Chemistry Reference
In-Depth Information
FIGURE 9.7.
Geometries of the
4
A
2
state and the
2
A
2
state of
p-6c
on the UB3LYP/6-311G
(
d
,
p
) level of theory. Both states show
C
2v
-symmetry and the bond lengths and angles, as well
as the calculated IR spectra are quite similar.
radicals. Both C
2v
symmetrical states show almost similar bond lengths and bond
angles. As a consequence, the calculated doublet and quartet infrared spectra are very
similar with only small frequency shifts. This makes it difficult to determine the spin
state of these nitreno radicals by infrared spectroscopy, in particular since only the
strongest bands are visible in the experiment. Hence, only EPR spectroscopy, which
is very sensitive to paramagnetic species, proves the quartet state of the nitreno
radicals. An interesting case is
p-
, where the doublet state has a nonplanar
C
S
symmetrical geometry, whereas quartet
p-
6b
is planar with
C
2v
symmetry. As a
result, the calculated IR spectra of the quartet and doublet states of
p-
6b
6b
are different
enough to allow a definitive assignment of the electronic state of
p-
6b
based only on
IR spectroscopy.
11
EPR signals of the nitreno radicals
p-
are observed during 308 nm irradiation
(XeCl excimer laser) of the precursors at 4-15 K. Annealing at 30 K results in the
disappearance of these signals, and subsequent 308 nm irradiation at 4 K in their
reappearance.
The EPR spectra of
p-
6
can be reproduced by simulation of quartet high-spin
systems with suitable ZFS parameters, which allows the assignment of all exper-
imentally observed EPR transitions. The simulation of the EPR spectrum of
p-
6b
6b
0.043 cm
1
resulted in excellent
agreement with the experimental spectrum (Figs. 9.8 and 9.9). The assignment of the
transitions (Table 9.4) is based on the simulated spectrum. The field dependence of
the magnetic energy levels can be calculated for the principal axes using the solution
of the Hamilton matrix of a quartet state
34,35
with the parameters derived from the
simulation. The axial transitions for the external magnetic field parallel to
x
,
y
, and
z
,
and their positions in the spectrum depending on the ZFS parameters are shown in
Figures 9.10 and 9.11, respectively. The transition
z
4 appears in the spectrum if the
D
-value is larger than
with ZFS parameters
j
D
/hc
j¼
0.285 and
j
E
/hc
j¼
0.16 cm
1
and moves with increasing
D-
value to higher field
values (Fig. 9.11). As all
z
signals, the position of
z
4 in the spectrum does not change
