Molecular orbitals pentadienyl

Fig. 10.1. TT-Molecular orbitals and energy levels for the pentadienyl cation.

In the 1930 s HiickeP proposed, on the basis of molecular-orbital calculations, a theoretical criterion for aromaticity of cyclic polyenes, known as Hiickers rule, which states that cyclic polyenes should be aromatic if, and only if, they contain 4n- -2 Jt-electrons. At that time only two of such cyclic polyenes were known benzene and cyclo-pentadienyl anion, each having six rc-electrons and satisfying Huckel s rule. Since then, the validity of Hiickel s rule had not been challenged... 

Two groups have studied the bonding in pentadienyl-metal-tricar-bonyl complexes (119, 238) and are agreed that effective overlap between the pentadienyl nonbonding orbital and an orbital of suitable symmetry on the metal (Fig. 17) makes a major contribution to the stability of these complexes. However, the two types of molecular orbital calculation [one an extended Hiickel (119) and the other a parameter-free approximate Hartree-Fock calculation (255)] disagree about the precise ordering of energy levels in this type of complex. 

Figures 1.19 and 1.20 show the -n molecular orbitals for butadiene and pentadienyl. In each case the lowest-energy orbital has no vertical nodes, and each higher-energy orbital has one more vertical node than the orbital below it had, with the highest-energy orbital always having a node between every adjacent pair of atoms. Chains with an odd number of atoms have a nonbonding orbital, in which there is no contribution from alternate/) orbitals.

In the pentadienyl radical, predict the distribution of the unpaired electron (a) from the resonance model, and (b) from the molecular orbital model. 

Fig. 7. Comparison of Hiickel molecular orbitals of cyclopentadienyl and pentadienyl ligands.

Fig. 1. Representations of the -molecular orbitals of the cyclopentadienyl (left) and pentadienyl (right) groups...

Nonetheless, some useful information regarding methylation has been obtained from molecular orbital calculations. One CNDO/2 study on the allyl and pentadienyl anions indicated that in all cases the methyl group would serve to withdraw net electron density from the anions196. For the allyl anion, a 1-methyl substituent would withdraw a substantial 0.102 electrons, while at the formally uncharged 2 position the effect was consider-... 

Molecular orbital (MO) calculations indicate that the intermediate radical anions have more anionic character at the positions ortho or meta to a TT-donating substituent and para or ipso. ipso means at the substituent-containing carbon) to a 7r-accepting substituent. (See Birch, A. J. Hinde, A. L. Radom, L. J. Am. Chem. Soc. 1980, 102, 3370-3376). The pentadienyl anions formed after transfer of the second electron have more anionic character on the central carbon, and that is where the second protonation occurs. 

The interaction of atomic orbitals giving rise to molecular orbitals is the simplest type of conjugation. Thus in ethylene the two p orbitals can be described as being conjugated with each other to make the n bond. The simplest extension to make longer conjugated systems is to add one p orbital at a time to the n bond to make successively the n components of the allyl system with three carbon atoms, of butadiene with four, of the pentadienyl system with five, and so on. Hiickel theory applies, because in each case we separate completely the n system from the a framework, and we can continue to use the electron-in-the-box model. 

Molecular Orbitals and Organic Chemical Reactions Student Edition by Ian Fleming, Wiley, Chichester, 2009 on pentadienyl anions,... 

Qualitative representation of the molecular orbitals for linear 7t systems (a) pentadienyl and (b) hexatriene. (Adapted from reference 8.)... 

Sketch the nodal properties of the highest occupied molecular orbital of pentadienyl cation (CH2=CHCH=CHCH2 ). 

---