The high-resolution infrared spectrum of 11BF3 from 400 to 1650 cm−1

High-resolution infrared spectra of boron trifluoride, enriched to 99.5 at. % 11B, have been measured from 400 to 1650 cm−1. In that region we have identified and analyzed 16 absorption bands attributed to the three fundamental bands, two combination bands, 10 hot bands, and one difference band. All possible states were accessed in this region through direct transitions either from the ground state or as hot bands from thermally populated levels. The spectral resolution of the measurements varied from 0.0015 to 0.0020 cm−1. An improved set of ground state rotational constants and rovibrational constants for the infrared-active fundamental vibrations have been determined from over 32 000 assigned transitions. This study resulted in the first direct characterization of the infrared-inactive ν1 state of 11BF3 leading to values for ν1, α1B, and α1C of 885.843205(24), 0.000678548(53), and 0.000337564(66) cm−1, respectively. The Fermi resonance perturbation between the E′ states ν3 and 3ν4 (l = ±1) was further elucidated by observation of hot band transitions to both the 3ν4 (l = ±1) and 3ν4 (l = ±3) states. Several other resonances were also found including the weak rotational interaction, between the A1′ state 2ν2 and the E′ state of ν1 + ν4.