Chemistry Reference
In-Depth Information
FIGURE 9.11.
Simulation of the field dependence of the most intense axial transitions of a
quartet system with
D-
values between 0.05 and 0.285 cm
1
and
E
/
D
between 0 and 0.35. The
values
j
E
/hc
j¼
0.043 cm
1
of
p-
6b
are marked.
Source
: Reprinted from
Ref. 13 with permission from the American Chemical Society.
D
/hc
j¼
0.285 and
j
are calculated without any spin polarization. It has unfortunately been found
previously that inclusion of the spin polarization into the calculation of the SS
term (which is technically feasible of course) significantly degrades the agreement
with the experimental values.
52
Thus, at present, there does not seem to be a
satisfactory solution to this problem in a DFT framework.
The calculations demonstrate that the bulk of the
E
-value arises from one-center
contributions of the direct spin-spin coupling interaction. This information together
with the fact that
p-
simultaneously exhibits nitrene and carbene character then
leads to a simple and appealing geometric interpretation of the large
E
-value. We
assume for
p-
6b
N bond,
the
x
-axis perpendicular to the molecular plane and the
y
-axis, consequently, lies in
the molecular plane. In the nitrene part of the electronic structure, the two SOMOs
are comprised of
p
x
and
p
y
orbitals and their dipolar field is consequently directed
along
z
. For the carbene part of the 1-
4
A
2
ground state, the SOMOs are comprised of
p
z
and
p
x
contributions and consequently the resulting dipolar field points along
y
.
The calculations show that the magnetic
z
-axis of the quartet nitreno radical is
located parallel to the N
6b
a coordinate system where the
z
-axis points along the C
C bond, the
x
-axis is perpendicular to the molecular plane,
and the
y
-axis lies in the molecular plane (Fig. 9.13). Thus, the nitrene character
prevails over the carbene character, the main magnetic axis is still pointing along the
C
N bond in the
z
-direction. However, the carbene character is reflected by an
enhanced dipolar field in the
y
-direction, which is sufficiently strong in order to give
significantly magnetically inequivalent
y
- and
x
-directions. Thus, we come to the
