Observation of the Water-HNSO₂ Complex

Area of Science:

• Atmospheric Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Sulfamic acid (NH$_{2}$SO$_{3}$H, SFA) is crucial for atmospheric new particle formation (NPF).
• Its formation involves the SO$_{3}$-NH$_{3}$ reaction system, with proposed intermediates like SO$_{3}$···NH$_{3}$ and HNSO$_{2}$···H$_{2}$O.

Purpose of the Study:

• To provide the first spectroscopic evidence for the HNSO$_{2}$···H$_{2}$O complex.
• To characterize its different forms and bonding interactions.

Main Methods:

• Matrix-isolation infrared (IR) spectroscopy at 10 K in an Ar matrix.
• Isotopic labeling (D and $^{18}$O) for spectral confirmation.
• Quantum chemical calculations (CCSD(T)-F12a/aug-cc-pVTZ).

Main Results:

• Spectroscopic observation of HNSO$_{2}$···H$_{2}$O in two distinct forms.
• The major form features a six-membered ring stabilized by two hydrogen bonds (dissociation energy 7.6 kcal mol$^{-1}$).
• A less stable form exhibits a chalcogen bond (S···O) with a dissociation energy of 7.2 kcal mol$^{-1}$.

Conclusions:

• The study provides direct spectroscopic evidence for HNSO$_{2}$···H$_{2}$O, confirming its role as an atmospheric NPF intermediate.
• Understanding these complex structures enhances models of atmospheric chemistry and aerosol formation.