6 Synthesis and Selected Properties of Polymeric Sulfur Nitride, (Polythiazyl), (SN)x
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A. G. MacDIARMID, C. M. MIKULSKI, M. S. SARAN, P. J. RUSSO, M. J. COHEN, A. A. BRIGHT, A. F. GARITO and A. J. HEEGER Departments of Chemistry and Physics, University of Pennsylvania, Philadelphia, Penna. 19174
Analytically pure single crystals of polymeric sulfur nitride (polythiazyl), (SN) , suitable for solid state studies were prepared by the spontaneous room temperature polymerization of single crystals of pure S N . The polymer is relatively inert to oxygen and water. A single-crystal x-ray study revealed that an (SN) polymer molecule consists of an almost planar chain of alternating sulfur and nitrogen atoms in which the sulfur-nitrogen bond lengths are all very similar. The optical reflectance of crystalline films of (SN) from the near uv through the visible region is metal-like in character for light polarized parallel to the polymer chain axis. The covalent polymer has the optical and electrical properties of a low-dimensional metal. x
2
2
x
x
T i T e t a l s , b y d e f i n i t i o n , are those elements w h i c h possess c e r t a i n c h a r acteristic c h e m i c a l p r o p e r t i e s s u c h as ease i n f o r m i n g p o s i t i v e ions b y c h e m i c a l processes a n d c e r t a i n c h a r a c t e r i s t i c s o l i d state p h y s i c a l p r o p erties s u c h as h i g h e l e c t r i c a l c o n d u c t i v i t y ( w h i c h increases as the t e m p e r a t u r e is l o w e r e d ), h i g h reflectivity of l i g h t , g o o d t h e r m a l c o n d u c t i v i t y , d u c t i l i t y , a n d m a l l e a b i l i t y . U n t i l v e r y r e c e n t l y , i t w a s b e l i e v e d t h a t these c o l l e c t i v e properties w e r e u n i q u e to m e t a l l i c elements, b u t i t n o w appears t h a t m a n y of these properties m a y be f o u n d i n a s i m p l e i n o r g a n i c p o l y m e r that contains no m e t a l a t o m . Studies of the c o m p o u n d , p o l y m e r i c s u l f u r n i t r i d e ( p o l y t h i a z y l ), ( S N ) ^ , r e v e a l t h a t i t possesses m a n y of t h e a b o v e properties a n d m a y therefore b e t e r m e d a m e t a l even t h o u g h its m e t a l l i c characteristics are s t r o n g l y a n i s o t r o p i c .
I t is b e l i e v e d that this p o l y m e r
m a y be the f o r e r u n n e r of a w h o l e n e w class of p o l y m e r i c metals.
63 In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
64
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
S p a c e does not p e r m i t a c o m p l e t e d i s c u s s i o n of the b a c k g r o u n d i n f o r m a t i o n a v a i l a b l e o n ( S N ) * , b u t k e y l a n d m a r k s i n its s t u d y are s u m m a r i z e d v e r y briefly. P o l y m e r i c s u l f u r n i t r i d e w a s first p r e p a r e d i n 1910 b y B u r t ( 1 ) b y passing S N 4
4
v a p o r over s i l v e r gauze or q u a r t z w o o l at
1 0 0 - 3 0 0 ° C . I t w a s d e p o s i t e d outside the hot z o n e as v e r y s m a l l crystals or as a
film—when
t h i n , the film was b l u e b y t r a n s m i t t e d l i g h t , b u t , as i t
b e c a m e t h i c k e r , it b e c a m e o p a q u e a n d t o o k o n a b r o n z e m e t a l l i c luster b y reflected l i g h t . I n 1971 B o u d e u l l e et al. (2, 3, 4,5,6) f o r m e d as a p r i m a r y p r o d u c t w h e n S N 4
r e p o r t e d that S N , w h i c h w a s 2
4
2
v a p o r w a s passed o v e r h e a t e d
s i l v e r w o o l , w o u l d p o l y m e r i z e at l o w temperatures to g i v e p s e u d o single
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crystals c o m p o s e d of layers of p a r a l l e l fiberlike crystals of ( S N ) * .
The
crystals w e r e too s m a l l for s i n g l e - c r y s t a l x - r a y studies, b u t t h e y w e r e u s e d i n a p r e l i m i n a r y electron diffraction investigation.
Boudeulle
reported
r e c e n t l y t h a t the S - N b o n d lengths a l t e r n a t e d b e t w e e n short (1.58
A)
a n d l o n g (1.72 A ) b o n d s w h i c h are close to d o u b l e a n d single S - N b o n d lengths, r e s p e c t i v e l y . A l s o b o n d angles of 113.5° for S - N - S a n d 111.5° for N - S - N were reported. B y using more reliable single-crystal x-ray diffract i o n m e t h o d s , w e r e c e n t l y d e m o n s t r a t e d ( 7 ) t h a t these d a t a are n o t c o r rect (the S N 2
2
a n d ( S N ) * structures w e r e d e t e r m i n e d b y the M o l e c u l a r
S t r u c t u r e C o r p . , C o l l e g e Stations, T e x a s ) . I n 1973 L a b e s et al. (8,9)
s y n t h e s i z e d c r y s t a l l i n e b u n d l e s of i m p u r e
( S N ) a . fibers. A l t h o u g h the S : N a t o m i c r a t i o was 1:1, the m a t e r i a l c o n tained 5.48%
impurity (4.93%
0, 0 . 4 2 %
H , and 0.13%
C).
However,
m e t a l l i c - l i k e c o n d u c t i v i t y w a s o b s e r v e d i n d i r e c t i o n s p a r a l l e l to the ( S N ) ^ fibers, a n d this i n c r e a s e d s h a r p l y w i t h decrease i n t e m p e r a t u r e . S i x different samples h a d c o n d u c t i v i t i e s at r o o m t e m p e r a t u r e of 10, 89, 230, 640, 1470, a n d 1730 o h m " c m " . S i n c e the e l e c t r i c a l c o n d u c t i v i t y of a n a n i s o 1
1
t r o p i c substance c a n b e affected e n o r m o u s l y b y e v e n traces of i m p u r i t i e s , w e d e c i d e d t h a t i t w a s most i m p o r t a n t to a t t e m p t to synthesize a n a l y t i c a l l y p u r e crystals of ( S N ) * a n d to e x a m i n e the p h y s i c a l a n d c h e m i c a l p r o p e r t i e s of the m a t e r i a l . O n l y i n this w a y w o u l d i t b e possible to d e t e r m i n e w h e t h e r the m e t a l l i c - l i k e p r o p e r t i e s r e p o r t e d for ( S N ) * (8,9)
were
c h a r a c t e r i s t i c of the p u r e m a t e r i a l . T h e L a b e s ( 8 , 9 ) m e t h o d of p r e p a r i n g ( S N ) ^ u s u a l l y i n v o l v e d passing S N 4
4
v a p o r over h e a t e d s i l v e r w o o l a n d t h e n i m m e d i a t e l y c o n d e n s i n g
the i s s u i n g v a p o r s o n a c o l d finger at 0 - 8 ° C . H o w e v e r , w e f o u n d t h a t , i n o r d e r to synthesize r e p r o d u c i b l y large single crystals of a n a l y t i c a l l y p u r e i t is first necessary to p r e p a r e a b s o l u t e l y p u r e S N
(SN)
X
2
of S N ) b y p u m p i n g S N 4
2
4
silver w o o l at 2 2 0 ° C . c o l d finger, the S N 2
2
4
vapor formed from solid S N 4
4
2
(free of traces
at 8 5 ° C t h r o u g h
After being condensed on a l i q u i d nitrogen-cooled is s l o w l y s u b l i m e d f r o m the c o l d finger ( b y w a r m i n g
i t to r o o m temperature) i n t o a t r a p w i t h r e c t a n g u l a r w a l l s t h a t is i m m e r s e d
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
6.
M A C D i A R M i D
i n a 0 ° C bath. W h e n S N 2
2
65
Sulfur Nitnde
Polymeric
ET AL.
crystals of the correct shape a n d d i m e n s i o n s
h a v e f o r m e d , the r e c t a n g u l a r t r a p is r a i s e d to r o o m t e m p e r a t u r e . i n i t i a l l y colorless, t a b u l a r , m o n o c l i n i c crystals of S N 2
2
The
r a p i d l y t u r n intense
b l u e - b l a c k a n d b e c o m e p a r a m a g n e t i c ( g = 2.005). A f t e r s e v e r a l h o u r s , these crystals c h a n g e spontaneously to the b r i g h t , lustrous g o l d e n c o l o r of ( S N ) * ; h o w e v e r , c h e m i c a l a n d x - r a y studies ( 7 , 1 0 ) r e v e a l t h a t t h e y s t i l l c o n t a i n l a r g e a m o u n t s of u n p o l y m e r i z e d S N . H e n c e , p o l y m e r i z a t i o n m a y 2
p o s s i b l y start at the surface of a n S N 2
2
crystal and proceed inward. T h e
2
crystals are left at r o o m t e m p e r a t u r e for a p p r o x i m a t e l y t w o a d d i t i o n a l days, a n d t h e y are
finally
unpolymerized S N . 2
2
p u m p e d o n at 75 ° C to r e m o v e a l l traces of
T h e m a t e r i a l does n o t u n d e r g o a n y c h a n g e i n a p
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p e a r a n c e d u r i n g this t w o - d a y p e r i o d at r o o m t e m p e r a t u r e a n d the h e a t i n g t o 7 5 ° C . T h e (SN)
X
of S N 4
and S N
4
2
(JO, 11).
2
final
so o b t a i n e d gives n o E P R s i g n a l ; the absence
is d e m o n s t r a t e d b y a n x - r a y p o w d e r d i f f r a c t i o n s t u d y
T w o t y p i c a l analyses for the o b t a i n e d ( S N ) ^ are p r e s e n t e d i n
T a b l e I. Table I. Analysis Calcd. Found
(SN)* Crystals
0
S, %
N, %
C, %
H, %
0/ %
69.59 69.29° 70.18
30.41 30.56 30.20
0.00 0.00
0.00 0.00
0 0
e
Total,
%
100.00 99.85 100.38
d
Crystals were from two different preparations. Semiquantitative emission spec trograph ic analyses did not detect the presence of any metal impurities at the partsper-million level. Traces of oxygen are difficult to determine accurately experimentally. First set of data obtained by Galbraith Laboratories, Inc., Knoxville, Tenn. Reported as "none or trace." Second set of data obtained by Schwarzkopf Microanalytical Laboratory, Woodside, Ν. Y . Analyses were for sulfur and nitrogen only. α
6
c
d
6
T h e ( S N ) ; , . crystals f o r m e d i n this s t u d y h a v e a n e x t r e m e l y h i g h lustrous g o l d e n m e t a l l i c a p p e a r a n c e o n a l l faces, b u t the ends are d u l l , d a r k b l u e - b l a c k . T h i s is e x p e c t e d ( see b e l o w ). T h e lustrous surfaces of less w e l l f o r m e d crystals are h i g h l y s t r i a t e d ; this causes
considerable
scattering of l i g h t f r o m these surfaces at c e r t a i n i n c i d e n t angles a n d c o n s e q u e n t l y t h e y a p p e a r d a r k b l u e - b l a c k u n d e r these p a r t i c u l a r l i g h t i n g c o n ditions.
E l e c t r o n m i c r o g r a p h studies
(magnification
s p e c i a l l y chosen, i m p e r f e c t , single c r y s t a l of
100-1300)
of
a
( S N ) ^ r e v e a l e d t h a t the
crystals are c o m p o s e d of layers of fibers s t a c k e d p a r a l l e l to e a c h o t h e r a l o n g t h e l o n g axis (b axis) of the c r y s t a l . A t places w h e r e the c r y s t a l w a s s e p a r a t e d m e c h a n i c a l l y , l o n g fibrous strands of ( S N ) ^ are a p p a r e n t . T h e crystals are h i g h l y a n i s o t r o p i c , a n d t h e y m a y b e m e c h a n i c a l l y c l e a v e d v e r y e a s i l y a l o n g a p l a n e p a r a l l e l to the (SN),,. fibers. T h e crystals are also soft a n d m a l l e a b l e ; t h e y c a n be
flattened
r e a d i l y b y m i l d pressure
a p p l i e d p e r p e n d i c u l a r to the fibers to g i v e t h i n , lustrous, g o l d e n sheets.
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
66
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
T h e c r y s t a l l i n e m a t e r i a l also exhibits v e r y h i g h o p t i c a l a n i s o t r o p y i n t h e v i s i b l e p o r t i o n of the s p e c t r u m : o n l y t h a t c o m p o n e n t of i n c i d e n t l i g h t w h i c h is p o l a r i z e d p a r a l l e l to the ( S N ) * fibers is reflected ( R | ) w i t h h i g h (
m e t a l l i c reflectance. T h e c o m p o n e n t of i n c i d e n t a l l i g h t w h i c h is p o l a r i z e d p e r p e n d i c u l a r to the fibers is reflected ( R
±
) o n l y v e r y w e a k l y . T h u s , the
f a c e of a n ( S N ) * c r y s t a l m a y first b e e x a m i n e d w i t h n o n p o l a r i z e d l i g h t at a n i n c i d e n c e a n g l e of 90° w i t h the a n a l y z e r adjusted to g i v e m a x i m u m i n tensity of the reflected l i g h t . W h e n the a n a l y z e r is t h e n r o t a t e d t h r o u g h 9 0 ° , the i n t e n s i t y of the reflected l i g h t is r e d u c e d e n o r m o u s l y , a n d its c o l o r changes to a d a r k b l u e - b l a c k . It is therefore a p p a r e n t w h y the ends of the ( S N ) * crystals, w h i c h consist o n l y of ends of ( S N ) * fibers, a p p e a r b l u e - b l a c k — a t these surfaces a l l i n c i d e n t r a d i a t i o n is p e r p e n d i c u l a r to the Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch006
( S N ) * fibers; f u r t h e r m o r e , l i g h t scattering is extensive at these less s m o o t h surfaces. A l t h o u g h ( S N ) * crystals s l o w l y b e c o m e c o v e r e d w i t h a w h i t i s h - g r a y p o w d e r after t h e y s t a n d i n a i r f o r several m o n t h s , n o i m m e d i a t e r e a c t i o n w i t h a i r is a p p a r e n t ; h o w e v e r , the p o s s i b i l i t y of instantaneous f o r m a t i o n of a n o n v i s i b l e film of o x i d i z e d or h y d r o l y z e d m a t e r i a l c o a t i n g the ( S N ) * surface cannot yet be r u l e d out. D u r i n g seven days at r o o m t e m p e r a t u r e , crystalline (b)
( S N ) * is not
attacked b y
(a)
oxygen
w a t e r v a p o r at a pressure of 4.6 torr, or ( c )
water vapor.
at 1-atm
pressure,
oxygen saturated w i t h
It is not a t t a c k e d b y degassed d i s t i l l e d w a t e r w h e n
com-
p l e t e l y i m m e r s e d i n i t for 24 h r at r o o m t e m p e r a t u r e ; h o w e v e r , after six d a y s , a l t h o u g h there is no c h a n g e i n a p p e a r a n c e of the ( S N ) * , a v e r y s m a l l a m o u n t of w h i t e s o l i d m a t e r i a l m a y b e o b t a i n e d b y e v a p o r a t i n g the water.
W h e n i t is h e a t e d i n a sealed t u b e in vacuo
at a p p r o x i m a t e l y
1 4 0 ° C , ( S N ) * d e c o m p o s e s to sulfur, n i t r o g e n , a n d p o s s i b l y other as yet unidentified materials. W h e n ( S N ) * is h e a t e d w i t h constant p u m p i n g at ^
140 ° C , g o l d e n ,
lustrous, p o l y c r y s t a l l i n e cohesive films of ( S N ) * c a n b e c o n d e n s e d o n a v a r i e t y of substrates h e l d at temperatures of 0 ° - 5 0 ° C .
X-ray powder
patterns of scrapings of these s u b l i m e d films d e m o n s t r a t e that the (SN)-,. has the same c r y s t a l structure as the ( S N ) * crystals f r o m w h i c h t h e y w e r e s u b l i m e d a n d that the films are c o m p l e t e l y free of S N 4
4
and S N . Optical 2
2
reflectance a n d x - r a y studies of the films f o r m e d o n a glass surface r e v e a l e d that the ( S N ) * fibers a l w a y s l i e p a r a l l e l to the glass surface n o ( S N ) * fibers are o b s e r v e d p e r p e n d i c u l a r to the surface. e s t i n g p o s s i b i l i t i e s of e p i t a x i a l g r o w t h of
(II);
M a n y inter-
( S N ) * films therefore
appear
l i k e l y since f u l l y o r i e n t e d e p i t a x i a l films of ( S N ) * c a n b e d e p o s i t e d v a r i o u s substrates i n c l u d i n g M y l a r , T e f l o n , a n d p o l y e t h y l e n e method
by
on this
(12).
W e p r e p a r e d single crystals of a n a l y t i c a l l y p u r e ( S N ) * t h a t w e r e sufficiently large f o r s i n g l e - c r y s t a l x - r a y studies (10)
b y s o l i d state p o l y -
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
6.
M A CDIARMID
E TA L .
Polymeric Sulfur Nitnde
S(c) N(c)
67
S(a) N(a)
S(WN(W
S(d) NW)
1 m e r i z a t i o n of S N 2
2
crystals. T h e ( S N ) * crystals h a v e t h e same
g r o u p ( P 2 i / c ) as, a n d are p s e u d o m o r p h s of, t h e S N 2
2
space
crystals f r o m w h i c h
t h e y are d e r i v e d (10). T h e studies r e v e a l t h a t ( S N ) * consists o f a n a l m o s t Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch006
p l a n a r c h a i n o f a l t e r n a t i n g s u l f u r a n d n i t r o g e n atoms as i n 1. T h e r e are f o u r S N u n i t s p e r u n i t c e l l w h i c h has a = 4.153(6), b = 4.439(5), a n d c — 7.637(12) Α , β — 1 0 9 . 7 ( 1 ) ° , a n d D structure ( R =
c h a i n distances o f S ( a ) - N ( a ) S(a)-S(b)
=
c
=
=
3
1.593(5), S ( a ) - N ( b )
2.789(2), N ( a ) - N ( b )
=
T h e refined —
1.628(7),
2.576(7), a n d S ( a ) - N ( c )
2.864(5) A a n d b o n d angles o f S - N - S — 106.2(2)°.
2.30 g / c m .
0.11) has as its m a j o r feature ( S N ) * c h a i n s w i t h i n t r a —
and N - S - N —
119.9(4)°
T h e S - N b o n d lengths a r e a l l v e r y s i m i l a r , a n d t h e y c o r r e
s p o n d to a s u l f u r - n i t r o g e n b o n d o r d e r t h a t is i n t e r m e d i a t e b e t w e e n those e x p e c t e d f o r a single a n d a d o u b l e b o n d . C e r t a i n of t h e s u l f u r - s u l f u r i n t e r c h a i n distances (3.48 A ) b e t w e e n ( S N ) * c h a i n s l y i n g i n t h e same (102) p l a n e a r e less t h a n t h e s u m o f t h e v a n d e r W a a l s r a d i i o f t w o s u l f u r atoms (3.70 A ) . T h i s suggests t h e presence o f a w e a k b u t i m p o r t a n t i n t e r c h a i n i n t e r a c t i o n ( s h o w n as d o t t e d
Q
Figure I .
SULFUR
Ο
NITROGEN
Diagrammatic representation of (SN) chains in the 102 plane of an (SN) crystal X
X
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
68
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
lines i n F i g u r e 1 ) w h i c h is p r o b a b l y l a r g e l y r e s p o n s i b l e f o r c a u s i n g ( S N ) * to b e , at t h e v e r y least, a n a n i s o t r o p i c t w o - d i m e n s i o n a l m e t a l r a t h e r t h a n a one-dimensional metal. T h e o p t i c a l reflectance of t h i n films o f ( S N ) * w a s m e a s u r e d f r o m t h e n e a r U V (30,000 c m " ) t o t h e f a r I R ( 5 0 0 c m ' ) regions ( F i g u r e s 2 a n d 1
1
3 ). Reflectance c h a r a c t e r i s t i c of metals is o b s e r v e d i n t h e I R t h r o u g h t h e v i s i b l e regions, w i t h a w e l l d e f i n e d p l a s m a m i n i m u m at 22,000 c m ' corresponds (JJ).
1
that
to l i g h t p o l a r i z e d p a r a l l e l ( R ) to t h e p o l y m e r c h a i n axis M
Furthermore,
a n d R±
were
recently measured
u s i n g a face of a c a r e f u l l y p o l i s h e d s i n g l e c r y s t a l (12).
separately
T h e peaks i n
the I R at 9 9 5 a n d 685 c m " ( F i g u r e 2 ) a r e p r o b a b l y c a u s e d b y S - N 1
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v i b r a t i o n a l s t r e t c h i n g m o d e s . T h e e x p a n d e d p l a s m a e d g e p o r t i o n o f this c u r v e is p r e s e n t e d i n F i g u r e 3. I n b o t h c u r v e s , t h e d a s h e d l i n e is t h e least squares c o m p u t e r D r u d e fit ( J J ) to t h e m e a s u r e d reflectance.
Drude
reflectance is c o n c e r n e d o n l y w i t h t h e m e t a l l i c - l i k e reflectance of l i g h t , i.e. t h a t c o m p o n e n t of l i g h t t h a t is p a r a l l e l to t h e ( S N ) * fibers ( J J ) . I t w a s d e m o n s t r a t e d t h a t t h e difference b e t w e e n t h e c u r v e f o r m e a s u r e d reflectance a n d t h a t c a l c u l a t e d b y t h e D r u d e e q u a t i o n is a t t r i b u t a b l e to s u b s t a n t i a l i n t e r c h a i n c o u p l i n g ( J 2 ) . T h e m a x i m u m m e t a l l i c reflectance o b s e r v e d is a b o u t 4 5 % . T h i s v a l u e is a c t u a l l y v e r y m u c h greater t h a n i t first appears. T h u s , i t m a y b e r e c a l l e d t h a t t h e m a x i m u m m e t a l l i c reflec tance o f u n p o l a r i z e d l i g h t possible i n this system is 5 0 % since e a c h c r y s t a l d o m a i n i n t h e film w i l l reflect i n a m e t a l l i c f a s h i o n o n l y h a l f t h e i n c i d e n t l i g h t w h i c h falls u p o n i t , i.e. o n l y that c o m p o n e n t p o l a r i z e d p a r a l l e l to t h e (SN)*
fibers.
T h e r e a l m e t a l l i c reflectance is therefore o n t h e o r d e r of
9 0 % w h i c h is c h a r a c t e r i s t i c o f a m e t a l . 70
60
50
£
4
0
oc 30
20
10 04
06 08 10
20
3040 v
60 80100 (
°
m
*
200 300 χ Ι Ο ' Physical Review Letters
Figure 2. Reflectance spectrum (500-30,000 cm' ) of a thin film of (SN) . Dashed line: Drude fit to the measured reflectance (11). 1
X
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
6.
M A C D i A R M i D
Polymeric
ET AL.
Sulfur
69
Nitride
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Physical Review Letters
Figure portion Figure dashed
3. Expanded view of the plasma edge of the reflectance spectrum of (SN) in 2. Solid line: experimental spectrum; line: Drude fit to the expenmental spectrum ( 1 1 ) . X
F r o m the p l a s m a m i n i m u m ( p l a s m a f r e q u e n c y , ω ) , one c a n r e a d i l y ρ
c a l c u l a t e τ the e l e c t r o n i c r e l a x a t i o n t i m e b y t h e c l a s s i c a l D r u d e e q u a t i o n w h i c h is w e l l e s t a b l i s h e d for m e t a l films (11). 10"
15
T h i s gives a v a l u e of 1.9
X
sec f o r ( S N ) * . F r o m τ a n d ω , one c a n c a l c u l a t e the d c c o n d u c t i v i t y ρ
of a m e t a l b y the w e l l - k n o w n e q u a t i o n
(11):
dc c o n d u c t i v i t y =
ω τ/4π ρ
2
U s i n g this r e l a t i o n s h i p , a d c c o n d u c t i v i t y of 3 Χ
10 ohm" 3
1
cm"
1
is
c a l c u l a t e d for ( S N ) * a l o n g t h e fiber axis. T h i s v a l u e agrees w e l l w i t h t h e p r e l i m i n a r y d c c o n d u c t i v i t y ( f o u r - p r o b e m e t h o d ) of a single c r y s t a l of ( S N ) * a l o n g the d i r e c t i o n of the fiber axis. W e o b t a i n e d values of 3700 o h m " c m 1
- 1
1200-
at r o o m t e m p e r a t u r e ; the c o n d u c t i v i t y increases m a r k e d l y
w h e n the t e m p e r a t u r e is l o w e r e d to 4.2 K , i n c r e a s i n g a p p r o x i m a t e l y 50-205-fold.
T h u s , ( S N ) * has a c o n d u c t i v i t y at r o o m t e m p e r a t u r e o n the
same o r d e r of m a g n i t u d e as t h a t of a m e t a l s u c h as m e r c u r y (see I I ) . I t w a s r e c e n t l y r e p o r t e d (13)
t h a t ( S N ) * becomes s u p e r c o n d u c t i n g
at 0.26K. Table II. Substance (SN)* Bi Nichrome Hg Sb Fe Cu
Table
Electrical Conductivity Conductivity
(16)
at 20°C, 3.7 8.33 10.0 10.4 23.9 1.0 5.80
Χ Χ Χ χ Χ Χ Χ
10 10 10 10 10 10 10
ohm'
1
3 3 3 3 3 5
5
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
70
INORGANIC
COMPOUNDS
WITH
UNUSUAL
PROPERTIES
S i n c e the l e n g t h of a s u l f u r - n i t r o g e n single b o n d is e x p e c t e d to b e a p p r o x i m a t e l y 1.74 A a n d that of a s u l f u r - n i t r o g e n d o u b l e b o n d ~ 1.54 A (14, 15),
for a s u l f u r - n i t r o g e n b o n d o r d e r of 1.5, a n e x p e r i m e n t a l l e n g t h
of a p p r o x i m a t e l y 1.64 A w o u l d be e x p e c t e d . F r o m s i m p l e v a l e n c e concepts, the b o n d i n g b e t w e e n s u l f u r a n d n i t r o g e n atoms i n a n
bond (SN)*
c h a i n m a y therefore be c o n s i d e r e d as d e r i v e d , to a first a p p r o x i m a t i o n , f r o m the t w o extreme resonance forms
M
S-N Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch006
S=N
/
c
S
( 2 a n d 3 ) ( i n w h i c h a l l atoms
=N
X
S=N
s-N
S - N
e x h i b i t n o r m a l o x i d a t i o n states a n d valences ) to g i v e a resonance h y b r i d species ( 4 )
i n w h i c h a l l b o n d s are i n t e r m e d i a t e b e t w e e n d o u b l e a n d
single b o n d s . It s h o u l d be stressed that this d e s c r i p t i o n is o b v i o u s l y a n o v e r - s i m p l i f i c a t i o n since, a l t h o u g h a l l S - N b o n d s are i n t e r m e d i a t e
in
l e n g t h b e t w e e n single a n d d o u b l e b o n d s , t h e y are n o t a l l exactly e q u a l experimentally. A l t e r n a t i v e l y , since the N S m o n o m e r is the s u l f u r a n a l o g of
NO
( w h i c h has one u n p a i r e d electron i n a π* o r b i t a l ) , ( S N ) * m a y be r e g a r d e d as m a d e u p f r o m the p o l y m e r i z a t i o n of
•m o n o m e r u n i t s , w h i c h l i k e N O , m i g h t b e e x p e c t e d to h a v e a n i t r o g e n s u l f u r b o n d o r d e r of 2.5. P o l y m e r i z a t i o n t h e n gives 5; e a c h n i t r o g e n a t o m
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
6.
MACDIARMID
ET
Polymeric
AL.
Sulfur
71
Nitride
\ Ρ \
f
6 is d e p i c t e d as b e i n g associated w i t h e i g h t electrons, e a c h s u l f u r w i t h n i n e electrons. T h e n i n t h e l e c t r o n c a n b e r e g a r d e d as present i n one of t h e π* orbitals of e a c h - S ~ N - u n i t .
T h e s e π* o r b i t a l s o n adjacent - S ~ N -
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units c a n t h e n o v e r l a p to g i v e some m u l t i p l e - b o n d c h a r a c t e r to the other S - N l i n k a g e s , viz.
6.
It s h o u l d b e n o t e d t h a t this q u a l i t a t i v e t r e a t m e n t
differs f r o m the p r e v i o u s resonance h y b r i d a r g u m e n t — i t does not suggest t h a t the a m o u n t of m u l t i p l e b o n d c h a r a c t e r i n a l l S - N l i n k a g e s is neces sarily exactly the same.
T h e h a l f - f i l l e d o v e r l a p p i n g ττ* orbitals of
each
N S u n i t are e x p e c t e d to p r o d u c e a h a l f - f i l l e d c o n d u c t i o n b a n d i n t h e p o l y m e r m o l e c u l e i n a m a n n e r t h a t is s o m e w h a t s i m i l a r , f o r e x a m p l e , to the p r o d u c t i o n of a h a l f - f i l l e d c o n d u c t i o n b a n d i n m e t a l l i c L i * " p o l y m e r " b y the o v e r l a p p i n g of h a l f - f i l l e d 2s a t o m i c o r b i t a l s . F r o m these observations, i t appears t h a t ( S N ) * e x h i b i t s s t r o n g l y a l l the m a j o r c o l l e c t i v e properties c h a r a c t e r i s t i c of a m e t a l i n a d i r e c t i o n p a r a l l e l to the ( S N ) * class
of
metals.
covalent
fibers.
I t is, therefore, the first m e m b e r of a n e w
polymeric
materials—lower dimensional,
polymeric
I t is h i g h l y l i k e l y t h a t a w h o l e b r o a d n e w field of
polymeric
metals b a s e d o n d i a t o m i c n e u t r a l , c a t i o n i c , o r a n i o n i c m o n o m e r i c u n i t s t h a t c o n t a i n one u n p a i r e d e l e c t r o n c o u l d i n d e e d exist. A p r i m e r e q u i s i t e appears to b e t h a t the r e a l or h y p o t h e t i c a l m o n o m e r u n i t h a v e o n e u n p a i r e d electron. N e u t r a l species of this t y p e m i g h t b e sought f r o m b i n a r y c o m p o u n d s that c o n t a i n one element f r o m a n o d d - n u m b e r e d g r o u p i n the P e r i o d i c T a b l e a n d one f r o m a n e v e n - n u m b e r e d g r o u p , e.g. A s S . C a t i o n i c or a n i o n i c u n i t s c o u l d , i n p r i n c i p l e , b e o b t a i n e d b y r e m o v i n g o r a d d i n g a n e l e c t r o n , r e s p e c t i v e l y , to b i n a r y c o m p o u n d s i n w h i c h b o t h atoms w e r e from either even-numbered T a b l e , e.g.
or odd-numbered
(CS)~ and ( P N ) . +
groups
i n the
Periodic
I n this respect, i t m a y b e n o t e d t h a t a
species s u c h as ( C S ) * ' * is i s o e l e c t r o n i c w i t h ( N S ) . Species i s o e l e c t r o n i c X
w i t h ( N S ) * are, of course, n o t l i m i t e d to ions. isoelectronic w i t h N S a n d consequently
F o r e x a m p l e , H C S is
( H C S ) * — a n d its d e r i v a t i v e s ,
( R C S ) * , i f t h e y c o u l d b e s y n t h e s i z e d — m i g h t b e e x p e c t e d to b e metals. I t appears t h a t there is m u c h n e w c h a l l e n g i n g s y n t h e t i c c h e m i s t r y b o t h i n o r g a n i c a n d o r g a n i c - t o b e c a r r i e d o u t i n o r d e r to a s c e r t a i n t h e extent a n d i m p o r t a n c e of this n e w area of c h e m i s t r y t h a t is
concerned
w i t h c o v a l e n t p o l y m e r s w h i c h are metals.
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
72
INORGANIC
COMPOUNDS
WITH
UNUSUAL
PROPERTIES
Acknowledgment T h e authors w i s h to t h a n k J . M . T r o u p a n d B . A . F r e n z o f t h e M o l e c u l a r S t r u c t u r e C o r p . f o r m a n y h e l p f u l discussions c o n c e r n i n g t h e s i n g l e c r y s t a l x-ray studies.
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Literature Cited 1. Burt, F. P., J. Chem. Soc. (1910) 1171. 2. Boudeulle,M.,Douillard, Α., Michel, P., Vallet, G., C. R. Acad. Sci Ser. C (1971) 272, 2137. 3. Boudeulle,M.,Michel, P., Acta Crystallogr. A (1972) 28, S199. 4. Boudeulle, M., Douillard, Α., J. Microsc. Paris (1971) 11, 3. 5. Boudeulle, M., Ph.D. thesis, University of Lyon, 1974. 6. Boudeulle, M., Cryst. Struct. Commun. (1975). 4, 9. 7. MacDiarmid, A. G., Mikulski, C. M., Russo, P. J., Saran, M. S., Garito, A. F., Heeger, A. J., Chem. Commun. (1975) 476. 8. Walatka, Jr., V. V., Labes, M. M., Perlstein, J.H.,Phys. Rev. Lett. (1973) 31, 1139. 9. Hsu, C.H.,Labes, M. M.,J.Chem. Phys. (1974) 61, 4640. 10. MacDiarmid, A. G., Mikulski, C. M., Russo, P. J., Saran, M. S., Garito, A. F., Heeger, A. J., J. Am. Chem. Soc. (1975) 97, 6358. 11. Bright Α. Α., Cohen, M. J., Garito, A. F., Heeger, A. J., Mikulski, C. M., Russo, P. J., MacDiarmid, A. G., Phys. Rev. Lett. (1975) 34, 206. 12. Bright, Α. Α., Cohen, M. J., Garito, A. F., Heeger, A. J., Mikulski, C. M., MacDiarmid, A. G., Appl. Phys. Lett. (1975) 26, 612. 13. Greene, R. L., Street, G. B., Suter, L. J., Phys. Rev. Lett. (1975) 34, 577. 14. Lu,C.,Donohue, J., J. Am. Chem. Soc. (1944) 66, 818. 15. Sharma, B., Donohue, J., Acta Crystallogr. (1963) 16, 891. 16. "Handbook of Chemistry and Physics," 52nd Ed., p. E-72, The Chemical Rubber Co., Cleveland (1971-1972). RECEIVED January 24, 1975. Work supported in part by the National Science Foundation through the Laboratory for Research on the Structure of Matter and grants GH-39303 and GP-41766X, and by the Advanced Research Projects Agency through grant DAHC 15-72-C-0174.
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.