A DFT Evaluation of Molecular Reactivity of Volatile Organic Compounds in Support of Chemical Ionization Mass Spectrometry

21 December 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Gas-phase molecular properties of volatile organic compounds (VOCs) play an important role in the selection of gas-phase reagent ions for chemical ionization mass spectrometry (CI-MS). We apply hybrid density functional theory (DFT) to compute proton affinity (PA), ionization energy (IE), and global reactivity parameters for VOCs, which are widely regarded as the primary sources of taints and off-flavors in wine. Atomic polar tensor (APT) charges and total energies at the stationary point for neutral and protonated molecules are also computed. PA and IE values determine the CI-MS mode of reactions, either proton transfer or electron transfer from the reagent gas ions to VOCs. Global reactivity parameters, such as chemical potential (µ), chemical hardness (η), softness (σ), and electrophilic nature (ω) as obtained from frontier molecular orbitals, are considered useful in rationalizing the chemical reactivity patterns of the molecules. A benchmark calculation of indole molecule with MP2, B3LYP, and M06-2X DFT methods at thermodynamically and kinetically stable protonation sites further supports the applied DFT method. Since limited data are available on computed parameters, the reported values would support CI-MS quantification of trace-level VOCs not only in wine but also in various food products.

Keywords

volatile organic compounds
density functional theory
chemical ioniza- tion mass spectrometry
Global reactivity parameters
benchmark calculation

Supplementary materials

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Description
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Supporting Material
Description
Data on the total energy at the stationary point of the neutral and protonated complex together with the net atomic charge, bond length change post protonation are collected as supplementary material for all species occurring in this study.
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