INFRARED BANDS OF ISOTOPIC BENZENES - NU-13 AND NU-14 OF (C6D6)-C-13

The two low-frequency perpendicular infrared active fundamentals, ν14 and ν13, of the heaviest stable isotopic benzene molecule, 13C6D6, 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 KΔK ranging from -60 to +60 have been assigned in the ν14 band. The rRK branches for K = 36 to 42 are strongly affected by a second-order rotational resonance with the B2u state ν10, which is inactive in both the infrared and the Raman spectrum. In the ν13 band, subbands with KΔK = -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, B0 = 0.1489154 cm-1, DJ0 = 2.190 × 10-8cm-1, and DJK0 = -3.651 × 10-8cm-1, were determined from 1478 combination differences obtained from the two bands. This value for B0 was combined with the previously obtained ground state B0-values for 13C6H6 and 12C6D6, with the newly revised highly accurate B0 determined by Junttila et al. for the main isotopomer 12C6H6, and with the B0 recently obtained by Ruoff et al. for 12C6H3D3(1,3,5), in a determination of improved structural parameters for the benzene molecule, rCC = 1.3902 ± 0.0002 A ̊, and rCH = 1.0862 ± 0.0015 A ̊, using approximate corrections for zero-point vibrations. Accurate spectroscopic constants are reported for the upper states ν14 and ν13 of 13C6D6, and tentative values are also obtained for the constants of the perturbing B2u state ν10. © 1991.