Quantum Chemical Calculation of Mass Spectrometry via Automated Transition State Search

• Applicant: Professor Dr. Stefan Grimme
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 533949111

The Quantum Chemistry Electron Ionisation/Collision Induced Dissociation Mass Spectra (QCxMS, x = EI/CID) program has been developed for the routine calculation of mass spectra, as has already been detailed in a previous proposal. QCxMS combines elements of statistical theory with Born-Oppenheimer molecular dynamics (MD) utilizing efficient quantum mechanical methods. The accuracy and general applicability of QCxMS has been shown in more than ten project-related publications. Here we propose to overcome inherent, fundamental limitations of the method leading to a generally more accurate theoretical model of mass spectrometry. We propose a mostly new approach termed "QCxMS2", in which the spectrum is derived from automatically computed barriers of the various fragmentation reactions according to Rice-Ramsperger-Kassel-Marcus and Quasi-Equilibrium Theory (RRKM/QET). Compared to the old approach, based on extensive sampling via molecular dynamics simulations, for which only rather approximate semiempirical quantum mechanical methods are feasible, in QCxMS2, the reaction barriers can be systematically improved at the density functional theory or even wave function theory level. Computationally demanding MD simulations are completely avoided. This is expected to enable the calculation of mass spectra with unprecedented accuracy at moderate computational cost. The proposed workflow to achieve this in an automated way is developed and implemented in an open-source, freely available software. First, we will simulate the experimentally better defined EI-MS spectra and then later turn to the practically more important CID ("soft" ionization) data. Thorough benchmarking and applications in analytical structure elucidation workflows are intended to establish QCxMS2 as a routine tool in theoretical mass spectrometry.

• DFG Programme: Research Grants