ACETANILIDE : A CRYSTAL WITH HYDROGEN-BONDED CHAINS. OPTICAL AND NEUTRON SCATTERING INVESTIGATIONS

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ACETANILIDE : A CRYSTAL WITH

HYDROGEN-BONDED CHAINS. OPTICAL AND NEUTRON SCATTERING INVESTIGATIONS

M. Barthes, R. Almairac, J. Sauvajol, R. Currat, J. Moret, J. Ribet

To cite this version:

M. Barthes, R. Almairac, J. Sauvajol, R. Currat, J. Moret, et al.. ACETANILIDE : A CRYSTAL WITH HYDROGEN-BONDED CHAINS. OPTICAL AND NEUTRON SCATTERING INVESTIGA- TIONS. Journal de Physique Colloques, 1989, 50 (C3), pp.C3-209-C3-215. �10.1051/jphyscol:1989332�.

�jpa-00229473�

JOURNAL DE PHYSIQUE

Colloque C3,

s u p p l h m e n t

au n03, Tome 50, Mars 1989

ACETANILIDE : A CRYSTAL WITH HYDROGEN-BONDED CHAINS. OPTICAL AND NEUTRON SCATTERING INVESTIGATIONS

M. BARTHES, R. ALMAIRAC, J.L. S A W A J O L , R. CURRAT*, J. MORET and J.L.

RIBET

Groupe de Dynamique des Phases Condensees, USTL, F-34060 Montpellier Cedex, France

* ~ n s t i t u t Laue-Langevin, Grenoble, France

L 8 ~ c 6 t a n i l i d e (ACN) e s t u n c r i s t a l a n h a r m o n i q u e c o n t e n a n t d e s c h a i n e s m o l 6 c u l a i r e s r e l i g e s p a r d e s l i a i s o n s hydroggne.

D e s a n o m a l i e s o n t 6 t 6 o b s e r v g e s

A

b a s s e t e m p g r a t u r e d a n s l e s s p e c t r e s i n f r a r o u g e s e t Raman. P o u r e x p l i q u e r c e s a n o m a l i e s plusieurs m o d k l e s t h g o r l q u e s o n t 6 t 6 hlabor6s. f a i s a n t a p p e l r e s p e c t i v e m e n t

A

d e s s o l i t o n s d e D a v y d o v , d e s m o d e s l o c a l i s 6 s , d e s s o l i t o n s t o p o l o g i q u e s o u d e s r g s o n a n c e s d e Fermi.

N o u s p r 6 s e n t o n s d e s m e s u r e s o p t i q u e s a i n s i q u e d e s r h s u l t a t s d e d i f f u s i o n i n g l a s t i q u e d e s neutrons. L e s m o d k l e s t h g o r i q u e s sont e n s u i t e d i s c u t 6 s s u r la base d e c e s n o u v e a u x r6sultats.

ABSTRACT :

Crystalline acetanilide(ACN) i s an anharmonic solid characterised by s o f t hydrogen bonded c h a i n s o f molecules. Anomalies i n the I.R. and Raman mode spectra were recently observed a t l o w temperature. Several theoretical models have been proposed t o explain these a n o m a l i e s , involving a l t e r n a t i v e l y ,

Davydov-like s o l i t o n s , l o c a l i z e d m o d e s , topological defects o r F e r m i resonance.

We present optical measurements: b i r e f r i n g e n c e , Brillouin s c a t t e - ring and I.R. absorption in the 7 0 0 - 8 0 0 cm-' range. Inelastic neutron scatte- ring r e s u l t s a r e a l s o sumrnarized.On the basis o f these n e w r e s u l t s the diffe- rent theoretical models a r e discussed.

INTRODUCTIOW :

H

Acetanilide (C6H5-N-C-CH ) , or ACN, is an orthorhombic (P

3

A5),

organic, trans-

bcaor 2h

parent and well-ordered crystal, in which parallel chains of amide groups (...H-N-C=O...), related by hydrogen bonds, run through the crystal along the b direction. This one -dimen-- 4

sionnal array of hydrogen-bonds is specially interesting for the following reasons :

-

To understand the hydrogen-bond mechanism is important in polymer physics, with direct applications to hydrogen-bonded polymers.

-ACN is a candidate to support non-linear solitonic-type excitations, which in this case could be studied in a highly ordered cristalline system.

-

In ACN, the amide groups display hydrogen-bond distance, (-N-H

...

0-),very close to those found in polypeptides or alpha helix proteins. Since the physical properties of mate- rials containing hydrogen-bonded amide groups are very sensitive to bond distances, ACN is expected to be a useful model system in the search of physical features of extended polypep- tides, and perhaps of alpha-helix proteins.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989332

C3-2

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0

JOURNAL

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PHYSIQUE

So a r e c e n t i n t e r e s t h a s been f o c u s s e d on t h i s s y s t e m . New o p t i c a l i n v e s t i g a t i o n s ( 1 ) have d e m o n s t r a t e d t h e e x i s t e n c e o f s e v e r a l i n f r a r e d a b s o r p t i o n b a n d s . ( a n d Raman modes), h a v i n g a n anomalous t e m p e r a t u r e dependence : a t 1650cm-~ ( n e a r t h e amide

-I

o r C = O s t r e t - c h i n g mode a t 1665cm-l), a t 3250cm-'(a band a s s o c i a t e d t o t h e N-H s t r e t c h i n g mode) and some o t h e r s i n t h e f a r i n f r a r e d ( c o r r e s p o n d i n g t o l o w - l y i n g o p t i c a l modes).

C o n s e q u e n t l y , s e v e r a l t h e o r e t i c a l models i n v o l v i n g n o n - l i n e a r s o l i t o n i c - t y p e e x c i t a t i o n s have been d e v e l o p p e d i n view of e x p l a i n i n g t h e s e o p t i c a l a n o m a l i e s .

The f i r s t t h e o r y ( 2 ) i s based on a Davydov-like s o l i t o n , o r a n e n e r g e t i c i n t e r a c t i o n between t h e amide-I mode and low-frequency l a t t i c e phonons. T h i s i n t e r a c t i o n i n v o l v e s s h i f t s i n t h e a v e r a g e p o s i t i o n o f t h e ground s t a t e s o f t h e low-frequency v i b r a t i o n s which i n t u r n , a c t a s a p o t e n t i a l w e l l t o t r a p t h e amide-I e n e r g y . L a t e r , s e v e r a l o t h e r models were propo- s e d i n v o l v i n g a l o c a l i z e d mode due t o a c o u p l i n g o f a h i g h f r e q u e n c y i n t e r n a l mode w i t h a c o u s t i c o r low-frequency o p t i c phonons (31, t o p o l o g i c a l s o l i t o n s ( l a r g e a m p l i t u d e f l u c t u a - t i o n s between a l t e r n a t e c h a i n s c o n f i g u r a t i o n s

)(4),

o r c o u p l e d o s c i l l a t o r s ( 5 ) .

However, a n a l t e r n a t i v e e x p l a n a t i o n o f t h e o p t i c a l a n o m a l i e s was g i v e n ( 6 ) i n t e r m s o f t e m p e r a t u r e - t u n i n g o f a Fermi-resonance.

The c o n t r o v e r s y between t h e models o f n o n - l i n e a r i n t e r a c t i o n s and F e r m i r e s o n a n c e i s s t i l l a l i v e .

So, t h e c h a r a c t e r i z a t i o n o f t h e dynamical p r o p e r t i e s o f ACN may c o n t r i b u t e t o c l a - r i f y i n g t h e n a t u r e o f n o n - l i n e a r e x c i t a t i o n s i n low-dimensionnal anharmonic s o l i d s . To t h i s a i m , we have s t u d i e d some o p t i c a l p r o p e r t i e s ( b i r e f r i n g e n c e , B r i l l o u i n s c a t t e r i n g and a p a r t o f t h e i n f r a r e d s p e c t r u m ) and i n e l a s t i c n e u t r o n s c a t t e r i n g o f a c o u s t i c modes o f ACN. I n t h i s p a p e r , p r e l i m i n a r y r e s u l t s o f i n c o h e r e n t n e u t r o n s c a t t e r i n g w i l l a l s o be sum- m a r i z e d .

S y n t h e s i s and g r o w t h o f ACN and o f f u l l y - d e u t e r a t e d ACN s i n g l e c r y s t a l s were done i n o u r l a b o r a t o r y . L a t e r , we p r e p a r e d powdered s a m p l e s o f :/I/-C6H5NHCOCH ; /2/-C6H5NDCOCH

3 3;

/ 3/-C6D5NHCOCD / 4 / (C6D5NDCOCD )

.

3 3

OPTICAL MEASUREMENTS

To -heck t h e a b s e n c e o f any p h a s e t r a n s i t i o n which c o u l d be r e s p o n s i b l e f o r t h e o b s e r v e d o p t i c a l a n o m a l i e s , we f i r s t measured t h e DTA s i g n a l , and a l s o t h e t e m p e r a t u r e

-9 3

dependence o f t h e l i n e a r b i r e f r i n g e n c e i n t h e b , c p l a n e . (The b i r e f r f n g e n c e i s r e l a t e d t o t h e s p e c i f i c e h e a t , and i s a v e r y s e n s i t i v e t e c h n i q u e t o p o i n t o u t s t r u c t u r a l phase

F i g .

1

' G ~ Z Fig. 2

I

100 150 2 0 0 250

T(K)

t r a n s i t i o n s ) . Between 50K and 300K t h e b i r e f r i n g e n c e f o l l o w s a c l a s s i c a l monotonous tempera- t u r e dependence, e x c l u d i n g t h e e x i s t e n c e o f a p h a s e t r a n s i t i o n i n t h i s r a n g e ( f i g 1 ) . DTA measurements g i v e a l s o t h e same r e s u l t .

B r i l l o u i n s c a t t e r i n g e x p e r i m e n t s were t h e n i n i t i a t e d between 7OK and 300K. The d a t a show a s t r o n g a n i s o t r o p y , a s e x p e c t e d i n a c h a i n s r r a n t a i n i n g c r y s t a l . T h e l o n g i t u d i n a l sound

// //

v e l o c i t y a l o n g t h e c h a i n s ,

%

=3130m/s, i s h i g h e r t h a n C J = 2 6 4 5 m/s and =2530 m/s i n t h e p e r p e n d i c u l a r d i r e c t i o n s .

The t e m p e r a t u r e dependence o f t h e f r e q u e n c y o f t h e l o n g i t u d i n a l B r i l l o u i n modes a l o n g t h e d i - r e c t i o n o f t h e c h a i n s ( / / b ) , and p e r p e n d i c u l a r ( / / c ) , i s shown i n f i g . 2 . The i m p o r t a n t chan- g e w i t h t e m p e r a t u r e r e f l e c t s t h e a n h a r m o n i c i t y o f ACN. The w i d t h o f t h e modes were a l s o mea- s u r e d , a s w e l l a s t h e f r e q u e n c y o f t h e t r a n s v e r s e a c o u s t i c modes. P r e c i s e measurements on deu- terated compounds a r e underway. D e t a i l e d r e s u l t s w i l l be d i s c u s s e d e l s e w h e r e .

R e c e n t l y , i n r e l a t i o n w i t h o u r r e s u l t s of i n c o h e r e n t n e u t r o n s c a t t e r i n g , we have r e - i n v e s t i g a t e d one p a r t of t h e i n f r a r e d s p e c t r u m , namely t h e 700-800 cm-I r e g i o n , u s i n g t h e samples numbered / 1 / , / 3 / and 141. Our measurements r e v e a l e d t h e e x i s t e n c e o f a wide band, c e n t e r e d a t a b o u t 750cm-I

,

i n t h e s a m p l e s / 1 / and 1 3 1 ; t h i s band i s t h n s u n a l t e r e d when t h e whole m o l e c u l e i s d e u t e r a r e d e x c e p t t h e p r o t o n o f t h e hydrogen-bond ( f i g 3 a , b ) . On t h e c o n t r a r y , t h i s band i s s t r o n g l y s e n s i t i v e t o t h e d e u t e r a t i o n of t h i s l a s t p r o t o n

( s a m p l e / 4 / , f i g 3 , c ) .

It i s t h e p r o o f t h a t i t i s r e l a t e d w i t h some movement i n v o l v i n g t h e hydrogen-bond, and n o t , a s p r e v i o u s l y a s s i g n e d ( 7 1 , t o a C-H o u t - o f - p l a n e bending mode. I n s u c h a c a s e , it would be s h i f t e d t o l o w e r e n e r g y i n t h e P a r t i a l l y / 3 / , and f u l l y d e u t e r a t e d / 4 / s a m p l e s . An i n v e s t i g a - t i o n of t h e e v o l u t i o n o f t h i s band a t l o w - t e m p e r a t u r e (25K) ( f i g 4 ) i n d i c a t e s a s p l i t t i n g and

a n i n c r e a s e o f t h e i n t e n s i t y , a s i n t h e anomalous band a t 1650cm-l. T h i s anomalous t e m p e r a t u r e - d e p e n d e n c e w a s a l r e a d y o b s e r v e d by G . C a r e r i e t a l ( s e e ( 1 ) f i g 4 ) , i n t h e f u l l y

-

h y d r o g e n a t e d s a m p l e . .This band c o u l d be r e l a t e d t o a n N-H o u t o f p l a n e b e n d i n g mode, which i s e x p e c t e d i n t h i s r a n g e o f f r e q u e n c y ( 7 ) .

NEUTRON SCATTERING EXPERIMENTS

The e x i s t e n c e of a s t r o n g anharmonic c o u p l i n g between o p t i c a l l y a c t i v e h i g h - f r e q u e n - c y i n t e r n a l mode4 and low-Erequency l a t t i c e modes, a 3 p o s t u l a t e d by s e v e r a l a u t h o r s , s h o u l d l e a d t o o b s e r v a b l e a n o m a l i e s on t h e d i s p e r s i o n and t e m p e r a t u r e dependence o f t h e low-frequen- c y modes. I n o r d e r t o c h a r a c t e r i z e t h e s t a t i c and low-frequency r e s p o n s e o f ACN, a c o u s t i c phonon d i s p e r s i o n measurements were done a s a f u n c t i o n o f t h e t e m p e r a t u r e , u s i n g h i g h r e s o - l u t i o n c o h e r e n t i n e l a s t i c n e u t r o n s c a t t e r i n g . A l l measurements have been c a r r i e d o u t a t t h e

~ ~ ~ t i t ~ t ~ ~ ~ ~ - ~ a n g e v i n i n G r e n o b l e . F u l l y d e u t e r a t e d s i n g l e c r y s t a l s were used ( 8 ) .

JOURNAL

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The d i s p e r s i o n c u r v e s o f t h e a c o u s t i c phonons i n t h e ( 0 0 1 ) s c a t t e r i n g p l a n e a n d a f e w p o i n t s o f a n o p t i q a l b r a n c h h a v e b e e n o b t a i n e d ( f i g 5 )

-

Sound v e l o c i t i e s a r e c a l c u l a t e d f r o m t h e s l o p e s o f t h e a c o u s t i c d i s p e r s i o n c u r v e s , a n d t h e c o r r e s p o n d i n g e l a s t i c c o n s t a n t s a r e d e d u c e d . C22 i s a p p r o x i m a t e l y t w i c e a s l a r g e as C I 1 , r e f l e c t i n g t h e a n i s o t r o p y i n t h e b o n d i n g b e t w e e n t h e m o l e c u l e s . The n e u t r o n r e s u l t s y i e l d s l i g h t l y l o w e r s o u n d v e l o c i t i e s t h a n t h o s e d e d u c e d f r o m B r i l l o u i n s c a t t e r i n g m e a s u r e m e n t s . The t e m p e r a t u r e d e p e n d e n c e o f t h e TA ( 0 1 0 ) , TA ( 1 0 0 ) a n d LA ( 0 1 0 ) a c o u s t i c s l o p e s b e t w e e n 10K a n d 29% were a l s o

Y Y

s t u d i e d . L a r g e t e m p e r a t u r e s h i f t s a r e o b s e r v e d , r e l a t e d t o a n h a r m o n i c p r o c e s s e s . However, no a n o m a l o u s b e h a v i o u r which c o u l d be a s s i g n e d t o a c o u p l i n g w i t h n o n - l i n e a r e x c i t a t i o n s , c o u l d be d e t e c t e d .

A s e a r c h f o r a n i s o t r o p i c e l a s t i c o r q u a s i - e l a s t i c s c a t t e r i n g was p e r f o r m e d a t va- r i o u s p o i n t s i n t h e r e c i p r o c a l s p a c e a n d a t d i f f e r e n t t e m p e r a t u r e s . The p r e s e n c e o f t o p o l o - g i c a l d e f e c t s s h o u l d i n p r i n c i p l e g i v e r i s e

t o q u a s i - e l a s t i c s c a t t e r i n g , a l t h o u g h n o e s t i m a t i o n s o f t h e m a g n i t u d e o f t h i s e f f e c t was made. No q u a s i - e l a s t i c b r o a d e n i n g c o u l d be d e t e c t e d w i t h o u r i n s t r u m e n t n l e n e r g y window o f 4OGHz.

P r e l i m i n a r y r e s u l t s o f i n c o h e r e n t n e u t r o n s c a t t e r i n g e x p e r i m e n t s i n d i c a t e t h e p r e - s e n c e o f a s t r o n g a n d v e r y wide maximum i n t h e f r e q u e n c y d i s t r i b u t i o n o f phonons i n t h e f u l - l y h y d r o g e n a t e d / I / s a m p l e and i n t h e p a r t l y d e u t e r a t e d / 3 / s a m p l e , a t a b o u t 750cm-l ( f i g 6 ) . T h i s w i d e peak i s e x t r e m e l y weak i n s a m p l e s / 2 / a n d / 4 / , i n which t h e p r o t o n o f t h e h y d r o g e n - bond h a s b e e n s u b s t i t u t e d . T h e s e d a t a , r e l a t e d t o t h e a b o v e

-

m e n t i o n n e d t e m p e r a t u r e d e p e n - d e n t 750cm-' I R a b s o r p t i o n band p r e s u m a b l y i n d i c a t e t h e p r e s e n c e o f a l o c a l i z e d mode i n - v o l v i n g t h e hydrogen-bogd. D e t a i l e d a n a l y s i s o f t h e t e m p e r a t u r e b e h a v i o u r o f t h e phonon den- s i t i e s o f s t a t e s f o r s a m p l e s / l / / 2 / / 3 / / 4 / w i l l be p u b l i s h e d e l s e w h e r e .

DISCUSSION

The l o w - f r e q u e n c y d y n a m i c a l p r o p e r t i e s o f ACN a r e t y p i c a l o f a n a n h a r m o n i c c r y s t a l - l i n e s o l i d . The p r e s e n t r e s u l t s g i v e n o i n d i c a t i o n f o r o b v i o u s a n o m a l i e s i n t h e l o w - f r e q u e n - c y p a r t ( a c o u s t i c modes) o f t h e phonon s p e c t r u m . M o r e o v e r , no e v i d e n c e f o r a n o m a l o u s q u a s i - e l a s t i c s c a t t e r i n g was f o u n d . A q u a n t i t a t i v e d i s c u s s i o n o f t h e p r e s e n t r e s u l t s i n t h e c o n - t e x t o f r e c e n t t h e o r e t i c a l models i s d i f f i c u l t , a s t h e y a i m a t e x p l a i n i n g t h e a m i d e -I o p t i - c a l a n o m a l y , which i s c l e a r l y o b s e r v e d i n t h e h y d r o g e n e o u s compound, / I / , o n l y .

It i s p o s s i b l e however

,

t o d i s c a r d t h e t h e o r e t i c a l m o d e l s i n v o l v i n g t o p o l o g i c a l s o l i t o n s , w h i c h s h o u l d g i v e r i s e t o some s t r u c t u r a l d i s o r d e r , d i s c e r n i b l e i n q u a s i - e l a s t i c s c a t t e r i n g . It i s c l e a r a l s o t h a t a D a v y d o v - l i k e s o l i t o n i c e x c i t a t i o n d u e t o a c o u p l i n g b e t - ween i n t e r n a l modes w i t h a c o u s t i c phonons i s n o t e v i d e n t . A t t h i s s t e p , t h e r e m a i n i n g p o s - s i b l e e x p l a n a t i o n s o f t h e o p t i c a l a n o m a l i e s a r e e i t h e r a n e n e r g e t i c i n t e r a c t i o n b e t w e e n i n - t e r n a l modes a n d l o w - l y i n g o p t i c modes, o r a n a c c i d e n t a l t e m p e r a t u r e t u n i n g o f a F e r m i re- s o n a n c e .

The wide maximum o b s e r v e d a t a b o u t 750cm-' i n i n c o h e r e n t n e u t r o n s c a t t e r i n g i s a s - s o c i a t e d w i t h a n i n f r a r e d b a n d . B o t h a p p e a r o n l y i n / 1 / a n d / 3 / s a m p l e s , a n d n o t i n s a m p l e s i n which t h e amide-group h a s b e e n d e u t e r a t e d . The i n f r a r e d band e x h i b i t s a n a n o m a l o u s tem- p e r a t u r e d e p e n d e n c e , t h e e x p l a n a t i o n o f which may be r e l a t e d t o t h o s e o f t h e a m i d e -I band a t 1650cm- 1

.

Fig. 3

Fig. 4

JOURNAL

DE

PHYSIQUE

I n t h i s c o n t e x t , a mechanism i n v o l v i n g a t e m p e r a t u r e t u n i n g o f a Fermi r e s o n a n c e becomes l e s s c o n v i n c i n g b e c a u s e i t b a s i c a l l y r e s t s on t h e a s s u m p t i o n o f a n a c c i d e n t a l d e g e n e r a c y b e t w e e n two e n e r g y l e v e l s . Now, t h e e x p e r i m e n t a l d a t a c l e a r l y show t h a t a t l e a s t t h r e e i n t e r n a l modes b e l o n g i n g t o t h e a m i d e g r o u p ( n a m e l y t h e C-0 s t r e t c h i n g , t h e N-H s t r e t c h i n g , a n d t h e mode a t 750cm-I i n v o l v i n g some v i b r a t i o n o f t h e H-bond) a s s o c i a t e d w i t h I . R . b a n d s s h o w i n g a n a n o m a l o u s t e m p e r a t u r e d e p e n d e n c e , a r e p r o b a b l y l p d r e s s e d " w i t h low- f r e q u e n c y p h o n o n s . An a c c i d e n t a l e f f e c t would n o t o c c u r o n most o f t h e v i b r a t i o n modes o f t h e amid%-group. On t h e o t h e r h a n d , i t i s u n c l e a r how a F e r m i r e s o n a n c e ( o c c u r r i n g i n t h e

750-800cm-' r e g i o n ) c o u l d be r e l a t e d w i t h a wide maximum i n t h e phonon d e n s i t y o f s t a t e s . M o r e o v e r , a s t r o n g a r g u m e n t f a v o u r i n g t h e mechanism o f F e r m i r e s o n a n c e ( 6 ) r e s t s on t h e d i s a p p e a r a n c e o f t h e a n o m a l o u s e f f e c t o n d e u t e r a t e d s a m p l e s . Now, t h e a n o m a l o u s band h a s a l s o b e e n o b s e r v e d i n a n o t h e r c r y s t a l ( N - m e t h y l a c e t a m i d e ) c o n t a i n i n g c h a i n s o f amide g r o u p s ( 9 ) a l t h o u g h a F e r m i r e s o n a n c e o n t h e same mode as i n ACN would be u n l i k e l y . The w e a k e n i n g o f t h e hydrogen-bond i n d e u t e r a t e d ACN p r o d u c e s a d e c r e a s e o f t h e o n e - d i m e n s i o n n a l c h a r a c - t e r , a n d p e r h a p s t h e p r o p a g a t i o n o f n o n - l i n e a r e x c i t a t i o n s , e x p l a i n i n g a l s o t h e l a c k o f a n o m a l i e s .

On t h e o t h e r h a n d , t h e o r i e s i n v o l v i n g a s e l f - t r a p p i n g o f t h e a m i d e g r o u p e n e r g y by means o f l o c a l i z e d - s t a t e s ( 3 ) o r Davydov l i k e e x c i t a t i o n s i m p l y i n g a c o u p l i n g o f t h e i n t e r - n a l modes w i t h l o w - l y i n g o p t i c a l p h o n o n s ( 1 ) a r e n o t c o n t r a d i c t e d by e x p e r i m e n t s .

Fig. 6

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