Vibrational Spectra and Structure of Small Ring Molecules. XVI. Vibrational Analysis and Ring-Puckering Vibration of 1-Pyrazoline

The infrared spectrum of gaseous 1-pyrazoline has been measured from 33 to 4000 cm−1. The infrared and Raman spectra of the corresponding liquid sample have also been recorded. A vibrational assignment of the fundamentals is proposed which is consistent with the Cs equilibrium configuration. The assignment is based on the gas-phase band contours, the depolarization values, and group frequency correlations. A series of 13 pronounced Q branches was observed in the far-infrared spectral region, and they have been attributed to the strongly anharmonic ring-puckering vibration. Although the far-infrared frequencies could be reproduced by two different potential functions, a double minimum potential of the form V = 23.40(z4 − 4.40z2) where z is the reduced ring-puckering coordinate provides the most satisfactory interpretation of all the spectral data. The height of the inversion barrier is calculated to be 113 cm−1, and the equilibrium angle between the two dihedral planes of the puckered ring is 19.7° ± 1.0°. The results are compared with those obtained on cyclopentene.