Charge-Shifted Weak Noncovalent Interactions in the Atmospherically Important OCS Microhydrates

Abstract

Stratospheric aerosol, mainly comprising microhydrated carbonyl sulfide (OCS), is among the primary drivers of climate change. In this study, we investigate the effect of microhydration on the structure, energetics, and vibrational properties of the neutral OCS molecule using ab initio calculation, molecular electrostatic potential (MESP), topological analyses of electron density, and natural bond orbital (NBO) analyses. The complexation energy increases with the cluster size, and the first solvation shell of OCS consists of four water molecules that interact with the OCS moiety preferentially through SOCS center dot center dot center dot OW, OOCS center dot center dot center dot OW, and COCS center dot center dot center dot OW type of weak noncovalent interaction instead of the typical OOCS center dot center dot center dot H-OW and SOCS center dot center dot center dot H-OW H-bonds. These noncovalent interactions originate due to the electron shift from the water oxygen lone pair to the antibonding orbital of C=S [BD*(C=S)], sometimes via BD*(C=O), which substantially perturbs the bending mode of surrounding water molecules. The present study thus unravels the underlying relationship between the OCS atmospheric hydrolysis and the charge-shifted noncovalent interactions.