Quantum Chemical Calculation of Electron Ionization Mass Spectra

This thesis reports the computation of electron ionization (EI) mass spectra using a method that combines statistical theory and molecular dynamics. Due to the complexity of the unimolecular reaction space, not all competing fragmentation pathways can be fully treated in an ab initio way using a purely statistical framework. The main idea behind the present simulation protocol is to use approximate quantum chemical potential energy surfaces and simple internal energy distributions to discover the reaction pathways and barriers, and thus the relative rate constants automatically. This idea was proposed, implemented and published in late 2013 by my thesis supervisor Stefan Grimme, and termed QCEIMS. The first part of this thesis gives a brief overview over the physical chemistry of EI mass spectrometry and the most important theoretical methods that I have used. These involve finitetemperature density functional theory and the semi-empirical Geometries, Frequencies and Noncovalent Interaction eXtended Tight Binding Hamiltonian (GFN-xTB). The energies and forces computed at these levels of theory are the input for the subsequent Born-Oppenheimer molecular dynamics simulations. The second part deals with the application of finite-temperature density functional theory. The results show that the fractional occupation number weighted density ρ FOD can be used as a measure for static electron correlation in biradicals and related systems, and that the fractional occupation numbers can be useful for the first guess at a multiconfigurational wave function. Furthermore, potential energy surfaces along model reaction coordinates are explored and the transferability of the ρFOD concept to semi-empirical quantum chemistry is shown. The third part shows the main results of this work related to EI mass spectrometry. In Chapter 4, the literature is reviewed and the “Quantum Chemistry Electron Ionization Mass Spectra” (QCEIMS) method is presented. It is then evaluated concerning the assignment of the charge to a fragment using a series of ethanol homologues. A small mass spectrometric benchmark study is also included, showing that isomers can be distinguished by QCEIMS predicted EI mass spectra, provided their fragmentation pathways differ substantially. In Chapters 5, 6, and 7 QCEIMS applications to large drug molecules, the nucleobase adenine and other nucleobases, are presented. For each case, the fragmentation pathways are analyzed, thereby elucidating the structures of the fragment ions. Finally, in Chapter 8, predicted EI mass spectra for 23 compounds across the whole periodic table are shown. This has been made possible by V. Ásgeirsson’s implementation of GFNxTB into QCEIMS. This robust and efficient method performs remarkably well for organic molecules as well as organometallic compounds and main group inorganic systems while reducing the computational cost by a factor of 1,000 when compared to hybrid density functional calculations