Thermochemistry of organic, elementorganic and inorganic species. Part XXI: Enthalpies of formation for bi- and triradicals of main group elements’ halogenides

Abstract

General trends in thermochemistry of bi- and triradicals as the fragments of halogenated molecules of main group elements :EX (:EHX; :EX 2) and triradicals ·:EX, respectively, where X-halogens F–I were found for the first time. The enthalpy of formation for methylene CH 2 was drastically changed in present work with adopted values 79 ( 3B 1) and 88 ( 1A 1) compared with currently used values ∼93 and ∼102 kcal mol −1, respectively. This happened mainly because of the drastic change in the enthalpy of formation for ketene CH 2 C O molecule from currently used value −11.4 to −24 kcal mol −1 [A.V. Golovin, D.A. Ponomarev, V.V. Takhistov, This Journal 524 (2000) 259] which was the source of experimental determination of Δ H f 0 :CH 2. Other experimental data are provided for support of the lower values of the Δ H f 0 for :CH 2. It was established that halogens F–I stabilize all biradicals :EX (E = B–Tl, N–Bi) and :EHX (:EX 2) (E = C–Pb) compared with the free radicals of these elements. This was interpreted by essentially smaller thermodynamic stability of bi- compared with mono (free)radicals the former extracting larger stabilization at H → X replacement from the same halogen compared with more stable free radicals. The expected increase in stabilization of biradicals is observed when coming down the periodic table while due to lower stability of biradicals all halogens reveal similar (contrary to free radicals) stabilization effects. Finding unknown values and correcting some literature data on the enthalpies of formation for mono-, bi- and triradicals we could estimate their thermodynamic stabilities. It was established that only about 40 molecules, radicals and biradicals from more than 800 hydrides and halogenides possess higher Δ H f 0 values compared with their fragments with elements in lower valent states with H 2 (rarely HX) as a partner. This might be treated only in the sense that the parent species are less stable than the system [fragment + H 2 (or HX)] rather than as the support of the higher thermodynamic stability of an element in its lower valent state. The thermodynamic stability of such state may be found only by the comparison of the consecutive bond dissociation energies (BDEs) in parent molecules like PbH 3X → ·PbH 2X (BDE 1) → :PbHX (BDE 2) → ·:PbX (BDE 3) and only if BDE 2 < 0 one can declare the larger thermodynamic stability of element in lower valent state. But this does not happen with any species studied in this work. This means that thermodynamic stability quite definitely decreases in the row molecule > free radical > biradical > triradical > :E: (C–Pb) contrary to the current opinion of larger thermodynamic stability of lower valent states for, say, thallium or lead. The detailed analysis of many computational results related to thermochemistry of halogenated main group elements is performed with quite definite result that neither ab initio, DFT, semi-empirical methods (or their combination) can be trusted either in support of known or prediction of unknown values of the enthalpies of formation while the empirical approach, elaborated in this work, solves the problem of calculation of the enthalpies of formation and search of general trends in structure/enthalpy of formation for halogenated main group elements.