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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. October 2004, Volume 3, Number 10, 662-671 |
Prediction of R-H Homolytic Bond Dissociation Energies
from Gas-Phase IR Data on νCH Values
Aleksander Golovin, Dmitry Ponomarev, and Viatcheslav Takhistov
Internet Electron. J. Mol. Des. 2004, 3, 662-671
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Abstract:
The main goal of this work is the application of νCH IR frequencies
of gaseous R-H molecules to thermochemistry of carbon-centered
R• free radicals. For the first time the variable ΔνCH (in R-H
molecules) per 1 kcal mol-1 in bond dissociation energies are
suggested: 9 cm-1 for alkanes, 11 cm-1 for C-H bond in alkenes, 12
cm-1 in RC≡C-H and 14 cm-1 for RCH=O molecules. This being
accepted the exact ΔHf° values for several key aliphatic free
radicals (Me3C•, •CH2OH,
•CH2NH2, •CH2NHMe, •CH2NMe2)
and the novel values for 12 substituted ethynyl RC≡C• radicals are
suggested. For the latter, the specific role of polarizability of
substituent R in stabilization of radical center is demonstrated. The
limitations and difficulties in application of IR spectra to free
radicals' thermochemistry are analyzed. The absence of low lying
("adiabatic") C-H bond in many CH3X molecules and the
observation of these bonds at much (by 60-180 cm-1) higher
frequencies compared with expected ones makes questionable the
application of IR spectra to these purposes. However, the small
deviations by 20-40 cm-1 can be well treated by isodesmic
reactions control and Raman spectra for prediction of "lost"
frequency and ΔHf° of free radical as it was demonstrated for
•CH2OCH3 and Me3CCH2• radicals.
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