SUB-DOPPLER INFRARED-SPECTROSCOPY OF HCCCN-BF3 (V(1)) AND HCN-BF3 (V(1) AND 2V(1))

Using a molecular beam opthothermal spectrometer, we have recorded the sub-Doppler ir rovibrational spectra of the CH stretching vibrations of the C-3 upsilon Symmetric-top van der Waals molecules HCCCN-BF3 (up silon(1) fundamental) and HCN-BF3 (upsilon(1) fundamental and first ov ertone). At first sight, all spectra appear regular, showing no signs of extensive (local) perturbations as might have been expected from th e similarities in structure between HCN-BF3 and the stable molecule HC CCF3, of which the 2 upsilon(1) spectrum in particular presents a clea r example of intermediate case intramolecular vibrational energy relax ation. However, closer inspection reveals that the HCCCN-BF3 spectrum shows evidence of an isotope dependent vibrational predissociation rat e, the B-11 complex dissociating at least twice as fast as its B-10 co unterpart. The complexation induced red shift equals 2.70 cm(-1), and is in the same range as observed for the majority of the HCCCN and HCN complexes studied to date. In contrast, the HCN-BF3 spectra show inst rument limited linewidths, therewith providing only a lower limit to t he lifetime, but the complexation induced frequency shifts are clearly anomalous: the fundamental spectrum is red shifted by only 0.15 cm(-1 ) while the overtone spectrum is blue shifted by 0.66 cm(-1). These ob servations are rationalized by invoking long-range (anharmonic) vibrat ional interactions in both the HCCCN-BF3 and the HCN-BF3 molecule.