Interplay of experiment and theory: high resolution infrared spectrum and accurate equilibrium structure of BF2OHElectronic supplementary information (ESI) available: Tables S1-S4. See DOI: 10.1039/c5cp05444c

The high-resolution Fourier transform infrared (FTIR) spectrum of 11 BF 2 OH (difluoroboric acid) is analyzed taking into account numerous interactions. The ν 1 , ν 2 and ν 3 infrared bands are analyzed for the first time, whereas the parameters of the 6 1 , 7 1 , 8 1 and 9 1 states and for the 4 1 and 9 2 interacting states are redetermined. These results are used to check the quality of the ab initio force field. It is found that the ab initio rovibrational corrections are more accurate than the experimental ones. An earlier attempt to determine a semiexperimental structure did not allow us to obtain an accurate equilibrium structure. The reasons of this failure are investigated. This failure was mainly due to the lack of useful experimental information. Indeed, there is no isotopic substitution available for the fluorine atoms, and the boron atom is extremely close to the center of mass. Furthermore, the available isotopic substitutions (H → D and 16 O → 18 O) induce a large rotation of the principal axis system which amplifies the errors. However, the mixed estimation method has allowed us to determine a complete and reliable equilibrium structure. Thanks to this method, it is possible to determine an accurate structure, even in extremely difficult cases. An extensive analysis of the quality of structure calculations at the CCSD(T) level is also performed using basis sets up to five ζ quality. It was found that, at the convergence limit, the effects of the diffuse functions are practically disappearing, whereas the core-core and core-valence electron correlation effects are quite important for the bond lengths. Substitution of hydrogen by deuterium induces a large rotation of the principal axis system that amplifies the errors.