Principles of orbital interactions

When we use molecular orbital (MO) theory, the term orbital structure of a complex, or of any molecule, means the shape and the energetic order of the MO. Usually, these orbitals are expressed as Linear Combinations of Atomic Orbitals (LCAO) of the different atoms that make up the system being studied. The shape of an MO is determined by the relative magjiitudes and the signs of the different coefficients. The electronic structure is then obtained by placing electrons in these orbitals, filling first those which are lowest in energy. 

To construct the MO, it is often advantageous to decompose the molecular system being studied into two simpler sub-systems whose orbitals, either atomic or molecular, are already known. The MO of the complete system are then obtained by allowing the orbitals of the two fragments to interact. In this paragraph, we shall remind the reader of the principal rules which control the interaction between two orbitals on two fragments. For simplicity, we shall treat atomic orbitals, but this limitation will not affect the general nature of our conclusions in any way. 

Interaction between two orbitals with the same energy 

We shall often be interested later in this book by interactions involving either two or four electrons. In the first case (1-20), after the interaction the two electrons both occupy the bonding MO, producing a stabilization of the electronic energy equal to We deduce that 

In the case of a four-electron interaction (1-21), both the bonding and antibonding orbitals are doubly occupied. Since AE is larger than Air , the four-electron interaction is destabilizing, and it can be shown that the destabilization is proportional to the square of the overlap,.  

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Use principles of orbital interaction theory to explain the lower Lewis acidity of the allyl cation, [CH2CHCH2]+, compared to the methyl cation, [CH3]+. 

V. G, J. Am. Chem. Soc., 1995, 117, 12863-12864). Dynamic 13C NMR evidence indicates that the complex is fluxional. A structure was postulated, based on semi-empirical calculations, in which the NO+ is attached to a single n bond. Use principles of orbital interaction theory to discuss the structure and bonding of such a complex (see also Figure 4.8 and question 4, Chapter 11). 

Unlike carbonyl compounds, /nbcarbonyl compounds such as thioketones, RiR2C=S, are very reactive and often impossible to isolate in a pure state. Apply principles of orbital interaction theory to explain ... 

It was established that the oxygen transfer takes place via an Sn2-like transition state (a), rather than a pathway which resembles a 1,3-dipolar addition (b). Comment on the relative merits of the two transition states using principles of orbital interaction theory. A spiro structure similar to (a) for the transition state has been located by ab initio calculations (Bach, R. D. Owensby, A. L. Gonzalez, C Schlegel, H. B. McDouall, J. J. W., J. Am. Chem. Soc., 1991, 113, 2338). 

The operation of the anomeric effect and the stabilization of carbocations are beautifully illustrated in a conformational study of 2-oxanol (2-oxacyclohexanol) (Smith, B. J., /. Am. Chem. Soc., 1997, 119, 2699-2706). 2-Oxanol prefers the OH axial form by 12 kJ/mol and, upon protonation of the OH group, spontaneously loses water to form the oxonium ion. Use principles of orbital interaction theory to explain ... 

Sample problems and quizzes, grouped approximately by chapter, are presented in Appendix B. Many are based on examples from the recent literature and references are provided. Detailed answers are worked out for many of the problems. These serve as further examples to the reader of the application of the principles of orbital interaction theory. 

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