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reproduces
E
0
for [Eu
2
(L3)
3
]
6þ
,17236cm
1
. For the amide ligand L4, inducing a N
6
O
3
coordination environment, one calculates
E
0
¼
17 235 cm
1
, in line with the observed
value of 17 232 cm
1
. Substitution of pyridine by Cl and NCS apparently affects the neph-
elauxetic contribution of this moiety leading to overall effects larger by 9 and 13 cm
1
,
respectively. When the amide group is substituted with a carboxylate function (ligands
H
2
L
i
,
i
17 231 cm
1
, in excellent agreement with the range of
¼
5-11) one calculates
E
0
¼
5-8 (17 232-17 235 cm
1
). When the fused benzene ring of benz-
imidazole groups is substituted by aromatic rings, the nephelauxetic effect decreases lead-
ing to
E
0
energies which are 6-11 cm
1
smaller than predicted. In view of the limited
number of substitutions, this trend is difficult to quantify. Finally, the N
3
O
6
environment
provided by ligands H
2
L12a,b results in a calculated
E
0
¼
values observed for
i
¼
17 229 cm
1
, in reasonable
agreement with the experimental values of 17 226 and 17 221 cm
1
, respectively. From all
these data, one sees that the accuracy of the relationship is around 5% (i.e., discrepancies
range between 1 and 11 cm
1
, representing 1-8% of the total nephelauxetic effect) which,
given the average increments taken into consideration, is satisfactory. The small span of
nephelauxetic effects displayed by the helicates for which relevant experimental data are
available is unfortunately insufficient to check if the trend in these effects is indeed
reflected in
t
rad
values, especially taking into account the relatively large experimental
uncertainties on the latter parameter. More data will be needed in this respect.
6.2.3 Site-Symmetry Analysis
As briefly outlined in Section 6.2.1, analysis of the fine structure of the emission bands
taking into account group-theory selection rules gives access to the symmetry of the Eu
III
site. Such analysis has been carried out for helicates [Eu
2
(L
i
)
3
](
i
5-9, 11) in frozen
aqueous solutions at 10 K because emission lines are narrower at low temperature.
Paramagnetic induced NMR shifts and relaxation times point to [Eu
2
(L5)
3
] having an
averaged structure with
D
3
symmetry at room temperature, so that emission spectra
have been analysed with respect to this point group of symmetry. A typical spectrum is
shown on Figure 6.5 and we describe its partial analysis only. In
D
3
symmetry, the
¼
161cm
-1
31 cm
-1
31 cm
E
E
A
2
579
580
581
580
590
600
580
600
620
640
660
680
700
λ
/ nm
Figure 6.5 High-resolution emission spectrum of a frozen solution of [Eu
2
(L7)
3
] in water/
glycerol 90 : 10 (v/v) at 10 K.
Reproduced with permission from ref. [63];
#
2008 New York
Academy of Sciences.
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