Enthalpies of formation of alkyl radicals

Abstract

The data on the enthalpies of formation ( f H) for alkyl radicals (R • ) were examined. Taking into account the values of enthalpies of С-Х bond dissociation (Х = Н, Cl) in alkanes and their monochlorosubstituted derivatives,  f H of fifteen R • were determined for the first time. The values of  f H(R • ) were analyzed on the basis of quantitative structure-property relationships in terms of the additive groups model with the new extended set of fragments introduced according to the quantum theory "Atoms in Molecule." The parameters for predic tion of  f H of radicals for of the considered class were determined. The renewed and extended by more than 25% database on  f H of alkyl radicals was proposed.Key words: free radicals, alkyl radicals, enthalpy of formation, enthalpy of bond disso ciation, quantitative structure-property relationships, additive groups model. Data on enthalpies of formation ( f H at 298.15 K and 0.1 MPa) of alkyl radicals (R • ) are key data in the ther mochemistry of organic free radicals. As a rule, various calculation methods for prediction of  f H(R • ) are ap proved on the basis of these values. They also represent a necessary attribute of reference sets for the calibration of the corresponding correlations of quantitative structureproperty relationships (QSPR). On the basis of them, correlations of chemical bonds in organic compounds are determined. These data are also important for studying processes of hydrocarbon transformations involving R • .Alkyl radicals R • were almost the first organic inter mediates characterized 1-3 by  f H, and a range of values were disputed for a long time, 4,5 due to which the re searchers concluded that the initial values of  f H for CH 3 C • H 2 , CH 3 CH 2 C • H 2 , (CH 3 ) 2 C • Н, and (CH 3 ) 3 C • should be increased by 10-16 kJ mol -1 . The NIST data base presently contains 6 the values of  f H for eight alkyls, of which only one, for the initial member of the series ( f H(С • Н 3 ) = 146.7±0.3 kJ mol -1 ), was established 7 by IUPAC. According to literature data, 8,9  f H are known for 20 alkyl radicals; the corresponding values are present ed for 24 compounds 10 ; and  f H for 26 compounds are given. 11 These values were presented 8,9 without any analy sis and there is a contradiction between them and pub lished 10 values, since the authors 10 used earlier, underesti mated values for the indicated above members of the ho mological series. It should be mentioned that these values were qualified 10 as "doubtful" with errors ranging from 8 to 13 kJ mol -1 . According to published data, 8,9,11 the errors of the discussed values do not exceed 2-4 kJ mol -1 ; however, in our opinion, the real values of these errors are not lower than 4-6 kJ mol -1 (except for that for radi cal • СН 3 ).An analysis of the enthalpies (energies) of С-X bond (Х = H, Cl) dissociation (D) in alkanes (RH) and mono chloroalkanes showed that the values of  f H(R • ) can be determined on the basis of D for a series of earlier un studied alkyl R • . T...