measurements, performed using internal calibration, allowed
elemental composition and the number of double bond
equivalents (DBEs) to be reached for each targeted ion.
DBE corresponds to the number of double bonds and rings.
In case of an even-electron state ion, add 0.5 to convert the
reported DBE value to the actual number of double bonds.12
Analyst software version 2.1 was used for instrument control,
data acquisition and data processing. Direct sample introduc-
tion was performed at a 5 mL minÀ1 flow rate using a syringe
pump.
or the methoxycarbonyl radical. The relative rate of these
dissociations was observed to highly depend on the nature of
the radical trapped. MS/MS analysis of the spin adduct allows
unambiguous identification of the addend. This study also
showed an unexpected reactivity of these ketonitrones towards
ꢀCH3, resulting in the formation of EPR-silent methoxy-
amines. The feasibility of this approach should generate
a renewal of interest for ketonitrone spin traps, since
the results presented herein demonstrate the potential of
N-arylketonitrones in the identification of short-lived free
radicals.
Heats of formation
Heats of formation in the gas phase of some radicals could not
be found in the literature. A low performance calculation
(CS MOPAC, AM1) was thus performed. The followiÀn1g
Acknowledgements
L. C. acknowledges support from Spectropole, the Analytical
Facility of Aix-Marseille University, by allowing a special
access to the instruments purchased with European Funding
(FEDER OBJ2142-3341).
values were obtained: DH1f(ꢀCH3) = +130.6 kJ mol
,
DH1f(ꢀCH2OH) = À111.0 kJ molÀ1, DH1f[ꢀCH(CH3)OC2H5] =
À158.8 kJ molÀ1, DH1f[ꢀC(O)OCH3] = À210.0 kJ molÀ1
,
DH1f[ꢀC(CH3)(CO2CH3)2]
=
À616.0
kJ
À801.7 kJ molÀ1
molÀ1
,
DH1f[ꢀC(CH2OH)(CO2CH3)2]
=
,
aÀn1d
DH1f{ꢀC[CH(CH3)OC2H5](CO2CH3)2} = À820.0 kJ mol
.
References
Comparison with referenced data indicates the absolute calcu-
lated values are very approximate (DH1f(ꢀCH3) = +145.7 kJ
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molÀ1 15
,
DH1f(ꢀCH2OH) = À9.0 kJ mol
,
À1 16 DH1f[ꢀCH(CH3)-
OC2H5] = À81.2 kJ molÀ1 14
,
DH1f[ꢀC(O)OCH3] = À169.6 kJ
molÀ1 12
,
DH1f[ꢀC(CH3)(CO2CH3)2] = À607.6 kJ molÀ1 13,14
,
)
but could be used to validate the relative stability scale.
Conclusions
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First, the versatility of their synthesis pathway would permit
to prepare a much wider range of nitrones in this series,
bearing different functional groups and showing various pro-
perties.7 For example, it could be possible to prepare
N-arylketonitrones that could be covalently linked to natural
or synthetic macromolecules, or able to cross biological mem-
branes and enter the cells. In addition, the spin adducts
obtained after trapping free radicals by ketonitrones do not
disproportionate, which enhances their lifetime. Thus, the
various carbon-centred radical spin adducts n-Y considered
in this study were found to be stable for weeks in organic
media. Note however that, generally speaking, the efficiency of
spin trapping reactions is altered by the presence of bulky
groups born on either the nitronyl-carbon or the free radical
centre. These considerations let us think that N-arylketoni-
trones could be very efficient tools for qualitative studies, but
their use for free radical quantification should be avoided.
Though EPR signals of 1–3 various spin adducts were not
characteristic of the radical trapped, EPR analysis permitted
to prove the presence of paramagnetic species in the medium.
Collision-induced dissociation of the various spin adducts
mainly proceeds via three pathways, consisting of the elimina-
tion of the arylnitroso fragment, the radical initially trapped,
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ꢁc
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New J. Chem., 2008, 32, 680–688 | 687