Theoretical Study of the Reaction of Hydrogen Selenide with the Cl^• Atom and the ^•OH Radical, and Differences with the Behavior of Other Hydrogen Chalcogenides

Hydrogen selenide, H₂Se, is the third-row analog of hydrogen sulfide, H₂S, and water, H₂O. While there is ample thermochemical and kinetic information about the reactions of the latter two species, few experimental or theoretical data are available on H₂Se. In this work, we use high-level post-Hartree-Fock methods to study the reaction of H₂Se with two of the most abundant atmospheric radical species, the Cl^• atom and the ^•OH radical, H₂Se + Cl^• → HSe^• + HCl H₂Se + ^•OH → HSe^• + H₂O We used the SVECV-f12 composite quantum chemical method to study the stability of adducts and transition states, as well as the barriers for the transformations. It was found that a correct representation of the barrierless adduct is crucial for a correct description of the reaction's kinetics, and we present in this paper the first theoretical determination of the reaction coefficient of H₂Se with Cl^• in the literature, obtaining a value of 5.7 × 10^-10 cm³ molecule^-1 s^-1, in excellent agreement with the experimental determination of 5.5 × 10^-10 cm³ molecule^-1 s^-1 at room temperature Additionally, using the same procedure, we obtained a value of 6.4 × 10^-11 cm³ molecule^-1 s^-1 for the reaction with ^•OH, in this case slightly smaller than the only previous estimation of 7.2 × 10^-11 cm³ molecule^-1 s^-1 obtained indirectly from similar reactions for sulfur compounds, in all cases at 298.15 K. Judging from the agreement of the theoretical and experimental rate coefficients in the case of the reaction with chlorine, we suggest that our value for the reaction with the hydroxyl radical is more accurate than the estimated one. A comparison of the dependence of the rate coefficients for H₂S and H₂Se as a function of the temperature shows some noticeable differences. A convex behavior of the T-dependence for the Cl^• reaction at high temperatures was found, instead of the concave behavior found for sulfur. Nevertheless, this is not important in atmospheric chemistry conditions, and a sufficiently linear region was found with the expression, <i>k</i>(Cl^•) = 1.6 × 10^-10 exp (0.7/<i>RT</i>) cm³ molecule^-1 s^-1. The reaction with ^•OH is even more complicated, with nonlinear tail at high (combustion) and low (stratosphere) temperatures, while the region important in tropospheric chemistry could be fitted with the Arrhenius equation <i>k</i>(^•OH) = 5.9 × 10^-12 exp (1.4/<i>RT</i>) cm³ molecule^-1 s^-1. Using our theoretically determined kinetic data, we were also able to calculate the atmospheric lifetime of H₂Se as 2.6 h, considerably shorter than that of H₂S (12.2 h).