Molecular orbital systems

In eq. 23 and 24, Xa is the atomic contribution for the molecule in question (the snm erf the Pascal constants), Xting represents the expected diamagnetic contribution of the metal-ligand w-molecular orbital system (analogous to the ring diamagnetism in benzene) and... 

It seems likely that the cationic CPC micelles, which have a large positive charge at or near the micellar surface, interact attractively with the n-molecular orbital system of benzene, and that this interaction contributes to the fact that the solubilization constant for benzene in CPC is approximately twice as large as that in SDS micelles. A preferential interaction between cationic surfactants and aromatic solutes has been reported by several groups of investigators (25-27), and recent work in our laboratory shows that 1-hexadecyltrimethylammonium bromide micelles also solubilize benzene more effectively than do the anionic alkylsulfate surfactant micelles (28). Thus, the tendency of benzene molecules to solubilize near the surface of the cationic micelles, at low XB values, may lead to a partial saturation of surface "sites" by benzene, diminishing the ability of additional benzene molecules to bind near the surface. Such an effect could be responsible for the initial increase in activity coefficient that occurs, particularly in the CPC solutions, as Xg increases. 

Mesomeric effect The movement of % electrons in the % molecular orbital system. 

This model stresses the view of the hydrogen atom as a quantum particle acting independently of the heavy atoms and playing a role somewhat like the electrons in a molecular orbital. Systems containing formally symmetric hydrogen bonds could, in this model, have similar dissociation thresholds, and overall potential functions, almost independently of their chemical nature. 

They can ben rationalized using conjugated hydrocarbons as models (fig. 11). In this way, vinyl alcohol CH2=CH-OH is seen as allyl anion CH2=CH-CH2 The 7r molecular orbitals system is then filled with not 3 but 4 electrons. In the similar way, for a substituted enol, for instance CFH=CHOH, the model will be a butadiene with 2 extra electrons. Additional electrons in formerly empty molecular orbitals are, in our model, grounds for the structural changes. 

Very similar treatments can be applied to other planar molecules for which resonance is required by the localized electron model. For example, the NO3 ion can be described using the tt molecular orbital system shown In Fig. 9.48. In this molecule each atom is assumed to be sp hybridized, which leaves one p orbital on each atom perpendicular to the plane of the Ion. These p orbitals can combine to form the tt molecular orbital system. 

Independently of crystal tangle due to accidental conditions of growth, many crystals seem to be constituted by joining of two crystals, more or less extended, symmetrical with respect to a plane. In fact, the sample is a sole crystal —giant molecule— which possesses translation symmetries only in each part, not in the whole. Such crystals are called twin-crystals. The symmetry plane is the twin-plane. This plane corresponds necessarily to a reticular plane of each part. Nevertheless, the joining of the lattices of the two parts, which must be obtained without discontinuity in the molecular orbital system, provokes in general distortions in the neighborhood of the twin-plane. 

The champion of the molecular-orbital system was Robert Mulliken, whose father was a professor of chemistry at Massachusetts Institute of Technology (MIT). While still young Mulliken proofread a four-volume chemical treatise written by his father and helped him in the laboratory. He had little thought for any career other than chemistry. 

Molecular Orbitals for Octahedral Metal Carixmyls and Cyanides.— The coordinate stem adopted for tiie case of full metal valence orbitals are d, ( -h l)s, and ( + l)p. The carbon 2s mid 2pr orbitals will be used for ff-bonding for x-bonding, both the ligand x-bonding (x ) and x-antibonding (x ) molecidar orbitals will be combined urith the d, and p, metal orbitals to form the x-molecular orbital system for the complex. The sin e electron molecular orbitals all are assumed to have the form... 

We shall now discuss the <7-molecular-orbital system in some detail. Since the 2j level of Li is more stable than the 2p level, it is a good approximation to consider the o molecular orbital as composed mainly of the hydrogen Ij and the lithium 2s orbitals. 

Figure 9.47 (a) The tt molecular orbital system in benzene is formed by combining the six p orbitals from the six sp hybridized carbon atoms, (b) The electrons in the resulting TT molecular orbitals are delocalized over the entire ring of carbon atoms, giving six equivalent bonds. A composite of these orbitals is represented here. 

The material in this chapter is composed almost entirely of concepts. There are few new reactions or synthetic procedures. We concentrate here on the special stability of some planar, cyclic, and fully conjugated polyenes. The special stability called aromaticity is encountered when the cyclic polyene has a molecular orbital system in which all degenerate bonding molecular orbitals are completely fiUed. 

These especially stable molecular orbital systems are found in planar, cyclic, fully conjugated polyenes that contain 4 -I- 2 7t electrons (Hiickel s rule). 

The hydrogen atom is simple to describe—it merely consists of a proton and a single electron in a It atomic orbital. The pentadienyl molecular orbital system isn t difficult either. There are five n molecular orbitals and these can be constructed by taking combinations of five carbon orbitals. We assume again that it is the highest energy... 

Aromaticity (Section 13.5) The special stability of planar, cyclic, fully conjugated molecules with 4 + 2 it electrons. Such molecules will have molecular orbital systems with all bonding molecular orbitals filled and all antibonding molecular orbitals empty. Usually, there will be filled degenerate orbitals. 

Benzene (Section 13.1) The archetypal aromatic compound a planar, regular hexagon of sp hybridized carbons. The six 2p orbitals overlap to form a six-electron cycle above and below the plane of the ring. The molecular orbital system has three fully occupied bonding molecular orbitals and three unoccupied antibonding orbitals. 

Huckel s rule (Section 13.6) AH planar, cyclic, fully conjugated molecules with 4 + 2 re electrons wUl be aromatic (especially stable).The rule works because such molecules wUl have molecular orbital systems in which all bonding molecular orbitals are fiUl and aU antibonding molectular orbitals are empty. In such molecules, the bonding degenerate molecular orbitals are filled. 

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