The reaction paths of hydrogen trioxide (HO₃) with sulphur dioxide (SO₂) have been investigated on the doublet potential energy surface, theoretically. All species of the title reaction have been optimized at the PMP2(FC)/cc-pVDZ computational level. Energetic data have been obtained at the CCSD(T)//PMP2 level employing the cc-pVDZ basis set. No stable collision complexes have been found between the SO₂ and HO₃ molecules. Therefore, the SO₂ + HO₃ reaction starts without initial associations. The four possible paths, P₁ through P₄, have been obtained for the formation of SO₃ (D$_{3h}$) + HOO$^{\bullet}$ product. Our results show that these four paths include relatively high energy barriers to produce the final product of the SO₃ (D$_{3h}$) + HOO$^{\bullet}$. Therefore, the SO₂ + HO$_3\to$SO₃(D$_{3h}$) + HOO$^{\bullet}$ reaction is difficult to perform under atmospheric conditions. This means that the importance of SO₂ + HO$_3 \to$ SO₃ (D$_{3h}$) + HOO$^{\bullet}$ reaction increases with increasing temperature and, this reaction plays an important role in the SO₃(D$_{3h}$) production as the main molecule of the formation of acid rain at high temperatures.

Quantum chemical calculations are carried out in order to investigate the heterodimers of NF₃ with isoelectronic and isostructure species of N₂O and CO₂ in gas phase within the MP2 (Møller-Plesset perturbation) and CCSD(T) levels. Three and four minima are located on the potential energy surfaces of NF₃-CO₂ and NF₃-N₂O systems at the MP2/aug-cc-PVDZ level, respectively. Single-point energy calculations at the MP2/aug-cc-PVTZ and CCSD(T)/aug-cc-PVDZ levels confirm results obtained at the MP2/aug-cc-PVDZ level show that atmospheric roles of NF₃-N₂O and NF₃-CO₂ systems are similar. The atom in molecules theory was applied to analyse the nature of intermolecular interactions. Also, natural bond orbital (NBO) analysis has been used in order to obtain charge transfer quota among heterodimers.

The competition of hydrogen, halogen and pnicogen bonding to the stability of the atmospheric complexes is interesting, especially where the molecules by the most abundant greenhouse effect in the atmosphere are subject of interest. In the present work, we have computationally studied the addition of H₂O to the NF₃ and C₂FCl₂ molecules to reveal the electronic and structural features of the NF₃-H₂O and CF₂Cl₂-H₂O complexes through DFT, MP₂ and CCSD (T) methods. The interaction energies, geometry and electronic properties including charge transfer, energy gap, NEDA and AIM analyses of all the complexes werecalculated to discuss the nature and strength of intermolecular interactions. The results indicate that the role of halogen bonding is more obvious than that of hydrogen and pnicogen bonding, and compared with the NF₃,CF₂Cl₂ is more effectively stabilized by the H₂O molecules