Gas-Phase Photodissociation of Transition Metal Ion Complexes and

Nov 23, 1987 - DOI: 10.1021/bk-1987-0359.ch010. ACS Symposium Series , Vol. 359. ISBN13: 9780841214415eISBN: 9780841212060. Publication Date ...
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Chapter 10

Gas-Phase Photodissociation of Transition Metal Ion Complexes and Clusters 1

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Robert L. Hettich and Ben S. Freiser Department of Chemistry, Purdue University, West Lafayette, IN 47907 H i g h l i g h t s are presented from our recent work on the p h o t o d i s s o c i a t i o n of three c a t e g o r i e s of t r a n s i t i o n metal ion s p e c i e s : (I) ML (L=ligand), (II) ML +, and MFe (M=3d metal). The r e s u l t s i n d i c a t e that these species absorb broadly i n the u l t r a v i o l e t and v i s i b l e s p e c t r a l regions. Because of t h i s broad absorption, p h o t o d i s s o c i a t i o n thresholds are a t t r i b u t e d to thermodynamic f a c t o r s and y i e l d absolute bond energies. Isomer d i f f e r e n t i a t i o n i s demonstrated by observing d i f f e r e n c e s i n cross s e c t i o n s , s p e c t r a l band p o s i t i o n s , and fragmentation pathways. I n t e r e s t i n g l y , product ions generated by p h o t o d i s s o c i a t i o n are found i n some cases to d i f f e r s i g n i f i c a n t l y from those produced by c o l l i s i o n - i n d u c e d d i s s o c i ation. S o p h i s t i c a t e d pulse techniques, which permit the multistep synthesis and i s o l a t i o n of a wide v a r i e t y of organometallic i o n s , together with the long ion storage times, make FTMS i d e a l l y suited for these s t u d i e s . +

+

2

Over t h e p a s t s e v e r a l y e a r s , t h e a r e a o f g a s - p h a s e t r a n s i t i o n m e t a l i o n c h e m i s t r y has been g a i n i n g i n c r e a s i n g a t t e n t i o n from t h e s c i e n t i f i c community [ 1 - 1 6 ] . I t s a p p e a l i s m a n i f o l d : f i r s t , i t has b r o a d i m p l i c a t i o n s t o a s p e c t r u m o f o t h e r a r e a s such as a t m o s p h e r i c chemistry, c o r r o s i o n chemistry, s o l u t i o n organometallic chemistry, and s u r f a c e c h e m i s t r y ; s e c o n d l y , an a r s e n a l o f gas phase t e c h n i q u e s a r e a v a i l a b l e to study the thermochemistry, k i n e t i c s , and mechanisms o f t h e s e " u n u s u a l " s p e c i e s i n t h e a b s e n c e o f s u c h c o m p l i c a t i o n s a s s o l v e n t and l i g a n d

Current address: Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, T N 37831

0097-6156/87/0359-0155506.00/0 © 1987 American Chemical Society

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

156

FOURIER TRANSFORM MASS SPECTROMETRY

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e f f e c t s ; and, t h i r d l y , t h e h i g h l y c o n t r o l l e d n a t u r e o f t h e s e gas phase e x p e r i m e n t s p e r m i t s t h e s t u d y o f s p e c i f i c l i g a n d e f f e c t s and p r o v i d e s d a t a f o r d i r e c t c o m p a r i s o n t o t h e o r y , as new t h e o r e t i c a l t r e a t m e n t s emerge. How does t h e g a s - p h a s e c h e m i s t r y o f a b a r e and, therefore, highly c o o r d i n a t i v e l y unsaturated t r a n s i t i o n m e t a l i o n compare t o i t s s o l u t i o n c o u n t e r p a r t ? As a r e p r e s e n t a t i v e example, Beauchamp and c o w o r k e r s p r o p o s e d Scheme I f o r t h e d e h y d r o g e n a t i o n o f b u t a n e by N i [ 2 ] . The mechanism shown has a n a l o g i e s t o s o l u t i o n o r g a n o Scheme I

,

Y >

r

t

\

. > Λ

"

.

>

+

ll-Ni -||



H,

y/ m e t a l l i c c h e m i s t r y , but i s u n u s u a l i n t h a t o x i d a t i v e a d d i t i o n of the metal c a t i o n i s proposed t o occur at the C-C bond as opposed t o t h e C-H bond, w h i c h i s t h e e x a c t o p p o s i t e o f what i s g e n e r a l l y o b s e r v e d i n s o l u t i o n studies. O x i d a t i v e a d d i t i o n o f F e , C o , and N i t o C-C bonds i n l o n g e r c h a i n a l k a n e s a p p e a r s t o be g e n e r a l [ 3 - 7 ] . In o r d e r t o p r o p o s e a mechanism such as t h a t shown i n Scheme I, i t i s e v i d e n t t h a t not o n l y must t h e p r o d u c t i o n m a s s - t o - c h a r g e r a t i o ( e m p i r i c a l f o r m u l a ) be d e t e r m i n e d , but a l s o t h e s t r u c t u r e o f t h e i o n must be elucidated. C l e a r l y , a d i f f e r e n t mechanism i n v o l v i n g C-H i n s e r t i o n m i g h t have been p r o p o s e d i f t h e N i C . H p r o d u c t had been N i - b u t e n e i n s t e a d o f Ni(ethéne) . S e v e r a l e f f e c t i v e s t r u c t u r a l probes are a v a i l a b l e to the gas-phase i o n chemist i n c l u d i n g c o l l i s i o n - i n d u c e d d i s s o c i a t i o n [ 3 1 4 a , 4 b ] , i o n - m o l e c u l e r e a c t i o n s ( f o r metal i o n complexes, l i g a n d exchange [ 1 - 3 ] and H/D exchange [5] are commonly e m p l o y e d ) , and use o f s p e c i f i c a l l y l a b e l l e d n e u t r a l compounds [ 2 , 6 , 7 ] . E a c h o f t h e s e methods was a p p l i e d t o t h e r e a c t i o n shown i n Scheme I and c o n f i r m t h e presence of N i ( e t h e n e ) . Another important c o n s i d e r a t i o n i n f o r m u l a t i n g a mechanism i s w h e t h e r each s t e p , as w e l l as t h e o v e r a l l process, i s thermodynamically feasible. In Scheme I t h e f i r s t s t e p i n v o l v e s c l e a v i n g t h e c e n t r a l C-C bond and f o r m i n g two N i * - C bonds. Thus, t h e d e t e r m i n a t i o n of m e t a l - l i g a n d bond e n e r g i e s i s c r i t i c a l l y important. A g a i n , s e v e r a l p o w e r f u l g a s - p h a s e t e c h n i q u e s can be a p p l i e d to t h i s problem. Among t h e s e , t h e d e t e r m i n a t i o n o f e n d o t h e r m i c r e a c t i o n t h r e s h o l d s from ion-beam e x p e r i ments has y i e l d e d t h e m a j o r i t y o f t h e a b s o l u t e v a l u e s c u r r e n t l y i n t h e l i t e r a t u r e today [ 1 , 8 , 9 ] . Ligand d i s +

+

+

+

g

+

+

2

+

2

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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10.

HETTICH AND FREISER

Photodissociation of Transition Metal Ions

157

p l a c e m e n t r e a c t i o n s and e q u i l i b r i u m measurements have y i e l d e d a c c u r a t e r e l a t i v e m e t a l i o n - l i g a n d bond e n e r g i e s f o r a v a r i e t y o f m e t a l c a t i o n s and l i g a n d s [10,11]. Observation of exothermic i o n - molecule r e a c t i o n s can p r o v i d e l i m i t s on t h e s e e n e r g i e s and, most r e c e n t l y , c o m p e t i t i v e l i g a n d l o s s by c o l l i s i o n - i n d u c e d d i s s o c i a t i o n has shown p r o m i s e b o t h f o r q u a l i t a t i v e and q u a n t i t a t i v e bond i n f o r m a t i o n [ 1 2 ] . D e s p i t e t h e s u c c e s s e s o f t h e above m e n t i o n e d t e c h n i q u e s f o r s t r u c t u r e and bond e n e r g y a n a l y s e s , i n ­ h e r e n t u n c e r t a i n t i e s and l i m i t a t i o n s i n e a c h method make i t i m p o r t a n t t o d e v e l o p new and i n d e p e n d e n t t e s t s f o r comparison. I n t h i s r e g a r d our l a b o r a t o r y has i n i t i a t e d an i n t e n s i v e e f f o r t t o s t u d y t h e g a s - p h a s e p h o t o d i s s o c i a ­ t i o n of t r a n s i t i o n metal c o n t a i n i n g i o n s [13-16]. A l t h o u g h a good d e a l o f i n f o r m a t i o n i s a v a i l a b l e on t h e p h o t o d i s s o c i a t i o n of o r g a n i c i o n s [ 1 7 l 8 ] , r e l a t i v e l y l i t t l e work has been done on o r g a n o m e t a l l i c i o n s [19>20]. FTMS i s i d e a l l y s u i t e d f o r t h e s e s t u d i e s b e c a u s e o f t h e wide v a r i e t y o f i n t e r e s t i n g i o n s t h a t can be g e n e r a t e d and b e c a u s e o f t h e l o n g s t o r a g e t i m e s w h i c h p e r m i t i r r a d i a t i o n o f the i o n s . I n t h i s p a p e r some o f t h e h i g h ­ l i g h t s o f our work i s p r e s e n t e d t o g e t h e r w i t h a d e s c r i p ­ t i o n o f t h e m e t h o d o l o g y and t h e t y p e s o f i n f o r m a t i o n t h a t can be o b t a i n e d from t h e s e s t u d i e s . f

Experimental Section The t h e o r y and i n s t r u m e n t a t i o n o f F o u r i e r t r a n s f o r m mass s p e c t r o m e t r y (FTMS) have been d i s c u s s e d e x t e n s i v e l y i n t h i s book and e l s e w h e r e [21-23]· A l l e x p e r i m e n t s were p e r f o r m e d on a N i c o l e t p r o t o t y p e FTMS-1000 F o u r i e r t r a n s ­ f o r m mass s p e c t r o m e t e r p r e v i o u s l y d e s c r i b e d i n d e t a i l [24] and e q u i p p e d w i t h a 5.2 cm c u b i c t r a p p i n g c e l l s i t u a t e d between t h e p o l e s o f a V a r i a n 15 i n . e l e c t r o m a g n e t m a i n t a i n e d a t 0.85 T. The c e l l was c o n s t r u c t e d i n our l a b o r a t o r y and u t i l i z e s two 80$ t r a n s m i t t a n c e s t a i n l e s s s t e e l s c r e e n s as t h e t r a n s m i t t e r p l a t e s . This permits i r r a d i a t i o n w i t h a 2.5 kW Hg-Xe a r c lamp, used i n c o n j u n c t i o n w i t h a S c h o e f f e l 0.25 m monochromator s e t f o r 10 nm r e s o l u t i o n . M e t a l i o n s a r e g e n e r a t e d by f o c u s i n g t h e beam o f a Q u a n t a Ray Nd:YAG l a s e r ( e i t h e r t h e f u n d ­ a m e n t a l l i n e a t 1064 nm o r t h e f r e q u e n c y d o u b l e d l i n e a t 532 nm) i n t o t h e c e n t e r - d r i l l e d h o l e (1 mm) of a h i g h p u r i t y r o d o f t h e a p p r o p r i a t e m e t a l s u p p o r t e d on t h e t r a n s m i t t e r s c r e e n n e a r e s t to the l a s e r . The l a s e r i o n i z a t i o n t e c h n i q u e f o r g e n e r a t i n g m e t a l i o n s has been o u t l i n e d elsewhere [25]. D e t a i l s of the c o l l i s i o n - i n d u c e d d i s s o c i a t i o n (CID) e x p e r i m e n t s have been d e s c r i b e d [ 2 6 ] . A r g o n was used as t h e c o l l i s i o n gas a t a t o t a l p r e s s u r e o f ~4 χ 10" torr. The c o l l i s i o n e n e r g y o f t h e i o n s can be v a r i e d ( t y p i c a l l y between 0 and 100 e V ) . A B a y a r d - A l p e r t i o n i z a t i o n gauge was used t o m o n i t o r s t a t i c p r e s s u r e s . Each o f t h e c h e m i c a l s was o b t a i n e d c o m m e r c i a l l y and used w i t h o u t f u r t h e r p u r i f i c a t i o n , e x c e p t f o r m u l t i p l e

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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158

FOURIER TRANSFORM MASS SPECTROMETRY

freeze-pump-thaw c y c l e s t o remove n o n - c o n d e n s i b l e g a s e s . E l e c t r o n impact mass s p e c t r o m e t r y i n d i c a t e d no d e t e c t ­ able i m p u r i t i e s . A l l samples were a d m i t t e d t o t h e c e l l t h r o u g h a G e n e r a l V a l v e Corp. S e r i e s 9 p u l s e d s o l e n o i d v a l v e [ 2 7 ] . The p u l s e d v a l v e , w h i c h was t r i g g e r e d con­ c u r r e n t l y w i t h t h e l a s e r p u l s e used t o g e n e r a t e t h e m e t a l i o n s , i n t r o d u c e d t h e r e a g e n t gas i n t o t h e vacuum chamber t o a maximum p r e s s u r e o f a p p r o x i m a t e l y 10 torr. A l t h o u g h t h e p u l s e d u r a t i o n o f t h e v a l v e was 2 ms, t h e h i g h p r e s s u r e o f t h e r e a g e n t gas had a r i s e time o f about 200 ms and was pumped away by a h i g h speed 5 i n . d i f f u s i o n pump i n a p p r o x i m a t e l y 400 ms. Swept d o u b l e r e s o n a n c e p u l s e s were t h e n used t o i s o l a t e t h e i o n o f i n t e r e s t , w h i c h was s u b s e q u e n t l y t r a p p e d f o r 3 - 6 s e c o n d s ( d e t e r m i n e d by t h e i o n ' s c r o s s s e c t i o n f o r p h o t o d i s s o c i a ­ t i o n ) e i t h e r i n the presence or absence of r a d i a t i o n . For each i o n , two s e t s o f p h o t o d i s s o c i a t i o n s p e c t r a were t a k e n , one a t 2 χ 10 t o r r argon, t o permit c o l l i s i o n a l c o o l i n g , and a n o t h e r a t a b a c k g r o u n d p r e s s u r e o f "10" t o r r [ 2 8 ] . I n a l l c a s e s , d a t a from t h e c o l l i s i o n a l l y cooled ions are presented. P h o t o d i s s o c i a t i o n s p e c t r a were o b t a i n e d by m o n i t o r i n g t h e a p p e a r a n c e o f i o n i c p h o t o p r o d u c t s as a f u n c t i o n o f t h e wavelength of l i g h t . Shot-to-shot v a r i a t i o n of the l a s e r generated m e t a l p r e c u r s o r i o n s made m o n i t o r i n g t h e p h o t o disappearance of the parent i o n i m p r a c t i c a l . Assuming a o n e - p h o t o n p r o c e s s [ 1 7 , 2 9 ] , t h e p h o t o d i s s o c i a t i o n o f AB , eq. 1, can be d e s c r i b e d by f i r s t - o r d e r k i n e t i c s , eq. 2, where and are the a b s o l u t e c r o s s s e c t i o n s f o r f i

hv

+

d(AB )/dt

(1)

= -(σ

+

1

+ σ )Ι(ΑΒ )

(2)

2

r e

p r o d u c t i o n o f t h e p h o t o p r o d u c t s Ρ * and P?*' ~ s p e c t i v e l y , and I i s t h e p h o t o n f l u x . I n t e g r a t i n g eq. and s u b s t i t u t i n g (AB ) = (AB ) + (P ) ^ + +

+

h

p

1

+

( o )

t h e e x t e n t of p h o t o d i s s o c i a t i o n t o t h e 1. rrceilaait>eoss une c A ο c H υ u ι yu w u vu JL Ο Ο ν κ> λ. « . w λ. ν ** w ~ c r e s s s e cBtt i o n a t a g i v e n w a v e l e n g t h , eq. 3» where σ.

σ- + 1

-

i ss t h e t o t a l c r o s s s e c t i o n i + +• ln|1 + l * 2 oI t

σ d

0

['

P

and t i s t h e (3)

P

t

AB

+

J

i r r a d i a t i o n time. S o l v i n g eq. 3 f o r a and p l o t t i n g t h a t v a l u e as a f u n c t i o n o f w a v e l e n g t h , a p p r o p r i a t e l y c o r r e c t ­ i n g f o r b l a n k s (no l i g h t ) , y i e l d s t h e p h o t o d i s s o c i a t i o n spectrum of the i o n ( i . e . , r e l a t i v e a vs. wavelength). C l e a r l y , the c o r r e c t spectrum r e q u i r e s t h a t a l l of the p h o t o d i s s o c i a t i o n p r o d u c t s be d e t e c t e d . Photoappearance c u r v e s f o r t h e i n d i v i d u a l p h o t o p r o d u c t s can be o b t a i n e d , f c

fc

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

HETTICH AND FREISER

Photodissociation of Transition Metal Ions

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f o r example, by s o l v i n g eq. 4 f o r σ and p l o t t i n g a f u n c t i o n o f wavelength. Each s p e c t r u m r e p o r t e d

159

i t as i s an

average o f s e v e r a l t r i a l s . The r e p r o d u c i b i l i t y o f t h e peak i n t e n s i t i e s i s ± k0% and peak l o c a t i o n s i s ± 10 nm. P h o t o d i s s o c i a t i o n t h r e s h o l d s were c o n f i r m e d by u s i n g c u t ­ off f i l t e r s . To o b t a i n a b s o l u t e v a l u e s f o r t h e c r o s s s e c t i o n s o f t h e i o n s b e i n g examined, t h e p h o t o d i s s o c i a t i o n o f C H g ( f r o m t o l u e n e a t 20 eV) a t 410 nm [σ(410 nm) = 0.05 A ] [ 3 0 ] was compared t o t h e p h o t o d i s s o c i a t i o n of a given i o n a t i t s b o t h t a k e n under s i m i l a r experimental conditions. A l l cross s e c t i o n s determined i n t h i s manner have an e s t i m a t e d u n c e r t a i n t y o f ± 50% due t o i n s t r u m e n t a l v a r i a b l e s . +

λ

Μ

χ

ί

R e s u l t s and D i s c u s s i o n on

C h e m i c a l Systems As l i s t e d below, o u r i n i t i a l s t u d i e s have f o c u s s e d three general c a t e g o r i e s o f metal c o n t a i n i n g i o n s : I.

II. III.

MFe

+

(M = 3d S e r i e s )

P h o t o d i s s o c i a t i o n o f simple metal i o n - l i g a n d s p e c i e s ( I ) can p r o v i d e i n f o r m a t i o n on t h e i r a b s o r p t i o n c h a r a c t e r ­ i s t i c s (e.g., i s the a b s o r p t i o n o f l i g h t metal or l i g a n d l o c a l i z e d or o f a charge t r a n s f e r n a t u r e ? ) , s t r u c t u r e ( p a r t i c u l a r l y i n t h e c a s e o f i s o m e r s ) , and bond e n e r g y , as d i s c u s s e d i n g r e a t e r d e p t h below. The a d d i t i o n o f a s e c o n d l i g a n d ( I I ) a l l o w s one t o s t u d y t h e e f f e c t o f t h a t l i g a n d on t h e bond e n e r g y o f t h e f i r s t ( i . e . , a r e t h e r e any s y n e r g i s t i c a f f e c t s ? ) , as w e l l as i t s e f f e c t on t h e shape and c r o s s s e c t i o n o f t h e p h o t o d i s s o c i a t i o n s p e c t r a . As shown i n Scheme I , o x i d a t i v e a d d i t i o n o f a m e t a l i o n r e s u l t s i n f o r m a t i o n o f two new bonds t o t h e m e t a l c e n t e r . In f o r m u l a t i n g a s i m p l e e n e r g y d i a g r a m f o r such a p r o c e s s , i t i s g e n e r a l l y assumed t h a t t h e t o t a l l i g a n d b o n d i n g e n e r g y i s t h e sum o f t h e two i n d i v i d u a l m e t a l i o n - l i g a n d bond e n e r g i e s . W h i l e some i n f o r m a t i o n i s a v a i l a b l e on m u l t i p l e l i g a n d s y s t e m s [ 1 0 ] , i t i s q u i t e r a r e , and p h o t o d i s s o c i a t i o n holds promise f o r g r e a t l y expanding t h e d a t a base on t h i s q u e s t i o n . F i n a l l y , t h e a r e a o f c l u s t e r s i s a r a p i d l y g r o w i n g one. Our e f f o r t i n t h i s a r e a has been t o d e v e l o p a method f o r s y n t h e s i z i n g JLû s i t u a wide v a r i e t y o f d i m e r ( I I I ) [31-33] and t r i m e r i o n s [ 3 4 ] o f known c o m p o s i t i o n and s t u d y t h e i r g a s - p h a s e c h e m i s t r y .

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

160

FOURIER TRANSFORM MASS SPECTROMETRY

Once a g a i n , p h o t o d i s s o c i a t i o n s t u d i e s can p r o v i d e complementary i n f o r m a t i o n on t h e s e c h e m i c a l l y important species.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 30, 2017 | http://pubs.acs.org Publication Date: November 23, 1987 | doi: 10.1021/bk-1987-0359.ch010

Specific

ion

Synthesis

The u n i q u e f e a t u r e o f FTMS t o c o n t r o l b o t h t h e i o n and n e u t r a l p o p u l a t i o n s by u s i n g programmable p u l s e s e q u e n c e s makes i t an e x t r e m e l y p o w e r f u l t o o l f o r g e n e r a t ing i n a stepwise f a s h i o n s p e c i f i c o r g a n o m e t a l l i c fragment i o n s f o r s u b s e q u e n t c h e m i c a l and p h o t o c h e m i c a l studies. F i g u r e 1 shows a t y p i c a l p u l s e s e q u e n c e w h i c h b e g i n s w i t h an i o n i z a t i o n p u l s e . C o n c u r r e n t l y w i t h the l a s e r p u l s e , a p u l s e d v a l v e f i r e s t o admit a b u r s t of r e a g e n t gas. The m e t a l i o n s r e a c t w i t h t h e r e a g e n t gas t o f o r m t h e i o n o f i n t e r e s t , e i t h e r d i r e c t l y i n a primary or secondary r e a c t i o n , o r by a s u b s e q u e n t s t e p s u c h as c o l l i s i o n - i n d u c e d dissociation. F o r example, CoOH can be made by a d i r e c t primary r e a c t i o n of Co w i t h CH 0N0 ( r e a c t i o n 5) [ 1 3 ] , w h i l e m e t a l d i m e r s o f t h e f o r m M F e can be s y n t h e s i z e d +

+

+

Co

+

+

CH 0N0

>

3

CoOH

+

+

CH N0 2

(5)

i n a s t e p w i s e f a s h i o n from Fe(CO) by f i r s t g e n e r a t i n g MFe(CO) f o l l o w e d by c o l l i s i o n - i n d u c e d d i s s o c i a t i o n t o g e n e r a t e t h e b a r e M F e , r e a c t i o n s 6 and 7 [ 3 1 - 3 3 ] . Next, a s e r i e s o f i o n e j e c t i o n p u l s e s a r e used t o i s o l a t e t h e +

M

+

+

MFe(C0)

Fe(C0) + c

>

5

C

v

I

D

+

MFe(C0) _ +

> MFe

5

+

+

x

XCO

(5 -X)C0

(6) (7)

ion of i n t e r e s t . By t h i s t i m e t h e r e a g e n t gas has been pumped away, p e r m i t t i n g t h e i o n s t o be s t o r e d e f f i c i e n t l y f o r t h e r e l a t i v e l y l o n g t r a p p i n g t i m e s used f o r p h o t o dissociation. F o l l o w i n g an a p p r o p i a t e d e l a y p e r i o d ( w i t h o r w i t h o u t l i g h t ) , d e t e c t i o n y i e l d s t h e f u l l mass s p e c t r u m , and f i n a l l y a quench p u l s e e l i m i n a t e s a l l o f t h e i o n s from t h e c e l l and t h e whole s e q u e n c e i s r e p e a t e d .

Information

from

Photodisgocjatjon

I n o r d e r t o o b s e r v e p h o t o d i s s o c i a t i o n p r o c e s s 1, t h r e e c r i t e r i a must be met: f i r s t , t h e i o n must a b s o r b a p h o t o n ; s e c o n d , t h e p h o t o n must have s u f f i c i e n t e n e r g y t o c a u s e f r a g m e n t a t i o n ; and t h i r d , t h e quantum y i e l d f o r p h o t o d i s s o c i a t i o n must be n o n - z e r o . Figure 2 i s useful in u n d e r s t a n d i n g the i n f o r m a t i o n i n h e r e n t i n a photod i s s o c i a t i o n experiment. I f the f i r s t allowed e l e c t r o n i c s t a t e o f AB l i e s a t an e n e r g y above t h a t r e q u i r e d t o generate the p r o d u c t s P * and P * ( l e f t s i d e of F i g u r e 2 ) , the observed p h o t o d i s s o c i a t i o n onset i s s p e c t r o s c o p i c a l l y d e t e r m i n e d and y i e l d s o n l y an upper e n e r g y l i m i t f o r t h e p r o c e s s e s i n r e a c t i o n 1. In o t h e r words, even i f t h e r e i s s u f f i c i e n t e n e r g y i n t h e p h o t o n to c a u s e i o n f r a g m e n t a t i o n , c l e a r l y t h e p r o c e s s w i l l not o c c u r i f t h e i o n does not a b s o r b t h e p h o t o n . Alter+

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

HETTICH AND FREISER

Photodissociation of Transition Metal Ions

161

\Reagent Gas Pulse

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/Ionization Synthesis

CID Excitation

Ion Ejection Separations

Detection

Quench

Product jAnalysis Structure J ' Determination,

• Reaction Time

F i g u r e 1.

P u l s e sequence used to s y n t h e s i z e , i s o l a t e , and s p e c i f i c metal-containing ions.

AB

+



— P.\ P

?

+

- AB

F i g u r e 2.

study

+

Energy l e v e l diagram d e p i c t i n g cases where photod i s s o c i a t i o n t h r e s h o l d s a r e d e t e r m i n e d by s p e c t r o s c o p i c f a c t o r s ( l e f t s i d e ) and thermodynamic f a c t o r s (right side).

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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162

FOURIER TRANSFORM MASS SPECTROMETRY

n a t i v e l y , i f an a l l o w e d e l e c t r o n i c s t a t e f o r t u i t o u s l y l i e s a t t h e same e n e r g y as t h e l o w e s t e n e r g y p r o c e s s o r , as shown i n F i g u r e 2 ( r i g h t s i d e ) , t h e r e i s a h i g h d e n s i t y o f allowed s t a t e s having energies i n the v i c i n i t y of the thermodynamic t h r e s h o l d , t h e o b s e r v e d p h o t o d i s s o c i a t i o n o n s e t s h o u l d a c c u r a t e l y r e f l e c t t h e d i s s o c i a t i o n energy f o r p r o c e s s 1 p r o v i d e d t h e quantum y i e l d i s n o n - z e r o . Ons e t s f o r h i g h e r energy p r o d u c t s w i l l a l s o r e f l e c t t h e thermochemistry i f rapid i n t e r n a l conversion to a v i b r a t i o n a l l y e x c i t e d ground s t a t e o c c u r s r a n d o m i z i n g t h e energy. Thus, i f an i o n a b s o r b s l i g h t o v e r a broad r a n g e , say from t h e i r t o t h e uv, t h e n p h o t o d i s s o c i a t i o n c a n b e g i n t o o c c u r when t h e p h o t o n e n e r g y i s s u f f i c i e n t t o cause f r a g m e n t a t i o n . I n any e v e n t , t h e p h o t o d i s s o c i a t i o n s p e c t r u m i s an i n d i r e c t measure o f t h e t r u e g a s - p h a s e a b s o r p t i o n s p e c t r u m and, as such, p r o v i d e s a " f i n g e r p r i n t " of the ground s t a t e s t r u c t u r e o f t h e i o n . In g e n e r a l , t h e s t a t e d i a g r a m on t h e l e f t i n F i g u r e 2 i s a good d e s c r i p t i o n f o r o r g a n i c i o n s . As an example, t h e p h o t o d i s s o c i a t i o n s p e c t r u m o f b e n z o y l c a t i o n o b t a i n e d by m o n i t o r i n g p r o c e s s 8 has two maxima a t 260 nm and 310 nm C H CO 6

5

+

+ hv

C

H

> 6 5

+

+

0

0

(

8

)

1 1 1 a t t r i b u t e d to the t r a n s i t i o n s A^ 1u 1 2 . r e s p e c t i v e l y , and an o n s e t for p h o t o d i s s o c f a t i o n a t 350 nm o r 3.5 eV [ 1 8 ] . The o n s e t i s c o n s i d e r a b l y h i g h e r t h a n t h e a c t u a l e n t h a l p y o f 2.3 eV r e q u i r e d t o decarbonylate the benzoyl i o n . In c o n t r a s t t o t h e o r g a n i c i o n s , a l l o f o u r work t o d a t e on t h e c h e m i c a l s y s t e m s I, I I , and I I I , s u g g e s t t h a t t h e s e m e t a l c o m p l e x e s have a h i g h d e n s i t y o f low l y i n g e l e c t r o n i c s t a t e s and, t h e r e f o r e , a b s o r b b r o a d l y y i e l d i n g p h o t o d i s s o c i a t i o n o n s e t s w h i c h r e f l e c t t h e thermochemistry. I n p a r t i c u l a r , v a l u e s o b t a i n e d by o b s e r v i n g p h o t o d i s s o c i a t i o n o n s e t s a r e f o u n d i n g e n e r a l t o be i n good agreement w i t h t h o s e o b t a i n e d by o t h e r t e c h n i q u e s [15]. C e r t a i n l y , e x c e p t i o n s t o t h i s w i l l be f o u n d . F i n a l l y , a l t h o u g h the i n f o r m a t i o n c o n t e n t o f these photod i s s o c i a t i o n spectra i s great with regard to i o n s t r u c t u r e and t h e r m o c h e m i s t r y , t h e h i g h d e n s i t y o f low l y i n g e l e c t r o n i c s t a t e s makes band a s s i g n m e n t s v i r t u a l l y i m p o s s i b l e a t t h i s t i m e . I n f a c t such an i n t e r p r e t a t i o n w i l l p r o v i d e a r e a l c h a l l e n g e t o t h e o r i s t s f o r y e a r s t o come. B

a

n

d

A

B

(I)

P h o t o d i s s o c i a t i o n o f ML* One o f t h e f i r s t examples from o u r l a b o r a t o r y [13] which suggested t h a t p h o t o d i s s o c i a t i o n t h r e s h o l d s c o u l d y i e l d q u a n t i t a t i v e m e t a l - l i g a n d bond e n e r g i e s was from a c o m p a r i s o n o f t h e p h o t o d i s s o c i a t i o n s p e c t r a o f FeOH and F e C 0 o b t a i n e d by m o n i t o r i n g r e a c t i o n s 9 and 10, r e s p e c t i v e l y , and shown i n F i g u r e 3. The two s p e c t r a a r e r e m a r k a b l y s i m i l a r w i t h two a b s o r p t i o n maxima o b s e r v e d +

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

HETTICH AND FREISER

163

Energy (eV)

(a)

5.0

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Photodissociation of Transition Metal Ions

4.0

3.0



Wavelength (nm) (b)

Energy (eV)

5.0

4.0

3.0

20

Wavelength (nm)

Figure

3.

+

( a ) P h o t o d i s s o c i a t i o n spectrum o f FeOH o b t a i n e d by m o n i t o r i n g r e a c t i o n 9 as a f u n c t i o n o f wavelength. No p h o t o d i s s o c i a ­ t i o n i s observed a t wavelengths greater than 390 nm. (b) P h o t o d i s s o c i a t i o n spectrum o f F e C 0 g e n e r a t e d by e l e c t r o n i m p a c t on Fe(CO) . The o b s e r v a t i o n o f p h o t o d i s s s o c i a t i o n a t w a v e l e n g t h s g r e a t e r t h a n 660 nm was c o n f i r m e d by u s i n g a c u t o f f f i l t e r . (Reproduced from r e f . 13. C o p y r i g h t 1984 American Chemical S o c i e t y . ) +

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

164

FOURIER TRANSFORM MASS SPECTROMETRY

FeOH

+

+

hv

->

Fe

+

+

OH

(9)

FeCO

+

+

hv

>

Fe

+

+

CO

(10)

n e a r 290 nm and 335 nm, s u g g e s t i n g t h a t t h e t r a n s i t i o n s are metal l o c a l i z e d . The F e O H s p e c t r u m , however, has a p h o t o d i s s o c i a t i o n t h r e s h o l d o f 390 ± 1 0 nm, w h i l e F e C 0 i s o b s e r v e d t o have a low i n t e n s i t y l o n g w a v e l e n g t h a b s o r p t i o n t a i l from about 400 t o 700 nm. The c u t o f f o b s e r v e d a t 390 nm f o r F e O H y i e l d s an a b s o l u t e bond e n e r g y D ° ( F e - 0 H ) = 73±3 k c a l / m o l , w h i c h i s i n e x c e l l e n t agreement w i t h a v a l u e o f 76±5 k c a l / m o l o b t a i n e d by m e a s u r i n g t h e i o n i z a t i o n p o t e n t i a l o f FeOH [35] and a v a l u e o f 77±8 k c a l / m o l o b t a i n e d by b r a c k e t i n g t h e p r o t o n a f f i n i t y o f FeO [13]· A p p e a r a n c e p o t e n t i a l measurements on FeCO* from F e ( C 0 ) were i n t e r p r e t e d t o y i e l d D ( F e - C 0 ) s 60 k ? a l / m o l [ 3 6 ] . T h e s e same r e s u l t s , however, were l a t e r r e i n t e r p r e t e d t o y i e l d D ° ( F e - C 0 ) = 38 k c a l / m o l [ 3 7 ] . Observation of photodissociation re­ a c t i o n 10 a t "700 nm o r 41 k c a l / m o l p r o v i d e s a d d i t i o n a l support f o r the lower value. As shown i n T a b l e I , t h e e v i d e n c e i s m o u n t i n g t h a t p h o t o d i s s o c i a t i o n t h r e s h o l d s do i n f a c t y i e l d a c c u r a t e m e t a l - 1 i g a n d bond e n e r g i e s . I f there i s a disagreement between t h e p h o t o d i s s o c i a t i o n v a l u e and t h e v a l u e from an a l t e r n a t i v e technique, t h e p h o t o d i s s o c i a t i o n value tends to be l o w e r i n c o n t r a s t , f o r example, t o t h e b e n z o y l c a t i o n case d i s c u s s e d above. A r r i v i n g a t a l o w e r v a l u e by p h o t o d i s s o c i a t i o n suggests the p o s s i b i l i t y that the p r e c u r s o r i o n may be g e n e r a t e d w i t h e x c e s s i n t e r n a l energy [4b,38]. To c i r c u m v e n t t h i s p r o b l e m , once formed t h e p r e c u r s o r i o n s a r e p e r m i t t e d t o undergo t h e r m a l i z i n g c o l l i s i o n s w i t h a r g o n ( s e e e x p e r i m e n t a l s e c t i o n ) and t h e e f f e c t on t h e t h r e s h o l d , i f any, i s n o t e d . In addition, whenever p o s s i b l e , t h e p r e c u r s o r i o n o f i n t e r e s t was g e n e r a t e d from more t h a n one n e u t r a l s o u r c e . For the m a j o r i t y o f t h e s y s t e m s s t u d i e d , however, n e i t h e r t h e c o l l i s i o n a l c o o l i n g s t e p n o r t h e s y n t h e s i s from d i f f e r e n t n e u t r a l p r e c u r s o r s had a m e a s u r e a b l e e f f e c t on t h e threshold. One n o t a b l e a c c e p t i o n i s F e C H as d i s c u s s e d below. T r a n s i t i o n - m e t a l m e t h y l i d e n e s have been i m p l i c a t e d a s intermediates i n a v a r i e t y c f important c a t a l y t i c t r a n s ­ f o r m a t i o n s i n c l u d i n g o l e f i n m e t a t h e s i s , t h e Ζiegler-Natta p o l y m e r i z a t i o n o f o l e f i n s , o l e f i n homologation, and t h e heterogeneous F i s c h e r - T r o p s c h process. Likewise, the i n c r e a s i n g l i t e r a t u r e on b a r e M C H * i n t h e g a s phase h a s shown t h e s e s p e c i e s t o have I n t e r e s t i n g p h y s i c a l and chemical p r o p e r t i e s [39-42]. I t i s perhaps not s u r p r i s ­ ing, t h e r e f o r e , that the photochemistry of these s p e c i e s has p r o v e n t o be among t h e most i n t e r e s t i n g . Our i n i t i a l work on M C H * was f o r M = Fe and Co [14], b u t t h e s e s t u d i e s have r e c e n t l y been expanded t o i n c l u d e M = Rh, Nb, and L a [ 1 6 ] . The m e t h y l i d e n e s c a n be +

+

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+

+

+

+

+

2

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

HETTICH AND FREISER

Table

I.

Bond Energy

Photodissociation of Transition Metal Ions

165

Determinations +

D°(A -B)(kcal/mol) photodissociation

A+-B +

Fe -CH Fe -CH Fe -C Co -CH Co -CIT Co -C Nb -CH Nb -CH Nb -C Rh -CH Rh -CIT Rh -C La -CH La -CH La -C Fe -CH Co -CH^ Fe -0 Fe -S Co -S Ni -S V -C H +

+

+

+

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 30, 2017 | http://pubs.acs.org Publication Date: November 23, 1987 | doi: 10.1021/bk-1987-0359.ch010

+

+

+

+

+

+

+

98 112

r

+

+

+

+

+

3

+

+

+

+

r

+

Co -C*H* SC -FI Ti -Fe V -Fe Cr -Fe Fe -Fe Co -Fe Ni -Fe Cu -Fe Nb -Fe Ta -Fe +

96±5 (b) 115±20 ( c ) 89 (c) 85±7 (d)

82±5( ' a) 101±5CoC H 2

+

8

a

(23)

H

5 10

+

The p h o t o d i s s o c i a t i o n s p e c t r u m o f C o - p e n t e n e . 320 nm. F i g u r e 5, i n d i c a t e s peak maxima a t 320 nm (σ = 0.02 A ) and 370 nm. A l l f i v e photoproducts are observed at w a v e l e n g t h s out t o a t l e a s t 430 nm. I f the photoappearance of Co i s due s o l e l y t o r e a c t i o n 23, t h e n o b s e r v a t i o n of Co a t 430 nm w o u l d i m p l y D°(Co -C H J < 66 k c a l / m o l . T h i s v a l u e must be e x p r e s s e d w i t h c a u t i o n , however, s i n c e t h e p r i m a r y p h o t o p r o d u c t s can a l s o f u r t h e r d i s s o c i a t e to give Co . +

+

1

+

II.

Photodissociation

of

ML^

The i o n o f s t r u c t u r e I I can be made p r e s u m a b l y by r e a c t i o n 24 [ 3 ] . The h i g h s t a t i c p r e s s u r e o f a r g o n

(24)

s t a b i l i z e s the c o n d e n s a t i o n of C o n t o CoC-Hg . P h o t o d i s s o c i a t i o n o f CoC H * frOm r e a c t i o n 24 ( F i g u r e 6) y i e l d s o n l y tnrèe p h o t o p r o d u c t s , r e a c t i o n s 25-27, w h i c h a r e a l l o b s e r v e d a t l e a s t out t o 430 nm. The r e l a t i v e a b u n d a n c e s o f t h e s e p h o t o p r o d u c t s were t a k e n a t

61* C H 2

4

(25)

->C0C H < 3

hv

6

15*

C

H

3 6

(26)

—> COC H/ -> Co (27) 5 10> 24* .2 L . = 320 nm(a= 0.05 k ) · No CoC.Hg o r CoC H« are o U s e r v e d , s u p p o r t i n g t h e f a c t t h a t t h i s i o n e x i s t s as s t r u c t u r e I I . The s p e c t r a o f t h e s e two i s o m e r s I and I I a r e a r e s i m i l a r , but t h e 370 nm peak o b s e r v e d i n t h e s p e c t r u m o f C o - p e n t e n e i s a b s e n t i n t h e s p e c t r u m o f Co (propene) ( e t h y l e n e ) . The enhanced p h o t o d i s s o c i a t i o n 2

( C

H

Y

+

+

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.



HETTICH AND FREISER

Photodissociation of Transition Metal Ions 169

Energy (Kcal/mol) 90

80

70

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Effect of Pressure on FeCH£

Wavelength (nm) F i g u r e 4.

Long wavelength t h r e s h o l d r e g i o n f o r Fed^" d i s s o c i a t i n g to F e at various pressures.

1-

photo-

+

ENERGY (Kcol/mol) 110 ~i—ι

250

90 1

300

1

350

70 1

400

Γ

450

WAVELENGTH (nm) F i g u r e 5. P h o t o d i s s o c i a t i o n spectrum o f C0C5H-LQ r e a c t i o n 18.

g e n e r a t e d from

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

170

FOURIER TRANSFORM MASS SPECTROMETRY

c r o s s s e c t i o n o b s e r v e d f o r s t r u c t u r e I I ( i e . 0.05 A*) i s t y p i c a l o f a t r e n d we have o b s e r v e d t h a t p h o t o d i s s o c i a t i o n c r o s s s e c t i o n tends to i n c r e a s e w i t h the a d d i t i o n of a second l i g a n d [15]. The d i f f e r e n c e i n c r o s s s e c t i o n s a l o n e , however, would not be s u f f i c i e n t t o d i s t i n g u i s h t h e two i s o m e r s , A photoappearance t h r e s h o l d f o r Co , r e a c t i o n 27, c o u l d not be o b t a i n e d due t o t h e s e c o n d a r y d i s s o c i a t i o n o f CoC H / and CoC^H.."" t o y i e l d Co . R e c e n t l y we r e p o r t e d t h a t N i ( C H . ) c o u l d a l s o be d i s t i n g u i s h e d from t h r e e o t h e r i s o m e r s f N i - b u t e n e , Ni i s o b u t e n e , n i c k e l a c y c l o p e n t a n e c a t i o n ) on t h e b a s i s o f i t s u n i q u e p h o t o d i s s o c i a t i o n s p e c t r u m and p h o t o p r o d u c t s [15]· In p a r t i c u l a r N i i C - H j . ) * p h o t o d i s s o c i a t e s t o g i v e two p r o d u c t s o f about e q u a l abundance (by l o s s o f C H^ and C.Hg). Continuous e j e c t i o n of Ni -C H . allows r e a c t i o n 28 t o be m o n i t o r e d . As shown i n F i g u r e 7 , +

1

+

+

2

J

2

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 30, 2017 | http://pubs.acs.org Publication Date: November 23, 1987 | doi: 10.1021/bk-1987-0359.ch010

+

+

2

+

2

||-

N i * -||

+

hv

> Ni

+

+

2

i

C H 2

(28)

4

,

r e a c t i o n 28 i s o b s e r v e d f o r < 370 nm i m p l y i n g D°(Ni* "-2 p i i ) = 77±5 k c a l / m o l . The f a c t t h a t t h i s v a l u e i s net s i g n i f i c a n t l y d i f f e r e n t t h a n t w i c e D ° ( N i - C H ) = 37±2 k c a l / m o l s u g g e s t s t h a t s y n e r g i s t i c e f f e c t s a r e not very pronounced i n t h i s i o n . W h i l e t h i s may p r o v e t o be t y p i c a l , i t i s not e x p e c t e d t o be t h e r u l e . F o r example, p r e l i m i n a r y s t u d i e s on t h e b i s - b e n z e n e i o n s , ( 5 6)o ' suggest a n e g l i g i b l e s y n e r g i s t i c e f f e c t f o r M = V [l5J and Sc [ 4 4 1 , a n e g a t i v e e f f e c t f o r L a ( i . e . , D°(La* -C.H ) > D ( L a C H . - C.H.), and a p o s i t i v e e f f e c t f o r Cu ? i . e. , B°t C u - C . H, ) < D°( CuC. H."* - C H ) [44]. c

H

+

2

M

4

C

H

+

+

+

fi

A

+

+

6

6

III.

6

6

6

6

P h o t o d i s s o c i a t i o n o f MFe+

E x a m i n a t i o n o f t h e b o n d i n g and e n e r g e t i c s o f s m a l l b a r e c l u s t e r s has become a t o p i c o f c o n s i d e r a b l e i n t e r e s t in recent years. An e x c i t i n g a r r a y o f methods has been d e v e l o p e d t o g e n e r a t e c l u s t e r s o f v a r y i n g s i z e s and c o m p o s i t i o n f o r s t u d y i n t h e gas phase. Some of t h e s e new methods i n c l u d e s p u t t e r i n g t e c h n i q u e s [ 4 5 ] , gas e v a p o r a t i o n techniques [46], supersonic expansion techniques w i t h oven [47] and p u l s e d l a s e r s o u r c e s [ 4 8 ] , and m u l t i p h o t o n d i s s o c i a t i o n o f m u l t l n u c l e a r o r g a n o m e t a l l i c compounds [ 4 9 ] . As d e s c r i b e d above, our l a b o r a t o r y dem o n s t r a t e d t h e s y n t h e s i s o f s m a l l h o m o n u c l e a r and h e t e r o n u c l e a r t r a n s i t i o n m e t a l c l u s t e r i o n s ±n _s_i£_u. by u s i n g FTMS [ 3 1 - 3 3 ] . F o r example, r e a c t i o n s 6 and 7 have been used t o s y n t h e s i z e a wide v a r i e t y o f M F e s p e c i e s . The c l e a r a d v a n t a g e s o f t h i s method a r e t h a t not o n l y i s there a great deal of s e l e c t i v i t y i n generating s p e c i f i c c l u s t e r s , but once formed t h e f u l l power o f FTMS can be a p p l i e d t o s t u d y the c h e m i s t r y and p h o t o c h e m i s t r y o f t h e s e species in d e t a i l . T h i s a p p r o a c h i s t y p i f i e d by ext e n s i v e s t u d i e s on t h e r e a c t i v i t i e s o f C o F e [ 3 2 ] and VFe [33] with alkenes. I n a soon t o be p u b l i s h e d +

+

+

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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10. HETTICH AND FREISER

Photodissociation of Transition Metal Ions

ENERGY 110

100

171

( KCAL/MOL )

90

80

70

IK-II

6

hv.

Ni

0

+

Ο BLANK i

—-



e

Γ ι ι ι ι I 300

250

ι

ι

JL

ο · οι

350

ι

I

ι

ι

ι I ι

J

150

100

WAVELENGTH ( NM ) Figure

7.

T h r e s h o l d r e g i o n f o r N i ( C H J photod i s s o c i a t i n g t o N i . I n t h sie x p e r i m e n t , was c o n t i n u o u s l y e j e c t e d . Ν10 Η 2

+

-

Ί

2

4

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

L

172

FOURIER TRANSFORM MASS SPECTROMETRY

paper [ 1 6 ] , we expand on t h i s work by r e p o r t i n g on t h e p h o t o d i s s o c i a t i o n o f MFe (M = Se, T i , V, C r , Fe, Co, N i , Cu^ Nb, T a ) . To summarize t h o s e r e s u l t s , b o t h M and Fe a r e o b s e r v e d as p h o t o p r o d u c t s , w i t h t h e m e t a l h a v i n g t h e l o w e s t i o n i z a t i o n p o t e n t i a l p r e d o m i n a t i n g . The p h o t o d i s s o c i a t i o n s p e c t r a r e v e a l broad a b s o r p t i o n i n t h e u l t r a v i o l e t and v i s i b l e r e g i o n s w i t h a range o f c r o s s s e c t i o n s from 0.06 A f o r VFe t o 0.62 A f o r f o r CrFe . Bond e n e r g i e s o b t a i n e d by o b s e r v i n g p h o t o a p p e a r a n c e ons e t s a r e i n t h e range o f 48 k c a l / m o l f o r S c F e t o 75 kcal/mol f o r VFe ( s e e T a b l e I ) . T h e s e s t u d i e s c a n be r e a d i l y e x t e n d e d t o o t h e r d i m e r s e r i e s , s u c h as MV , M C r , and MCo g e n e r a t e d i n a n a l o g y t o r e a c t i o n s 6 and 7 from V ( C 0 ) , C r ( C 0 ) , and C o ( C 0 ) N 0 , r e s p e c t i v e l y , as w e l l as t o t r i m e r s [ 3 4 ] and n i g h e r o r d e r clusters. C l e a r l y , t h e s u r f a c e has j u s t been s c r a t c h e d ! +

+

+

+

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 30, 2017 | http://pubs.acs.org Publication Date: November 23, 1987 | doi: 10.1021/bk-1987-0359.ch010

+

+

+

+

6

6

3

Acknowledgment i s made t o t h e D i v i s i o n o f C h e m i c a l S c i e n c e s i n t h e O f f i c e o f B a s i c Energy S c i e n c e s i n t h e u n i t e d S t a t e s Department o f Energy (DE-AC02-80ER10689) f o r s u p p o r t i n g t h i s r e s e a r c h and t o t h e N a t i o n a l S c i e n c e F o u n d a t i o n ( C H E - 8 3 1 0 0 3 9 ) f o r c o n t i n u e d s u p p o r t o f FTMS methodology. The a u t h o r s a l s o w i s h t o thank M i c h e l l e Buchanan f o r h e r i n v i t a t i o n t o c o n t r i b u t e t o t h i s symposium.

References 1.

2. 3. 4.

5. 6. 7. 8. 9. 10. 11.

For a comprehensive review on gas-phase metal ion chemistry see: Allison, J . in Progress in Inorganic Chemistry, Ed. Lippard, S. J., Wiley - Interscience, New York, Vol. 34, 628, 1986. Halle, L.F.; Houriet, R.; Kappes, M.; Staley, R.H.; Beauchamp, J . L . J . Am. Chem. Soc. 1982, 104, 6293. Jacobson, D.B.; Freiser, B.S.; J. Am. Chem. Soc. 1983, 105, 5197. (a) Larsen, B.S.; Ridge, D.P. J . Am. Chem. Soc. 1984, 106, 1912. (b) Freas, R.B.; Ridge, D.P. J . Am. Chem, Soc., 1980, 102, 7129. (c) Armentrout, P.B.; Beauchamp, J.L.; J. Am. Chem. Soc., 1980, 102, 1736. Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 72. Houriet, R.; Halle, L.F.; Beauchamp, J . L . Organonetallics 1983, 2, 1818. Allison, J.; Freas, R.B.; Ridge, D.P., J. Am. Chem. Soc., 1979, 101, 1332. Armentrout, P.B.; Beauchamp, J . L . J . Am. Chem. Soc. 1981, 103, 784. Aristov, N.; Armentrout, P.B. J . Am. Chem. Soc 1984, 106, 4065. Jones, R.W.; Staley, R.H. J. Am. Chem. Soc. 1982, 104, 2296. Uppal, J . S . ; Staley, R.H. J . Am. Chem. Soc. 1982, 104, 1235.

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10. HETTICH AND FREISER

12. 13. 14. 15.

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16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

26. 27. 28.

29. 30. 31. 32. 33. 34. 35. 36. 37.

Photodissociation of Transition Metal Ions 173

McLuckey, S.A., Schoen, A.E.; Cooks, R.G. J. Am. Chem. Soc. 1982, 104, 848. Cassady, C . J . ; Freiser, B.S. J . Am. Chem. Soc. 1984, 106, 6176. Hettich, R . L . ; Freiser, B.S. J . Am. Chem. Soc. 1986, 108, 2537. Hettich, R . L . ; Jackson, T . C . ; Stanko, E . M . ; Freiser, B.S. J . Am. Chem. Soc. 1986, 108, 5086. Hettich, R . L . ; Freiser, B.S. J. Am. Chem. Soc., in press. Dunbar, R.C. "Gas Phase Ion Chemistry;" Bowers, M.T., E d . ; Academic Press, Inc.; New York, 1984; Vol. 3, Chap. 20. Freiser, B.S.; Beauchamp, J . L. J . Am. Chem. Soc. 1976, 98, 3136. Dunbar, R.C.; Hutchinson, B.B. J . Am. Chem. Soc. 1974, 96, 3816. Burnier, R.C.; Freiser, B.S. Inorg. Chem. 1979, 18, 906. Comisarow, M.B. Adv. Mass Spec. 1980, 8, 1698. Gross, M . L . ; Rempel, D.L. Science 1984, 226, 261. Lande, Jr.,D.A.; Johlman, C . L . ; Brown, R.S.; Weil, D.A.; Wilkins, C.L. Mass Spec. Rev. 1986, 5, 107. Cody, R.B.; Burnier, R.C.; Freiser, B.S. Anal. Chem. 1982, 54, 96. Cody, R.B.; Burnier, R.C.; Reents, Jr., W.D.; Carlin, T.J.; McCrery, D.A.; Lengal, R.K.; Freiser, B.S. Int. J . Mass Spec. Ion Phys. 1980, 33, 37. Cody, R.B.; Burnier, R.C.; Freiser, B.S. Anal. Chem. 1982, 54, 96. Carlin, T.J.; Freiser, B.S. Anal. Chem. 1983, 55, 571. The photodissociation spectra of a l l of the ions showed no pressure dependence, except for slight quenching of the low energy t a i l region, indicating that photodissociation in these cases is probably due to one-photon excitation and that the precursor ions do not have substantial internal energy. Freiser, B . S . ; Beauchamp, J . L . Chem. Phys. Lett. 1975, 35, 35. Dunbar, R.C. Chem. Phys. Lett. 1975, 32, 508. Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1984, 106, 4623. Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 1581. Hettich, R . L . ; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 6222. Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1984, 106, 5351. Murad, E. J . Chem. Phys. 1980, 73, 1381. Distefano, G.J. Res. Natl. Bur. Stand, Sect. A. 1970, 74A, 233. Halle, L.F.; Armentrout, P.B.; Beauchamp, J . L . Organometallics 1982, 1, 963. Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

174

38.

39.

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40. 41. 42. 43.

44. 45.

46.

47.

48.

49.

FOURIER TRANSFORM MASS SPECTROMETRY

Metal ions can be made with excess internal and/or kinetic energy. See, for example, (a) Kang, H . ; Beauchamp, J . L . J . Phys. Chem. 1985, 89, 3364. (b) Halle, L.F.; Armentrout, P.B.; Beauchamp, J . L . J . Am Chem. Soc., 103, 962 (1981). (a) Stevens, A . E . ; Beauchamp, J . L . J . Am. Chem. Soc. 1980, 100, 2584. (b) Stevens, A . E . ; Beauchamp, J . L . J . Am. Chem. Soc. 1979, 101, 6449. Armentrout, P.B.; Halle, L.F.; Beauchamp. J . L . J . Am. Chem. Soc. 1981, 103, 6501. (a) Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 2605. (b) Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 4375. (a) Carter, A . E . ; Goddard III, W.A. J . Am. Chem. Soc. 1986, 108, 2180. (b) Shim, I . ; Gingerich, K.A. J . Chem. Phys. 1982, 76, 3833. A l l heats of formation (and other supplementary values) are taaken from: Rosenstock, H.M.; Draxl, K.; Steiner, B.W.; Herron, J . T . J . Phys. Chem. Ref. Data. Suppl. 1 1977, 6. Lech, L . M . ; Tews, E . C . ; Huang, Y . ; Freiser, B.S. unpublished results. (a) Katakuse, I . ; Ichihara, T . ; F u j j i t a , Y . ; Matsuo, T . ; Sakurai, T . ; Matsuda, H. Int. J . Mas Spec. Ion Proc. 1985, 67, 229. (b) Freas, R.B.; Campana, J . E . J . Am. Chem. Soc. 1985, 107, 6202. (a) Sattler, K . ; Muhlbach, J.; Recknagel, E. Phys. Rev. Lett. 1980, 45, 821. (b) Abe, H.; Schulze, W.; Tesche, B. Chem. Phys. 1980, 47, 95. (c) Godenfeld, I . ; Frank, F . ; Schulze, W.; Winter, B. Int. J . Mass Spec. Ion Proc. 1986, 71, 103. (a) Riley, S . J . ; Parks, E . K . ; Mao, C.R.; Poppo, L . G . ; Wexler, S. J . Phys. Chem. 1982, 86, 3911. (b) Bowles, R.S.; Park, S.B.; Otsuka, N.; Andres, R.P. J. Mol. Catal. 1983, 20, 279. (a) Bondybey, V . E . ; English, J . H . J . Chem. Phys. 1982, 74, 6978. (b) Morse, M.D.; Hansen, G.P.; Langridge-Smith, P.R.R.; Zheng, L . S . ; Geusic, M . E . ; Michalopoulos, D . L . ; Smalley, R.E. J . Chem. Phys. 1984, 80, 5400. (c) Smalley, R.E. Laser Chem. 1983, 2, 167. Leopold, D.G.; Vaida, V. J . Am. Chem. Soc. 1983, 105, 6809.

RECEIVED May 12,

1987

Buchanan; Fourier Transform Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.