Cation-7! forces

De Andrade, J., Does, E. S., and Stassen, H., Computational study of room temperature molten salts composed by l-alkyl-3methylimidazolium cations-force field proposal and validation, /. Phys. Ghent., B, 106,13344-13351, 2002. 

Unsaturated amides are easily converted to imidates by 0-alky la ting reagents such as trimethyl-oxonium telrafluoroborate or trimethylsilyl triflate. Nucleophilic attack by the thus formed imino group on an halonium ion followed by hydrolysis gives a lactam. Thus an unsaturated dimethyl imidate was treated with bromine in dichloromethane at 0 °C and cyclized with total regio- and stereoselectivity to give 1, since the tertiary cation forces the cyclization towards the formation of a six-membered ring. The trans addition to the double bond was shown by successive conversion to a m-aziridine 2126. 

The relative sizes of the atoms or ions. The radius ratio (usually r+/r but sometimes r /r+, where r+ is the radius of the cation and r is the radius of the anion) is generally used to measure this. Small cations will fit in the tetrahedral or octahedral holes of a close-packed anion lattice. Somewhat larger cations will fit in the octahedral holes, but not in tetrahedral holes, of the same lattice. Still larger cations force a change in structure. This will be explained more fully in Section 7-1-4. 

Figure 7.14 Double exchange, schematic a cation, left, transfers a spin-up electron to an oxygen p orbital, centre (arrow), which simultaneously transfers a spin-up electron, right (arrow), to an empty orbital on the receiving cation, forcing ferromagnetic alignment of spins. The interaction is illustrated for HS MrP (3d", t ) and HS Mn (3d, ti) cations...

Electrolyte cations—Forced to move away from anodic dissolution sites because of charge repulsion. A small fraction of hops can be allowed in the wrong direction in order to incorporate more random motion into the motion of the cations, and to allow anions the opportunity of escaping out of small pits on the crystal surface. 

From a consideration of molecular geometries, the 7-norbornyl system (119) is regarded as a better model for 3-nortricyclyl (120) than is 2-norbomyl p-nitrobenzo-ate. A comparison firstly of the ratio of the rates of solvolysis of (120) and (119), where major relative rate enhancements occur with increasing electron demand by the aromatic ring, and secondly of the values so obtained with those for (96) and (97), has led to the conclusion that the stabilization of cyclopropylcarbinyl is not the result of a-bridging. Thus, for example, the rigidity of the 3-nortricyclyl cation forces... 

Study of Room Temperature Molten Salts Composed by l-Alkyl-3-methylimidazolium Cations—Force Field Proposal and Validation. 

Very probably, in the first case, chelation of the potassium cation forces the molecule into a bowl-shaped conformation favoring the convex interaction with the double bond. This effect is excluded in the presence of a crown ether. 

Madeluag constant For an ionic crystal composed of cations and anions of respective change z + and z, the la ttice energy Vq may be derived as the balance between the coulombic attractive and repulsive forces. This approach yields the Born-Lande equation,... 

Wliether the potentials are derived from quantum mechanical calculations or classical image forces, it is quite generally found that there is a stronger barrier to the adsorption of cations at the surface than anions, in agreement with that generally. ... 

Rutland M W and Parker J L 1994 Surface forces between silica surfaces in cationic surfactant solutions adsorption and bilayer formation at normal and high pH Langmuir 0 1110-21... 

The ability of living organisms to differentiate between the chemically similar sodium and potassium ions must depend upon some difference between these two ions in aqueous solution. Essentially, this difference is one of size of the hydrated ions, which in turn means a difference in the force of electrostatic (coulombic) attraction between the hydrated cation and a negatively-charged site in the cell membrane thus a site may be able to accept the smaller ion Na (aq) and reject the larger K (aq). This same mechanism of selectivity operates in other ion-selection processes, notably in ion-exchange resins. 

An alternative drivirg force could involve a donor - acceptor interaction. The electron-poor pyridine ring that is coordinated to the copper cation can act as electron acceptor with respect to the aromatic ring of the -amino acid. The fact that donating substituents on the amino acid increase the efficiency... 

Atoms combine with one another to give compounds having properties different from the atoms they contain The attractive force between atoms m a compound is a chemical bond One type of chemical bond called an ionic bond, is the force of attraction between oppositely charged species (ions) (Figure 1 4) Ions that are positively charged are referred to as cations, those that are negatively charged are anions... 

Section 1 2 An ionic bond is the force of electrostatic attraction between two oppo sitely charged ions Atoms at the upper right of the periodic table espe cially fluorine and oxygen tend to gam electrons to form anions Elements toward the left of the periodic table especially metals such as sodium tend to lose electrons to form cations Ionic bonds m which car bon IS the cation or anion are rare... 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Phase transfer catalysis succeeds for two reasons First it provides a mechanism for introducing an anion into the medium that contains the reactive substrate More important the anion is introduced m a weakly solvated highly reactive state You ve already seen phase transfer catalysis m another form m Section 16 4 where the metal complexmg properties of crown ethers were described Crown ethers permit metal salts to dissolve m nonpolar solvents by surrounding the cation with a lipophilic cloak leav mg the anion free to react without the encumbrance of strong solvation forces... 

In addition to hydrogen bonding between the two polynucleotide chains the double helical arrangement is stabilized by having its negatively charged phosphate groups on the outside where they are m contact with water and various cations Na" Mg and ammonium ions for example Attractive van der Waals forces between the... 

Extensive intercalation of polar molecules takes place in this substance in an irreversible manner, and marked hysteresis results (Fig. 4.28). The driving force is thought to be the interaction between the polar molecules and the exchange cations present in the montmorillonitic sheets, since non-polar molecules give rise to a simple Type B hysteresis loop with no low-pressure hysteresis. 

---