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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. June 2005, Volume 4, Number 6, 367-380 |
Thermochemistry of Organic and Heteroorganic Species. Part XVI.
Application of IR Spectra of Unsaturated Aliphatic Molecules to the
Thermochemistry of Vinylic and Allylic Free Radicals
Dmitry Ponomarev and Viatcheslav Takhistov
Internet Electron. J. Mol. Des. 2005, 4, 367-380
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Abstract:
The application of IR-spectroscopy supplemented by method of
isodesmic (formal) reactions which characterize the relative
stabilities of free radicals is demonstrated as a useful tool for
getting novel data of the enthalpies of formation for certain types
of free radicals. In a result, ΔHf° values for
23 XC(•)=CH2 and 17
XC(•)=O novel radicals were obtained and for 8 more drastically
corrected from literature νCH values in
XCH=CH2 and XCH=O
molecules, respectively. On the example of H-CH2C(X)=O
molecules it was demonstrated the possibility to estimate the
enthalpies of formation of ground state free radicals from νCH
values for excited C-H bonds. Using the results on the latter
radicals the ΔHf° values for 7 novel
•CH2C(X)=CH2 radicals were
estimated with the aid of isodesmic reactions. In all these
calculations the variable ΔνCH value at H→X replacement
equivalent to 1 kcal mol-1 in C-H bond dissociation energy was
applied: 9 cm-1 for H-CH2R bond, 14 cm-1 for XC(-H)=O and 11
cm-1 for vinylic C-H bond. The strong destabilization effect in both
vinylic type XC(•)=O and XC(•)=CH2 free radicals at H→X
replacement (X - substituent with lone pair or π-electrons) was
found. For XC(•)=O radicals even with such groups as RO or R2N,
traditionally treated as strong electron-donating, destabilization
effect at H→X replacement was demonstrated. The origin of this
effect was suggested in the absence of overlapping of free radical
center with lone pair or π-electrons of substituent X. Stabilization
(destabilization) of vinylic free radicals is found to be the result of
electronegativity/polarizability interplay of a substituent.
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