The structures of 5,6,9,10-tetradehydrobenzocyclooctene have been optimized at the RHF/6-31G^**, MP2/6-31G^**, B3LYP/6-31G^**LEVELS. aT THE B3LYP/6-31G^** level, the IR spectrum, the Rama spectrum and the NMR spectrum have been calculated. The calculated results are well consistent with the experimental data, theoretically proving the existence of 5,6,9,10-tetradehydrobenzocyclooctene. The isodesmic reaction and natural bond orbital analysis for 5,6,9,10- tetradehydrobenzocyclooctene have been obtained at the B3LYP/6-31G^** leve. The results show that 5,6,9,10-tetradehydrobenzocyclooctene has a relatively rigid planar structure. Because of the weakened conjugated actions between the carbon-carbon double bonds in the benzene ring and its strong strain in molecule, 5,6,9,10- tetradehydrobenzocyclooctene decomposes easily.

Key words: CYCLOOCTATETRAENE P, BENZO-, CYCLOOCTENE P, AB INITIO CALCULATION