ORGANIC QUANTUM CHEMISTRY .21. STRUCTURE AND SPECTRUM OF CYCLOOCTADECANONAENE ([18]ANNULENE)

An SCF modification of a previously described version of the semiempirical ASMO-CI method of Pariser and Parr has been applied to the calculation of the π-electronic transitions of a number of unsaturated hydrocarbons. This method generally predicts the electronic transitions to within 0.3 eV of the observed values. This method has been applied to various conceivable structures for the title compound and suggests that the structure best reconciled with the observed spectrum is one having alternating long and short bonds. Energy calculations, which include the compression energy of the a system and van der Waals' interactions, also indicate that this structure is the favored one. The structure observed in the crystalline state is demonstrated to be both incompatible with the observed spectrum and energetically unfavorable, and it is concluded that it differs markedly from the structure in the gas phase. The potential curves for bond-length changes for the ground and excited states are d scussed. © 1969, American Chemical Society. All rights reserved.