8/3/2019 J.M. Mestdagh, J.P. Visticot and P.F. Bernath- Comment on "Influence of the radical size on the Ca^*+ ROH->CaOH

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E L S E V I E R

19May 1995

Chemical Physics Letters 237 (1995) 568-569

C H E M I C A LP H Y S I C SL E T T E R S

C o m m e n tC o m m e n t o n " I n f l u e n c e o f t h e r a d i c a l s i z e

o n t he C a * + R O H C a O H * + R ( R = C H 3, C 2 H 5 and C 3 H 7)r e a c t i o n c r o s s s e c t i o n "

J . M . M e s t d a g h a, j . p . V i s t i c o t a, P . F . B e r n a t h ba C E A / D R E C A M / S e r c i c e d e s P ho t o n s, A t o m e s e t M o l d c u l es C . E .N . S ac la y , 9 1 19 1 G i f - s u r -Y v e t te C e de x , F r a n c e

h Dep ar tme nt o f Chemis try , Unit ,ers i O, o f Water loo, Water loo, O ntar io , Canad a N2L 3G 1Received 20 January 1995

Abs t r a c t

The recent results of Esteban, Garay, Garcla-Tijero, Verdasco and Gonzfilez Urefia reexamined in the light of previousinvestigations on the same class of reactions.

I n the t i t l e Le t te r [ 1] , Es teban e t a l . have r epor tedr eac t ions of e lec t r on ica l ly exc i ted ( 3s4d ~D 2 and3 s 3 p 3 p j ) c a l c i u m a t o m s , w i t h a v a r i e t y o f a l c o h o l s( m e t h a n o l , e t h a n o l a n d n - p r o p a n o l ) . T h e r e a c t i o n sw e r e i n v e s t i g a t e d u n d e r b e a m - g a s c o n d i ti o n s . T h eo n l y c h e m i l u m i n e s c e n c e s i g n a l t h a t w a s o b s e r v e do r i g i n a t e d f r o m t h e A 2 I ] a n d B 2 ~ s t a te s o f c a l -c i u m m o n o h y d r o x i d e , C a O H . T h e c h e m i l u m i n e s -c e n c e c r o s s s e c t i o n i s v e r y s m a l l , o f t h e o r d e r o f10 3 to 10 -2 ~2.

These r esu l t s a r e in t r igu ing , s ince they a r e a t oddsw i t h t h e a l r e a d y k n o w n g a s - p h a s e c h e m i s t r y o f t h ea lka l ine ea r th e lements .

T h e c h e m i s t r y o f a l k a l i n e e a r t h a l c o h o l m i x t u r e shas been inves t iga ted bo th in the s ing le co l l i s ion[ 2 ,3] and in the mul t ip le co l l i s ion [ 4] r eg im es w i ths p e c i a l e m p h a s i s o n t h e r e a c t i o n s o f b a r i u m a t o m s .I n r e a c t i o n s o f m e t h a n o l , b o t h w i t h g r o u n d s t a t e( 6s 2 I S 0) and meta s tab le ( 6s5d 1D 2) b a r ium a tom s ,o n l y t h e B a O C H 3 + H e x i t c h a n n e l w a s o b s e r v e d ,d e s p i t e t h e m u c h l a r g e r e x o e r g i c i t y f o r f o r m a t i o n o f

e i t h e r B a O H + C H 3 o r B a O + CH 4. T h e s e o b s e r v a -t i o n s h a v e s u g g e s t e d t h a t t h e f o r m a t i o n o f t h e r e a c -t i on p r o d u c t s r e s u lt s f r o m a m i g r a t i o n o f th e H a t o mt h a t i s b o n d e d t o o x y g e n , a p r o c e s s t h a t i s s t r o n g l ye n h a n c e d b y e l e c t r o n i c e x c i t a t i o n o f t h e i n c o m i n gb a r i u m a t o m [ 2 ] . S i m i l a r r e s u l ts h a v e b e e n r e p o r t e dby de Pujo e t a l . [ 3 ] in an inves t iga t ion of chemi lu-m i n e s c e n c e i n B a ( 6 s 6 p ~ P ~ ) r e a c t io n s w i t h w a t e r ,a l c o h o l s a n d e t h e r s . H e r e a g a i n , m i g r a t i o n o f t h e Ha t o m t h a t p r o m o t e s t h e i n s e r t i o n o f B a i n t o t h e O Hb o n d o f t h e a l c o h o l g r o u p w a s t h e k e y s t e p i n th er eac t ion mechanism. F ina l ly , Br az ie r e t a i . [ 4 ] havei n v e s t i g a t e d t h e r e a c t i o n s o f c a l c i u m , s t r o n t i u m a n db a r i u m w i t h m e t h a n o l , 1 - p r o p a n o l, 2 - p r o p a n o l , 1 -b u t a n o l , 2 - b u t a n o l , 2 - m e t h y l - 2 - p r o p a n o l , a c e t o n e a n da c e t a l d e h y d e . O n c e a g a i n , t h e o n l y c h e m i l u m i n e s -c e n t p r o d u c t o b s e r v e d i n r e a c t i o n s w i t h t h e a l c o h o l si s t he a l k a l i n e -e a r t h a l k o x i d e , M O R ( M = a l k a l i n eear th , R = a lky l r ad ica l ) .

The Le t te r o f Es teban e t a l . [ 1 ] i s sur pr i s ing s incen o p r e v i o u s s t u d y r e p o r t s t h e o b s e r v a t i o n o f a n

0009-2614/95/$09.50 1995 Elsevier Science B.V. All rights reservedS S D I 0009-2614(95)00346-0

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J,M. Mestdagh et al. / Chemical Physics Letters 237 (1995) 568-569 5 6 9T a b l e 1E n e r g e t i c s o f t h e C a + C H 3 O H s y s t e mRe a c t i o n A H ( e V )C a ( I D 2 ) + C H 3 O H ~ C a O H ( A ) + C H 3 - 1 .0Ca ( 1D 2 ) + C H 3 O H ~ C a O H ( B ) + C H 3 - 0 . 8Ca ( 1D 2 ) + CH 3 O H ~ Ca O C H 3 ( A ) + H - 0 . 4C a ( ~D 2 ) + C H 3 O H ~ C a O C H 3 ( B ) + H - 0 . 2C a ( 3 P j ) + C l t 3 O H ~ C a O H ( A ) + C H 3 - 0 .2C a ( 3 p j ) + C H 3 O H ~ C a O H ( B ) + C H 3 + 0 . 1C a ( 3 1~ t )+ C H 3 O H ~ C a O C H 3 ( A ) + H + 0 . 4Ca ( 3 P I ) + CH 3 O H - -~ Ca O C H 3 ( B) + H + 0 . 6

a l k a l i n e - e a r t h h y d r o x i d e b y r e a c t i o n w i t h a l c o h o l s .T h e o n l y w o r k s o f a r w h e r e a l k a l i n e - e a r t h h y d r o x -i d e s h a v e b e e n o b s e r v e d , o f t e n t o g e t h e r w i t h t h eo x i d e s a n d t h e a l k o x i d e s , i s w h e n t h e m o l e c u l e t h a tr eac t s has a c lea r O H leav ing gr oup , a s i s the casew i th w a te r [ 2 ,5] and a lky l hydr oper oxides [ 6] .

T h e w o r k o f E s t e b a n e t a l . n e e d s t o b e d i s c u s s e din de ta i l in or der to s ee w he ther i t can f i t w i th thep i c t u r e o f t h e a l k a l i n e e a r t h / a l c o h o l r e a c t i o n s d i s -cussed above , o r w he ther i t i s in conf l ic t w i th i t .U n f o r t u n a t e l y , s u c h a d i s c u s s i o n c a n n o t b e f o u n d i nthe Le t te r o f Es teban e t a l . [ 1 ] .

I n f ac t the s i tua t ion i s s imple i f one cons ider s thee n e r g e t i c s o f t h e c a l c i u m - m e t h a n o l r e a c t i o n s l i s t e di n T a b l e 1 . T h e b o n d s t r e n g t h s u s e d t o c o n s t r u c t t h et a b l e a r e t a k e n f r o m R e f . [ 7 ] ( C H 3 - O H a n d C H 3 0 -H ) a n d R e f. [8 ] ( C a - O H ) . T h e u n k n o w n C a - O C H 3b o n d s t r e n g t h w a s t a k e n t o b e e q u a l t o t h a t o fC a - O H . T h e e n e r g y o f t h e r e l e v a n t e l e c t r o n i c a l l yexc i ted leve l s a r e t aken f r om Ref . [ 9 ] f or Ca , andf r o m R e f . [4 ] f o r C a O H a n d C a O C H 3 .

E x a m i n a t i o n o f T a b l e 1 s h o w s t h a t t h e o n l yc h e m i l u m i n e s c e n t c h a n n e l t h a t i s o p e n f o r t h e r e a c -t ion o f C a ( 3 p j ) i s t h e f o r m a t i o n o f C a O H ( A ) , a n da l s o t h a t o f C a O H ( B ) i f t h e c o l l i s io n e n e r g y ( 0 . 1 4e V ) a v a i la b l e in t he C a - C H 3 O H e x p e r i m e n t o f E s t e-ban e t a l . i s inc luded . For the r eac t ion of Ca( 1D 2)f l u o re s c e n t e m i s si o n f r o m b o t h C a O H a n d C a O C H 3c o u l d b e o b s e r v e d . H o w e v e r , d e P u j o e t a l . [ 3 ] h a v es h o w n t h at t h e b o n d e n e r g y o f B a - O C H 3 i s su b s t an -t i a ll y s m a l l e r th a n t h a t o f B a - O H ( 3 .8 v e r s u s 4 . 6e V ) . I f th e s a m e s i t u a t i o n i s e n c o u n t e r e d w i t h c a l -c i u m , t h e n t h e p r o d u c t i o n o f e l e c tr o n i c a l l y e x c i t e dC a O C H 3 i s a l so e n e r g e t i c a l ly c l o s e d f o r r e a c t io n o fC a ( I D 2 ) , a s i s t h e c a s e f o r r e a c t i o n o f C a ( 3 p j ) .

F r om these ener ge t ic cons ider a t ions , i t i s thus no ts u r p r i s i n g t h a t t h e c h e m i l u m i n e s c e n c e o b s e r v e d i nt h e w o r k o f E s t e b a n e t a l . o r i g i n a t e s f r o m C a O H .N ow , le t us t ake in to cons ider a t ion the r eac t ionm e c h a n i s m d i s c u s s e d a b o v e f o r t h e a l k a l i n ee a r t h / a l c o h o l r e a c t i o n s . T h e r e a c t i o n i n v o l v e s a nH - a t o m m i g r a ti o n t h a t e v e n t u a ll y p r o d u c e s C a O C H 3a s th e m a j o r p r o d u c t . W e c o n c l u d e t h a t i f C a O H i sf o r m e d i n t h e r e a c t i o n i t c a n o n l y c o r r e s p o n d t o am i n o r r e a c t io n p r o d u c t , a f r a c ti o n o f w h i c h i s e le c -t r o n i c a l l y e x c i t e d . T h i s c o n c l u s i o n a g r e e s w i t h t h es m a l l c h e m i l u m i n e s c e n t c r o s s s e c t i o n s r e p o r t e d b yE s t e b a n e t a i . f o r t h e r e a c t i o n o f c a l c i u m w i t hm e t h a n o l . A s i m i l a r p i c t u r e h o l d s f o r t h e o t h e r a l c o -hols inves t iga ted by Es teban e t a l . [ 1 ] .

W h y w a s t h e m i n o r h y d r o x i d e c h a n n e l n o t o b -se r ved in Ref s . [ 3 ,4] ? Ther e a r e tw o main r easons :f i r s t ly , the ener ge t ics in those cases d id no t p r ec ludet h e c h e m i l u m i n e s c e n t e m i s s i o n o f t h e m a j o r a l k o x i d ep r o d u c t , a n d s e c o n d l y t h e e m i s s i o n s p e c t r a o f e l e c -t r o n i c a l l y e x c i t e d c a l c i u m h y d r o x i d e a n d c a l c i u malkoxides a r e c lose toge ther [ 4] . For these tw o r ea -s o n s , t h e w e a k c h e m i l u m i n e s c e n c e o f C a O H , w a sh i d d e n b y t h a t o f th e d o m i n a n t c a l c i u m a l k o x i d epr oduc t .

To conf i r m the pr esen t in te r pr e ta t ion , a l a se r - in -d u c e d f l u o r e s c e n c e s t u d y o f t h e g r o u n d s t a te r e a c t i o np r o d u c t s w o u l d b e h i g h l y d e s i r a b l e .

References[ 1 ] M . E s t e b a n , M . G a r a y , . I . M . G a r c l a - T i j e r o , E . V e r d a sc o a n d

A . G o n z f i l e z U r e f i a , Ch e m . Ph y s . L e t t e r s 2 3 0 ( 1 9 9 4 ) 5 2 5 .[ 2 ] H . F . D a v i s , A . G . Su i t s , Y . T . L e e , C . A l c a r a z a n d J . - M .

M e s t d a g h , J . C h e m . P h y s . 9 8 (1 9 9 3 ) 9 5 9 5 .[ 3 ] P . d e Pu j o , O . Su b l e m o n t i e r , J . - P . V i s t i c o t , J . Be r l a n d e , J .

C u v e l l i er , C . A l c a r a z , T . G u s t a v s s o n , J . -M . M c s t d a g h a n d P .M e y n a d i e r , J . C h e m . P h y s . 9 9 ( 1 9 9 3 ) 2 5 3 3 .

[ 4 ] C . R . B r a z i e r , L . C . E l l i n g b o e , S . K i n sc y - N i e l se n a n d P . F .B e r n a t h , J . A m . C h e m . S o c . 1 0 8 (1 9 8 6 ) 2 1 2 6 .

[ 5 ] S . K i n se y - N i e l se n , C . R . B r a z i e r a n d P . F . Be r n a t h , J . Ch e m .P h y s . 8 4 ( 1 9 8 6 ) 6 9 8 .

[ 6 ] B . S . C h e o n g a n d J . M . P a r s o n , J . C h e m . P h y s . 1 0 0 ( 1 9 9 4 )2 6 3 7 .

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[ 8] M . W . C h a s e , J. L . C u r n u t t , R . A . M c D o n a l d a n d A . N . S y v e r u d ,J A N A F t a b l e s , J . P h y s . C h e m . R e f . D a t a 7 ( 1 9 7 8 ) 7 9 3 .

[ 9] C .E . M o o r e , A t o m i c e n e r g y l e v e ls , N R S D S - N B S , C i r c u la rN o . 4 6 7 , V o l . 2 ( U S G P O , W a s h i n g t o n , 1 9 5 2 ) .