Conjugated ions molecular orbitals

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons He pointed out that cycloheptatrienyl cation also called tropyhum ion contained a completely conjugated closed shell six tt electron sys tern analogous to that of benzene... 

Ultraviolet-visible spectroscopy (UV = 200 - 400 nm, visible = 400 - 800 nm) corresponds to electronic excitations between the energy levels that correspond to the molecular orbital of the systems. In particular, transitions involving n orbital and ion pairs (n = non-bonding) are important and so UV/VIS spectroscopy is of most use for identifying conjugated systems which tend to have stronger absorptions... 

In the case of mercuration (soft electrophile), attack at the 2-position is favoured. These observations accord with predictions based on molecular orbital calculations, that hard electrophiles (nitronium ions) should attack at C-4 and soft electrophiles (e.g. HgS04) at C-2 (68JA223). Furthermore, very hard electrophiles (e.g. S03) are predicted to attack at C-3. This is hard to verify because pyridine 1-oxide reacts at C-3 as its conjugate acid (or... 

It is apparent that the molecular orbital theory is a very useful method of classifying the ground and excited states of small molecules. The transition metal complexes occupy a special place here, and the last chapter is devoted entirely to this subject. We believe that modem inorganic chemists should be acquainted with the methods of the theory, and that they will find approximate one-electron calculations as helpful as the organic chemists have found simple Hiickel calculations. For this reason, we have included a calculation of the permanganate ion in Chapter 8. On the other hand, we have not considered conjugated pi systems because they are excellently discussed in a number of books. 

Using a simple resonance approach, we might incorrectly expect both of the cyclopentadienyl ions to be unusually stable. Shown next are resonance structures that spread the negative charge of the anion and the positive charge of the cation over all five carbon atoms of the ring. With conjugated cyclic systems such as these, the resonance approach is a poor predictor of stability. Hiickel s rule, based on molecular orbital theory, is a much better predictor of stability for these aromatic and antiaromatic systems. 

Q Explain how to construct the molecular orbitals of a conjugated cyclic system similar to benzene and cyclobutadiene. Use the polygon rule to draw the energy diagram, and fill in the electrons to show whether a given compound or ion is aromatic or antiaromatic. 

It appears from the description of radical ions in Sects. 1 and 3 that redox reactions can significantly change the chemical and physical properties of conjugated 7r-systems. Whether the extended jc-species are treated within molecular orbital theory or within band-structure theory, the inherent assumption in these concepts is that an electron transfer is reversible and does not promote subsequent chemical reactions. While inspection of cyclic voltammetric waves and the spectroscopic characterization of the redox species provide reliable criteria for the reversibility of an electron transfer and the maintenance of an intact (T-frame, it is generally accepted that electron transfer, depending on the nature of the substrate and on the experimental conditions, can also initiate chemical reactions under formation or cleavage of er-bonds [244, 245],... 

Since the highest occupied molecular orbital of the triaminocyclopropenylium ion is greatly lowered by the conjugatively electron-donating amino groups, the ion undergoes facile one-electron oxidation either electrochemically ° or by the action of sulfuric acid ° or anti-mony(V) chloride. The produced radical dication, which is red-violet in color, is so stable that it can be directly observed by ESR ° and also isolated as the bis(hexachloroantimonate) salts 2 in more than 90% yield. " ... 

While nonbonded electron pairs in molecules do not enter into covalent bonding in the usual sense, they may exhibit a secondary kind of valency by being transferred into vacant molecular orbitals in suitable acceptor molecules. This results in the transformation of a coordination complex in which the bond formed between the electron-pair donor and the acceptor is said to be a coordinate covalent or dative bond. Brpnsted basicity is the simplest example of coordinate covalent bond formation. A Brpnsted base donates a pair of nonbonded electrons to a vacant Is orbital of a hydrogen ion to form the conjugate acid. The o-bond formed between the base and the hydrogen ion results in the loss of identity of the nonbonded pair previously localized on the base. The formation of coordination complexes has significance in the interpretation of spectra of compounds having nonbonded electron pairs. 

Here is a selection of some of the more important conjugated radicals and radical ions, to some of which we shall refer in later chapters. They all show how the patterns of molecular orbitals deduced in this chapter are supported by ESR measurements. The numbers are the coupling constants lflHl in gauss. 

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