Structure, stability and reactivity parameters of (CH)(8) isomers and their cation and anion radical counterparts: A theoretical study

Among the hydrocarbons with (CH)(2k) (k = 1, 2, 3, 4,...) structural formul a, (CH)(8) where k = 4, possesses nineteen local minima at semiempirical le vel with comparable heats of formation. AM1 procedure is found to be better than MNDO and PM3 in evaluating the heats of formation and geometries base d on comparison with available experimental data of the neutral isomers. Ve rtical and adiabatic electron affinities and ionization potentials have bee n calculated for all the available isomers. The relative energy ordering is mainly controlled by the electronic factors rather than the strain. Koopma n's theorem is not expected to yield reliable answers for ionization potent ials, as the orbital relaxation seems to be very high for some isomers. The major geometric distortions are due to either Jahn-Teller distortions or s trong vibronic interactions and the consequent reordering of the skeleton. Relaxation energies from vertical to adiabatic states are roughly proportio nal to the geometric deformations that occur upon ionization or addition of an electron.