The Infrared Spectrum and Internal Rotation Barrier in HF--BF3
K Nauta, R E. Miller, Gerald T. Fraser, Walter J. Lafferty
The rotationally resolved infrared spectrum of the HF stretch fundamental in combination with one quantum of internal rotationexcitation has been observed for the HF--BF3 complex using a color-center-laser optothermal molecular-beam spectrometer.This study builds on a previous microwave investigation of the rotational spectrum of the ground vibration-internal rotor state (m= 0) by Phillips et al. [J. Am. Chem. Soc. 117, 12549 (1995)] which revealed a nearly symmetric top, with the HF fluorine lyingalong the BF3 three-fold symmetry axis (C3). The microwave study also indicated that the HF axis is nearly orthogonalwith the BF3 C3 axis, with the proton undergoing nearly free rotation about this axis. In the present investigation, theinfrared band origin is observed at 3957.54397(21) cm-1 and is red-shifted by only 3.9 cm-1 from the HF monomer origin.A consequence of the nearly free rotation is that the band resembles the perpendicular band of a symmetric top, with both the m = 1 first excited and m = 0 ground internal-rotor states well characterized by symmetric-top Hamiltonians. The magnitude of theCoriolis interaction in the doubly degenerate m = 1 internal rotor state observed in combination with the HF stretch is |2Az| =0.61258(6) cm-1 and corresponds to a barrier to internal rotation of 54 cm-1 in the HF stretch excited state.