A molecular orbital study of ethylene and the all-trans conjugated polyenes: C4H6, C6H8, C8H10 and C10H12
Bock, CW; George, P; Trachtman, M
We have carried out calculations on ethylene, trans 1,3-butadiene and on all-trans 1,3,5-hexatriene, 1,3,5,7-octatetraene and 1,3,5,7,9-decapentaene at the 6-31G level, on 1,3-butadiene and 1,3,5-hexatriene at the 6-31G* level, and on 1,3-butadiene at the 6-311G** level, with full geometry optimization. As the chain length is increased, the terminal CC bond shows an increase in length, the adjacent CC bond a decrease, the next CC bond along the chain an increase, and the next CC bond a decrease, the change in each case tending towards a limiting value. The terminal CC bond is shorter than the others, and the CC bond next to the end is longer than the others. The HC bonds in the middle of the longer chains also show a progressive increase towards a limiting value. The terminal C-atoms serve as both σ- and π-charge acceptors, whereas the other C-atoms are σ-charge acceptors but π-charge donors. The σ-charge transfer tends to level off at a value of about −0.182 in the middle of the chain, while the π-charge transfer drops almost to zero. The total overlap populations between the C-atoms of the CC and CC bonds are larger the shorter the bond length, and vice-versa, following straight line relationships. The breakdown of the total overlap population into contributions from atom centers, proximal and distal bonded atoms, and antibonded atoms, finds significant antibonded atom contributions amounting to 10–14% of that from proximal bonded atoms. Isodesmic bond separation energies and homodesmotic group separation energies are evaluated to assess the conjugation energy (stabilization energy) in the polyenes, and comparisons are made with the corresponding energies for benzene.