Thermodynamic Properties of Gaseous Selenium Species of Atmospheric Interest

Selenium (Se) is an essential element for fauna, flora, and human health. Up to a third of Se can cycle through the atmosphere. The volatile organic Se species include dimethyl selenide (CH₃SeCH₃), dimethyl diselenide (CH₃SeSeCH₃), and methaneselenol (CH₃SeH), and will undergo rapid atmospheric oxidation. To better constrain the fate of atmospheric Se compounds, high-level ab initio electronic structure calculations were performed to estimate the thermodynamic properties of 11 gaseous Se species (HSe^•, H₂Se, CH₂Se, CH₃SeH, ^•CH₂SeH, CH₃Se^•, CH₃SeSe^•, CH₃SeCH₃, CH₃Se^•CH₂, CH₃SeSeCH₃, and CH₃SeSe^•CH₂) using their atomization reactions. Several corrections were applied to provide highly accurate calculated standard enthalpies of formation at 298 K, Δ_fH°_298 K. Standard molar entropy at 298 K, S°_298 K, and heat capacity, C_p(T) over the temperature range 300-1500 K, from vibrational, translational, and external rotation contributions were computed using statistical thermodynamics based on the vibrational frequencies and structures obtained at the CCSD(T)/aVTZ level of theory. Hindered rotational contributions to S°_298 K and C_p(T) were calculated from the energy levels, where the internal rotation potential was calculated at the MP2/aVTZ level of theory. The bond dissociation energies at 298 K for H-Se, Se-Se, and C-Se bonds in the Se molecules were derived from their calculated Δ_fH°_298 K values. The same protocol was applied to O and S species for comparison with Se. Their Δ_fH°_298 K, S°_298 K, and C_p(T) values were in good agreement with the corresponding available literature data. This work provided the first thermodynamic properties for the organic Se species. The data obtained in this work could be used in chemical-transport models to assess the fate of atmospheric Se and its speciation unravelling the Se biogeochemical cycles.