ORGANIC CHEMISTRY AT HIGH PRESSURE : CAN UNSATURATED BONDS SURVIVE 10 GPa ?

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ORGANIC CHEMISTRY AT HIGH PRESSURE : CAN UNSATURATED BONDS SURVIVE 10 GPa ?

M. Nicol, G. Yin

To cite this version:

M. Nicol, G. Yin. ORGANIC CHEMISTRY AT HIGH PRESSURE : CAN UNSATURATED BONDS SURVIVE 10 GPa ?. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-163-C8-172.

�10.1051/jphyscol:1984830�. �jpa-00224330�

JOURNAL DE PHYSIQUE

Colloque C8, supplement au n ° l l , Tome 45, novembre 1984 page C8-163

ORGANIC CHEMISTRY AT HIGH PRESSURE : CAN UNSATURATED BONDS SURVIVE 10 GPa ?

M. N i c o l and G.Z. Yin

Department of Chemistry and Biochemistry, University of California- Los Angeles, Los Angeles, California 90024, U.S.A.

Abstract - When solutions of naphthalene in mixed alkanes or alcohols are irradiated at 315 nm or shorter wavelengths, naphthalene is destroyed by a series of reactions whose rates are greatly accelerated at high pressures.

Analyses of the photoproducts recovered from diamond-anvil high pressure cells by gas chromatography-mass spectrometry demonstrate that several reactions are involved: 1) sensitized photolysis of solvent molecules to alkyl and alkoxy radicals; 2) reduction of naphthalene to tetrahydronaphthalene and hydronaphthyl radicals; 3) polymerization of the hydronaphthyl and alkyl radicals to dimers, trimers and higher polymers; 4) photoaddition of solvent radicals to naphthalene; and 5) H-D exchange between naphthalene and the solvents. The dependence of rate of disappearance of naphthalene on the excitation intensity shows that the primary photochemical step involves two-photons and triplet naphthalene intermediates that sensitize production of the free radicals which, at high pressures, are efficient consumers of unsaturated bonds. Implications of these and other phenomena described in the high pressure literature for the stabilities of unsaturated organic compounds at high pressures are discussed.

The chemistry of organic materials at high pressures is of interest for producing diamonds, high-strength plastics, and other hard materials; selectively accelerating chemical syntheses; understanding explosives; and making new metals and superconductors. Unusual transformations have been reported for specific systems, including: cooking proteins [1]; polymerizing CO [2], CS2 [3], and benzene [4];

synthesizing tetrathiofulvane directly from CS? and C0H2 [5]; and converting polyacetylenes from semiconductors to insulators [D. Schiferl and K. Syassen, personal communications]. Except for reaction kinetics at relatively low pressures, these phenomena have not been correlated in a systematic way. Indeed, sub-microgram samples are sp difficult to analyze that organic and photochemistry are essentially unexplored at pressures above 3 GPa.

Résumé - Quand le naphthalène en solution dans des mélanges d'alcanes ou d'al- cools est irradié à des longueurs d'onde < 315 nm, il est détruit par une série de réactions qui sont fortement accélérées à haute pression. Les analyses G O M S des photoproduits recouvrés après montée à très haute pression (> 5GPa) montrent que plusieurs réactions sont impliquées : 1) la photolyse sensibilisée des molé- cules de solvant en radicaux alkyles et alkoxyles ; 2) la réduction du naphtha- lène en tétrahydronaphthalène et radicaux hydronaphthyles ; 3) la polymérisation des radicaux hydronaphthyles et alkyles en dimères, trimères et polymères ; 4) la photoaddition des radicaux du solvant sur le naphthalène ; et 5) l'échange H-D entre le naphthalène et les molécules de solvant. La variation de l'intensité d'excitation en fonction de la vitesse de disparition du naphthalène montre que l'étape photochimique primaire est la formation biphotonique des intermédiaires triplets du naphthalène sensibilisant la production des radicaux libres qui, à haute pression, sont des consommateurs efficaces de liaisons insaturées. Les con- séquences de ces phénomènes (ainsi que d'autres décrits dans la littérature) sur la stabilité des composés organiques insaturés, à haute pression, sont discutées.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984830

C8-164 JOURNAL DE PHYSIQUE

R e c e n t l y , we [6,71 o b s e r v e d t h a t n a p h t h a l e n e became u n u s u a l l y p h o t o r e a c t i v e a t p r e s s u r e s of t h e o r d e r of 5 GPa. T h i s was u n e x p e c t e d s i n c e , w i t h few e x c e p t i o n s , a r o m a t i c h y d r o c a r b o n s a r e g e n e r a l l y s t a b l e t o u l t r a v i o l e t ( u v ) r a d i a t i o n

(8-221.

The few p h o t o a d d i t i o n s and p h o t o r e d u c t i o n s of a r o m a t i c h y d r o c a r b o n s i n " i n e r t " s o l v e n t s t h a t h a v e been r e p o r t e d [23-261 o c c u r w i t h low y i e l d s and a t low t e m p e r a t u r e s . I n o r d e r t o c h a r a c t e r i z e t h e h i g h p r e s s u r e r e a c t i o n , we d e v e l o p e d t h e t e c h n i q u e s t o r e c o v e r t h e p h o t o p r o d u c t s f o r mass s p e c t r o s c o p i c a n a l y s i s t h a t a r e b r i e f l y reviewed h e r e . T h e s e a n a l y s e s showed t h a t m a j o r p r o d u c t s w e r e d e r i v a t i v e s o f t e t r a l i n (1,2,3,4-tetrahydronaphthalene) [ 7 1 ; t h a t i s , most of t h e p r o d u c t s r e s u l t from a f o u r - e l e c t r o n r e d u c t i o n . C o n s i d e r a t i o n o f t h e p r o b a b l e m e c h a n i s m s of t h e s e r e a c t i o n s l e d u s t o e x a m i n e more g e n e r a l l y t h e c h e m i s t r y o f t h e u n s a t u r a t e d carbon-carbon bond a t h i g h p r e s s u r e s and t o f o r m u l a t e t h e t h e s i s of t h i s p a p e r , t h a t u n s a t u r a t e d C-C bonds become thermodynamically and k i n e t i c a l l y u n s t a b l e w i t h r e s p e c t t o s a t u r a t e d p r o d u c t s a t p r e s s u r e s of t h e o r d e r of 1 0 GPa.

I

- EXPERIMENTAL

The p h o t o c h e m i s t r y was e x c i t e d w i t h e i t h e r a sychronously-pumped, cavity-dumped d y e l a a e r e x c i t a t i o n s o u r c e a t 270 o r 290 mu o r t h e uv out.put of 100-W s h o r t a r c mercury lamp, i s o l a t e d by a Corning 7-54 f i l t e r . P r o g r e s s of t h e r e a c t i o n was m o n i t o r e d by measuring t h e f l u o r e s c e n c e s p e c t r a o r l i f e t i m e s of t h e s o l u t i o n .

A

diamond-anvil h i g h p r e s s u r e c e l l was

m o d i f i e d f o r t h e s e e x p e r i m e n t s b y a t t a c h i n g w i t h epoxy two s t a i n l e s s s t e e l r i n g (3.0-m d i a m e t e r and 0.6- h i g h ) a r o u n d t h e d i a m o n d s t o f o r m m i c r o c o n t a i n e r s w i t h volumes of a b o u t 5 u 1 ( F i g u r e 1 ) f o r r e c o v e r i n g r e a c t i o n p r o d u c t s . N a p h t h a l e n e ( F i s h e r N128 o r A l d r i c h n a p h t h a l e n e - d g ) s o l u t i o n s ( i n a b s o l u t e e t h a n o l , P u b l i c k e r I n d u s t r i e s , I n c .

^;

m e t h a n o l , Ma1 l i n c k r o d t

AR ; p e n t a n e ,

M a l l i n c k r o d t spectAR; i s o p e n t a n e , A l d r i c h s p e c t r o p h o t m e t r i c g r a d e ) , t h a t had been d e a e r a t e d , were l o a d e d i n t o t h e c e l l by p l a c i n g t h e c e l l i n s i d e a small s t a i n l e s s s t e e l box, f l o o d i n g t h e l o v e r p a r t of t h e c e l l i n c l u d i n g t h e lower diamond and g a s k e t w i t h t h e s o l u t i o n , and s e a l i n g t h e sample w i t h t h e upper diamond. These o p e r a t i o n s were performed i n s i d e of a n i t r o g e n - f i l l e d g l o v e box t o minimize t h e e x p o s u r e of t h e s o l u t i o n t o oxygen. The r e s u l t i n g sample c o n t a i n s o n l y a v e r y s m a l l a m o u n t o f n a h t h a l e n e , a b o u t 8 0 nanograms o r 4 x 1 0 12' m o l e c u l e s .

F i g u r e 1. C r o s s - s e c t i o n of diamond a n v i l c e l l t h a t shows t h e l o c a t i o n s l o c a t i o n of t h e m i c r o c o n t a i n e r s ( c r o s s - h a t c h e d ) used t o c o n f i n e t h e l i q u i d s o l v e n t t h a t e n t r a p s t h e sample when t h e c e l l i s opened.

The s m a l l samples and h i g h v o l a t i l i t y of n a p h t h a l e n e and t h e s o l v e n t s a t room t e m p e r a t u r e f o r c e d u s t o d e v e l o p s p e c i a l p r o c e d u r e s f o r c o l l e c t i n g t h e p r o d u c t s . These p r o c e d u r e s were performed i n a c o l d room ( a t -150

C )

where t h e c e l l . s o l v e n t , and t o o l s h a d been s t o r e d f o r a t l e a s t two h o u r s . The c o l l e c t i o n p r o c e d u r e began by r i n s i n g t h e a r e a s u r r o u n d i n g t h e diamond a n v i l s w i t h 5 - ~ 1 a l i q u o t 6 of s o l v e n t , e i t h e r u v - g r a d e hexane ( B u r d i c k 6 J a c k s o n Lab., I n c . ) o r methanol. These s o l v e n t s were i n j e c t e d w i t h a s y r i n g e i n t o t h e m i c r o c o n t a i n e r s , sucked back by t h e same s y r i n g e , and t r a n s f e r r e d t o a 5- d i a m e t e r Pyrex t u b e . T h i s o p e r a t i o n was r e p e a t e d u n t i l a b o u t 0.1 ml was c o l l e c t e d . The r i n s i n g s were combined and - a n a l y z e d by GC-MS i n o r d e r t o d e t e c t i m p u r i t i e s and r e s i d u a l n a p h t h a l e n e s u r r o u n d i n g t h e diamonds.

Next, i n a n a l m o s t c o n t i n u o u s s e r i e s of o p e r a t i o n s , 5-u1 of f r e s h s o l v e n t was

i n j e c t e d i n t o m i c r o c o n t a i n e r s i n o r d e r t o s u r r o u n d t h e diamonds w i t h s o l v e n t ( F i g u r e 1 ) ; t h e p r e s s u r e was s l o w l y r e l e a s e d ; t h e u p p e r p i s t o n was l i f t e d , removed and t u r n e d o v e r ; and a few more m i c r o l i t e r s of t h e s o l v e n t were q u i c k l y d e l i v e r e d t o e a c h m i c r o c o n t a i n e r t o cover t h e t i p s of t h e diamonds. The g a s k e t was t h e n r i n s e d w i t h a b o u t 1 0 ~1 of s o l v e n t by squeeze-suck c y c l e s of a p i p e t , and e a c h diamond and i t s m i c r o c o n t a i n e r were r i n s e d w i t h t h e same p i p e t . The r i n s e s o l u t i o n s were t r a n s f e r r e d t o a second Pyrex t u b e .

The s o l u t i o n s i n t h e Pyrex t u b e s t h e n were r e d u c e d i n volume t o a b o u t 2 ~1 by f l o w i n g a g e n t l e s t r e a m o f h e l i u m f r o m a p i p e t o v e r t h e s o l u t i o n . T h e s e c o n c e n t r a t e d s o l u t i o n s were i m m e d i a t e l y drawn i n t o a 1 0 - ~ 1 s y r i n g e and i n j e c t e d i n t o a KRATOS MS-25 g a s chromatograph-mass s p e c t r o m e t e r (GC-MS) w i t h a 0 . 2 %

nrm x

30 m DB5 f u s e d s i l i c a c a p i l l a r y column (J

& W

S c i e n t i f i c , I n c . ) . The oven t e n p e r a t u r e program f o r t h e GC column u s u a l l y was: i n i t i a l t e m p e r a t u r e , 3Z0

C

f o r 1 0 min; ramp r a t e , 160 C/min; f i n a l t e m p e r a t u r e . 2800

C

f o r 1 0 min. A Data G e n e r a l Nova computer s y s t e m was i n t e r f a c e d w i t h t h e GC-MS f o r d a t a a n a l y s i s . The d e t e c t i o n l i m i t of t h e KRATOS MS-25 GC-MS, a b o u t 2 ng, i s a d e q u a t e f o r t h e photoproduct a n a l y s e s . Gas chromatographs w e r e o b t a i n e d w i t h a Hewlett-Packard 5710A g a s chromatograph (GC) w i t h a hydrogen f l a m e i o n i z a t i o n d e t e c t o r and a 0.2% mm x 30 m SE-52 f u s e d s i l i c a c a p i l l a r y column (J

6 W

S c i e n t i f i c , I n c . ) . Mass s p e c t r a (MS) were r e c o r d e d on an AEI MS902 mass s p e c t r o m e t e r .

I1

- ^RESULTS

C a l i b r a t i o n and Backnround S t u d i e s . GC-MS e l u t i o n p a t t e r n s of a sample of t h e hexane s o l v e n t u s e d t o r i n s e t h e diamond c e l l and a n a p h t h a l e n e sample r e c o v e r e d from t h e diamond c e l l w i t h o u t e x p o s u r e t o uv r a d i a t i o n a r e shown i n F i g u r e 2. The h e a v i e r components i n t h e uv-grade hexane a r e r e t a i n e d p r e f e r e n t i a l l y when t h e s o l u t i o n i s c o n c e n t r a t e d . Thus, t h e Cn i m p u r i t y peaks a r e more i n t e n s e i n t h e c o n c e n t r a t e t h a n i n t h e o r i g i n a l hexane. Q u a n t i t a t i v e s t u d i e s w i t h o u t

w

e x p o s u r e showed t h a t between 26 and 3 3 ng o u t of 80 2 1 0 ng of n a p h t h a l e n e loaded i n t o t h e diamond c e l l c o u l d be r e c o v e r e d .

Hinh P r e s s u r e P h o t o c h e m i s t r v . When 5

x

loe2

M

n a p h t h a l e n e i n 1:l p e n t a n e : i s o p e n t a n e was compressed t o 4.7 GPa and i r r a d i a t e d f o r 3 0 m i n u t e s a t 310 nm w i t h a mercury a r c , two new p h o t o p r o d u c t bands a p p e a r e d i n t h e e m i s s i o n s p e c t r a

[61.

The broad (300-to-550 nm) band h a s a 5 n s e c l i f e t i m e , w h i l e t h e s t r u c t u r e d band a s s h o r t e r w a v e l e n g t h s (280-310 mn) h a s a l i f e t i m e of 12-to-15 n s e c . A f t e r s h o r t e x p o s u r e s , t h e growth of t h e i n t e n s i t y of t h e broad band ( r e l a t i v e t o t h e n a p h t h a l e n e e m i s s i o n ) c o u l d be p a r t i a l l y r e v e r s e d by h e a t i n g t h e sample t o 180 OC f o r one hour. However, a f t e r i r r a d i a t i o n f o r two h o u r s , t h e n a p h t h a l e n e m i s s i o n c o u l d n o t b e r e s t o r e d by h e a t i n g . T h i s b e h a v i o r s u g g e s t s . t h a t . some weakly-bound n a p h t h a l e n e d i m e r s form d u r i n g t h e e a r l y s t a g e s of t h e p h o t o c h e m i s t r y . At t h e same time, n a p h t h a l e n e r e a c t s w i t h s o l v e n t m o l e c u l e s t o form r e d u c t i o n and a d d i t i o n p r o d u c t s . However, w i t h prolonged i r r a d i a t i o n , a l l of t h e n a p h t h a l e n e , even t h e weakly-bound d i n e r s , r e a c t s i r r e v e r s i b l y t o form a t l e a s t two d i f f e r e n t t y p e s of p r o d u c t s . These p r o d u c t s were i d e n t i f i e d by MS and GC-MS methods.

The l i g h t e r p r o d u c t s were d e t e r m i n e d by GC-MS a n a l y s i s of t h e t y p e i l l u s t r a t e d i n F i g u r e 3-5. These d a t a were c o l l e c t e d from a 5 x 1 0 - ~

M

s o l u t i o n of n a p h t h a l e n e i n 1:l p e n t a n e : i s o p e n t a n e t h a t had been i r r a d i a t e d w i t h a mercury a r c f o r 3 hours and c o l l e c t e d w i t h methanol. Naphthalene i s c l e a r l y a b s e n t . The mass s p e c t r a f o r t h r e e p h o t o p r o d u c t s , GC peaks 8487, 528 and 532, have t h e same abundance maxima ( a t m/z 129 and 1301, t h e mono- and d i - h y d r o n a p h t h y l i o n s . The h i g h e r mass f r a g m e n t s i n t h e s p e c t r a show t h a t n a p h t h a l e n e r e d u c t i o n o c c u r s by a d d i t i o n of s o l v e n t m o l e c u l e s . Each n a p h t h a l e n e m o l e c u l e i n c o r p o r a t e d i n t h e s e a d d i t i o n p r o d u c t s r e t a i n s o n l y one a r o m a t i c r i n g ; t h u s , t h e e m i s s i o n band a t 280-to-310 w can be a t t r i b u t e d t o them.

Although i t was d i f f i c u l t t o d e t e r m i n e t h e t o t a l amount of p r o d u c t s r e c o v e r e d by

t h i s p r o c e d u r e , e x p e r i e n c e w i t h r e c o v e r i n g samples t h a t had n o t r e a c t e d s u g g e s t e d

t h a t t h e s e " l i g h t " p r o d u c t s r e p r e s e n t e d a b o u t

95I of t h e i n i t i a l n a p h t h a l e n e .

J O U R N A L DE PHYSIQUE

ELUTION TIME/ MIN:SEC(: 0:03)

F i g u r e 2.

GC-MS

e l u t i o n p a t t e r n s of ( a ) c o n c e n t r a t e d hexane u s e d t o r i n s e t h e s u r r o u n d i n g s of t h e d i a m o n d s a n d ( b ) n a p h t h a l e n e r e c o v e r e d i n h e x a n e f r o m d i a m o n d c e l l w i t h o u t p h o t o c h e m i s t r y .

SCAN NUMBER

Another t y p e of p r o d u c t was found by c a r e f u l l y examining t h e diamond t i p and g a s k e t a f t e r t h e l i g h t e r m o l e c u l e s had been c o l l e c t e d . These s u r f a c e s were t h i n l y c o a t e d w i t h a w h i t e p r e c i p i t a t e t h a t was i n s o l u b l e i n e i t h e r hexane o r methanol. Because epoxy was u s e d t o form t h e m i c r o c o n t a i n e r s , s t r o n g e r s o l v e n t s c o u l d n o t be u s e d t o r i n s e them. By mass s p e c t r a l a n a l y s i s of s i m i l a r d e p o s i t s o b t a i n e d from a t m o s p h e r i c p r e s s u r e e x p e r i m e n t s , t h e s e p r o d u c t s w e r e i d e n t i f i e d a s h i g h m o l e c u l a r weight compounds i n which s e v e r a l s o l v e n t and a r o m a t i c m o i t i e s a r e l i n k e d . The permanent, broad (300-to-550 nm) e m i s s i o n s p e c t r a a r e a t t r i b u t e d t o t h i s m a t e r i a l .

P h o t o c h e m i s t r y Atmospheric P r e s s u r e and Room T e m ~ e r a t u r e . When a b r o a d , b r i g h t

w h i t e e m i s s i o n was o b s e r v e d d u r i n g an u n i n t e n t i o n a l l y prolonged 315-nm i r r a d i a t i o n

of a s o l u t i o n of n a p h t h a l e n e i n m e t h a n o l c o n t a i n e d i n a Pyrex t u b e , e x p e r i m e n t s

were conducted w i t h n a p h t h a l e n e o r naphthalene-dg i n 4: 1 methano1:ethanol o r 1: 1

p e n t a n e : i s o p e n t a n e

[ 7 1 .

F o r a l l c o m b i n a t i o n s of n a p h t h a l e n e a n d s o l v e n t

( n a p h t h a l e n e o r n a p h t h a l e n e - d g , a l c o h o l s o r a l k a n e s , d e a e r a t e d o r a e r a t e d ) , a

p r e c i p i t a t e w i t h a t h e same b r i g h t f l u o r e s c e n c e formed, and most of i t d e p o s i t e d o n

t h e t u b e w a l l . ( S i m i l a r d e p o s i t s d i d n o t form i n q u a r t z t u b e s which s u g g e s t s t h a t a n

e a r l y s t e p i n t h e f o r m a t i o n of t h e d e p o s i t c a n b e r e v e r s e d

by

s h o r t wavelength uv.)

ELUTION TIME / M I N : S E C

SCAN NUMBER

Figure 3 .

GC-MS

e l u t i o n pattern of a 5

x

M a o l u t i o n of naphthalene i n 1 : l pentane:isopentane c o l l e c t e d by methanol from the diamond c e l l a f t e r a 3-hour i r r a d i a t i o n with an Hg a r c .

Figure

4.

Masa spectrum f o r

GC

peak 8487 of Figure 3 , trihydronaphthalene.

JOURNAL DE PHYSIQUE

F i g u r e 5. Mass s p e c t r u m f o r GC peak f 5 2 8 of F i g u r e 3 , pentyl-bi-dihydronaphthalene.

The c h e m i s t r y of t h e s e d e p o s i t s was s t u d i e d by i r r a d i a t i n g samples w i t h a new, t i g h t l y - f o c u s s e d mercury a r c f o r one o r more d a y s , d i l u t i n g t h e r e s u l t i n g s o l u t i o n f i v e f o l d w i t h a d d i t i o n a l s o l v e n t , and i n j e c t i n g 500-nl a l i q u o t s of t h e d i l u t e d s o l u t i o n s i n t o t h e GC-MS s y s t a n . These a n a l y s e s , which a r e d e s c r i b e d i n d e t a i l i n R e f e r e n c e s 9 a n d 2 9 , showed t h a t , i n 4 : l m e t h a n o l : e t h a n o l , a l m o s t a l l o f t h e n a p h t h a l e n e p h o t o r e a c t e d w i t h i n 2 4 h o u r s , w h e r e a s , i n 1:l p e n t a n e : i s o p e n t a n e , o n l y s m a l l p o r t i o n o f n a p h t h a l e n e r e a c t e d d u r i n g 3 0 - t o - 6 0 h o u r s . ( F o r c o m p a r i s o n , r e a c t i o n a t h i g h p r e s s u r e s were c o m p l e t e w i t h i n a c o u p l e of h o u r s and were more r a p i d i n t h e a l k a n e s o l v e n t s . )

The p r o d u c t a n a l y s e s c l e a r l y d e m o n s t r a t e d t h a t t h e c h e n i s t r y b e g i n s when n a p h t h a l e n e i s photoreduced t o a h y d r o n a p h t h y l o r a l k y l n a p h t h y l r a d i c a l . These r a d i c a l s t h e n combine w i t h e a c h o t h e r t o produce dimers o r r e a c t f u r t h e r w i t h o t h e r n a p h t h a l e n e o r s o l v e n t m o l e c u l e s t o form a d d u c t s . A l l of t h e n a p h t h a l e n e p h o t o p r o d u c t s found i n t h e s o l u t i o n i s e s t i m a t e d t o c o n t a i n l e s s t h a n h a l f of t b e o r i g i n a l n a p h t h a l e n e . More t h a n h a l f of t h e n a p h t h a l e n e ends up i n t h e p r e c i p i t a t e .

S i m i l a r p h o t o p r o d u c t s were o b t a i n e d w i t h naphthalene-dg i n p e r p r o t o n a t e d p e n t a n e s ; however, a l l of t h e GC-MS f r a g m e n t a t i o n p a t t e r n s were broadened. These p a t t e r n s i n d i c a t e d t h a t , d u r i n g t h e p h o t o r e a c t i o n , one t o t h r e e d e u t e r i u m 6 exchange w i t h h y d r o g e n s f r o m t h e s o l v e n t s . O t h e r f r a g m e n t a t i o n p a t t e r n s s u g g e s t e d t h a t d e u t e r a t e d p e n t a n e r a d i c a l s , which came from t h e

H-D

exchange r e a c t i o n , r e a c t e d w i t h p r o t o n a t e d p e n t a n e t o form d e u t e r a t e d C10.

The p r e c i p i t a t e s would n o t e l u t e a t t h e h i g h e s t t e m p e r a t u r e of t h e GC column and

c o u l d n o t b e a n a l y z e d by GC-MS. They were c h a r a c t e r i z e d by d i r e c t mass s p e c t r a ,

nmr, and luminescence s p e c t r o s c o p y . D i r e c t mass s p e c t r a were measured a s t h e s e

s o l i d s s l o w l y e v a p o r a t e d w h i l e t h e y were h e a t e d from 200 t o 400° C. F i g u r e 6 shows a

mass s p e c t r u m of t h e p r e c i p i t a t e s from a 36-hour i r r a d i a t i o n of n a p h t h a l e n e i n

4 : l

methano1:ethanol. The f r a g m e n t a t i o n p a t t e r n c l e a r l y shows monomers (m/z 1291, d i m e r s

(m/z 258), t r i m e r s (m/z 387) and t e t r a m e r s (m/z 516) of h y d r o n a p h t h a l e n e s w i t h

e x t e n s i v e i o n s e r i e s of -(CH2)- o r -(CH2)0H. The dominant i o n s , from m/z 128 t h r o u g h

132, imply m u l t i p l e r e d u c t i o n s . M u l t i p l e p e a k i n g t h r o u g h o u t t h e s p e c t r a i m p l i e s

t h a t t h e p r e c i p i t a t e i s a m i x t u r e of polymerized h y d r o n a p h t h a l e n e s and s o l v e n t

m o l e c u l e s . P r e c i p i t a t e s from a l k a n e s o l u t i o n s had s i m i l a r s p e c t r a .

F i g u r e 6 . Mass s p e c t r a of t h e p r e c i p i t a t e s o b t a i n e d from a

5

x 10-2 M s o l u t i o n of n a p h t h a l e n e i n 4: 1 me thano1:e than01 a f t e r a 36-hour i r r a d i a t i o n w i t h a n Hg a r c u n d e r oxygen-free c o n d i t i o n s .

T h i s i n t e r p r e t a t i o n was confirmed by t h e nmr s p e c t r a and f l u o r e s c e n c e s p e c t r a of t h e p r e c i p i t a t e s . The broad f l u o r e s c e n c e s p e c t r a were s i m i l a r t o t h o s e of a r o m a t i c polymers ( s e e , f o r example, t h e s p e c t r u m of p o l y s t y r e n e i n e t h y l a c e t a t e 1281 o r p o l y - 2 - v i n y l n a p h t h a l e n e a t 1 0 - 3

M

i n 2 - v i n y l n a p h t h a l e n e 1 2 9 1 ) . T h e s e b r o a d m i s s i o n bands r e s u l t from i n t r a - c h a i n o v e r l a p p i n g and c o i l i n g of t h e a r o m a t i c r i n g s t h a t enhance i n t e r a c t i o n s among t h e p i o r b i t a l s .

I11

- PHOTOCHEMICAL MECHANISM

T h a t t h e i n i t i a l s t e p s o f t h e p h o t o p r o c e s s f o r m s s o l v e n t r a d i c a l s by e n e r g y t r a n s f e r from a t r i p l e t s t a t e of n a p h t h a l e n e e x c i t e d by s e q u e n t i a l a b s o r p t i o n of two photons h a s been s u g g e s t e d by s e v e r a l s t u d i e s 125, 30-341 and by t h e measured 2.0 0.3-power dependence of t h e r a t e of r e a c t i o n o n t h e i n t e n s i t y of e x c i t a t i o n . [71 However, d i r e c t i o n i z a t i o n of t h e e x c i t e d t r i p l e t a r o m a t i c o r t h e d i r e c t H a b s t r a c t i o n by t h e e x c i t e d t r i p l e t a r o m a t i c from t h e s o l v e n t s c a n n o t b e e x c l u d e d . Whether t.he e f f i c i e n c y of t h e two-photon pumping p r o c e s s i n c r e a s e s o r d e c r e a s e s w i t h p r e s s u r e d e p e n d s c r i t i c a l l y u p o n p r e s s u r e d e p e n d e n c e s of t h e r a t e s o f i n t e r s y s t e m c r o s s i n g from t h e s i n g l e t s t a t e e x c i t e d by t h e f i r s t p h o t o n and of n o n - r a d i a t i v e d e c a y of t h e i n t e r m e d i a t e t r i p l e t . O f f e n ' s s t u d i e s o f r i g i d s o l u t i o n s 1351 imply t h a t t h e pumping e f f i c i e n c y i n c r e a s e s s l i g h t l y w i t h i n c r e a s i n g p r e s s u r e s . However, t h o s e r e s u l t s may n o t be t r a n s f e r a b l e t o f l u i d s where t h e r e d u c t i o n i n d i f f u s i o n a l quenching of t h e i n t e r m e d i a t e t r i p l e t w i t h i n c r e a s i n g s o l v e n t v i s c o s i t y may i n c r e a s e t h e pumping r a t e .

The s u b s e q u e n t p r o c e s s e s have n o t been c a r e f u l l y c o n s i d e r e d i n e a r l i e r s t u d i e s , but a l i k e l y r e a c t i o n scheme i s i l l u s t r a t e d i n F i g u r e 7. A f t e r a s o l v e n t molecule d i s s o c i a t e s t o a hydrogen atclm and s o l v e n t r a d i c a l , a t l e a s t one of t h e s e r a d i c a l s a d d s t o n a p h t h a l e n e m o l e c u l e ( A H ) t o f o r m a h y d r o n a p h t h y l r a d i c a l , 'AH2, o r a l k y l n a p h t h y l r a d i c a l , .AHR. The volume of a c t i v a t i o n and o v e r a l l volume change f o r t h e d i s s o c i a t i o n s t e p s h o u l d be p o s i t i v e , and t h e r a t e and y i e l d of t h i s s t e p would be e x p e c t e d t o d e c r e a s e w i t h i n c r e a s i n g p r e s s u r e s . T h i s a d v e r s e e f f e c t might be compensated by c l o s e c o u p l i n g of t h e d i s s o c i a t i o n t o t h e a d d i t i o n s t e p w i t h i t s n e g a t i v e volume of a c t i v a t i o n and o v e r a l l volume change.

The r 6 v e r s i b l e d i m e r i z a t i o n of n a p h t h a l e n e a l s o may be i m p o r t a n t e a r l y i n t h e

p h o t o c h e m i s t r y . Bonds between t h e m o l e c u l e s of t h e dimer d e s t a b i l i z e t h e pi-bonds

of e a c h n a p h t h a l e n e m o i e t y . Thus, d i m e r s s h o u l d p h o t o r e a c t more r e a d i l y with

r a d i c a l s t h a n monomers. F u r t h e r m o r e , d i m e r p r o d u c t i o n would be enhanced a t h i g h

p r e s s u r e . I n s u c h a c i r c u m s t a n c e , t h e h y d r o n a p h t h y l n a p h t h a l e n e r a d i c a l (.AIIz-AH)

would be t h e key i n t e r m e d i a t e . A f t e r p h o t o r e d u c t i o n o r p h o t o a d d i t i o n , t h e dimer

n i g h t d i s s o c i a t e ; o r t h e r e l a t i o n might. p e r s i s t t o t h e f i n a l p r o d u c t .

J O U R N A L DE PHYSIQUE

POLYMER

F i g u r e 7. P r o p o s e d s c h e m e of t h e two-photon i n d u c e d r a d i c a l r e a c t i o n s .

Formation of r e d u c e d n a p h t h y l r a d i c a l s open u p s e v e r a l r a d i c a l c h a i n p r o p a g a t i n g and t e r m i n a t i n g r e a c t i o n s t h a t consume n a p h t h a l e n e . For i n s t a n c e , 'AH2 may r e a c t w i t h an H a t m o r .R t o form a d d u c t s , AH3 o r AH2R. These a d d u c t s c a n r e a c t f u r t h e r w i t h

H

a t o m s o r s o l v e n t r a d i c a l s t o f o r m t e t r a h y d r o n a p h t h l e n e ( A H 5 ) , t r i h y d r o n a p h t h y l a d d u c t s

(AH@),

o r m u l t i a d d u c t s (AH2RR'). Two .AH2 r a d i c a l s may combine t o g i v e h y d r o n a p h t h y l d i m e r s (AH2-AH2). Hydronaphthyl r a d i c a l s a l s o may t i t t a c k n a p h t h a l e n e t o form dimer r a d i c a l s of reduced n a p h t h a l e n e s t h a t may a t t a c k o t h e r n a p h t h a l e n e s t o f o r m t r i m e r r a d i c a l s a n d e v e n t u a l l y p o l y m e r s , a n d t h e r e d u c t i v e p h o t o a d d u c t s may r e p r e s e n t s i d e r e a c t i o n s t o t h e main polymer c h a i n . H atoms from t h e d i s s o c i a t i o n of p r o t o n a t e d s o l v e n t s c a n a t t a c k naphthalene-dg and r e p l a c e

D

atoms t o form n a p h t h a l e n e - d i ( i < 81, and t h e

D

a t m s c a n r e a c t w i t h t h e s o l v e n t m o l e c u l e s . Two s o l v e n t r a d i c a l s a l s o c a n combine t o y i e l d l o n g e r m o l e c u l e s which may e x p l a i n t h e l a r g e amounts of C10 a l k a n e s found i n some p e n t a n e r u n s . W i t h t h e g l a r i n g e x c e p t i o n o f d e r i v a t i v e s o f d i h y d r o n a p h t h a l e n e , m o s t o f t h e p r o d u c t s t h a t c a n be d e r i v e d from t h e scheme i n F i g u r e 7 were d e t e c t e d by

GC-MS

a n a l y s e s . T h i s s u g g e s t s t h a t d i h y d r o n a p h t h a l e n e d e r i v a t i v e s t h a t formed were e f f i c i e n t l y c o n v e r t e d t o t e tr a h y d r o n a p h t h a l e n e d e r i v a t i v e s . T h i s c a n be r e a d i l y u n d e r s t o o d i n t e r m s of t h e e n t h a l p i e s and volumes of a c t i v a t i o n and r e a c t i o n f o r t h e r e l e v a n t s t e p s :

AH + HR

---> AH2R

(1)

AH2R +

HR'

---> AH3RR' ( 2 )

I f

.both HR and

HR'

add t o t h e same r i n g of t h e o r i . g i n a 1 n a p h t h a l e n e , t h e s t a n d a r d

e n t h a l p i e s of r e a c t i o n s f o r t h e s e s t e p s a t a t m o s p h e r i c p r e s s u r e s h o u l d be a b o u t + l o 0 and -130 kJ/mol, r e s p e c t i v e l y , s i n c e t h e d i f f e r e n c e between t h e r e s o n a n c e e n e r g i e s of n a p h t h a l e n e and benzene must be s u p p l i e d i n r e a c t i o n ( 1 ) . A t low p r e s s u r e s , both s t e p s have p o s i t i v e e n t h a p l i e s of a c t i v a t i o n ; b u t , a g a i n i n t h i s r e s p e c t , s t e p ( 1 ) s h o u l d be more e n d o e r g i c t h a n s t e p

( 2 ) .

Thus, s i n c e t h e p h o t o e x c i t a t i o n s u p p l i e s r a d i c a l s of s u f f i c i e n t energy f o r t h e more e n d o e r g i c s t e p (11, t h e s i m i l a r b u t l e s s e n d o e r g i c s t e p ( 2 ) s h o u l d proceed f a c i l e l y under t h e same c o n d i t i o n s t o c o n v e r t d i h y d r o d e r i v a t i v e s t o t e t r a h y d r o d e r i v a t i v e s . F u r t h e r c o n v e r s i o n f ran t e t r a t o h e x a h y d r o d e r i v a t i v e s i n v o l v e s an even more endothermic s t e p i n which t h e resonance energy of benzene must be s u p p l i e d . T h i s may n o t be p o s s i b l e f o r t h e e x c i t a t i o n c o n d i t i o n s of t h e s e experiments.

P r e s s u r e s h o u l d s t r o n g l y a f f e c t t h e k i n e t i c s of t h e s e addition-propagation s t e p s of t h e mechanism. Each i n t e r m o l e c u l a r a d d i t i o n d e c r e a s e s t h e molar volume of t h e system by between -10 and -20 cm3 because s h o r t i n t e r m o l e c u l a r bonds r e p l a c e long i n t e r m o l e c u l a r bonds. The volumes of a c t i v a t i o n s h o u l d have s i m i l a r s i g n s and magnitudes. Thus, f o r each s t e p , t h e e n t h a l p i e s of a c t i v a t i o n and r e a c t i o n d e c r e a s e by between -10 and -20 kJ/GPa; and t h e r a t e of each s t e p i n c r e a s e s by a b o u t a f a c t o r of e 4 p e r g i g a p a s c a l . Thus, by p r e s s u r e s of t h e o r d e r of 1 0 GPa, s t e p ( 1 ) s h o u l d be exothermic. At somewhat h i g h e r p r e s s u r e s , c o n v e r s i o n of t h e t e t r a h y d r o d e r i v a t i v e s t o non-aromatic s p e c i e s c a n n o t be n e g l e c t e d .

These o b s e r v a t i o n s a l s o can e x p l a i n h i g h p r e s s u r e phenomena a s d i v e r s e a s t h e shock-induced p o l y m e r i z a t i o n s of benzene and n a p h t h a l e n e r e p o r t e d by Drenin

[351

o r t h e l o s s of m e t a l - l i k e r e f l e c t i v i t y of t h e p o l y a c e t y l e n e s , f o r examples. During shock compression, p r e s s u r e lowers t h e a c t i v a t i o n b a r r i e r s and l o c a l h e a t i n g by t h e h i g h s t r e s s g r a d i e n t s a c t i v a t e s m o l e c u l e s o v e r t h i s lower b a r r i e r t o produce r a d i c a l s t h a t consume u n s a t u r a t e d bonds u n t i l t h e sample i s quenched. The mobile c h a r g e s of t h e p o l y a c e t y l e n e s a l s o a r e r a d i c a l s t h a t w a n t t o c r o s s - l i n k n e i g h b o r i n g c h a i n s . P r e s s u r e i n c r e a s e s t h e c o n c e n t r a t i o n of t h e s e r e a g e n t s by d e c r e a s i n g t h e bandgap and a l s o f a c i l i t a t e s t h e i n t e r c h a i n r e a c t i v i t y by b r i n g i n g a d j a c e n t c h a i n s c l o s e r . I f t h e i n t r a c h a i n m o b i l i t y of t h e r a d i c a l s i n c r e a s e s w i t h p r e s s u r e , t h e r e a c t i v i t y a l s o w i l l b e enhanced s i n c e , w i t h i n any t i m e i n t e r v a l , t h e r e a c t i v e r a d i c a l s w i l l sample a l a r g e r number of p o t e n t i a l r e a c t i o n s i t e s .

T h u s , t h e i m p l i c a t i o n s o f t h i s w o r k a r e g e n e r a l . J u s t a s g r a p h i t e i s thermodynamically u n s t a b l e w i t h r e s p e c t t o diamond a t p r e s s u r e a of t h e o r d e r of 5 GPa, a r o m a t i c

C-C

b o n d s become t h e r m o d y n a m i c a l l y u n s t a b l e w i t h r e s p e c t t o p r o d u c t i o n of l a r g e s a t u r a t e d molecules a t p r e s s u r e s of t h e o r d e r of 5 t o 10 GPa.

(Of c o u r s e , t h e i s o l a t e d , u n s a t u r a t e d double and t r i p l e C-C bonds of t h e e t h y l e n e s and a c e t y l e n e s a r e u n s t a b l e even a t a t m o s p h e r i c p r e s s u r e . ) Furthermore, t h e r e i s no q u a l i t a t i v e d i f f e r e n c e whether t h e an u n s a t u r a t e d bond i s s a t u r a t e d by a d d i t i o n of an u n s a t u r a t e d o r a s a t u r a t e d s p e c i e s . U n t i l somewhat h i g h e r p r e s s u r e s , u n s a t u r a t e d bonds may remain k i n e t i c a l l y s t a b l e . N e v e r t h e l e s s , u n s a t u r a t e d C-C bonds w i l l n o t s u r v i v e p r e s s u r e s much h i g h e r than 10 GPa.

I V

- ACKNOWLEDGMENTS

The GC, MS, and GC-MS a n a l y s e s r e p o r t e d h e r e would n o t have been p o s s i b l e w i t h o u t t h e a b l e a s s i s t a n c e and a d v i c e of D r . D i l i p Senharma and Mr. John Wells of the I n s t r u m e n t a t i o n F a c i l i t y of t h e Department of Chemistry and Biochemistry. We art?

g r a t e f u l t o Drs.

K.D.

Bayes, M.A. El-Sayed, C.S. F o o t e , J . Joussot-Dubien,

M.

Lamotte. D. S c h i f e r l , K. Syassen f o r h e l p f u l comments and s u g g e s t i o n s . T h i s work

was s u p p o r t e d by NSF g r a n t DMR80-25620 a s supplemented by a UCLA F o u n d a t i o n - C o l l e g e

of L e t t e r s and S c i e n c e s F e l l o w s h i p ( t o GZY) and i n s t r u m e n t a t i o n g r a n t s from NSF

(C8379-10965, CHE77-09271, CHE76-05926, and GP323041, t h e U n i v e r s i t y Research

Committee, and S p e c t r a - P h y s i c s , Inc.

JOURNAL DE PHYSIQUE

V

- REFERENCES

1. BRIDGMAN P.W.,

J.

B i o l . Chem. 19 ( 1 9 1 4 ) 511.

2. XATZ A.L., SCHIFERL D., a n d MILLS R.L.,

J.

Phys. Chem. 88 (1984) 3176.

3 . BRIDGMAN P.W.,

J.

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