 0 mixtures (Scheme 24). As mentioned above, the absence of neutral nucleophile molecules, i.e., CH3 OH, in the reaction medium ensures that the 0-labeled ethers 45 and 58 of Scheme 24 arise exclusively from the intracomplex solvolysis of R)-56 and R)-51, respectively. 

Dipole-dipole interactions tend to be even weaker than ion-dipole interactions and to fell off more rapidly with distance (l/r3). Like ion-dipole forces, they are directional in the sense that there are certain preferred orientations and they are responsible for the association and structure of polar liquids. 

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