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Entropies and heat capacities of free radicals
Author(s) -
O'Neal H. E.,
Benson S. W.
Publication year - 1969
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550010208
Subject(s) - radical , chemistry , thermochemistry , isopropyl , pyrolysis , heat capacity , thermodynamics , kinetic energy , atmospheric temperature range , computational chemistry , organic chemistry , physics , quantum mechanics
Methods are presented for rapidly estimating the entropies and heat capacities of free radicals from the known S 0 and C p 0of structurally similar compounds. The methods consist of estimating the differences due to changes in mass, vibration frequencies, spin, symmetry, and changes in rotational barriers. Tables of contributions to S 0 and C p 0by different frequencies over the temperature range 300–1500°K are presented to facilitate the tabulation of the above differences. Conjugated radicals, such as benzyl and allyl, are included. It is shown that the greatest uncertainties in the estimates arise from uncertainties in the barriers to rotation in the radicals. The results are applied to kinetic data on the pyrolysis of branched hydrocarbons and the reverse reactions of radical recombination. Major discrepancies exist in these data which can be nearly reconciled by postulating improbably high rotational barriers of 8 kcal for CH 3 rotation in isopropyl and t ‐butyl radicals. It is shown that radical thermochemistry can be fitted into group schemes and tables of groups values are given for the rapid estimation of Δ H f 0 , S 0 , and C p 0for different organic radicals, including those containing sulfur, oxygen, and nitrogen.