Dipolar attractions

The preference for the endo TS is considered to be the result of interaction between the dienophile substituent and the tt electrons of the diene. These are called secondary orbital interactions. Dipolar attractions and van der Waals attractions may also be involved.12 Some exo-endo ratios for thermal D-A reactions of cyclopentadiene are... 

Stein and Rundle167 have presented a theory for the dipolar attraction between amylose and iodine in the solid state on the basis of the above... 

The proposed exciplex orientation, suggested to dictate the regioselectivity of the photoaddition, is consistent with the orientation of the dipolar attraction between the electronically n,7r excited triplet enone and the ground state alkene. Generally, electron acceptor substituents on the alkene provide preferential formation of the head-to-head (H,H) products, whereas electron donor substituents provide preferential formation of the head-to-tail (H,T) photoproducts55 (Scheme 16). 

Also called dipolar attraction, this is the attraction between the opposite (partial) charges of polar molecules. Obviously, dipolar attractions occur only between polar molecules. A dipole-dipole attraction is shown below for a pair of hydrogen chloride molecules. The dipole-dipole attractive force is indicated with a blue dashed line. 

Dipolar attractions result from the attraction between permanent molecular dipoles. Hydrogen bonding is a special type of dipolar interaction, but only occurs when a hydrogen atom is directly bonded to O, F, or N. London forces are temporary dipolar attractions that result from nonsymmetrical distribution of electrons in an atom or molecule. 

Figure 2.21. Interactions between analytes and polar sorbents via dipolar attraction or hydrogen bonding.

Snyder s next step was to define three selectivity parameters representing the three forces. To measure proton donation he chose dioxane as the probe for proton acceptance, ethanol and for dipolar attraction, nitromethane. Specifically, the selectivity parameter for measuring proton donation, xd, is... 

The resonance picture of an amide shows its strongly polar nature. Hydrogen bonds and dipolar attractions stabilize the liquid phase, resulting in higher boiling points. 

The three intermolecular forces are dipolar attractions, van der Waals forces (also called London forces), and hydrogen bonding. The effects of dipoles (Section 13.5) are considered first, followed by discussions of van der Waals forces and hydrogen bonding. 

Intermolecular forces van der Waals forces Dipolar attractions Hydrogen bonding... 

A molecule with more than one polar bond in which all the effects of the polar bonds do not cancel out has a dipole moment—an electrical dissymmetry that causes an inter-molecular attraction between this molecule and other similar ones. This attraction is called a dipolar attraction, and it lowers the ability of the substance to exist in the gas phase. However, if the polar bonds in a molecule are oriented so that their effects are canceled out, as in carbon dioxide, then a molecule with no dipole results (Section 13.5). 

Intermolecular attractions include dipolar attractions, as well as van der Waals forces and hydrogen bonding. van der Waals forces are similar to dipolar attractions but result from instantaneous dissymmetry of charge, which may disappear the next instant. The more electrons in the molecule, the greater is the van der Waals force. However, van der Waals forces tend to be lower in magnitude than dipolar attractions. 

Pig. 31. The fraction of intact parents that recoil from the surface as a function of the impact energy. MD simulations for a cluster of 8 NH3 molecules interacting only by a central van der Waals potential and by a potential containing in addition a dipolar attraction. The extra attraction shifts the shattering transition to a higher energy. 

A particularly strong type of dipolar attraction results when the positive center is associated with the hydrogen atom attached to an electronegative atom, most commonly nitrogen or oxygen, as in the following examples ... 

There is a somewhat similar phenomenon in which the presence of a dipole within a molecule induces a temporary dipole, either elsewhere in the molecule or in another molecule. The induced dipole is then attracted to the inducing charge or dipole, and another small attractive force comes into play that is not included in the molecular orbital picture at the most simple level of calculation, but is included when larger basis sets are used. Weak dipolar attractions like these, both the static and the induced, are not strong, and so nonpolar molecules are not well solvated by polar molecules the polar solvent molecules would rather solvate each other and the nonpolar molecules are left to their own devices. As it happens they do not repel each other as much as one might expect. 

II. Thus only the slightly different dipolar attractions of I and II to the silica gel of the dry column result in the separation of the two components. 

Describe the dipolar attractions known collectively as van der Waals forces. 

The nature of the extracting phase can be varied to allow extraction of different classes of compounds. Figure 18.3 illustrates bonded phases based on van der Waals forces, hydrogen bonding (dipolar attraction), and electrostatic attraction. 

C, however, Ca + does not have a dehydrating effect on Li+ since these ions possess similar enthalpies of hydration in saturated aqueous solutions. The nuclear magnetic relaxation rates and shifts of Li+ and Cs+ in aqueous solutions containing Fe + and various counter-anions are interpreted in terms of a dipolar attraction between Li+ and the unpaired electrons on the Fe + ion, and the formation of an ion pair between Cs+ and ferric halide complex. An increase of pressure in the range 0—1000 bar results in an enhancement in the hydration of the ions Na+ and K+ in their aqueous chloride solutions. The enhancement is more pronounced at 20 than at 45 °C. These conclusions... 

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