Far-Infrared Spectra of Ring Compounds. II. The Spectrum and Ring-Puckering Potential Function of Cyclopentene

Abstract

The infrared spectrum of cyclopentene has been recorded in the range 18–300 cm−1. Two series of thirteen and nine Q branches, respectively, were observed, both of which arise from the strongly anharmonic energy levels of the B1 ring-puckering vibration. The longer, more intense series is assigned to molecules in the ground vibrational state of all the other vibrations, and the shorter, weaker series to molecules in the first excited state of the A2 ring-twisting vibration. Each set of Q branches can be fitted to transitions between the energy levels of a double-minimum potential function of the form V=a(x4—bx2), where x is the B1 ring-puckering coordinate. The height of the barrier opposing planarity is calculated to be 232±5 cm−1, and the equilibrium angle between the two dihedral planes of the puckered ring to be 23.3°±1°.