University of Groningen
One-Dimensional Stacking of Bifunctional Dithia- and Diselenadiazolyl Radicals
Andrews, M.P.; Douglass, D.C.; Fleming, R.M.; Glarum, S.H.; Haddon, R.C.; Marsh, P.;
Oakley, R.T.; Palstra, T.T.M.; Schneemeyer, L.F.; Trucks, G.W.
Published in:
Journal of the American Chemical Society
DOI:
10.1021/ja00009a051
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Publication date:
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Citation for published version (APA):
Andrews, M. P., Douglass, D. C., Fleming, R. M., Glarum, S. H., Haddon, R. C., Marsh, P., Oakley, R. T.,
Palstra, T. T. M., Schneemeyer, L. F., Trucks, G. W., Tycko, R., Waszczak, J. V., Young, K. M.,
Zimmerman, N. M., & Cordes, A. W. (1991). One-Dimensional Stacking of Bifunctional Dithia- and
Diselenadiazolyl Radicals: Preparation and Structural and Electronic Properties of 1,3-
[(E2N2C)C6H4(CN2E2)] (E = S, Se).
Journal of the American Chemical Society
,
113
(9).
https://doi.org/10.1021/ja00009a051
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J.
Am. Chem. SOC.
1991,
113,
3559-3568
3559
One-Dimensional
Stacking
of
Bifunctional
Dithia-
and
Diselenadiazolyl
Radicals:
Preparation
and
Structural
and
Electronic
Properties
of
1,3-[
(
E2N2C)C6H4(CN2E2)]
(E
=
S,
Se)
M.
P.
Andrews,l' A. W. Cordes,**lb D. C. Douglass,la
R.
M. Fleming,'.
S.
H. Clarum,"
R.
C.
Haddon,*,l'
P.
Marsh,l*
R.
T. Oakley,*Jc T. T. M. Palstra,"
L.
F. Schneemeyer,"
G.
W.
Trucks,I'
R.
Tycko,"
J.
V.
Waszczak,18
K.
M. Young,lC and
N.
M. Zimmerman'.
Contribution from AT& T Bell Laboratories, Murray Hill, New Jersey 07974, Department of
Chemistry and Biochemistry, University
of
Arkansas, Fayetteville, Arkansas
72701,
and Guelph
Waterloo Centre for Graduate Work in Chemistry, Guelph Campus, Department
of
Chemistry
and Biochemistry, University
of
Guelph, Guelph, Ontario
N1
G
2
W1, Canada.
Received October
1.
1990
Abstract:
The preparation and solid-state characterization of the 1,3-phenylene-bridged bis(dithiadiazoly1) and bis(diselenadiazolyl)
diradicals
1
,3-[(E2N2C)C6H,(CN2E2)] (E
=
S,
Se) are reported. The isomorphous crystals of
1,3-[(E2N2C)C6H4(CN2E2)]
so
obtained are tetragonal, space group I4,/a. Stacks of diradical molecules, linked vertically
in
a zigzag fashion through
alternaie ends by long E---E contacts (mean 3.140/3.284
8,
for E
=
S/Se), are arranged in pinwheellike clusters about the
4, and 4 axes, producing complex patterns of interstack E- --E contacts. Both compounds show the presence of spin defects
in
the lattice, and there is a very large enhancement in the paramagnetism of the sulfur compound at high temperatures. The
selenium compound is a semiconductor, with a room temperature conductivity of
2
X
lo4
S
cm-l. Solid-state
NMR
experiments
find enhanced relaxation times, which have their origin
in
the presence of a mobile paramagnetic defect. Extended Hiickel
band structure calculations show the materials to
be
semiconductors, with band gaps of about 1.0/0.8 eV
for
E
=
S/Se. Although
the compounds adopt a columnar structure, the calculations indicate significant interactions between the stacks and the materials
exhibit well-developed three dimensionality. The enhanced paramagnetism in the sulfur compound is attributed to the presence
of thermally generated phase kinks
in
the lattice, whereas the selenium compound
is
classified as an intrinsic semiconductor.
Introduction
We are interested in the design of low-dimensional molecular
materials constructed from neutral as opposed to ionic radicals.
While the latter materials, which include simple charge transfer
salts, e.g., TTF TCNQ,2 and also the Bechgaard-type salts based
on
donors such as TMTSF and BEDT-TTF,3 have afforded much
insight into the general strategy of molecular metal design, the
former hold several potential advantages?
A
variety of systems
based
on
the originally proposed phenalenyl "building block" have
been in~estigated,~ but the comparison of theory and experiment
has been hindered by the absence of structural data.
Recent
advances in the design of thermodynamically stable hetrocyclic
thiazyl and selenazyl radicals? however, have prompted the ex-
ploration
of
such systems as molecular building blocks.7 Within
(I)
AT&T Bell Laboratories. (b) University of Arkansas. (c) University
of
Guelph.
(2)
See,
for
example: (a) Garito, A. F.; Heeger, A. J.
Acc. Chem.
Res.
1974,
7, 232.
(b)
Torrance, J. B.
Acc. Chem.
Res.
1979, 12, 79.
(c) Perlstein,
J. H.
Angew.
Chem.,
Int.
Ed.
1977,
16,
519.
(3)
(a) Wudl, F.
Acc.
Chem.
Res.
1984, 17, 227.
(b) Williams, J. M.;
Beno,
M.
A.;
Wang, H. H.; bung, P. C. W.; Emge, T. J.; Geiser,
U.;
Carlson,
K. D.
Acc.
Chem.
Res.
1985,
18,
261.
(c) Williams, J. M.; Wang, H. H.;
Emge, T. J.; Giescr, U.;
Beno,
M. A.;
Lung,
P. C. W.; Carlson, K. D.;
Thorn,
R.
J.; Schultz, A. J.; Whangbo, M.-H.
Prog. Inorg.
Chem.
1987,35,
51.
(d)
Inokuchi, H.
Angew.
Chem.,
Inr.
Ed.
1988, 27, 1747.
(4)
(a) Haddon,
R.
C.
Narure (London)
1975,256,394.
(b) Haddon. R.
C.
Ausr.
J.
Chem.
1975, 28, 2343.
(5)
(a) Haddon,
R.
C.; Wudl, F.; Kaplan,
M.
L.; Marshall, J. H.; Cais,
R.
E.; Bramwell,
F.
B.
J.
Am. Chem.
SOC.
1978,100,7629.
(b)
Kaplan, M.
L.; Haddon,
R.
C.; Hirani, A.
M.;
Schilling, F. C.; Marshall, J.
H.
J.
Org.
Chem.
1981, 46, 675.
(c) Haddon,
R.
C.;
Chichester,
S.
V., Stein,
S.
M.;
Marshall, J. H.; Mujsce,
A.
M.
J.
Org.
Chem.
1987.52, 71
1.
(d) Canadell,
E.; Shaik,
S. S.
Inorg.
Chem.
1987,26, 3797.
(e) Nakasuji, K.; Yamaguchi.
M.;
Murata,
1.;
Yamaguchi, K.; Fueno, T.; Ohya-Nishiguchi, H.; Sugano, T.;
Kinwhito, M.
J.
Am. Chem.
SOC.
1989,
Ill,
9265.
(6)
(a) Oakley,
R.
T.
Prog. Inorg.
Chem.
1988, 36, 299.
(b) Preston, K.
F.;
Sutcliffe, L. H.
Magn. Reson.
Chem.
1990,
28,
189.
this context the 1,2,3,5-dithiadiazolyI and
1,2,3,5-diselenadiazolyl
radicals
1
(E
=
S,
Se) are attractive candidates. Synthetic
strategies to such species are well-developed, and the cumulative
evidence of
ESR,
photoelectron and theoretical studies has pro-
vided a clear picture of their electronic structure;* the unpaired
electron in these derivatives is confined to the a2 distribution
2.
Consistently, hyperfine coupling constants to nitrogen and ioni-
zation potentials are relatively invariant to substituent effects.
Our initial approach to the construction of molecular conductors
from such radicals has been to design systems in which two radical
centers are incorporated into a single molecule.
Recently we
described the preparation and solid-state characterization of the
1,4-phenylene bridged 1,2,3,5-dithiadiazolyI and 1,2,3,5-disele-
nadiazolyl diradicals
3
(E
=
S,
Se).9
These materials are iso-
(7)
(a) Wolmershluser,
G.;
Schnauber, M.; Wilhelm, T.
J.
Chem.
SOC.,
Chem. Commun.
1984, 573.
(b) Wolmershauser,
G.;
Schnauber, M.; Wil-
helm, T.; Sutcliffe,
L.
H.
Synrh. Met.
1986,
14,
239.
(c) Dormann, E.;
Nowak, M. J.; Williams,
K.
A.; Angus,
R.
O.,
Jr.; Wudl,
F.
J.
Am. Chem.
SOC.
1987,
109,
2594.
(d) Wolmershiuser,
G.;
Wortmann,
G.;
Schnauber,
M.
J.
Chem.
Res.,
Synop.
1988, 358.
(e) Wolmershiuser,
G.;
Johann,
R.
Angew. Chem.,
Inr.
Ed.
1989,28,920.
(f)
Hayes, P. J.; Oakley,
R.
T.; Cordes,
A. W.; Pennington, W.
T.
J.
Am. Chem.
SOC.
1985, 107, 1346.
(g)
Boert,
R.
T.;
Cordes,
A.
W.; Hayes, P. J.; Oakley, R. T.; Reed,
R.
W.
Inorg.
Chem.
1986,
25,
2445.
(h) Oakley, R.
T.;
Reed, R. W.; Cordes, A. W.; Craig,
S.
L.;
Graham,
J.
B.
1987,
109,
7745.
(i)
Boer&
R.
T.; French, C. L.; Oakley,
R.
T.;
Cordes, A. W.; Privett, J.
A.
J.; Craig,
S.
L.; Graham, J. B.
J.
Am.
Chem.
SOC.
1985,107,7710.
G)
Awere, E.
G.;
Burford, N.; Haddon,
R.
C.;
Parsons,
S.;
Passmore, J.; Waszczak, J.
V.;
White, P.
S.
Inorg.
Chem.
1990,
29,
4821.
(k) Wolmershiuser,
G.;
Kraft,
G.
Chem.
Ber.
1990,
123, 881.
(8)
Vegas,
A.;
Perez-Salazar,
A.;
Banister, A. J.; Hey,
R.
G.
J.
Chem.
Soc.,
Dalton Trans.
1980, 1812.
(b) Hofs, H.-U.; Bats, J. W.; Gleiter,
R.;
Hart-
mann,
G.;
Mews,
R.;
Eckert-MaksiC, M.; Oberhammer, H.; Sheldrick,
G.
M.
Chem.
Ber.
1985,
118,
3781.
(c) Fairhurst,
S.
A,;
Johnson, K. M.; Sutcliffe,
L.
H.; Preston,
K.
F.; Banister,
A.
J.; Hauptmann,
Z.
V.;
Passmore, J.
J.
Chem.
Soc.,
Dalton Trans.
1986,
1465.
(d) Boer& R. T.; Oakley,
R.
T.;
Reed,
R. W.; Westwood,
N.
P. C.
J.
Am. Chem.
Soc.
1989,
Ill,
1180.
(e) Banister,
A.
J.;
Hansford, M.
I.;
Hauptmann,
2.
V.; Wait,
S.
T.;
Clegg. W.
J.
Chem.
SOC.,
Dalton Trans.
1989, 1705.
(f)
Cordes, A. W.; Goddard, J. D.; Oakley,
R.
T.;
Westwood,
N.
P. C.
J.
Am. Chem.Soc.
1989,
Ill,
6147.
(9)
(a) Cordes,
A.
W.; Haddon,
R.
C.;
Oakley, R.
T.;
Schneemeyer, L. F.;
Waszczak,
J.
A.;
Young,
K.
M.; Zimmerman, N. M.
J.
Am. Chem.
Soc.
1991,
113, 582.
(b) Del Bel Belluz, P.; Cordes, A. W.; Kristof, E. M.; Kristof, P.
V.;
Liblong,
S.
W.; Oakley, R. T.
J.
Am. Chem.
SOC.
1989,
111,
9276.
0002-7863/91/ 15 13-3559$02.50/0
0
1991 American Chemical Society
3560
J.
Am. Chem.
SOC.,
Vol.
113,
No.
9,
1991
Andrews et
al.
Figure
1.
ESR spectrum
of
4
(E
=
S)
in
CHCI,.
morphous and pack as interleaved arrays of discrete diradical
dimers. The sulfur compound is an insulator but the selenium
derivative exhibits a room temperature pressed pellet conductivity
of about
S
cm-l. Extended Hiickel band structure calculations
reveal the structures to have a significant degree of three di-
mensionality.
As
a continuation of this work we have prepared and charac-
terized the isomeric 1,3-phenylene bridged diradicals
4
(E
=
S,
Se). Herein we report the crystal and molecular structures of these
compounds and also provide details of their magnetic and con-
ductivity properties. The results are discussed in the light of
extended Huckel band structure calculations.
Results
and Discussion
Preparation
of
1,3-[(E2N2C)C6H,(CN2E2)]
(E
=
S,
Se).
The
dichloride salts
1,3-[(E2N2C)C6H4(CN2E2)]C12
(E
=
S,
Se) were
prepared as previously described for the corresponding 1,4-de-
rivatives. The necessary starting material, 1,3-phenylenebis-
[tris(
trimethylsilyl)amidine],
can be prepared by treatment of
1,3-dicyanobenzene with lithium
bis(trimethylsilyl)amide,
followed
by transmetalation of the intermediate N-lithio derivative with
trimethylsilyl chloride. Reduction of the crude dichlorides with
triphenylantimony in acetonitrile affords the diradicals
4
as black,
virtually insoluble powders. As before, the products were purified
by vacuum sublimation at 140
oC/10-2
Torr (E
=
S)
and
180
OC/104 Torr
(E
=
Se).
Solution-State
ESR
Measurements.
ESR
parameters have
been
reported
for
a wide range of monofunctional dithiadiazolyls
1
(E
Table
I.
Summary of Bond Lengths
(A)
and Angles (deg) and
Intermolecular Contacts
(A)
in
4
(E
=
S,
Se)
E=S
E
=
Se
av
range av range
d(E-E)
2.084
d(E-N)
1.627
d(N-C)
1.340
E-E-N
94.6
E-N-C
114.3
N-C-N
122
dl
d2
d3
d4
d5
dl
d8
d9
dl0
dl
I
dl2
d13
d14
dl5
dir
d6
2.078-2.090
1.616-1.638
1.330-1.348
94.0-94.9
113.8-1 14.8
121.9-122.5
3.104
3.179
3.139
3.136
4.004
3.914
3.960
3.984
3.448
3.71 1
3.509
3.691
3.280
3.817
3.959
4.509
2.335 2.323-2.352
1.78 1.72-1.83
1.34 1.30-1.39
91 89-92
115 110-120
128 121-1 35
3.181
3.368
3.31 1
3.277
4.086
3.875
4.007
4.090
3.456
3.714
3.530
3.662
3.891
4.613
3.486
3.628
s4
6
s2
S6
57
Figure
2.
Atom numbering scheme used for both structures. The view
shown
(for
E
=
S)
is
normal to the
C3-C8
plane.
=
S).8wvf
The ESR spectrum of
1
(E
=
Se, R
=
Ph) has also
been reported; its
g
value (2.0394) is predictably larger,gb ap-
proaching those found for [SN2Se2]'+ and [SeN2Se2]'+.10
As
in the case of the 1,4-diradicals
3,
ESR data for
4
is limited to
E
=
S;
for E
=
Se the material is sufficiently insoluble to reduce
signal intensity below the detection limit. The ESR signal of
4
(E
=
S),
recorded
on
a saturated solution in chloroform, has a
g
value of 2.01
1.
As illustrated in Figure
1,
the spectrum shows
a temperature dependence similar to although more extensive than
that seen for
3
(E
=
S).
The observed changes are characteristic
of very weak intramolecular" exchange coupling
(Jex)
between
two radical centers.'* At
-60
OC
the spectrum resembles that
expected for noninteracting radical centers, Le., a simple five-line
pattern with
uN
=
0.51 mT. Above this temperature the spectrum
begins to reveal the influence of exchange coupling between the
two radical centers; at
60
OC
the spectrum approaches the fast
exchange limit in which
J,,
>
aN.
(10)
(a) Awere,
E.
G.; Passmore,
J.;
Preston,
K.
F.;
Sutcliffe,
L.
H.
Can.
J.
Chem.
1988,
66,
1776.
(b)
Awere,
E.
G.;
Passmore,
J.;
White,
P.
S.;
Klapdtke,
T.
J.
Chem.
SOC.,
Chem.
Commun.
1989,
1415.
(1
1)
The low solubility of the diradicals precludes any possibility
of
in-
termolecular exchange interactions.
(12)
(a) Glarum,
S.
H.;
Marshall,
J.
H.
J.
Chem.
Phys.
1967,
47,
1374.
(b)
Briere, P.; Dupeyre, R.-M.; Lemaire,
H.;
Morat,
C.;
Rassat,
A.;
Rey,
P.
Bull.
Chim.
Soc.
Fr.
1965,
3290.
(c) Reitz, D.
C.;
Weissman,
S.
I.
J.
Chem.
Phys.
1960,
33,
700.
(d) Eaton,
G.
R.; Eaton,
S. S.
Acc.
Chem.
Res.
1988,
21,
107.
Stacking
of
Bifunctional Radicals
J.
Am.
Chem.
SOC., Vol.
113,
No.
9,
1991
3561
A
C
Figure
3.
xy
projection
(x
horizontal)
of
the cell packing. The
4,
axes
are perpendicular to this plane at the locations noted; the
4
points shown
are at elevations
of
z
=
0.375
and
0.875.
Crystal
and
Molecular
Structures.
The two diradicals
4
(E
=
S,
Se) have isomorphous structures; the crystals are tetragonal,
space group
14,/a.
A
summary of pertinent intramolecular and
intermolecular bond length and angle formation is provided in
Table
I.
There are two crystallographically independent diradical
units in the unit cell. An
ORTEP
drawing of a single asymmetric
unit for E
=
S
is shown in Figure 2. The mean internal structural
parameters of the diradicals are similar to those seen in
39
and
indeed in related monofunctional radicals. The
S-S
and Sese
bonds are slightly longer than in cationic structures, as are, to
a lesser degree, the
S-N
and
Se-N
distances. Taken collectively
the differences between the two oxidation states can be interpreted,
in simple
MO
terms, in terms of the occupation
(upon
reduction)
of the antibonding a2 orbital
2.
Within the unit cell of
4
(E
=
S,
Se) (Figure 3) there are four
pinwheellike clusters of vertically stacked arrays of diradical units
parallel to the
c
axis. Each of the vertical stacks consists of
molecular "plates" slightly offset from the ideal regular spacing
sequence, Le., the system has suffered a Peierls distortion. The
plates, however, do not couple into discrete dimers, as in the case
of the corresponding 1,4-diradical structures. Instead there is a
subtle rocking of opposite ends of each molecule
so
as to produce
a zigzag arrangement of short (mean of
d,
-
d4
=
3.14013.284
A
for
E
=
S/Se) and long (mean of
d5
-
de
=
3.966/4.014
A
for
E
=
S/Se)
E---E
contacts (see Figure 4 and Table
I).
This
rocking motion of the plates does not significantly perturb the
phenyl groups, for which the long and short centroid-to-centroid
distances are 3.57513.543
A
and 3.68713.596
A
(E
=
S/Se). The
planes of the two phenyl rings are, moreover, parallel to within
3.3/1.7O
(E
=
S/Se).
The lattice symmetry affords two symmetrically distinct
groups
of intercolumnar radical-radical contacts. The two groups are
distributed around, and defined by, the 4, axes (for example, at
x
=
0.75,
y
=
0)
and the
4
points (for example, at
x
=
0.50,
y
=
0.25,
z
=
0.375) (see Figure 3). The origins of the subtle
differences in the clustering of the radical dimers about these
symmetry elements can be understood with reference to Scheme
I,
which illustrates the effects of different Peierls distortions of
the idealized array A composed
of
the nearest chalcogen-chalcogen
contacts distributed consistently with the presence of both 4, and
4
symmetry elements. All three structures B,
C,
and D share a
common feature, namely, bond alternation (and unit
cell
doubling)
in the
c
direction. The bond alternation waves along the four
columnar components are, however, characterized by different
phase factors.
In
case
B
one set of 4,-related atoms moves upward
S8
SI)
I.
I
'A-
'8
a4
s2
P
Figure
4.
Zigzag
coupling
along
vertical stacks
of
diradicals.
Scheme
I
O=S
C
B
D
in unison, generating two distinct spiral arrays
of
close interco-
lumnar contacts about the 4, axis; this arrangement is common
to both the
sulfur-
and selenium-based compounds and is shown
(for E
=
S)
in Figure
5.
The short E-
-
-E contacts dlo-dIz are
listed in Table
I.
In
structures
C
and
D
alternate sets of atoms clustered about
4
points are elongated (in
C)
or
compressed (in D) approximately
along
c
in a manner that conserves the
4
symmetry. Depending
on whether the motion is expansive
or
compressive the short
intercolumnar contacts generated will be either between symmetry
unrelated (in
C)
or
related (in D) positions. In the sulfur-based
structure the assembly of
4
points is
of
type
C,
with short
S_2---S8'
contacts
(d13
and
d14),
while in the selenium derivative the 4 points
generate a structure of type D, with close Se2-
-
-Se2
(d,&
and
Se8-
-
-Se8
(d16)
approaches. The actual atomic arrangements
about these points in both structures
is
illustrated in Figure
6
(A
and
B).
3562
J.
Am. Chem.
Soc.,
Vol.
113,
No.
9,
1991
Andrews
et
ai.
Figure
5.
S-
-
-S
(Se-
-
-Se) contacts around the
4,
screw
axis.
The hollow
bonds
connect like atoms related
by
the screw axis,
with
spacings
d9
for
S3
(Se3)
and
dlo
for
S5
(Se5).
Solid-state Magnetic Measurements.
The measured magnetic
susceptibilities of
4
(E
=
S,
Se) as a function of temperature are
shown in Figures
7
and 8. The low-temperature susceptibilities
of
both compounds show Curie behavior, due to the presence
of
a small concentration of defects, which are associated with un-
paired electrons. The concentration of paramagnetic defects from
a Curie-Weiss fit to the data presented in the figures was found
to
be
0.97% (E
=
S)
and
0.51%
(E
=
Se) on a per molecule basis;
these values are similar to those found previously
for
3
(E
=
S).9a
The
0
values were 7 and
-1
K
and the measured diamagnetism
amounted to -160 and -164 ppm emu mol-' for E
=
S
and Se,
respectively.
ESR measurements on single crystals of the compounds pro-
duced very similar
results,
although there is some variation between
crystals in the concentration of paramagnetic defects. In the case
of
the samples whose ESR spectra (E
=
S)
appear in Figures
9
and
IO
the concentration
of
spins was found to be
0.50%
and
0.82%, respectively. The high-temperature behavior of the sus-
ceptibilities is most unusual, particularly in the case of the sulfur
compound. Both compounds show an increased paramagnetic
contribution to the susceptibility as the temperature is raised and
ultimately undergo decomposition. By assuming that the para-
magnetism may be described by a Curie relationship, the mole
fraction of spins is plotted as a function of temperature in Figure
11,
as determined from the bulk magnetic susceptibility and
single-crystal ESR studies. The bulk magnetic susceptibility and
single-crystal ESR results show good agreement between room
temperature and
200
"C. Although not obvious from the figure,
the concentration
of
spins, as determined from both the magnetic
susceptibility and ESR measurements, initially decreases above
room temperature for E
=
S
but not for the selenium compound,
which increases monotonically. In the case
of
the sample shown
in Figure
10,
the room temperature fraction
of
spins of 0.82% fell
to 0.52% at
50
"C, before starting a monotonic increase. This
n
sea
I
d14
B
Figure
6.
S-
-
-S
(A)
and
Se-
-
-Se
(B)
contacts about
the
4
points.
The
two
S-containing
radical
units
are
located
between
the
4
points
such
that
the
two
shortest contacts are S2--
38'
(the
solid
and
hollow bonds). The
Se
units
are
closer
to
the
4
points
such
that
the
shortest
contacts are
Se2-
-
-Se2
and
Se8-
-
-Se8 (hollow
bonds).
may suggest that initially some of the defects in the lattice of the
sulfur compound are annealed above room temperature, before
thermal energy begins to break the radical-radical bonds and
increase the concentration of paramagnetic defects. It may also
be seen in Figure
11
that the ESR line width at first broadens,
which would suggest that just above room temperature the spins
initially become more localized, before undergoing an increase
in motion around
360
K
as the concentration of defects begins
a substantial increase and the ESR line width narrows again. At
about 410
K
the line width broadens once more, presumably as
a result
of
dipolar interactions between pairs of electrons as the
concentrations of spins becomes significant. The break in the rise
in spin concentration of the sulfur compound as shown in Figures
7
and
11
at about 450
K
cannot be easily explained; careful
inspection of the data from the selenium compound suggests a
similar but vastly smaller feature at about 480
K.
Magnetic studies of the sulfur compound are reversible up to
about 170 OC, although molecular decomposition does not appear