STRUCTURE AND CONFORMATION OF CYCLOPENTENE, CYCLOHEPTENE AND TRANS-CYCLOOCTENE

The molecular geometries of different conformations of cycloalkenes, C nH2n-2, with n = 5, 7 and 8 were optimized by restricted Hartree-Fock calculations using the 6-31G basis set followed by second-order Molle r-Plesset perturbation theory (MP2) treatment of electron correlation. For cyclopentene, C5H8, the potential function for the ring-puckering motion was constructed, followed by solving for the vibrational eigen values in terms of distributed Gaussian bases. Good agreement was obta ined with the observed frequencies in the far-infrared spectrum of the molecule. For cycloheptene, C7H12, geometries were optimized for the chair, boat, twist-boat, and three possible transition states. The cha ir-chair interconversion mechanism was investigated and compared with available experimental evidence and with the results of a previous mol ecular mechanics calculation. The computed potential barrier compares well with NMR evidence, but the conformation of the relevant transitio n state is found to be different from the one assumed in the experimen tal study. The structure of the smallest isolable trans-cycloalkene, t rans-cyclooctene, C8H14, was optimized, yielding a structure in reason able agreement with a previous gas phase electron diffraction study. T he agreement includes the pyramidality of the olefinic carbon atoms wh ich was also compared with available X-ray data on related compounds. The bond angles and torsion angles were in better agreement with the e xperiment than were those obtained in earlier molecular mechanics stud ies, although it is remarkable how well that method works for these hi ghly strained cyclic systems. (C) 1998 Elsevier Science B.V.