Theoretical study of atmospheric clusters: HNO3–HCl–H2O

Abstract

Nitric acid, hydrochloric acid and water can form stable aggregates with atmospheric implications, for instance at the surface of polar stratospheric clouds. The structure, stability and chemical properties of these ternary complexes are studied by means of high level theoretical calculations (hybrid DFT B3LYP method along with aug-cc-pVQZ basis set). From the many possible systems that these molecules could form, only 15 are found to yield stable structures, well characterized by a minimum in their potential energy surfaces. These aggregates are studied in detail. They can be collected in three families, according to the role played by each species in the different hydrogen bonding links that result and provide the basis for the stabilization of the clusters. Water and HCl can be H-donors to almost every O atom of HNO(3), which in turn can donate its H atom to the other two molecules. Two special cases are found, one in which H(2)O plays a central role, and another with a three-dimensional structure, in contrast to the basically planar frame of the other clusters. Bonding properties are investigated for the whole series using AIM methods. The elongation of the H-Cl bond as a consequence of the aggregate creation is inspected in detail, as it may provide a clue to the lability of this molecule with implications in atmospheric processes. The Gibbs free energy calculated for these clusters shows that some of them could form spontaneously in the range of temperatures of the stratosphere.

Interaction in the Ternary Complexes of HNO3···HCl···H2O: A Theoretical Study on Energetics, Structure, and Spectroscopy

“…It has been shown by Gomez et al for one of the ternary complexes studied also in this article that the counterpoise-corrected surface intermolecular distances are a little longer than the values evaluated by the standard gradient optimization. 33 This difference was found to be 3 Â 10 À3 Å with B3LYP/aug-cc-pVQZ. Furthermore, it was reported by Hobza et al for the water dimer that this difference gets smaller with larger basis set.…”

Proton transfer and autoionization in HNO3·HCl·(H2O)n particles

“…[22][23][24] Go´mez et al 22 reported fifteen structures characterized by true minima located on the IPES, the NC1W-1 aggregate being the most stable one among them. These structures are especially suitable for studying proton transfer parameter variations in the smallest clusters.…”

“…This fact was also found in previous papers studying ternary clusters. 22,23 Finally, the third factor involved in proton transfer in HNO 3 is related to the existence of a proton donor acting over the acidic oxygen atom, and also to which molecule that donor may be. For instance, proton transfer does not occur when the proton donor is water, as in NC4W-5 or NC4W-1, whereas in NC4W-4 a HCl donor on the acidic oxygen has a marked effect, making r PT become positive.…”

“…22, the systems under study are designated NCnW-m, where N, C and W represent nitric acid, hydrogen chloride and water molecules, n indicates how many water molecules participate in the cluster, and m is just a running index for several species within a given NCnW set. Thus, for instance, NC3W-4 specifies the fourth aggregate (in increasing energy order) of the HNO 3 ÁHClÁ(H 2 O) 3 family, which happens to be a very special structure that we will discuss later.…”

“…We are not aware of any investigations based on ternary or higher order systems apart from three recent papers by Go´mez et al, 22,23 focused on HNO 3 ÁHClÁH 2 O, and by Balcı and Uras-Aytemiz. 24 Yet, this is an important point since more often than not two or more acids are present in many systems of chemical interest.…”