Enthalpies of formation of amidyl free radicals

Keywords: oxygen-containing heterocyclic radicals, structure-property relationship, methods of calculation, cyclic stress energy, enthalpy of formation.Study of the conversions of oxygen-containing organic compounds of cyclic structure has great value for understanding aspects of ecology, chemical technology, and biochemistry. The important role of free radicals in this determines the extreme urgency of a detailed study of their structure and physicochemical properties. Among the latter, thermochemical characteristics are the most informative.Experimental investigations of the thermochemical properties of radicals are linked with serious difficulties of a methodological and technical nature arising from the high chemical reactivity of radicals. Information on the thermodynamic properties of oxygen-containing organic radicals of a cyclic nature are extremely sparse and are of a preliminary nature [1, 2]. Among them only seven oxygen-containing heterocyclic radicals have been characterized by values of the standard enthalpies of formation (Δ f H o ).Calculated assessments of Δ f H o for the indicated radicals using phenomenological methods are hindered by the ambiquity of the cyclic stress energies E c [1]. Quantum-chemical methods are extremely resourceful at present but are also efficient only for the simplest radicals with less than 5-7 atoms [3]. Consequently until now extending the number of oxygen-containing heterocyclic radicals characterized by their Δ f H o values has come from fixation of their E c values, and the corresponding quantitative structure-property correlations have been established on these data.An approach was proposed in [4-10] for the resolution of this problem and has been carried out in practice for a wide class of radicals, including cyclic radicals [6-10]. In the present work this approach to the determination of Δ f H o of radicals, based on known values of the bond dissociation energies (D) and on quantitative

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“…The efficiency of such an approach has been convincingly demonstrated for organic radicals of various structure in [4][5][6][7][8][9][10].…”

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Formation enthalpies of organic oxygen-containing heterocyclic radicals

Orlov

Chernova

Туровцев

2011

Chem Heterocycl Comp

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Heteroatom-Centred Radicals for the Synthesis of Heterocyclic Compounds

Dénès

2018

Topics in Heterocyclic Chemistry

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Enthalpies of formation of alkyl radicals

et al. 2014

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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...

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Enthalpies of formation of amidyl free radicals

Cited by 6 publications

References 9 publications

Formation enthalpies of organic oxygen-containing heterocyclic radicals

Formation enthalpies of organic oxygen-containing heterocyclic radicals

Heteroatom-Centred Radicals for the Synthesis of Heterocyclic Compounds

Enthalpies of formation of alkyl radicals

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