THE ACCURATE DETERMINATION OF SPECTROSCOPIC CONSTANTS FOR A DARK VIBRATIONAL-STATE - THE CORIOLIS COUPLED NU-4 AND NU-6 BANDS OF D11BF2

The high resolution infrared spectrum of deuterated difluoroborane, DB F2, has been recorded from 700 to 912 cm-1. At low resolution, the onl y observable bands in this region belong to the c-type, out-of-plane d euterium motion, nu4, of the two B isotopomers. Upon analysis of the n u4 fundamental in D (BF2)-B-11, a large perturbation was observed due to a DELTAK = +/- 1 Coriolis interaction with a dark vibrational state . The dark state was presumed to be the unseen, a-type, in-plane D-B r ocking fundamental, nu6. A novel method of upper state energy level in terlacing was used to determine A, B + C and T0 for the dark excited s tate, 6(1), the band origin, rotational and centrifugal distortion con stants for the 4(1) state, and zeta4,6b (almost-equal-to 2zeta4,6b B). Based upon these constants, nu6 transitions were predicted and locate d in the region of the perturbation. Experimentally determined Corioli s, rotational, and centrifugal distortion constants are reported. The experimental results are compared with the results from ab initio calc ulations.