Quantum chemical study of atmospheric aggregates: HCl•HNO3•H2SO4

A systematic ab initio optimization of monohydrates of HCl•HNO3•H2SO4 aggregates

Hydrates of HCl, HNO₃ and H₂SO₄ involved in polar stratospheric clouds capture the attention of researchers due to the mixtures composed with them. The molecular aggregates generated with these strong acids show different behaviors, geometries and nucleation reactions at atmospheric temperatures. Here is presented a systematic ab initio optimization study of monohydrates of HCl•HNO₃•H₂SO₄ using the Density Functional Theory, by means of geometry optimizations carried out with B3LYP hybrid method and aug-cc-pVTZ basis set, a high level of theory, within Gaussian 09 program. This systematic optimization procedure consists to situate systematically the H₂O molecule around the cluster in study, on the favorable positions to develop higher quantity of hydrogen bonds as possible, in order to obtain major quantity of different electronic structures of these monohydrates. Applying this systematic optimization methodology over previously optimized complexes of HCl, HNO₃ and H₂SO₄, the present theoretical approach provides thirty-two different optimized electronic structures of monohydrates that were yielded from seven initial groups of (HCl•HNO₃•H₂SO₄)-complex, placing the H₂O in eight positions around them. Moreover, their Infrared spectra have been predicted for all (HCl•HNO₃•H₂SO₄)-monohydrates achieved. Likewise, It is shown the outcomes of the electronic energies, relative Gibbs free energies, Infrared spectra, the wavenumbers of hydrogen bonds, inter-monomeric parameters, electronic structures of (HCl•HNO₃•H₂SO₄)-monohydrates. These monohydrates could be considered precursors of the atmospheric heterogeneous nucleation reactions. These results can be useful to experimentalists of Catalysis, Astrophysics, Corrosion of metals and ceramics, aromatic compounds reactions, even environmental pollution and industrial smog.

A DFT study on the mechanism of the sulfonic acid + alcohol esterification reaction

Four alternative mechanisms for the benzenesulfonic acid + methanol esterification reaction have been studied at the B3LYP/aug-cc-pVTZ level. The participation of a pentacoordinate sulfur intermediate (in either neutral or protonated form) can be disregarded according to energy considerations. Instead, results show a low activation barrier for the S_N1 pathway (through a sulfonylium cation intermediate) and a moderate barrier for the S_N2 path (involving protonated methanol as an alkylating reagent).

Facet shapes and thermo-stabilities of H₂SO₄•HNO₃ hydrates involved in polar stratospheric clouds

The nucleation, ice crystal shapes and thermodynamic stability of polar stratospheric clouds particles are interesting concerns owing to their implication in the ozone layer destruction. Some of these particles are formed by conformers of H2O, HNO3, and H2SO4. We carried out calculations using density functional theory (DFT) to obtain optimized structures. Several stable trimers are achieved -divided in two groups, one with HNO3 moiety, second with H2SO4 moiety- after pre-optimization at B3LYP/6-31G and subsequently optimization at B3LYP/aug-cc-pVTZ level of theory. For both most stable conformers five H2O molecules are added to their optimized trimers to calculate hydrated geometries. The OH stretching harmonic frequencies are provided for all aggregates. The zero-point energy correction (ZEPC), relative electronic energies (∆E), relative reaction Gibbs free energies ∆(∆G)k-relative, and cooling constant (K cooling ) are reported at three temperatures: 188 K, 195 K, and 210 K. Shapes given in our calculations are compared with various experimental shapes as well as comparisons with their thermo-stabilities.

Relative stabilities of HCl•H2SO4•HNO3 aggregates in polar stratospheric clouds

Strong acids such as HCl (C), HNO3 (N) and H2SO4 (S) acquire relevance in Polar Stratospheric Clouds (PSCs) and aerosols in which nucleation processes occur. Ab initio quantum chemical studies of aggregates were performed for these strong acids. Structures were calculated using DFT methods with the B3LYP hybrid functional and aug-cc-pVTZ basis set. As an initial constraint, an H2SO4 moiety was placed in all candidate structures. A total of 11 optimized structures was found: a global minimum (CSN-a) plus ten local minima on the Potential Energy Surface (PES). The global minimum aggregate gave four hydrogen bonds, yielding a hexagonal ring in its structure. HNO3 acts as proton donor in all clusters; nevertheless, using trans-H2SO4 as the proton donor yielded the most stable structures, whereas HCl acts mainly as a proton donor/acceptor. Real harmonic frequencies, IR spectra, and inter-monomeric parameters were obtained. CSN-a symmetric stretching modes were shifted to 2805.56 cm(-1) and 3520.00 cm(-1) for H-Cl modes, while O-H modes shifted to 3256.87 cm(-1) and 3362.47 cm(-1). On the other hand, relative stabilities improved for 5 of the 11 aggregates when the temperature decreased from 298 K to 210 K, 195 K and 188 K. The aggregate CSN-f remained unstable only at 210 K. Moreover, the relative Gibbs free energy, ΔG(0-298K) was -9.26 kcalmol(-1) with respect to CSN-a; relative reaction Gibbs free energy [Δ(ΔG)] values ranged from 0.0 at 298 K, to -6.9 kcalmol(-1) at 188 K. It seems that CSN aggregates remain slightly more stable than CNS aggregates with a HNO3 moiety when the temperature decreases from 298 to 188 K. Five structures remained relatively stable under both study conditions.