INFRARED BANDS OF ISOTOPIC BENZENES - NU-13 OF C6D6-(C-12) AND NU-12 OF C6H6-(C-13)

In a continuation of the investigation of spectra of isotopically substituted benzene molecules, two perpendicular bands due to fundamentals of the species E1u, viz., ν13 of 12C6D6, centered at 1335.221 cm-1, and the CH stretching band ν12 of 13C6H6, were measured with a high-resolution Fourier transform instrument having a maximum optical path difference of 442 cm. Subbands with KΔK = -60 to +60 were assigned in the ν1312C6D6 band, and a set of spectroscopic constants was obtained from a least-squares fit to >2100 lines of this band for the ν13 state and for the 2ν4 + ν20 (E2u) combination, which was found to affect the K = 6 to 9pP and pQ branches by an x,y-Coriolis perturbation. The CH stretching region 3000-3100 cm-1 exhibits a strong band centered at 3065.204 cm-1, due to the ν12 vibration, and a weak satellite band near 3006.33 cm-1, assigned to the ν13 + ν16 E1u 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 KΔK = -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 ν12 and ν13 + ν16 states. An anharmonic perturbation of the low-K subbands due to an unidentified E1u combination state was also taken into account. The availability of accurate rotational constants B0 for the ground vibrational states of 12C6H6, 13C6H6, and 12C6D6 has made it possible to obtain improved internuclear distances, rm (CC) = 1.3894 A ̊, rm (CH) = 1.0859 A ̊, for the benzene molecule. © 1990.