First Principles Calculation of Electron Impact Mass Spectrometry of Molecules

• Applicant: Professor Dr. Stefan Grimme
• Subject Area: Theoretical Chemistry: Molecules, Materials, Surfaces ; Organic Molecular Chemistry - Synthesis and Characterisation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2013 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 253235332

• Final Report Year: 2020

Final Report Abstract

The Quantum Chemistry Electron Ionized Mass Spectra (QCEIMS) program was developed to automatically calculate standard 70 eV electron ionization (EI) mass spectra. It employs Born-Oppenheimer molecular dynamics (BO-MD) to simulate the fragmentation processes of molecules. Within this project, a total of seven publications were released that describe the development of the method and elucidated the fragmentation patterns of various molecules. It was shown that QCEIMS can, in addition to simple dissociations, correctly calculate rearrangement reactions which are of particular importance in many systems. In addition to the calculation of positive ions, a negative-ion mode was introduced. Since QCEIMS is based on quantum chemical methods, the underlying theory level for the PES is important for the accuracy of the results. Both, density functional theory (DFT) and semi-empirical quantum mechanical (SQM) methods are implemented. Since DFT methods are usually too cost-intensive, the default mode in QCEIMS is based on the SQM level which was thoroughly tested in the project. The in-house developed tight-binding GFN1-xTB and GFN2-xTB methods show particularly good results in extensive performance evaluations compared to experimental spectra. In two of the publications, it was shown that the GFNn-xTB (n=1,2) methods require low computational costs to produce good spectra involving organic, organometallic and main group molecules. Recently, the software was extended to calculate collision-induced dissociation (CID) spectra. In this mode, collisions between a protonated precursor ion and a neutral colliding gas are simulated. Several technical aspects were considered, such as impact energy, collision angle and the number of collision events. These parameters can be freely selected in the program and their influences on the impact processes can be examined. This allows a detailed insight into the physical processes occurring in a CID experiment which otherwise are difficult to describe otherwise due to often unclear experimental conditions. Initial steps for a fundamental revision of the entire approach (QCEIMS2) are undertaken where the use of MD for the fragmentation is entirely avoided. Instead, reactions are automatically obtained by constrained geometry optimization, reaction path searches and calculation of barriers. This enables the use of a more accurate PES without sacrificing the efficiency of the method. Overall, the QCEIMS method was developed to help in the elucidation of mass spectra also uncovering the underlying reaction mechanisms, without relying on any further information. Hence, it is complementary to existing data base driven or machine learning techniques. Already now the program is used by other researchers to identify hazardous substances and to generate reference spectra for the determination of unknown compounds.

Publications

• ”Elucidation of Electron Ionization Induced Fragmentations of Adenine by Semiempirical and Density Functional Molecular Dynamics”, J. Phys. Chem. A, 118 (2014), 11479-11484
  C. A. Bauer and S. Grimme
  (See online at https://doi.org/10.1021/jp5096618)
• ”First principles calculation of electron ionization mass spectra for selected organic drug molecules”, Org. Biomol. Chem., 43 (2014), 8737-8744
  C. A. Bauer and S. Grimme
  (See online at https://doi.org/10.1039/c4ob01668h)
• ”Automated quantum chemistry based molecular dynamics simulations of electron ionization induced fragmentations of the nucleobases uracil, thymine, cytosine, and guanine”, Eur. J. Mass Spectrom., 21 (2015), 125-140
  C. A. Bauer and S. Grimme
  (See online at https://doi.org/10.1255/ejms.1313)
• ”How to Compute Electron Ionization Mass Spectra from First Principles”, J. Phys. Chem. A, 120 (2016), 3755-3766
  C. A. Bauer and S. Grimme
  (See online at https://doi.org/10.1021/acs.jpca.6b02907)
• ”Unimolecular Decomposition Pathways of Negatively Charged Nitriles by Ab Initio Molecular Dynamics”, Phys. Chem. Chem. Phys., 18 (2016), 31017-31026
  V. Ásgeirsson, C. A. Bauer and S. Grimme
  (See online at https://doi.org/10.1039/c6cp06180j)
• ”Quantum chemical calculation of electron ionization mass spectra for general organic and inorganic molecules ”, Chem. Sci., 8 (2017), 4879–4895
  V. Ásgeirsson, C. A. Bauer and S. Grimme
  (See online at https://doi.org/10.1039/c7sc00601b)
• ”Calculation of Electron Ionization Mass Spectra with Semiempirical GFNn-xTB Methods”, ACS Omega 4 (2019), 15120–15133
  J. Koopman and S. Grimme
  (See online at https://doi.org/10.1021/acsomega.9b02011)