Cations intermolecular hydrogen bonds

The solubilization of polysaccharides such as chitin and cellulose apparently results from the disruption of strong intermolecular hydrogen bonding by the lithium ions in the N,N-dimethylacetamide. Interestingly under identical conditions, cations such as Na+, K" ", Cs+, Ca+, Ba" "" " showed no tendency to solvate the above polymers. Additionally, some specificity was shown for the anion type, i.e., Br-, Cl-, and NO3-. These trends are under further investigation. 

Structural Data for Intermolecular Hydrogen-Bonded Cation Pairs... 

It may thus be concluded that variations in Kd can to a large extent be rationalized in terms of charge effects, but that intramolecular hydrogen bond interactions (a type) may contribute significantly to stabilization of the dinuclear species. Intermolecular hydrogen bond interactions may stabilize the mononuclear species, but in the case of +2 charged cations such interactions are measurable only in very concentrated solutions. The possible kinetic consequences of these inter- and intramolecular hydrogen bond interactions are discussed in Sections VII and VIII. 

Only a few thermodynamic studies have been made of intermolecu-lar condensation reactions between mononuclear and relatively simple oligomeric species, and kinetic studies of such reactions have first been reported only quite recently. Charge effects seem to play a dominant role, although another important factor may be the formation of intermolecular hydrogen-bonded cation pairs. 

However, preorganization by reversible interactions via hydrogen bonds does not necessarily need a further template such as an anion. Direct intermolecular hydrogen bonds between derivatives of isophthalic acid and 2,6-pyridine diacid were used to synthesize a series of new catenanes [17,18], rotaxanes [18c,19], and trefoil knots [20]. The interaction of dibenzylammonium cations and dibenzo[24]crown[8] [21] led to the preparation of rotaxanes by the attachment of stoppers to the thread ends (Scheme 5.5) [22], or similarly by clipping of the crown ether chain wrapped around the ammonium dumbbell [23]. Recently, several novel catenanes were synthesized via the macrocydic connection of three ammonium cations threaded through three crown ether rings attached to a triptycene core [24]. 

Ando, S., Ando, I., Shoji, A., andOzak, T., Intermolecular hydrogen-bonding effect on C NMR chemical shifts of glycine residue carbonyl cations of peptides in the solid state, J. Am. Chem. Soc. 110, 3380-3386(1988). 

In addition to dipole-dipole interactions, polar protic solvents are capable of intermolecular hydrogen bonding, because they contain an O - H or N - H bond. The most common polar protic solvents are water and alcohols (ROH), as seen in the examples in Figure 7.6. Polar protic solvents solvate both cations and anions well. 

D. Intermolecular Hydrogen Bonds of Aniline Cation Infrared Depletion... 

Intermolecular NMR-spectroscopic Nuclear Overhauser Enhancements of [C4mim] [BFJ-water mixtures led to the suggestion that, in addition to anion-water interactions, solvation (hydration) of the cation also occurs [96], Water replaces the cation-anion hydrogen bonds and also reduces cation-cation associations. Conformational changes in the structure of the butyl chain with respect to the plane of the imidazolium ring were suggested, which were evidenced in various studies in 2003 [114-117],... 

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