Infrared Spectroscopy of Stepwise Hydration Motifs of Sulfur Dioxide.

Experimental characterization of microscopic events and behaviors of SO2-H2O interactions is crucial to understanding SO2 atmospheric chemistry but has been proven to be very challenging due to the difficulty in size selection. Here, size-dependent development of SO2 hydrate structure and cluster growth in the SO2(H2O)n (n = 1-16) complexes was probed by infrared spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser. Spectral changes with cluster size demonstrate that the sandwich structure initially formed at n = 1 develops into cycle structures with the sulfur and oxygen atoms in a two-dimensional plane (n = 2 and 3) and then into three-dimensional cage structures (n ≥ 4). SO2 is favorably bound to the surface of larger water clusters. These stepwise features of SO2 hydration on various sized water clusters contribute to understanding the reactive sites and electrophilicity of SO2 on cloud droplets, which may have important atmospheric implications for studying the SO2-containing aerosol systems.