Orbitals planar relationships

In this chapter, elimination reactions were presented both independently and in association with their related nucleophilic substitution mechanisms. Furthermore, the processes by which molecules undergo both El and E2 eliminations were presented and explained using bonding and nonbonding orbitals and their required relationships to one another. While much emphasis was placed on the planar relationships of orbitals required for both elimination reaction mechanisms, the special case of frans-periplanar geometries were described as necessary for efficient E2 eliminations to occur. 

One of the early reported cases of distinctive mass spectra of stereoisomers was the El-induced behaviour of deacetylcyclindrocarpol 23a and of its epimer at C-19, 23b (Scheme 20). The more pronounced loss of the hydrogen atom from position 19 of the molecular ion of 23b, as compared with that of 23a (10.6% versus 1.7%), was attributed to the antiperi-planar relationship of the 19-C-19-H bond and the p-orbital of the adjacent nitrogen atom in 23b, in contrast to the epimer 23a. 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

First, note that there is a parallel relationship between high-spin tetrahedral and spin-paired planar d, as compared with the octahedral and planar situations just described. Analogous to Fig. 7-4, we have Fig. 7-5. Do not be confused about the reversed labelling of the xy and orbitals at the extremes of Fig. 7-4 and... 

We use different hybridization schemes to describe different arrangements of electron pairs. For example, to explain a trigonal planar electron arrangement, we mix one s-orbital with two p-orbitals and so produce three sp2 hybrid orbitals. They point toward the corners of an equilateral triangle (Fig. 3.19a). A linear arrangement of pairs requires two hybrid orbitals, so we mix an s-orbital with a p-orbital to obtain two sp hybrid orbitals (Fig. 3.19b). Table 3.2 summarizes the relationship between electron arrangement and hybridization type. No matter how many atomic orbitals we mix together, the number of hybrid orbitals is always the same as the number of atomic orbitals we started with, so N atomic orbitals produce N hybrid orbitals. 

Jellinek has pointed out also that the tetragonal structure of coop-erite, PtS, is related to NiAs. In diamagnetic PtS (d8), four d-orbitals are doubly occupied and one is empty. Therefore, two anions from a hypothetical octahedron have been removed, and the cation is in a square-planar coordination the anion is coordinated by four cations that form a deformed tetrahedron. The cooperite structure is shown in Figure 7. The relationship between the NiS and PtS structure is similar to that between... 

It is important to note that the proportional relationship between Amax, Amid, and Amin for these couplings is the same for 100% spin density, and for the present case with approximately 50% spin density. When this is so it indicates that there is no rocking motion at the radical site. This is good evidence therefore that the radical site is essentially planar. The best evidence for radical planarity comes from the analysis of the direction cosines associated with each principal values of the hyperfine coupling tensor. The direction of Amin (Table 18-2) is known to be associated with the direction of the >C-H bond, while the direction associated with the Amid indicates the direction of the n-clcctron orbital. These directions are easily calculated from the crystal structure, and are included in Table 18-2. One sees that the direction associated with Amid deviates only 2.0° from the computed perpendicular to the ring plane, while the direction of Amin, deviates only 2.8° from the computed direction of the C6-H bond. The errors listed on these values are at the 95% confidence level. This is very clear evidence that the radical shown here is planar in the solid-state. Any torsional motion of the C6-H would lead to asymmetries of the hyperfine coupling tensor, and would not produce the observed agreement between the direction cosines and the known directions obtained from the crystal structure. 

Ab initio molecular orbital calculations have played a central role in the analysis and interpretation of X-ray photoemission data obtained on the PMDA-ODA polyimide surface 1 4. The repeat unit of the PMDA-ODA polyimide is shown in Figure 1 and is constructed from planar pyromellitimide (PMDA) and diphenyl ether segments. An understanding of the XPS data and its relationship to the surface chemistry prior to the deposition of any metal is crucial with respect to the interpretation of changes in the XPS data which signify important metal-polymer chemistry that occurs upon formation of the interface. 

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