Coherent Raman spectra of the ν1 mode of 10BF3 and 11BF3

High resolution (0.001 cm−1) coherent anti-Stokes Raman spectroscopy (CARS) was used to directly examine the ν1 symmetric stretching mode of the planar symmetric D3h molecules 10BF3 and 11BF3. Simulations of the spectra were done using ν1 rovibrational parameters deduced from published infrared hot-band and difference-band studies and the close similarity to the observed CARS spectra confirms the validity of the infrared constants. No significant perturbations by Fermi resonance or Coriolis interactions with nearby states are observed, in marked contrast to the case of sulfur trioxide, a similar D3h molecule recently studied. In the harmonic approximation, the 10BF3 and 11BF3ν1 Q-branches would be identical since the isotopic substitution is at the center of mass but, interestingly, the ν1 stretching frequency for 11BF3 is found to be 0.198 cm−1higher than for the lighter 10BF3 isotopomer. This counterintuitive result is reproduced almost exactly (0.200 cm−1) by ab initio calculations (B3LYP/cc-pVTZ) that included evaluation of cubic and quartic force constants and xij anharmonicity constants. The ab initio computations also predict to within 1% the ΔB, ΔC changes in the rotational constants in going from the ground state to the v1 = 1 vibrational level. The results illustrate nicely the complementary interplay of modern infrared, Raman, and ab initio methods in obtaining and analyzing rovibrational spectra.