Infrared bands of isotopic benzenes: ν13 and ν14 of 13C 6D 6

Abstract

The two low-frequency perpendicular infrared active fundamentals, ν 14 and ν 13, of the heaviest stable isotopic benzene molecule, 13C 6D 6, with band origins at 808.5478 and 1295.7546 cm −1, respectively, were measured with a high-resolution Fourier transform spectrometer, in an effort to obtain further accurate data needed for a determination of the structural parameters and of a force field for the benzene molecule. Subbands with KΔ K ranging from −60 to +60 have been assigned in the ν 14 band. The r R K branches for K = 36 to 42 are strongly affected by a second-order rotational resonance with the B 2u state ν 10, which is inactive in both the infrared and the Raman spectrum. In the ν 13 band, subbands with KΔ K = −62 to + 59 have been observed. No perturbations by extraneous states have been detected in this band. Rotational and centrifugal constants for the ground vibrational state, B 0 = 0.1489154 cm −1, D J 0 = 2.190 × 10 −8cm −1, and D JK 0 = −3.651 × 10 −8cm −1, were determined from 1478 combination differences obtained from the two bands. This value for B 0 was combined with the previously obtained ground state B 0-values for 13C 6H 6 and 12C 6D 6, with the newly revised highly accurate B 0 determined by Junttila et al. for the main isotopomer 12C 6H 6, and with the B 0 recently obtained by Ruoff et al. for 12C 6H 3D 3(1,3,5), in a determination of improved structural parameters for the benzene molecule, r CC = 1.3902 ± 0.0002 A ̊ , and r CH = 1.0862 ± 0.0015 A ̊ , using approximate corrections for zero-point vibrations. Accurate spectroscopic constants are reported for the upper states ν 14 and ν 13 of 13C 6D 6, and tentative values are also obtained for the constants of the perturbing B 2u state ν 10.