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

- We show temperature dependence of free energies for sulfuric acid and ammonia cluster formation.
- For (H2SO4)n(NH3) (n = 1-6), the most stable clusters are at n = 6 for all temperatures from 200-300 K.
- For (H2SO4)(NH3)m (m = 1-6), the most stable clusters correspond to m = 6 at 200 K and down to m = 2 at 300 K.

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.