Determinant Factor for Thermodynamic Stability of Sulfuric Acid-Amine Complexes.

Atmospheric amines are thought to play significant roles in the nucleation of sulfuric acid-mediated aerosol particles. Their enhancing effects on the stabilization of the related complexes have formerly been correlated with the amine base strength, but there are a few exceptions reported. In this work, the influence of seven alkylamines on the thermodynamic stability of sulfuric acid-amine complexes has been theoretically investigated, e.g., ethylamine, propylamine, isopropylamine, tert-butylamine, dimethylamine, ethylmethylamine, and trimethylamine. For all primary and secondary amine-mediated complexes, a dual hydrogen bond configuration is generally suggested in the most stable isomer. The stabilization of this special structure predicted by the electrostatic potential distribution on the molecular surface of amines exactly agrees with the base strength sequence, providing crucial evidence for the previous deduction of correlation between the base strength and the enhancing effect. Meanwhile, the considerable van der Waals interactions are found between the free hydroxyl of sulfuric acid and the β-methyl group of amine, resulting in the extra stability for sulfuric acid-dimethylamine and sulfuric acid-ethylmethylamine complexes. Therefore, the electrostatic potential distribution of amines is the essential determinant factor for the thermodynamic stability of the relevant complexes. Our conclusions provide new insight into a way to evaluate the enhancing abilities of amines in aerosol particle nucleation.