The analysis of combination and overtone states of 11BF3 from 1650 to 4600 cm−1

High-resolution (0.0015-0.0035 cm−1) infrared spectra of isotopically enriched 11BF3 have been examined in detail. The analysis of the combination and overtone states within the region of study, from 1650 to 4600 cm−1, led to the assignment of over 25,000 transitions. The major perturbations were due to the Fermi resonances between states possessing one quantum of v3 and three quanta of v4. With corrections through the quadratic rotational terms, the equilibrium Be and Ce values have been determined; 0.3462679(7) cm−1 and 0.1731151(6) cm−1, respectively. An improved set of equilibrium rotational constants for 10BF3, consistent with this analysis of 11BF3 are also given. The averaged equilibrium values for both isotopomers lead to a B-F bond distance of re = 130.704 ± 0.005 pm. All of the quadratic anharmonic constants, with the exception of x33 were independently determined from experiment. For the first time for BF3, a normal force field analysis was performed that utilized the experimentally determined, fundamental harmonic vibrational frequencies.