Orbital interaction analysis

The energy of the cr H may be lowered by two distinct mechanisms. A change in the hybridization of the C spn orbital toward smaller n, that is, more s character, is accompanied by a lowering of the energy (or increase of electronegativity) of the hybrid orbital. The orbital interaction analysis, shown in Figure 10.4, predicts two consequences, a lowering of the cr orbital and increased polarization of the cr orbital toward H. Both... 

Orbital Interaction Analysis. An orbital interaction diagram for the Diels-Alder reaction is shown in Figure 12.5a. The geometry of approach of the two reagents which ensures a maximum favorable interaction between the frontier MOs (dashed lines) preserves a plane of symmetry at all separations. The MOs are labeled according to whether they are symmetric (S) or antisymmetric (A) with respect to reflection in the plane. Simultaneous overlap of both HOMO-LUMO pairs is a necessary feature of all peri-... 

Orbital Interaction Analysis. An orbital interaction diagram for the Cope rearrangement is shown in Figure 12.7a. The reaction may be initiated by electron donation from... 

Use orbital interaction analysis to explain stabilization of a carbocationic center by a cyclopropyl group. What kind of substituent (X , C, or Z) is cyclopropyl Explain. Predict the orientation of the planar cationic center relative to the cyclopropyl ring. 

Use orbital interaction analysis to suggest a reason that tropylium cation C7H7 is such a stable cation. This may be done in either of two ways by considering the interaction of a simple carbocation with 1,3,5-hexatriene or the interaction of an allyl cation with butadiene, both held in the 7-membered planar ring geometry of tropylium. In either case, attention must be paid to orbital symmetry. 

Use orbital interaction analysis to derive the bonding molecular orbitals of ethyl -benzenium ion. Consider ethylbenzenium ion to be the result of the interaction of a phenyl group, C6H5, and ethylene, C2H4, with the appropriate number of electrons. (Direct evidence for the existence of ethylbenzenium ion was obtained by Fornarini,... 

Use orbital interaction analysis to derive the bonding molecular orbitals of bisho-mocyclopropenyl cations, such as 4. The X-ray structure of the hexafluoroanti-monate salt of 4 was recently determined by Laube, T., J. Am. Chem. Soc., 1989, 111, 9224. 

As a second example of orbital interaction analysis of the COs, let us now consider the tetracyanoplatinate Pt(CN)4 chains. Although K2[Pt(CN)4] is an insulator, the salt can be cocrystallized with elemental bromine to produce partially oxidized salts, K2 [Pt(CN)4]Brj-3H20. These salts are metallic and have attracted a great deal of attention. The crystal structmes of these partially oxidized salts exhibit linear chains with a staggered arrangement of Pt(CN)4 units as shown in (52). All Pt atoms are equivalent and the Pt-Pt distance becomes shorter when the partial oxidation of Pt increases. 

The examples discussed in this section are qnite simple and have been chosen in order to illustrate some basic aspects of the orbital interaction analysis of the structure of sohds. This type... 

Real solid-state materials are often structurally complex and there can be a large number of bands to be considered. In addition, for 2D or 3D materials it is practically impossible to examine ei (k) for all the regions of the BZ. Fnrthermore, inside the BZ the syimnetry is nsnally qnite low and there can be many avoided crossings between bands. Hence, althongh in principle one can always perform an orbital interaction analysis of several COs, a particnlar orbital of the repeat unit can be spread ont between several bands. Under snch conditions, it can be very difficnlt to single ont an orbital or gronp of orbitals responsible for the structure or properties of the solid. However, since after all, solids are very big molecnles, we shonldbe able to develop qnahtative arguments similar to those snccessfiiUy nsed in molecnlar chemistry to explain the structure, properties, or reactivity of sohds. 

Complexes with one and two amines are common. For example, bis-(trimethylamine) alane, A1H3(N(CH3)3)2, is a white crystalline solid with a low vapor pressure. Using ammonia as a model for trimethylamine, apply orbital interaction analysis to describe the bonding in the 1 1 and 1 2 complexes. Theoretical studies on the ammonia complexes of AIH3 have led to the conclusion that there is little dative bonding (as judged by the amount of charge transfer) (Marsh, C. M, B. Schaefer III, H. F., J. Phys. Chem., 1995, 99, 14309-14315). Comment on the theoretical results. 

---