The infrared spectrum of cyclopropane: Vibration-rotation bands of cyclopropane-D 6

Abstract

All six infrared active fundamental bands of cyclopropane-D 6 were studied at high resolution (∼0.2 cm −1) and vibrational and rotational constants derived. Of the parallel fundamentals, ν 6 was reanalyzed with much greater precision than in a previous study, and an analysis of ν 7 was performed. Two of the four perpendicular fundamentals exhibit analyzable rotational structure − ν 9, which has a clearly defined Q branch structure, and ν 10, which has the appearance of a parallel band, and analyzes as such within the limits of the rigid rotor approximation. Accordingly, the value of ζ 10 is estimated to lie within ±0.015 of the value of ( 1 − B′ C′ ) required to give a zero subband spacing in a perpendicular band. Of the remaining two perpendicular fundamentals, ν 8 is not resolved but exhibits a well defined PQR contour; ν 11 closely resembles a parallel band, the J structure of which exhibits definite in- and out-of-phase characteristics, but which does not analyze satisfactorily in terms of parallel band equations. These bands were analyzed by computer simulation. For ν 8, an approximate value for ζ 8 is obtained only, and no information on the rotational constants α 8 C and α 8 B . For ν 11, only one set of the variables ζ 11, α 11 C , α 11 B was found which reproduces the observed contour essentially perfectly. For the perpendicular bands, the theoretical ζ sum rule is well obeyed. Two of the perpendicular bands exhibit ℓ-type resonance perturbations around their band centers. The possible effects of ignoring such perturbations on the precision of the results are discussed.