Observation of the Water-HNSO 2 Complex.

Sulfamic acid (NH 2 SO 3 H, SFA) is supposed to play an important role in aerosol new particle formation (NPF) in the atmosphere, and its formation mainly arises from the SO 3 -NH 3 reaction system in which weakly bonded donor-acceptor complexes such as SO 3 ···NH 3 and isomeric HNSO 2 ···H 2 O have been proposed as the key intermediates. In this study, we reveal the first spectroscopic observation of HNSO 2 ···H 2 O 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 SO 3 ···H 2 O in containing a pure chalcogen bond (S···O) with a dissociation energy of 7.2 kcal mol -1 . The characterization of HNSO 2 ···H 2 O with matrix-isolation IR spectroscopy is supported by D- and 18 O-isotope labeling and quantum chemical calculations.