Huckel rule description

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.)... 

If one organic compound has dominated the historical literature of the last few years, that compound must be benzene. Most probably, this is because its structure in some respects marks a transition from the most austere form of classical organic chemistry, in which carbon was tetravalent and tetrahedral, to a continuing series of changes from oscillating molecules, through partial valencies to MO descriptions, and Huckel s rules of aromaticity. It is the case par excellence of a single substance whose history intersects all major streams of chemical theory - except perhaps the periodic law - and which also has enormous industrial and economic importance. 

The extended Huckel calculations (45) start from an unbridged geometry where the OC—Mo—CO and Mo—Mo—C bond angles are both 90° while the Mo—Mo—Cp bond angle is set to 125.3°. This leads to a bond description including characteristic five below two frontier orbitals shown in Fig. 3. The five highest occupied molecular orbitals are 7r, 8, ir, 8, and a and allow a correlation with the triple bond assigned on the basis of the 18-electron rule. 

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