Spectra and Structure of Small Ring Compounds. XXV. Microwave Spectrum of Methylenecyclopentane

摘要

The microwave spectrum of methylenecyclopentane has been recorded from 18.0 to 26.5 GHz. BothQhyphen;branch andRhyphen;branch assignments have been made for the ground state. Five vibrationally excited states of the ringhyphen;puckering mode and one vibrationally excited state of the ringhyphen;twisting mode have been assigned. From the relative intensity measurements, it is concluded that the first excited state of the ringhyphen;puckering vibration has a frequency of63 plusmn; 10thinsp;cmminus;1and that the vibration is nearly harmonic with a barrier of pseudorotation of 1.8 kcalsol;mole. The two outhyphen;ofhyphen;plane ring vibrations are essentially independent and the methylenecyclopentane ring is not significantly different in rigidity than that of cyclopentanone. The components of the dipole moment were determined by the Stark effect to bemgr;aequals;0.60 plusmn; 0.01thinsp;Dandmgr;cequals;0.00 plusmn; 0.06thinsp;D. All the observed data are consistent with a permanently twisted ring of C2symmetry.