Strong Hydrogen Bonded Molecular Interactions between Atmospheric Diamines and Sulfuric Acid

Abstract

We investigate the molecular interaction between methyl substituted N,N,N',N'- ethylenediamines, propane-1,3-diamine, butane-1,4-diamine and sulfuric acid using computational methods. Molecular structure of the diamines and their dimer clusters with sulfuric acid is studied using three density functional theory methods (PW91, M06- 2X and ωB97X-D) with the 6-31++G(d,p) basis set. A high level explicitly correlated CCSD(T)-F12a/VDZ-F12 method is used to obtain accurate binding energies. The reaction Gibbs free energies are evaluated and compared with reactions involving ammonia and atmospherically relevant monoamines (methylamine, dimethylamine and trimethylamine). We find that the complex formation between sulfuric acid and the studied diamines provides similar or more favourable reaction free energies than dimethylamine. Diamines which contain one or more secondary amino groups are found to stabilize sulfuric acid complexes more efficiently. Elongating the carbon backbone from ethylenediamine to propane-1,3-diamine, or butane-1,4-diamine further stabilizes the complex formation with sulfuric acid by up 4.3 kcal/mol. Dimethyl substituted butane-1,4-diamine yields a staggering formation free energy of -19.1 kcal/mol for the clustering with sulfuric acid, indicating that such diamines could potentially be a key species in the initial step in the formation of new particles. For studying larger clusters consisting a diamine molecule with up to four sulfuric acid molecules we benchmark and utilize a domain local pair natural orbital Coupled Cluster (DLPNO-CCSD(T)) method. We find that a single diamine is capable of efficiently stabilizing sulfuric acid clusters with up to four acid molecules, whereas monoamines such as dimethylamine is capable of stabilizing at most 2-3 sulfuric acid molecules.