Interaction topologies of the S O chalcogen bond: the conformational equilibrium of the cyclohexanol SO cluster

The conformational landscape of the cyclohexanol SO 2 cluster was revealed in the gas phase using chirped-pulsed broadband rotational spectroscopy and quantum chemical calculations. Four isomers stabilized by a dominant S O chalcogen bond and cooperative C-H O&z.dbd;S and O-H O&z.dbd;S secondary weak hydrogen bonds were observed, with a near-parallel orientation of the S&z.dbd;O and O-H bonds. Isomers formed by equatorial- gauche cyclohexanol are more stable than the isomers containing axial cyclohexanol. The multiple conformations of cyclohexanol and the versatile binding properties of SO 2 , simultaneously operating as nucleophile and electrophile through its π-holes and non-bonding electrons lead to a complex conformational behavior when the cluster is formed. The long (2.64-2.85 Å) attractive S O interaction between SO 2 and cyclohexanol is mainly electrostatic and the contribution of charge transfer is obvious, with an NBO analysis suggesting that the strength of the S O interaction is nearly two orders of magnitude larger than the hydrogen bonds. This study provides molecular insights into the structural and energetic characteristics that determine the formation of pre-nucleation clusters between SO 2 and a volatile organic compound like cyclohexanol. The versatile SO 2 engages simultaneously in a S O chalcogen bond and C-H O hydrogen bonds when interacting with cyclohexanol.