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15 Steric Compression Control

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A Quantitative Approach to Reaction Selectivity in Solid State Chemistry 1

S A R A ARIEL, SYED ASKARI, JOHN R. S C H E F F E R , J A M E S TROTTER, and L E U E E N WALSH Department of Chemistry, University of British Columbia, Vancouver, Canada V6T 1Y6

The solid state photochemistry of eight closely related α, β-unsaturated cyclohexenones is correlated with their crystal and molecular structures as determined by X-ray diffraction methods. It is suggested that the observed changes in reactivity are caused by differences in the packing arrange ments near the reaction site which either sterically impede or allow certain key atomic motions along the reaction coordinate. Computer simulation of these motions coupled with calculations of the resulting non-bonded steric compression energies support the theory. Steric compression also explains the case of a molecule which fails to undergo [2+2] photocycloaddition when irradiated in the solid state despite an almost perfect crystal lattice alignment of the potentially reactive double bonds. The packing diagram suggests that photodimerization would lead to unfavorable steric interactions between the reacting molecules and their stationary lattice neighbors. Computer simulation of the early stages of the photo-dimerization coupled with estimates of the resulting steric compression energies corroborate the theory. I t i s w e l l e s t a b l i s h e d , i n a q u a l i t a t i v e sense, t h a t c h e m i c a l r e a c t i o n s o c c u r r i n g i n c r y s t a l s are s u b j e c t t o r e s t r i c t i v e f o r c e s , not found i n s o l u t i o n , which l i m i t the a l l o w a b l e range o f atomic and m o l e c u l a r motions a l o n g the r e a c t i o n c o o r d i n a t e . T h i s o f t e n l e a d s t o d i f f e r e n c e s , e i t h e r i n the product s t r u c t u r e s o r t h e product r a t i o s , i n going from s o l u t i o n t o the s o l i d s t a t e . This was f i r s t demonstrated i n a s y s t e m a t i c way by Cohen and Schmidt i n 1964 i n t h e i r s t u d i e s on the s o l i d s t a t e p h o t o d i m e r i z a t i o n o f cinnamic a c i d and i t s d e r i v a t i v e s ( 1 ) . T h i s work l e d t o t h e f o r m u l a t i o n o f the famous topochemical p r i n c i p l e which s t a t e s , i n 7

Author to whom correspondence should be directed. 0097-6156/85/0278-Ό243$06.00/0 © 1985 American Chemical Society

Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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e f f e c t , t h a t r e a c t i o n s i n c r y s t a l s tend to be l e a s t motion i n character. I n 1975, Cohen i n t r o d u c e d the concept of the r e a c t i o n c a v i t y i n s o l i d s t a t e c h e m i s t r y ( 2 ) . The r e a c t i o n c a v i t y was d e f i n e d as the space o c c u p i e d by the r e a c t i n g s p e c i e s and bounded by the s u r r o u n d i n g , s t a t i o n a r y m o l e c u l e s . Cohen viewed the t o p o c h e m i c a l p r i n c i p l e as r e s u l t i n g from the p r e f e r e n c e f o r c h e m i c a l p r o c e s s e s to o c c u r w i t h m i n i m a l d i s t o r t i o n of the r e a c t i o n c a v i t y , e i t h e r i n the f o r m a t i o n of v o i d s w i t h i n i t or e x t r u s i o n s from i t ( F i g u r e 1 ) . The next advance i n the u n d e r s t a n d i n g of the f o r c e s which u n d e r l i e the t o p o c h e m i c a l p r i n c i p l e was due t o McBride ( 3 ) . He i n t r o d u c e d the concept of l o c a l s t r e s s to e x p l a i n the d e t a i l s of the mechanisms by which d i a c y l p e r o x i d e s decompose i n the s o l i d s t a t e . McBride showed t h a t l e a s t motion can be overcome i n t h e s e cases by a n i s o t r o p i c s t r e s s e s e q u i v a l e n t to many tens of k i l o b a r s of p r e s s u r e e x e r t e d by the product carbon d i o x i d e m o l e c u l e s trapped i n u n f a v o r a b l e l a t t i c e p o i t i o n s . Most r e c e n t l y , G a v e z z o t t i (4) has a n a l y z e d t h e o r e t i c a l l y c e r t a i n s o l i d s t a t e p r o c e s s e s i n terms of the volume of the c o n s t i t u e n t m o l e c u l e s and the s i z e and l o c a t i o n of the empty and f i l l e d spaces i n the c r y s t a l l a t t i c e . With a statement t h a t w i l l be seen to be d i r e c t l y p e r t i n e n t to the r e s u l t s of our i n v e s t i g a t i o n , he c o n c l u d e s t h a t "a p r e r e q u i s i t e f o r c r y s t a l r e a c t i v i t y i s the a v a i l a b i l i t y of f r e e space around the r e a c t i o n s i t e " . What i s l a c k i n g a t t h i s p o i n t i n t h e o r i e s r e l a t i n g l a t t i c e r e s t r a i n t s and c h e m i c a l r e a c t i v i t y i s the i d e n t i f i c a t i o n of s p e c i f i c s t e r i c i n t e r a c t i o n s which a l t e r r e a c t i v i t y and an e s t i m a t i o n of t h e i r magnitude. T h i s r e q u i r e s an e x t e n s i v e database of s t r u c t u r e - r e a c t i v i t y i n f o r m a t i o n f o r a s e r i e s of c l o s e l y r e l a t e d compounds. T h i s we have from our s t u d i e s on the s o l i d s t a t e p h o t o c h e m i s t r y and X-ray c r y s t a l l o g r a p h y of a l a r g e number of v a r i o u s l y s u b s t i t u t e d b i c y c l i c dienones of g e n e r a l s t r u c t u r e ]^ ( 5 ) . I n t h i s s e r i e s , we r e c e n t l y observed a photorearrangement

which d i d not conform to the normal r e a c t i v i t y observed f o r these systems i n the s o l i d s t a t e and which c o u l d not be accounted f o r u s i n g t r a d i t i o n a l s t e r e o e l e c t r o n i c arguments. I n t h i s paper we demonstrate t h a t i n a l l l i k e l i h o o d , t h i s change i n r e a c t i v i t y i s caused by s p e c i f i c c r y s t a l p a c k i n g e f f e c t s near the r e a c t i o n s i t e which are unique to the compound which behaves a b n o r m a l l y . We suggest the term s t e r i c compression c o n t r o l f o r t h i s e f f e c t and e s t i m a t e i t s magnitude u s i n g non-bonded r e p u l s i o n energy c a l c u l a t i o n s . We a l s o demonstrate t h a t the concept of s t e r i c compression c o n t r o l can be a p p l i e d to b i m o l e c u l a r r e a c t i o n s ([2+2] p h o t o c y c l o a d d i t i o n s ) i n the s o l i d s t a t e . Enone Photorearrangements. As w i l l be seen, the s t e r i c compression i s a s s o c i a t e d w i t h the s u b s t i t u e n t s a t t a c h e d to the


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α,β-unsaturated double bond i n b i c y c l i c dienones of g e n e r a l s t r u c t u r e J,. We t h e r e f o r e s e l e c t compounds f o r comparison i n which R i s constant and e q u a l t o methyl throughout the s e r i e s . T a b l e I o u t l i n e s t h e e i g h t compounds, ter te» s t u d i e d ; each has had i t s m o l e c u l a r and c r y s t a l s t r u c t u r e determined by X-ray d i f f r a c t i o n methods (6-11). 5

T a b l e I . R e a c t a n t s , Hydrogen A b s t r a c t i o n D i s t a n c e s and S t e r i c Compressions i n S o l i d S t a t e Photorearrangements

Enone

R

R

l

R

2

H..C

3

Œ

(Â)

te Ui

te te te te te te

CH CH CH

3

3

3

H H H CH CH

CH CH CH CH CH

3

3

3

3

3

H 3

3

CH CH

3

3

H OH OH H OH H H OAc

OH H CH

3

OH H OH OAc H

2.78 2.88 2.86 2.82 2.74 2.92 2.74 2.79

H. · C β S t e r i c Compression Accom panying P y r a m i d a l i z a t i o n (Â) Cp a c

b

2.75 2.92 2.81 2.78 2.85 2.84 2.70 2.84

b

yes yes yes no yes yes yes yes

yes yes yes yes yes yes no yes

Yes i n d i c a t e s a hydrogen-hydrogen c o n t a c t upon p y r a m i d a l i z a t i o n of 2.2 Â), whereas p y r a m i d a l i z a t i o n i n a l l o t h e r cases l e d to new c o n t a c t s a v e r a g i n g ( a t t h e i r minimum) 1.6±0.3 Â. These r e s u l t s are summarized i n T a b l e I . As an example, F i g u r e 4 shows s t e r e o d i a g r a m s of enone \& b e f o r e and a f t e r p y r a m i d a l i z a t i o n at C Q . The s t e r i c compression accompanying f u l l 5 5 ° p y r a m i d a l i z a t i o n i s i n d i c a t e d by the d o t t e d l i n e s and c o n s i s t s of hydrogen-hydrogen c o n t a c t s of 1.71 and 1.87 Â; p y r a m i d a l i z a t i o n a t C i s unimpeded.

a

a

a

S t e r i c Compression E n e r g i e s . An e s t i m a t e of the s t e r i c compression e n e r g i e s accompanying p y r a m i d a l i z a t i o n may be reached u s i n g one or more of the s e v e r a l s e m i - e m p i r i c a l e q u a t i o n s which r e l a t e i n t e r a t o m i c d i s t a n c e and non-bonded r e p u l s i o n energy. Two of the b e t t e r known e q u a t i o n s are the Lennard-Jones 6-12 p o t e n t i a l f u n c t i o n (19) and the Buckingham p o t e n t i a l as p a r a m e t e r i z e d by A l l i n g e r f o r h i s MM2 f o r c e f i e l d program ( 2 0 ) . These two f u n c t i o n s are p l o t t e d g r a p h i c a l l y i n F i g u r e 5 f o r i n t e r a c t i o n s i n v o l v i n g hydrogen atoms. U s i n g t h i s p l o t we can e s t i m a t e the s t e r i c compression energy f o r enone \& f u l l y p y r a m i d a l i z e d a t 0β ( c o n t a c t s of 1.71 and 1.87 Â). T h i s amounts to 11.7 k c a l / m o l e (MM2) or 12.7 k c a l / m o l e ( 6 - 1 2 ) . M e t h y l Group R o t a t i o n . M e t h y l group r o t a t i o n , which can be r a p i d i n the s o l i d s t a t e , can o b v i o u s l y a l t e r the s t e r i c compression contacts. I n i t i a l l y , the computer s i m u l a t i o n of p y r a m i d a l i z a t i o n was c a r r i e d out k e e p i n g the m e t h y l groups i n t h e i r o r i g i n a l , ground s t a t e r o t a t i o n a l o r i e n t a t i o n s . Having determined the minimum i n t e r m o l e c u l a r c o n t a c t s a t t e n d i n g p y r a m i d a l i z a t i o n , we then r o t a t e d the i n t e r a c t i n g methyl groups i n b o t h d i r e c t i o n s by 30° and redetermined the c o n t a c t s . The r e s u l t s of such a computer experiment f o r r o t a t i o n of the p y r a m i d a l i z e d Comethyl group of enone \& are g i v e n i n F i g u r e 6 which i s a p l o t of i n t e r m o l e c u l a r hydrogen-hydrogen c o n t a c t v e r s u s angle of r o t a t i o n . T h i s shows t h a t r o t a t i o n of t h i s methyl group i n e i t h e r d i r e c t i o n does not r e l i e v e the s t e r i c compression caused by p y r a m i d a l i z a t i o n . For example, r o t a t i o n i n the p o s i t i v e d i r e c t i o n , w h i l e s l i g h t l y r e l i e v i n g the 1.71 Â c o n t a c t , s t r o n g l y decreases the 1.87 Â c o n t a c t . R o t a t i o n i n the o p p o s i t e d i r e c t i o n i s no b e t t e r ; the 1.71 Â c o n t a c t i s decreased s l i g h t l y w h i l e the 1.87Â i s r e l i e v e d . I n a d d i t i o n , a t h i r d c o n t a c t , 2.20 Â, which i s unimportant at 0°, becomes a s i g n i f i c a n t a t a p p r o x i m a t e l y -20°. A similar rotation


ARIEL

ETAL.

Steric Compression

Control

F i g u r e 4. Stereodiagrams of enone l g , b e f o r e (above) and a f t e r (below) p y r a m i d a l i z a t i o n a t the β-carbon atom. The s t e r i c compression c o n t a c t s which develop a r e shown by the d o t t e d lines.

1.5

1.7

Η ..·Η β

1.9

2.1

2.3

2.5

2.7

Contact

F i g u r e 5. Hydrogen-hydrogen non-bonded r e p u l s i o n e n e r g i e s v e r s u s d i s t a n c e f o r MM2 ( d o t t e d l i n e ) and 6-12 ( s o l i d l i n e ) p o t e n t i a l f u n c t i o n s . The e n e r g i e s a t 1.71 and 1.87 Â a r e shown. Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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s i m u l a t i o n was c a r r i e d out f o r the n o n - p y r a m i d a l i z e d m e t h y l group f o r the i n t e r a c t i n g p a i r i n the case of enone Again, rotation was found to be i n e f f e c t i v e i n r e l i e v i n g the hydrogen-hydrogen s t e r i c compression c o n t a c t s . M e t h y l r o t a t i o n was t e s t e d f o r a l l e i g h t enones s t u d i e d . A l t h o u g h the hydrogen-hydrogen c o n t a c t s v a r i e d w i t h r o t a t i o n (as a b o v e ) , i n no case d i d r o t a t i o n a l t e r t h e c o n c l u s i o n s reached on the b a s i s of the 0° r o t a t i o n c o n t a c t s . M e c h a n i s t i c I n t e r p r e t a t i o n » We i n t e r p r e t the s t e r i c c o m p r e s s i o n r e s u l t s k i n e t i c a l l y i n terms of the r e l a t i v e a c t i v a t i o n e n e r g i e s f o r hydrogen atom t r a n s f e r . I t i s w e l l e s t a b l i s h e d t h a t hydrogen a b s t r a c t i o n i s the r a t e d e t e r m i n i n g s t e p i n o t h e r hydrogen t r a n s f e r - i n i t i a t e d photorearrangements such as the N o r r i s h type I I r e a c t i o n ( 2 1 - 2 5 ) . The s i t u a t i o n i s summarized i n F i g u r e 7. I n the absence of any s t e r i c c o m p r e s s i o n , hydrogen a b s t r a c t i o n by the β-carbon has a lower a c t i v a t i o n energy than hydrogen t r a n s f e r t o the α-carbon atom f o r reasons a l r e a d y d i s c u s s e d . Steric compression accompanying hydrogen t r a n s f e r to both C and Co r a i s e s both a c t i v a t i o n e n e r g i e s , but m a i n t a i n s the o r d e r i n g of Co below C (enones J \$L> U, * S t e r i c compression at C but not Co (enone l & ) i n c r e a s e s the normal a c t i v a t i o n energy d i f f e r e n c e r e s u l t i n g i n a b s t r a c t i o n by Cg b e i n g even more f a v o r e d than b e f o r e . I n the anomalous case of enone however, s t e r i c compression o c c u r s o n l y a t Co w i t h the r e s u l t t h a t a b s t r a c t i o n by Co has a h i g h e r a c t i v a t i o n energy than a b s t r a c t i o n by C thus a c c o u n t i n g f o r the observed change i n r e g i o s e l e c t i v i t y . An a d d i t i o n a l i n t e r e s t i n g p o i n t concerns the p h o t o c h e m i c a l i n e r t n e s s of enone O r i g i n a l l y t h i s u n r e a c t i v i t y was a s c r i b e d s o l e l y t o the u n u s u a l l y l o n g hydrogen a b s t r a c t i o n d i s t a n c e s i n v o l v e d ( T a b l e I ) , but i t now can be seen to be due i n a d d i t i o n to the s t e r i c compression which would accompany a b s t r a c t i o n a t e i t h e r c a r b o n . At t h i s p o i n t , w h i l e the main f e a t u r e s of the t h e o r y a r e c l e a r , i t i s not w o r t h w h i l e t o t r y to c a l c u l a t e the a c t u a l a c t i v a t i o n energy d i f f e r e n c e s f o r each enone based on the hydrogen-hydrogen c o n t a c t s accompanying p y r a m i d a l i z a t i o n i n each c a s e . The main r e a s e n f o r t h i s i s the r e l a t i v e l y l a r g e ( b u t normal) e x p e r i m e n t a l e r r o r s i n d e t e r m i n i n g the hydrogen atom c o o r d i n a t e s from the room temperature c r y s t a l l o g r a p h i c d a t a . As i s apparent from F i g u r e 5, compression energy i s a v e r y s e n s i t i v e f u n c t i o n of d i s t a n c e below 2 Â. Neutron d i f f r a c t i o n s t u d i e s would p e r m i t more a c c u r a t e q u a n t i f i c a t i o n of the t h e o r y . a

atu

a

a

a

S t e r i c Compression I n h i b i t i o n o f [2+2] P h o t o c y c l o a d d i t i o n . F o l l o w i n g the p i o n e e r i n g work of Schmidt and co-workers on the s o l i d s t a t e p h o t o d i m e r i z a t i o n r e a c t i o n s of the c i n n a m i c a c i d s ( 2 6 ) , a v e r y l a r g e body of e v i d e n c e CH ο has accumulated which demonstrates ^ΛψΑ. t h a t i n t r a m o l e c u l a r [2+2] ί Τ Η 8 (Ε = CO CH ) p h o t o c y c l o a d d i t i o n i s the v i r t u a l l y ^•^Vr *v 2 3 i n e v i t a b l e r e s u l t of a c r y s t a l CH CH p a c k i n g arrangement which o r i e n t s l a t t i c e n e i g h b o r s so t h a t the p o t e n t i a l l y r e a c t i v e double bonds l i e 3

3


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2.6

2.5 2.4 2.20 A contact 2.3 2.2

Η· · ·Η Contact (A)

2.1 2.0 1.71 A contact

1.9

1.7 1.87 A contact -40°

-30° -20° -10°

0°

10°

20°

30°

40°

Angle of Rotation

F i g u r e 6· Hydrogen-hydrogen c o n t a c t s v e r s u s angle of r o t a t i o n f o r t h e p y r a m i d a l i z e d m e t h y l group of enone ^g,. Of t h e n i n e t o t a l c o n t a c t s , o n l y t h e t h r e e shown a r e below 2.2 Â.

Steric Compression at C Only a

Steric Compression at Both Carbons Steric Compression at 0β Only No Steric Compression

F i g u r e 7. R e l a t i v e a c t i v a t i o n e n e r g i e s f o r hydrogen t r a n s f e r t o C o r Cg as a f u n c t i o n o f s t e r i c compression accompanying pyramidalization. a


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i n a p a r a l l e l arrangement a t c e n t e r t o c e n t e r d i s t a n c e s o f 4.1 Â or l e s s (27-34). We were thus v e r y s u r p r i s e d t o observe t h e complete l a c k of photochemical r e a c t i v i t y of enone when i r r a d i a t e d i n the s o l i d s t a t e . Compound 8 c r y s t a l l i z e s i n a l a t t i c e arrangement which i s i d e a l f o r i n t e r m o l e c u l a r [2+2] photoc y c l o a d d i t i o n ( F i g u r e 8) ( 3 5 ) . The p o t e n t i a l l y r e a c t i v e double bonds a r e o r i e n t e d i n a head t o t a i l f a s h i o n and are p a r a l l e l , d i r e c t l y above one another and o n l y s l i g h t l y o f f s e t along t h e double bond a x i s (0.52 Â); t h e c e n t e r t o c e n t e r d i s t a n c e i s 3.79 Â. N e v e r t h e l e s s , p h o t o l y s i s o f s i n g l e c r y s t a l s o f f o r up t o 40 hours a t -16° t o -18°C ( t o prevent m e l t i n g ) w i t h a L i c o n i x H e l i u m Cadmium 325 nm CW l a s e r showed l e s s than 1% r e a c t i o n by c a p i l l a r y gas chromatography. That t h i s l a c k of r e a c t i v i t y i s not an i n t r i n s i c p r o p e r t y of enone was shown by t h e f a c t t h a t i t s s o l u t i o n phase i r r a d i a t i o n a t the same wavelength l e a d s t o essent i a l l y q u a n t i t a t i v e y i e l d s o f the cage compound r e s u l t i n g from i n t r a m o l e c u l a r [2+2] c y c l o a d d i t i o n . What i s the source o f t h i s remarkable l a c k o f r e a c t i v i t y i n the s o l i d s t a t e ? The p a c k i n g diagram shown i n F i g u r e 8 r e v e a l s the probable answer. As t h e p o t e n t i a l l y r e a c t i v e molecules X and X s t a r t t o move towards one another i n the i n i t i a l stages o f [2+2] p h o t o c y c l o a d d i t i o n , each e x p e r i e n c e s i n c r e a s i n g l y severe s t e r i c compression of two of i t s m e t h y l groups ( d o t t e d l i n e s ) . The key f e a t u r e of t h i s s t e r i c compression i s t h a t i t i s developed, n o t between the p o t e n t i a l r e a c t a n t s X and X ( a f t e r a l l , a c e r t a i n amount of s t e r i c compression between r e a c t a n t s must always accompany bond f o r m a t i o n between them), but between X and Ϋ and X and Y. Thus molecules Y and Ϋ a c t as s t a t i o n a r y impediments t o photo d i m e r i z a t i o n i n e x a c t l y t h e same way as t h e s t a t i o n a r y l a t t i c e n e i g h b o r s i n h i b i t e d hydrogen a b s t r a c t i o n i n the case of enones

Computer S i m u l a t i o n o f P h o t o d i m e r i z a t i o n . These i d e a s were t e s t e d by computer s i m u l a t i o n o f the s o l i d s t a t e [2+2] p h o t o c y c l o a d d i t i o n . Two mechanisms were c o n s i d e r e d : (1) M o l e c u l e s X and X move toward each o t h e r i n 0.24 Â increments a l o n g the double bond c e n t e r t o c e n t e r v e c t o r ( d u a l motion mechanism) and (2) molecule X remains f i x e d w h i l e molecule X moves toward i t i n 0.48 Â i n c r e ments a l o n g t h e c e n t e r t o c e n t e r v e c t o r ( s i n g l e motion mechanism). In both c a s e s , t h e c o o r d i n a t e s of molecules Y and Ϋ were l e f t unchanged d u r i n g the h y p o t h e t i c a l d i m e r i z a t i o n . The new hydrogenhydrogen c o n t a c t s were then determined a t each stage o f t h e d i m e r i z a t i o n s . By v i r t u e of the symmetry of t h e system, a l l f o u r hydrogen-hydrogen c o n t a c t s a r e i d e n t i c a l , and the c o n t a c t s developed by moving X toward X are t h e same as those developed by moving X toward X. The r e s u l t s a r e shown g r a p h i c a l l y i n F i g u r e 9. T h i s i s a p l o t o f t h e t o t a l s t e r i c compression energy (MM2) versus double bond c e n t e r t o c e n t e r d i s t a n c e f o r both the d u a l motion and s i n g l e motion d i m e r i z a t i o n pathways. I n both c a s e s , the s t e r i c compression accompanying d i m e r i z a t i o n i s of s u f f i c i e n t magnitude to account r e a s o n a b l y f o r t h e l a c k o f d i m e r i z a t i o n . F o r example, at a c e n t e r t o c e n t e r d i s t a n c e of 2.35 Â ( d u a l motion mechanism), the hydrogen-hydrogen c o n t a c t i s 1.9 Â and the t o t a l MM2 r e p u l s i o n


15.

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F i g u r e 8· S t e r e o p a c k i n g diagram of compound Molecules X and X a r e r e l a t e d through a c e n t e r of symmetry. T r a n s l a t i o n of X a l o n g a_ generates Y, and t r a n s l a t i o n of X a l o n g -a_ g e n e r a t e s Ϋ.

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

C e n t e r t o Center Double Bond D i s t a n c e (A)

F i g u r e 9. MM2 s t e r i c compression energy v e r s u s double bond c e n t e r to c e n t e r d i s t a n c e f o r s i n g l e and d u a l motion photo d i m e r i z a t i o n pathways. Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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energy i s 13.2 k c a l / m o l e . T h i s corresponds t o a p r e - d i m e r i z a t i o n geometry i n which 2p - 2p o r b i t a l o v e r l a p i s s m a l l . T h i s i s based on R o b e r t s ' c a l c u l a t i o n s of the o v e r l a p i n t e g r a l S ^ v e r s u s d i s t a n c e f o r 2ρ-σ and 2p-π bonding ( 3 6 ) . U s i n g these d a t a , we e s t i m a t e t h a t a t a c e n t e r t o c e n t e r s e p a r a t i o n o f 2.35 Â ( o f f s e t 0.33 Â ) , the ρ-orbital o v e r l a p between molecules X and X i s l e s s than 20% of maximum. F u r t h e r movement o f X and X towards each o t h e r becomes p r o h i b i t i v e l y expensive owing t o t h e f a c t t h a t t h e hydrogen-hydrogen r e p u l s i o n energy r i s e s v e r y s t e e p l y below 1.9 Â ( F i g u r e 5 ) . As i n t h e case o f enones ^ a j " l J i , these o v e r a l l c o n c l u s i o n s a r e not a l t e r e d when r o t a t i o n of t h e i n t e r a c t i n g m e t h y l groups i s taken i n t o a c c o u n t . A f i n a l p o i n t concerns t h e i n t e r e s t i n g p r e d i c t i o n t h a t a t c e n t e r t o c e n t e r d i s t a n c e s above 2.1 Â the d u a l motion photod i m e r i z a t i o n i s l e s s s t e r i c a l l y h i n d e r e d than t h e s i n g l e motion pathway. T h i s can be e x p l a i n e d w i t h t h e a i d of F i g u r e 10 which i s an i d e a l i z e d drawing of t h e p a c k i n g arrangement f o r compound g, showing the methyl-methyl i n t e r a c t i o n s . Simply p u t , the sum o f the f o u r i n t e r a c t i o n s developed i n moving both r e a c t a n t s toward each o t h e r by a d i s t a n c e d i s l e s s than t h e sum o f t h e two much more severe i n t e r a c t i o n s which r e s u l t when one o f t h e r e a c t a n t s i s moved toward the o t h e r by a d i s t a n c e 2d. Summary. We have shown t h a t the course of both u n i m o l e c u l a r and b i m o l e c u l a r s o l i d s t a t e c h e m i c a l r e a c t i o n s can be i n f l u e n c e d p r o f o u n d l y by c e r t a i n s p e c i f i c s t e r i c i n t e r a c t i o n s which d e v e l o p between he r e a c t i n g m o l e c u l e s and t h e i r s t a t i o n a r y l a t t i c e b e i g h b o r s . We suggest t h e term s t e r i c compression c o n t r o l f o r t h i s e f f e c t and p r e d i c t t h a t i t w i l l f i n d g e n e r a l u t i l i t y i n u n d e r s t a n d i n g c h e m i c a l p r o c e s s e s i n t h e s o l i d s t a t e . Our r e s u l t s p r o v i d e s t r o n g support f o r Cohen's r e a c t i o n c a v i t y ( 2 ) and G a v e z z o t t i ' s volume a n a l y s i s (4) view of s o l i d s t a t e s p e c i f i c i t y . We a r e i n the p r o c e s s of t e s t i n g t h e concept of s t e r i c compression c o n t r o l u s i n g a wide v a r i e t y of s o l i d s t a t e systems.

F i g u r e 10. I d e a l i z e d p a c k i n g diagram f o r compound Steric compression between f o u r m e t h y l groups develops as [2+2] photod i m e r i z a t i o n proceeds. A l l s u b s t i t u e n t s o t h e r than those i n v o l v e d i n the hydrogen-hydrogen i n t e r a c t i o n s have been o m i t t e d for c l a r i t y . Fox; Organic Phototransformations in Nonhomogeneous Media ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Acknowledgment. We thank the N a t u r a l S c i e n c e s and E n g i n e e r i n g Research C o u n c i l of Canada f o r f i n a n c i a l s u p p o r t .

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RECEIVED January 10, 1985


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