Microwave Spectroscopy Study Supported By Quantum Chemistry Calculations Of Limona Ketone, A Key Oxidation Product Of Limonene

Vegetation is a major source of emission of Biogenic Volatile Organic Compounds (BVOCs), which play an important role in atmospheric chemistry. Apart from methane and isoprene, monoterpenes (\chem{C_{10}H_{16}}) are emitted in significant amounts by plants. Among these, $\alpha$-pinene, $\beta$-pinene, and limonene are the predominant species\footnote{P. S. Monks et al., Atmospheric Composition Change – Global and Regional Air Quality. Atmos. Environ. 2009, 43 (33), 5268–5350.}. Recently, limonene and limonene oxide have been studied extensively with Fourier transform microwave spectroscopy (FTMW)\footnote{J.R. Moreno et al., Conformational Relaxation of S-(+)-Carvone and R-(+)-Limonene Studied by Microwave Fourier Transform Spectroscopy and Quantum Chemical Calculations. Struct. Chem. 2013, 24 (4), 1163–1170.}$^,$\footnote{D. Loru et al., Conformational Flexibility of Limonene Oxide Studied By Microwave Spectroscopy. ChemPhysChem 2017, 18 (3), 274–280.}. Limona ketone (\chem{C_9H_{14}O}) is a major oxidation product of limonene, and it was shown that it is considered as precursor of Secondary Organic Aerosol (SOA) formation\footnote{N.M. Donahue et al., Secondary Organic Aerosol from Limona Ketone: Insights into Terpene Ozonolysis via Synthesis of Key Intermediates. Phys. Chem. Chem. Phys. 2007, 9 (23), 2991–2998.}. Thus, BVOCs have considerable impact on numerous environmental processes, climate, and health, and it is important to determine their gas phase structure and to predict their interaction sites and patterns with surrounding molecular systems. Within this context, the rotational spectrum of limona ketone was recorded and analyzed over the 3.8-19.3 GHz range using FTMW spectrometer\footnote{M. Tudorie et al., Magnetic Hyperfine Coupling of a Methyl Group Undergoing Internal Rotation: A Case Study of Methyl Formate. J. Chem. Phys. 2011, 134 (7), 074314.}. The rotational spectrum analysis was supported by quantum chemical calculations, and transitions were assigned to the most stable equatorial conformer. The spectrum showed clearly that the lines were split. This splitting is due to internal rotation of methyl group, where the A and E states lines were assigned and fitted at instrumental accuracy, and the experimental barrier of the methyl torsion was determined