Spin-density distribution of the p-O2 N·C6 F4 ·CNSSN free radical by ...

scientifichighlights modelling
Spin-density distribution
of the p-O 2
N·C 6
F 4·CNSSN
free radical by experiment
and ab initio calculations.
The X·C 6
F 4
CNSSN• radicals, where X is an anion, are very promising as
building blocks in the search for organic magnets free of metallic compounds,
a field of increasing interest in the scientific community. Knowledge
of the spin-density distribution in the CNSSN• dithiadiazolyl radical
ring (DTDA for short) constitutes a major step towards the understanding
of the magnetic and electronic properties of the rich magnetism
of DTDA derivatives. The O 2
N·C 6
F 4
CNSSN• radical was chosen as the
most favorable DTDA derivative to study the spin distribution in this kind
of free radical by polarized-neutron diffraction with support by ab initio
calculations based on density functional theory. Both the experiment
and the ab initio calculations show that almost all the spin density is localized
on the sulfur and nitrogen atoms of the DTDA ring with a small negative
spin density on the carbon atom of the DTDA ring and negligible
spin density over the rest of the radical.
In recent years, there has been
considerable interest in the study of the
magnetic behaviour of free radicals [1].
Part of this interest has focused on the
development and study of organic materials
that exhibit spontaneous magnetization
on decreasing temperature in
order to develop organic magnets free
of metallic elements. Such materials can
exhibit peculiar and probably unprecedented
properties not shown by the traditional
inorganic magnetic materials
based on metallic or ionic lattices: low
density, transparency, photo-responsiveness,
electrical insulation, biocompatibility,
low-temperature fabrication, easy
processability etc. Furthermore, a most
interesting characteristic of these materials
is the possibility of performing
slight chemical modifications to change
these physical properties.
The dithiadiazolyl radicals X·C 6
F 4
CNSSN•
(X = Br, O 2
N, CN,…) present a very interesting
variety of magnetic behaviours.
The p-NC-C 6
F 4
CNSSN• radical has been
reported as a weak ferromagnet with
an ordering temperature of 36 K [2], the
highest observed in a purely organic
N1
N1
O1
C1
F1
F1
C2
C2
C3
C3
F2
F2
C4
J. Luzon, G.J. McIntyre
and M.R. Johnson (ILL)
J. Campo and F. Palacio
(ICMA, Zaragoza)
C.M. Pask and J.M. Rawson
(University of Cambridge, UK)
magnet to date. By replacing the CN
group by Br a paramagnetic system
possessing very weak antiferromagnetic
interactions is obtained. And,
finally, if the CN group is substituted by
NO 2
to give the p-O 2
N·C 6
F 4
CNSSN• radical,
ferromagnetic ordering is observed
below T c
= 1.3 K [3].
These different magnetic behaviours
are due to the change in the molecular
packing and, consequently, a substantial
modification of the magnetic
interaction pathways. Knowledge of
the spin-density distribution in the
dithiadiazolyl radical ring allows us a
better understanding of the rich magnetism
of DTDA derivatives and hence
helps in the design of new organic ferromagnets
with higher T C
based on the
DTDA ring.
In the work reported here we have
studied the spin-density distribution
in the p-O 2
N·C 6
F 4
CNSSN• radical by the
complementary techniques of polarized-neutron
diffraction and ab initio
calculations.
-1.20
Figure 1: Experimental spin-density reconstruction of the p-O 2
N·C 6
F 4
CNSSN• radical projected
along the perpendicular to the CNSSN mean plane.
C5
N2
N2
S1
S1
µ B
/A 2
2.00
1.20
0.40
0.09
0.06
0.05
0.03
0.00
-0.03
-0.05
-0.06
-0.09
-0.40
102 one hundred and two | one hundred and three

µB N2 S1 C5 C1 C2 C3 C4 F2
BLYP-molecule 0.226 0.308 0.065 -0.003 0.001 -0.004 0.005 0.000
PW91-molecule 0.220 0.312 -0.061 -0.003 0.001 -0.004 0.002 0.000
VWM-molecule 0.225 0.310 -0.062 -0.003 0.001 -0.004 0.002 0.000
BLYP-crystal 0.214 0.317 -0.061 -0.003 0.002 -0.004 0.004 0.001
This experiment 0.247 0.281 -0.055
Table 1: Ab initio spin-density populations in the p-O 2
N·C 6
F 4
CNSSN• radical. See figure 1 for the
atomic labelling.
The polarized-neutron diffraction
experiment was performed on the D3
lifting-counter diffractometer at the
ILL at 1.5 K in an applied field of 9 T.
A multipolar model of the spin density
(figure 1) was fitted to the measured
flipping ratios. Almost all the spin density
is localized in the nitrogen and sulfur
p orbitals perpendicular to the DTDA
ring (0.281 µ B
and 0.247 µ B
, respectively).
These orbitals generate the Single
Occupied Molecular Orbital (SOMO).
Some negative density is observed
between the sulfur and nitrogen atoms
that can be due to covalence and polarization
effects. In addition, a small negative
density is observed (-0.055 µ B
) on
the carbon site of the DTDA ring due to
polarization effects. The spin density
over the rest of the radical is below
the limits of the experimental accuracy.
To support the experimental modelling,
ab initio calculations were performed
at the generalized gradient
approximation level, using several
exchange-correlation functionals
(PW91, BLYP, VWM) with a double
numerical basis set including functions
and numerical integrations, as implemented
in the Dmol3 software based on
density functional theory. Both periodic
(crystal) and molecular calculations
were made.
The ab initio calculations (figure 2 and
table I) confirm the experimental results
and, in particular, they show that the
negative spin density in the plane of
the DTDA ring is not a spurious effect of
the multipolar model used. This negative
spin density could play a very important
role in the ferromagnetic behavior of the
O 2
N·C 6
F 4
CNSSN• radical. The spin populations
in the rest of the molecule could
also be obtained: in the carbon atoms
there are alternate negative and positive
low spin density populations due to
polarization effects. Only a suggestion
of the latter was observed experimentally,
which emphasizes the importance
of combining experiment and ab initio
calculations in such studies.
Figure 2: Spin-density distribution of the p-O 2
N·C 6
F 4
CNSSN• radical calculated with the Dmol3
package (BLYP functional, molecular calculation). The red surface is the contour of 0.0002 µ B
/Å 3
and the blue one is the contour of –0.0002 µ B
/Å 3 .
References: [1] "Magnetic Properties of Organic Materials" Ed. By P. Lahti, Marcel Dekker Inc New York (1999)
[2] F. Palacio, G. Antorrena, M. Castro, R. Burriel, J.M. Rawson, J.N.B. Smith, N. Bricklebank, J. Novoa and C. Ritter, Phys. Rev.
Lett. 79 (1997) 2336-9
[3] J. M. Rawson, C.M. Pask, F. Palacio, P. Oliete, C. Paulsen, A. Yamaguchi and R.D. Farley (To be published)
103