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F. Wang, L.-D. Lei and L.-M. Wu
the C O bond lying on a horizontal plane related to the
pꢀ-orbital. It leads to higher stability of nitroxides containing
acyl group, as observed in 2, 3, 6, 10, 13, 15, 16, 17, 18 and 19.
Figure 9. EPR spectrum generated by the UV irradiation of
NO-saturated pure tert-pentanal.
As a result, the lower spin density on nitrogen atom leads
to smaller ˇ-H HFSCs. However, an attempt to rationalize
the conformation of such radicals on the basis of EPR spectra
still requires a large amount of spectroscopic data and more
experiments and calculations.
coming from photofragmentation reaction of tert-pentanal,
expressed in Eqn (3). The BDE of the ꢂCH3ꢃ3C—CHO bond
is 81.6 kcal moleꢀ1. The other fragment is tert-butyl. (b) The
second one marked with open circles is a 0.791 mT 1 : 1 : 1
triplet with a g-value of 2.0064. This leads us to assign an
acyl group and another group without ˛-H atoms as being
attached to the nitroxide function. They are most likely
tert-butanoyl and tert-butyl in this case. The corresponding
radical species is numbered as the nitroxide 19. (c) The third
radical marked with arrows is a 1.555 mT 1 : 1 : 1 triplet
signal with a g-value of 2.0058, which is clearly di-tert-butyl
nitroxide. Therefore, we inferred the structures of nitroxide
18 and 19 as shown.
Acknowledgement
The authors thank the Natural Science Foundation of China for the
financial support (grant No. 20072013).
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CONCLUSION
The UV photochemical behavior of aliphatic aldehydes
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by NO trapping is mainly related to both a bimolecular
photoreduction to form a ketyl and an acyl radical and a
unimolecular fission at the C—CHO bond to give a formyl
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In the present work, ˇ-proton HFSCs estimated for
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9 and 11, exhibited smaller 14N-HFSCs (0.7–0.8 mT) with spin
delocalization induced by electron-withdrawing acyl groups.
Such stronger spin delocalization may be formed by overlap
of pꢀ-orbitals across the nitroxide function and the carbonyl
group. The most stable conformation may be controlled by
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Copyright 2004 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2005; 43: 156–165