Is There an\nAdduct of Pyridine <i>N</i>‑Oxide\nand Boron Trifluoride in the Gaseous State? Gas Electron Diffraction\nvs Mass Spectrometry

A study of saturated vapor over the pyridine <i>N</i>-oxide-boron trifluoride (PyO-BF<sub>3</sub>) adduct was\ncarried\nout at <i>T</i> = 448(5) K by a synchronous gas electron\ndiffraction/mass spectrometry (GED/MS) experiment. Due to the absence\nof ions in the mass spectrum, indicating the presence of a structure\nwith an O–B dative bond, several models of vapor composition\nwere tested by the GED method. It was found that the dominant molecular\nform (up to 100%) in vapor is the PyO-BF<sub>3</sub> adduct. Using\nthe DFT/M06-2X/aug-cc-pVTZ method, geometric optimization of the molecular\nion [PyO-BF<sub>3</sub>]<sup>+</sup> was carried out, which showed\nits intrinsic instability and dissociation into a [PyO]<sup>+</sup> cation and a BF<sub>3</sub> molecule. This study certainly demonstrates\nthe significant advantage of the GED method to determine the qualitative\nand quantitative gas-phase composition of dative-bonded adducts and\nother noncovalent complexes as well, whereas the interpretation of\nmass spectra may be ambiguous due to the possible intrinsic instability\nof ions containing a dative bond. The nature of the O–B bond\nis discussed in terms of the natural bond orbitals (NBOs) and the\nquantum theory of atoms in molecules (QTAIM). A comparison of structural\nand energetic parameters for PyO-BF<sub>3</sub> and the previously\nstudied BF<sub>3</sub> adducts allows the theoretical comprehension\nof the nature of the O–B bond to be extended and to explain\nthe different thermal stabilities of these compounds.