Properties and reactivity in groups of the periodic system: Ion-molecule reactions HX+HX(center dot+) (X=F, Cl, Br, I, At)

The radical cations of hydrogen halogenide dimers (HX)(2)(.+) (X = F-At) and products of their decomposition have been investigated, using beyond-Hartree-Fock methods (MP2, QCISD, and CCSD(T)). Both all-electron and pseudopotentials techniques were considered. The (HF)(2)(.+) system differs significantly from the remaining (HX)(2)(.+) systems. The energy barrier height for transfering the H-bonded system to the hemibond structure is very low, causing the spontaneous conversion of the H-bonded structures to the hemibond ones, with the exception of the (HF)(2)(.+) dimer where the inverse situation exists. The characteristics for the overall process HX + HX(.+) reversible arrow H(2)X(+) + X(.) were obtained from the experimental heats of formation and from calculated characteristics. The Delta H-298 values based on the pseudopotentials are satisfactory for the F- and Cl-containing systems; however, the agreement is less satisfactory for heavier systems.