A theoretical study of temperature dependence of cluster formation from sulfuric acid and ammonia

We have performed density functional theory (BL3YP) and ab initio (MP2) calculations to investigate the energetics of the cluster formation for (NH3)m(H2SO4) and (NH3)(H2SO4)(n) (m, n = 1-6) in the atmospherically-relevant temperature range between 200 and 300 K. For (NH3)(m)(H2SO4) clusters, the binding increases from m = 1 to 6 at 200 K, while the most stable complex at 300 K is the cluster with m = 2. For (NH3)(H2SO4) n clusters, the binding is more stable for those with larger n. There is a strong temperature dependence for the (NH3)(m)(H2SO4) cluster formation; the lowest free energy shifts from m = 6 at T = 200 K to m = 5 around T = 240 K and further to m = 2 at T >= 280 K. The temperature effects on (NH3) (H2SO4)(n) clusters are much less stronger, while there is still a similar trend which favors larger n in the entire temperature range from 200 to 300 K. (C) 2014 Elsevier B.V. All rights reserved.