Observation of the Water-HNSO₂ Complex

Sulfamic acid (NH₂SO₃H, SFA) is supposed to play an important role in aerosol new particle formation (NPF) in the atmosphere, and its formation mainly arises from the SO₃-NH₃ reaction system in which weakly bonded donor-acceptor complexes such as SO₃···NH₃ and isomeric HNSO₂···H₂O have been proposed as the key intermediates. In this study, we reveal the first spectroscopic observation of HNSO₂···H₂O in two forms in a solid Ar matrix at 10 K. The major form consists of two intermolecular H bonds by forming a six-membered ring structure with a calculated dissociation energy of 7.6 kcal mol^-1 at the CCSD(T)-F12a/aug-cc-pVTZ level of theory. The less stable form resembles SO₃···H₂O in containing a pure chalcogen bond (S···O) with a dissociation energy of 7.2 kcal mol^-1. The characterization of HNSO₂···H₂O with matrix-isolation IR spectroscopy is supported by D- and ¹⁸O-isotope labeling and quantum chemical calculations.