Ab-initio calculation of the rates of the reactions between volatile organic compounds in wine and cations for mass spectrometry

Wine is a complex mixture housing many aroma and flavor compounds giving it a unique texture and bouquet. These volatile organic compounds (VOCs), if present near the sensory threshold limits, may contribute positively to wine quality; however, excessive amounts can detract from quality, and are considered as a fault in wine. It is believed that nearly 10% of the world’s wine is affected from various types of faults. The most common and potent wine taint is 2,4,6-trichloroanisole (2,4,6-TCA), commonly known as cork-taint molecule resulting from the cork stopper of wine bottles. 2,4,6- TCA produces intense ’musty’, ’mouldy’ ’earthy’ smelling in wine. Similar off-flavor smells are associated to compounds including geosmin and 2-methoxy3,5-dimethylpyrazine. We studied 74 such VOCs frequently present in wine. Determining concentration of VOCs in wine requires detection techniques to be fast, in real time, and with a detection limit as low as few parts per trillion by volume. The most frequently used techniques based on direct injection mass spectrometry, namely proton transfer reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry (SIFT-MS), are being successfully employed in the measurements of VOCs concentrations. Quantification using these techniques usually relies on compound-by-compound instrument calibration. The calibration procedures are generally laborious and time consuming. The theoretical evaluation of the rate coefficients of ion-molecule reactions occurring in PTR-MS/SIFT-MS flow (drift) tubes is a practical alternative to calibration. In this thesis, we compute and report the rate coefficients for ion-molecule reactions, relevant for different experimental conditions, such as varied temperature and electric fields inside the drift tube. We have used well-established models based on capture cross-section collision and classical trajectories. These models rely on physical properties such as electric dipole moment and polarizability of the volatile molecules. To compute these quantities we resorted to ab initio density functional theory