A microwave spectroscopic and ab initio study of keto–enol tautomerism and isomerism in the cyclohexanone–water complex

Cyclohexanone and water are two important components of secondary organic aerosol (SOA), and understanding the intermolecular interactions between these two species can provide insight to the initial formation mechanism of SOA particles. In this work, we have investigated the keto-enol tautomeric and conformational changes of the cyclohexanone monomer and its monohydrate by Fourier-transform microwave spectroscopy and ab initio calculations. Chirped-pulse and cavity-based FTMW spectrometers in the region of 7-14 GHz were used to measure rotational spectra of the most stable species prepared in a cold molecular beam. The experimental and theoretical results suggest that the chair conformer of the keto tautomer is the most stable structure. We have measured and assigned rotational spectra of ten isotopologues, including all six single 13C substitutions observed in natural abundance and four different isotopic species of water (H2O, D2O, DOH and HOD). The experimental structure of cyclohexanone-water was determined directly using this isotopic information. The analysis reveals the existence of both canonical and secondary hydrogen bonding, which we confirm using QTAIM analyses. To further elucidate the hydrogen bonding characteristics in ketone-water clusters, cyclohexanone-water is compared to a variety of other hydrated ketones, namely formaldehyde, acetone, cyclobutanone, and cyclopentanone, through utilization of the symmetry adapted perturbation theory (SAPT) energy decomposition method. The results of this study shed light on the effects of water on keto-enol tautomerization, and the role of hydrogen bonding in ketone monohydrates and the formation of related SOA particles.