Chemistry Reference
In-Depth Information
CH
2
Ph
O
O
N
O
N
N
O
O
9
X-ray: Aldabbagh
26
10
X-ray: Aldabbagh
26
Scheme 5.6
Aldabagh
et al
.
26
used nitroxides
9
and
10
(Scheme 5.6) to control the free radical polymerization of
styrene and discussed the results in terms of structural parameters. According to X-ray crystal structures,
the six-membered ring of
10
deviates significantly from planarity as compared to the planar five-membered
ring of
9
. Thus,
10
possesses a more exposed oxyl group leading to a higher rate of radical trapping (k
c
)
and a lower rate of NO-C bond homolytic cleavage in the corresponding alkoxyamine (Section 5.6.1,
Scheme 5.31).
Nitronyl nitroxides, imino-nitroxides, and, to a lesser extent, nitroxides and dinitroxides are frequently
used as stable spin-building blocks to prepare magnetic materials.
27
The X-ray structure and magnetic
behavior of numerous crystalline materials formed with these building blocks have been reported.
28
Discussing the results of these numerous studies is beyond the scope of this chapter and only a few
reports describing the X-ray structure of non-coordinated new nitroxides or dinitroxides (Scheme 5.7)
are mentioned.
All-organic liquid crystals containing a chiral five-membered cyclic nitroxide unit within the rigid core
(Scheme 5.8) have been prepared and their X-ray structures and magnetic properties have been studied.
29
The X-ray structures of other nitroxides used in miscellaneous applications have also been determined;
some
30
are shown in Scheme 5.9.
5.2.3 Quantum mechanical (QM), molecular dynamics (MD) and molecular
mechanics (MM) calculations
The EPR parameters of a nitroxide are strongly dependent on its geometry and the nature of the embedding
environment, particularly the polarity and the presence or absence of molecular oxygen dissolved in the
solution. Nitroxides are widely used as reporter molecules to gain different kinds of information: motion of
biomolecules (spin labeling), oxygen content of a medium (oximetry), pH values, formation of inclusion
complexes,
31
and so on. As a result, it is important to fully understand how the molecular geometry and the
solvent influence the magnitude of the hyperfine coupling constants (hccs) of a nitroxide. Kikuchi
et al
.
32
explored the conformational space of small nitroxides by means of Monte Carlo (MC) simulations, the hccs
being then obtained by averaging results of quantum calculations on MC configurations. Barone
et al
.
33
developed various approaches to study the magnetic properties of large nitroxide systems
in vacuo
and in
condensed phases. They showed that within the density functional theory (DFT) approach,
33e
the popular
B3LYP hybrid functional coupled with the standard basis 6 - 31
G(d,p) was appropriate to reproduce the
experimental geometries and hccs of nitroxides accurately. However, they also developed some special
basis sets, like the EPRII, EPRIII. and N07D.
33g
With respect to environmental effects on the magnitude of hccs, a supermolecule composed of the
solute and the solvent molecules strongly coordinated to the aminoxyl group must be considered. Then,
adding bulk solvent effects in a suitable way, for instance, by the Polarizable Continuum Model (PCM),
+
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