Calculation of Electron Ionization Mass Spectra with Semiempirical GFNn-xTB Methods

In
this work, we have tested two different extended tight-binding
methods in the framework of the quantum chemistry electron ionization
mass spectrometry (QCEIMS) program to calculate electron ionization
mass spectra. The QCEIMS approach provides reasonable, first-principles
computed spectra, which can be directly compared to experiment. Furthermore,
it provides detailed insight into the reaction mechanisms of mass
spectrometry experiments. It sheds light upon the complicated fragmentation
procedures of bond breakage and structural rearrangements that are
difficult to derive otherwise. The required accuracy and computational
demands for successful reproduction of a mass spectrum in relation
to the underlying quantum chemical method are discussed. To validate
the new GFN2-xTB approach, we conduct simulations for 15 organic,
transition-metal, and main-group inorganic systems. Major fragmentation
patterns are analyzed, and the entire calculated spectra are directly
compared to experimental data taken from the literature. We discuss
the computational costs and the robustness (outliers) of several calculation
protocols presented. Overall, the new, theoretically more sophisticated
semiempirical method GFN2-xTB performs well and robustly for a wide
range of organic, inorganic, and organometallic systems.