Abstract

We have recorded the fundamental spectra of HCN−BF₃ and HCCCN−BF₃ in the region of the CH stretching vibration. For HCN−BF₃ we also recorded the overtone spectrum. The spectra we observe, for both B isotopes, are those of a symmetric top with the symmetry axis coincident with the HCN(HCCCN)−B molecular axis. The HCCCN−BF₃ spectrum is shifted by −2.70 cm¹ with respect to the spectrum of free HCCCN. This red shift is similar in size to shifts measured for other HCCCN complexes. However, the shifts observed for HCN−BF₃ are very different from those of other HCN complexes: the fundamental is red-shifted by only 0.1 cm^-1 and the over tone is shifted by 0.66 cm^-1 to the blue. This may be due to a long-range anharmonic vibrational interaction. We do not observe any localized perturbations, in contrast to the case of trifluoropropyne, in which both the ν₁ and 2ν₁ spectra show extensive couplings to background states. The width of the lines is attributed entirely to unresolved K structure, implying a vibrational predissociation lifetime larger than roughly 20 ns. At the same time this indicates a value of (ΔA−ΔB) for these systems, at least 10 times smaller than for trifluoropropyne.

© 1992 Optical Society of America