Butadiene molecular orbitals determined

The stereochemistry of the cyclobutene isomerizations and the reverse processes of this type, involving the formation of a bond between the ends of a linear system containing a number of 7i--electrons, has been discussed by Woodward and Hoffmann (1965). They term such processes electrocyclic and consider that their steric course is determined by the symmetry of the highest occupied molecular orbital of the open-chain isomer. In an open-chain system containing 4 7T-electrons (such as butadiene), the symmetry of the highest occupied ground-state orbital is such that bonding interaction between the ends of the chain must involve overlap between orbital envelopes on opposite faces of the system, and this can only occur in a conrotatory process ... 

Ultraviolet (UV) spectroscopy is a method cd structure determination applicable specifically to conjugated systems. When a coi jugated molecule is irradiated with ultraviolet light, ener gy absorption occurs and a it electron is promoted from the highest occupied molecular orbital tHOMOl to the lowest unoccupied molecular orbital (L.UMO1. For I,3< butadiene, radiation of 217 nm ia required. As a general rule, the... 

Using full Huckel theory with a secular determinant, derive the wavefunctions and molecular orbital energies for allyl and bu tadiene given in Figure 14.15. For allyl, you will derive a cubic equation whose roots x are 2,0, and 2. For butadiene, you will ha veto solve a fourth-order polynomial, the roots of which are X = 0.618,-0.618,1.618, and -1.618. 

Using the Internet, find a Huckel determinant solver and determine the relative energies of the tt molecular orbitals for (a) the butadiene molecule, which has four tt electrons, and (b) the cyclobutadiene molecule, which also has four tt electrons. Can you explain why cyclobutadiene is labeled with the word antiaromaticl... 

Use this expression to determine a reasonable empirical estimate of the parameter )3 for the series consisting of ethene, butadiene, hexatriene, and octatetraene given that light-induced absorptions from the HOMO to the LUMO occur at 61 500,46 080,39 750, and 32 900 cm h respectively, (b) Calculate the delocalization energy of octatetraene (see Exercise 10.36). (c) In the context of this Huckel model, the % molecular orbitals are written as linear combinations of the carbon 2p orbitals. The coefficient of the jth atomic orbital in the fcth molecular orbital is given by... 

Determine the symmetry of each molecular orbital of 1,3-butadiene and suggest another guideline that could be added to your list of ways to compare the energies of molecular orbitals. 

Theoretical.—Molecular orbital calculations have been reported for the thiiranium ions derived from ethylene, propylene, isobutene, and butadiene. The most stable system involves a p-configuration of the valence electrons of sulphur. Differences in the extension of the lowest unoccupied MO s on the ethylene carbon atoms were said to be important in determining the regio-specificity of ring-opening addition reactions with chloride ion. Theoretical calculations on C2H4SH indicate that a bridged thiiranium structure would be more stable as an intermediate than an open structure in electrophilic additions of RSX to olefins. The barrier to pyramidal inversion at the sulphur atom was calculated to be 78.11 kcal mol . 

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