Opposed dipoles

The reacting aldehyde displaces the oxazolidinone oxygen at the tetravalent boron in the reactive TS. The conformation of the addition TS for boron enolates is believed to have the oxazolidinone ring oriented with opposed dipoles of the ring and the aldehyde carbonyl groups. 

Fig. 2.2 Self-Consistent Reaction Field (SCRF) model for the inclusion of solvent effects in semi-empirical calculations. The solvent is represented as an isotropic, polarizable continuum of macroscopic dielectric e. The solute occupies a spherical cavity of radius ru, and has a dipole moment of p,o. The molecular dipole induces an opposing dipole in the solvent medium, the magnitude of which is dependent on e.

As well as these permanent dipole moments, random motion of electron density in a molecule leads to a tiny, instantaneous dipole, which can also induce an opposing dipole in neighbouring molecules. This leads to weak intermolecu-lar attractions which are known as dispersive forces or London forces, and are present in all molecules, ions and atoms - even those with no permanent dipole moment. Dispersive forces decrease rapidly with distance, and the attractions are in proportion to 1/r6, where r is the distance between attracting species. 

Figure 1.19 Schematic drawing of the cation-rr interaction showing the contact between the two. The quadrupole moment of benzene, along with its representation as two opposing dipoles, is also shown.

The synthesis of the C1-C9 fragment 120 began with an auxiliary controlled aldol reaction of the chloroacetimide 121, where chlorine is present as a removable group to ensure high diastereoselectivity in what would otherwise have been a non-selective addition (Scheme 9-39). The Lewis acid-catalyzed, Mukaiyama aldol reaction of dienyl silyl ether 122 with / -chiral aldehyde 123 proceeded with 94%ds, giving the 3-anti product 124, as predicted by the opposed dipoles model [3]. Anti reduction of the aldol product and further manipulation then provided the C1-C9 fragment 120 of the bryostatins. 

Benzo[c]cinnoline is a weakly basic compound (Section IV,A), crystallizing from water or cyclohexane in yellow, monoclinic needles of m.p. 156" C. It is very soluble in alcohols, ether, etc. An X-ray crystallographic study has shown the molecule to be only approximately planar, forming molecular pairs with opposed dipoles in the solid state dimerization also takes place in solution at low temperature. It is found that bonds 1—2, 3—4, 7—8, and 9—10 are shorter than the others of the benzene rings, indicating a degree of bond fixation with 1 as the preferred Kekule structure. 

CO2 and NjO are isoelectronic, linear triatomics with similar molecular weights, melting temperatures and quadrupole moments. Although NjO has no inversion symmetry, it has been shown to resonate between two bonding configurations with opposing dipole moments N=N =0 and N= N =0 [88]. As a result, the net dipole moment of nitrous oxide is negligible... 

Oxazolidinones have proven to be extremely useful auxiliary groups in a variety of synthetic reaction types. Thus, the Evans auxiliaries are useful in the control of configuration in enolate alkylations and concerted cycloadditions, to name a few of the more important applications. Sibi and his collaborators at North Dakota State University have pioneered the use of these auxiliary groups in radical transformations mediated by Lewis acids [21-24]. Consider the general conformational questions that arise in a carboximide, such as 12, derived from an oxazolidinone (Eq. 18). The conformer 12 is disfavored by steric factors while 13 and 14 have similar steric demands. In the absence of any chelating Lewis acid, one expects that 13 would be of lower energy than 14 because of the opposed dipoles of the anti carbonyls in this conformation. 

This behavior is a result of the interaction of adsorbed layers with opposing dipole polarities. It should not be too surprising in view of the Rasor plot (see Fig. 8), which shows that, the higherthebare work function of a substrate, the lower is the cesiated work function for a given ratio T/Tr. 

The ammonium salt, (NH4)H2P04, although resembling the potassium salt in many of its properties, does not become ferroelectric at low temperatures. Both K and NH4 salts contain a random system of H bonds, but at low temperatures the H atoms order themselves differently. In the ammonium salt, the two H atoms are associated with one upper and one lower corner of each 02P(OH)2 tetrahedron in such a way as to produce opposing dipoles in adjacent tetrahedra, perpendicular to the c axis. There is no resulting moment and the substance is antiferroelectric (Figure 12.31b). 

This area of research has seen an explosion of interest in recent years and has been well reviewed by others [39 1]. This extra element in the assembly of these mesogens is responsible for a number of interesting polar properties in this class of materials [42], As shown in Figure 4(b), the oxo-metal compounds provide a dipole moment to many of these mesogens. This allows them to transfer their oxygen atoms between members of the stack to switch the macroscopic polarity [42], Some have shown ferroelectric behavior [42] that is possibly due to the inability to pack opposing dipoles in a hexagonal lattice [43]. 

Another example of a nonpolar molecule is the CCI4 molecule, which has four polar bonds symmetrically arranged around the eentral C atom. Each of the C—Cl bonds has the same polarity, but because they have a tetrahedral arrangement, their opposing dipoles cancel out. As a result, a molecule of CClj is nonpolar. 

The renormalized vibrations are given in Figure 6.22, along with illustrations of the modes. In contrast to 1,4-difluorobenzene, there is no cancellation of opposing dipoles for any of the modes so, in line with the selection rules, all are IR active. The earlier analysis using the right-hand column of the character table also indicated that aU four will also be Raman active. 

The answer to the first question is easy there is a so-called surface selection rule such that only vibrations with a dipole perpendicular to the metal surface are infrared active. This is because a dipole moment change in an absorbed molecule induces an opposed dipole in the metal. This opposition leads to a zero nett dipole for dipoles parallel to the metal surface but an enhanced dipole perpendicular. (See the diagram). 

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