Interaction topologies of the S⋯O chalcogen bond: the conformational equilibrium of the cyclohexanol⋯SO2 cluster

Abstract

The conformational landscape of the cyclohexanol⋯SO₂ 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⋯OS and O–H⋯OS secondary weak hydrogen bonds were observed, with a near-parallel orientation of the SO 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 SO₂, 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 SO₂ 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 SO₂ and a volatile organic compound like cyclohexanol.