A Density Functional Theory Study of the Hydrates of 2NH_3:H_2SO_4 and Its Implications for the Formation of Atmosphere Particles

CT Density functional theory was used at the B3LYP/6-311++G(d,p) level of theory to study the hydrates of 2NH_3:H_2SO_4:nH_2O for n = 0～4. Neutrals of the most stable clusters, when n = 0 and 1, spontaneously formed and were determined to be hydrogen-bonded molecular complexes of monomerie species.Double ions (clusters containing a NH_4~+ cation and a HSO_4~- anion) or even ternary ions (clusters with two NH_4~+ cations and one SO_4~(2-) anion) spontaneously formed in the most stable clusters of 2NH_3:H_2SO_4:nH_2O (n = 2, 3, 4). The energetics of binding and ineremental association was also calculated. Double ions are not energetically favorable until 2NH_3:H_2SO_4:2H_2O because of the about equal free energies for forming the neutral (the most stable) and double ion (the second stable) isomers. The free energy of incremental association from free H_2O and 2NH_3:H_2SO_4:nH_2O has a maximum at n = 2 at room temperature with △G≈ -2 kcal/mol.The comparison of incremental association energies between 2NH_3:H_2SO_4:nH_2O, NH_3:H_2SO_4:nH_2O and H_2SO_4:nH_2O clusters revealed that NH_3 plays an important role in the atmospheric particle nucleation.