Structure and conformation of cyclopentene, cycloheptene and trans-cyclooctene

The molecular geometries of different conformations of cycloalkenes, CnH2n-2, with n = 5, 7 and 8 were optimized by restricted Hartree-Fock calculations using the 6-31G* basis set followed by second-order Moller-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 eigenvalues in terms of distributed Gaussian bases. Good agreement was obtained 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 chair-chair interconversion mechanism was investigated and compared with available experimental evidence and with the results of a previous molecular mechanics calculation. The computed potential barrier compares well with NMR evidence, but the conformation of the relevant transition state is found to be different from the one assumed in the experimental study. The structure of the smallest isolable trans-cycloalkene, trans-cyclooctene, C8H14, was optimized, yielding a structure in reasonable agreement with a previous gas phase electron diffraction study. The agreement includes the pyramidality of the olefinic carbon atoms which was also compared with available X-ray data on related compounds. The bond angles and torsion angles were in better agreement with the experiment than were those obtained in earlier molecular mechanics studies, although it is remarkable how well that method works for these highly strained cyclic systems. (C) 1998 Elsevier Science B.V.