Infrared bands of isotopic benzenes: ν13 of 12C 6D 6 and ν12 of 13C 6H 6

Abstract

In a continuation of the investigation of spectra of isotopically substituted benzene molecules, two perpendicular bands due to fundamentals of the species E 1 u , viz., ν 13 of 12C 6D 6, centered at 1335.221 cm −1, and the CH stretching band ν 12 of 13C 6H 6, were measured with a high-resolution Fourier transform instrument having a maximum optical path difference of 442 cm. Subbands with KΔ K = −60 to +60 were assigned in the ν 13 12C 6D 6 band, and a set of spectroscopic constants was obtained from a least-squares fit to >2100 lines of this band for the ν 13 state and for the 2 ν 4 + ν 20 ( E 2 u ) combination, which was found to affect the K = 6 to 9 p P and p Q branches by an x,y-Coriolis perturbation. The CH stretching region 3000–3100 cm −1 exhibits a strong band centered at 3065.204 cm −1, due to the ν 12 vibration, and a weak satellite band near 3006.33 cm −1, assigned to the ν 13 + ν 16 E 1 u combination, which is made visible by Fermi resonance with the fundamental. The coupling constant of the Fermi interaction was estimated to be ∼10.0 cm −1 from the ratio of the integrated intensities of the two observed bands. Subbands with KΔ K = −41 to +48 were assigned in the main band whose rotationless origin is shifted up by 1.75 cm −1 due to the resonance. A set of spectroscopic constants is reported for the ν 12 and ν 13 + ν 16 states. An anharmonic perturbation of the low- K subbands due to an unidentified E 1 u combination state was also taken into account. The availability of accurate rotational constants B 0 for the ground vibrational states of 12C 6H 6, 13C 6H 6, and 12C 6D 6 has made it possible to obtain improved internuclear distances, r m (CC) = 1.3894 A ̊ , r m (CH) = 1.0859 A ̊ , for the benzene molecule.