Thermochemistry of organic, elementorganic and inorganic species. Part XX. Enthalpies of formation for free radicals of main group elements’ halogenides

Abstract

General trends in thermochemistry for free radicals as the fragments of halogenated molecules of main group elements were found for the first time applying the series of isodesmic (working) reactions following the Benson’s comparison of relative stabilities of free radicals in equations R + R ′ H → RH + R ′ + Q , where Q is stabilization (destabilization) energy. The enthalpies of formation for parent molecules of main group elements’ halogenides necessary for application of this and similar equations were presented earlier [A.V. Golovin, V.V. Takhistov, J. Mol. Struct., 784 (2006) 47.]. For fluorinated free radicals EF (E = Be–Ra), EF 2 ( EHF) (E = B–Tl, N–Bi) and EF 3 ( EH 2F, EHF 2) (E = C–Pb) the gradual decrease in stabilization by fluorine atoms was found when coming down the periodic table. This turned to destabilization for Tl, Sb, and Bi, and IVth group of elements (excluding carbon) with increasing destabilization in the row Si < Ge < Sn < Pb. The destabilization of free radicals by other halogens for IVth group of elements decreased in the row F > Cl > Br > I which was interpreted by involvement of polarizability (PAZ) effect of halogens increasing in this direction. For finding the enthalpies of formation for silicon and germanium-centered free radicals the data on ν ≡ E–H frequencies in IR-spectra were applied. Quite definite tendencies in structure/enthalpy of formation interrelationship were found for chlorinated, brominated and iodinated free radicals of IInd and IIIrd group of elements. In the VIth group the situation with F → Cl → Br → I replacement in stabilization of free radical center appeared completely different compared with II–IV groups. Owing to the high electronegativity of HO-group and low thermodynamic stability of HO radical all halogens highly stabilize OX radical and perform it in the row F < Cl–Br < I which is explained by increase of PAZ effect in this direction. All halogens are suggested to stabilize other EX free radicals (E = S–Po) but essentially less compared with OX free radicals. The tendencies in stabilization/destabilization for Vth group free radicals appeared to be intermediate between those for IVth and VIth groups.