considered were structures for which all carbon-carbon bonds were assumed to be eqal and for which they were allowed'to alternate (i.e., double and single ...
JOURNAL DE PHYSIQUE Colloque C3, suppl6ment au n06, Tome 44, juin 1983
page C3-455
A T H E O R E T I C A L A N A L Y S I S O F ELECTRON ZEEMAN AND H Y P E R F I N E TENSORS FOR POLYACETY L E N E M.T.
Y.
.Jones(') H. ~ h o r n a n n ' ~ ) ,H. ~ i r n ' ~ )L.R. , ~ornkiewicz~~)
D a l t ~ n ' ~ ) B.H. ,
~obinson(~)and
(11 Department o f Chemistry, University o f Missouri-St. Louis, S t . Louis, MU 63222. U.S.A. (21 Department o f Chemistry, U n i v e r s i t y o f Southern C a l i f o r n i a , Los Angeles, CA 90089, U.S.A. (3) Department o f Chemistry, U n i v e r s i t y o f Washington, S e a t t l e , WA 98195, U.S.A. (41 IBM, T . J . Watson Research Center, Yorktown, NY 10598, U.S.A.
WsumB - On a isold l e s constituants des tenseurs Glectronicpes Zeeman pur des Bchantillons orient& au hasard ou m e l l e m e n t de p l y a c 6 t y l k e e t des c m p s d s de polyacgtylsne au moyen d'une spectroswpie EPR d'ondes m i l l h & triques. On a me-& l e s tenseurs hyperfins I3c e t I H avec une spectroscopic de r5sonance d i t e ENDOR e t TRIPLE. Tandis que l e s rgsultats des exgriences s'accordent en qualit5 avec l e s valeurs calcul&es par les t h h r i e s de soliton, crest-Fi-dire qu'on a is016 une fonction deplacse dlune onde dl&lectron n , l e s variations de densitd de spin observ6es dans l e s exgriences l e long de l a c h a k e du p l y h e ne s'accordent n i en qualit6 n i en quantit6 avec l e s thsories actuelles de soliton. Par contraste, nos mesures sont bien en accord avec l e s calculs d'orbitale mol6culaire d i t s F3MO e t McLachlan,qui suppsent que 116chantillon parinnagnCticpe e s t un p l y h e radical impair, l i n s a i r e e t neutre. Abstract - The components of t h e electron Zeeman tensors have been determined f o r randomly a n d partially oriented samples of p l y a c e t y l e n e and c o s i t e s of polyacetylene by millimeter w a v e EPR spectroscopy. T h e c o m p l e t e and ' H hyperfine tensors have been measured by ENDOR and TRIPLE resonance spectroscopy. While t h e experimental results a r e in qualitative a g r e e m e n t with t h e predictions of soliton theories in t h a t a delocalized d - e l e c t r o n wave function is observed, t h e experimentally observed spin density variations along t h e polyene backbone a r e in qualitative and quantitative disagreement with t h e currently existing soliton theories. In contrast, EWMO a n d McLachlan molecular orbital calculations, based on t h e assumption t h a t t h e paramagnetic e n t i t y being observed is a n odd, linear, neutral, polyene radical, reproduce t h e experimental observations very well.
A.
Electron Zeeman Tensor
The components of t h e electron Zeeman (g) tensor have been experimentally determined f o r randomly and partially oriented samples of cis- a n d trans-polyacetylene and t r a n s composites of polyacetylene by millimeter wave EPR spectroscopy which was used t o provide greater a c c u r a c y in t h e g-tensor measurements. T h e experimental results a r e summarized below. The value of g,, (the component observed when t h e magnetic f i e l d is para1leI t o t h e s t r e t c h e d direction) is 2.004. T h e value of g~ ( t h e m a g n e t i c field is pfrpendicular t o t h e s t r e t c h e d direction) is 2.003. This is in agreement with S. Kuroda e t a1 who reported t h a t g i ) ) gl. The components of t h e g-tensors f o r all t h r e e systems a r e identical a t liquid helium temperature. They a r e independent of t e m p e r a t u r e in t h e c a s e of cis-polyacetylene up t o room temperature. F o r trans-polyacetylene and trans-polyacetylene composites t h e r e a r e changes which c a n be explained in t e r m s of motional effects. The averages of t h e components of t h e tensor (i.e., t h e isotropic value) a r e independent of temperature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983390
JOURNAL DE PHYSIQUE
C3-456
The g-tensors f o r several p l y a c e t y l e n e fragments were c a l c u l a t e d using t h e Energ3 Weighted Maximum Overlap (EWMO) technique f i r s t proposed by Linderberg a n d Ohrn. Briefly, s o m e of t h e important conceptual a s p e c t s of t h e EWMO method a r e t h a t only a t o m i c p a r a m e t e r s (i.e., a t o m i c orbitals, a t o m i c orbital energies, a t o m i c s p i n - o r 3 t coupling energies, etc.) a n d t h e molecular geometry s e r v e a s input. See Jones and d e Boer a n d t h e references contained t h e r e i n f o r m o r e details regar d n g t h e EWMO method. Briefly, t h e g-tensors w e r e calculated f o r odd cis- a n d trans-polyacetylene f r a g m e n t s which varied in size from (CH)5 through (CH) 5. Calculations for larger s y s t e m s were n o t m a d e because t h e components of t h e g-tens8r a r e independent of s i z e f o r 1-119. Also, considered were structures for which all carbon-carbon bonds were assumed t o be e q a l a n d f o r which they w e r e allowed'to a l t e r n a t e (i.e., double and single bonds, respectively). T h e calculated g-tensors thus obtained a r e independent of t h e assumed s t r u c t u r e but in a g r e e m e n t with t h e experimental results (i.e., g,, = 2.0034 and gl = 2.0028). B.
Hyperfine Tensors
1 The c o m p l e t e 1 3 c and H hyperfine tensors have been measured by ENDOR and TRIPLE resonance spectroscopy and a r e shown below in t a b l e 1. T h e values of t h e hyperfine t e n s o r s for c i s - p l y a c e t y l e n e a r e independent of t e m p e r a t u r e from 3.8 t o 3 0 0 ' ~ . In c o n t r a s t , t h e y depend on t e m p e r a t u r e f o r trans-polyacetylene and t r a n s p o l y a c e t y l e n e composites. Although, a t very low temperatures, all s y s t e m s yield similar values f o r t h e hyperfine tensors. Again t h e s e t e m p e r a t u r e dependent e f f e c t s c a n b e explained in t e r m s of motional effects. T h e details of t h e ENDOR technique2 used t o evaluate t h e absolute values of t h e hyperfine tensors have been presented elsewhere. T h e important result of t h o s e experiments is t h a t only t w o different values of spin-density a r e observed t o exist on t h e carbon backbone of t h e polyacetylene (i.e., values of 0.06 and 0.02). TRIPLE resonance experiments show t h a t t h e signs of t h e spin-densities at t h e s e t w o different s i t e s a r e opposite t o e a c h other. Since t h e numbers of carbon a t o m s (at a given paramagnetic s i t e ) which possess t h e t w o different spin densities a r e essentially equal and, by definition, t h e sum of t h e spin-density t a k e n over all of t h e carbon a t o m s a t a given pramagnetic s i t e rnust equal +1, t h e smaller value (0.02) rnust c a r r y t h e negative sign. table 1 'roton
and 1 3 c Hf s Tensors f o r
Cis-Rich Poliacetylene Measured a t 135'
Proton Tensor Ax = +0.5 MHz A A
Y Y
3~ Tensor
A
= -1.3 MHz
= +2.1 MHz
Ax = -1.3 MHz
= +1.1 MHz
Az = +2.1 MHz
