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Doubly charged ions in the field ionization mass spectrum of 1-heptene
International Journal of Mass Spectrometr~ and Ion Physics, 28 (1978) 26~--273 © E l s e v i e r S c i e n t i f i c P u b l i s h i n g C o m p a n y , A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
I~TJBLY CEL~_RGED IoNS IN THE SPECTRUM OF 1-iIET~rENE
FIELD
IONIZATION
PIERRE TECON, DANIEL STAHL and TINO GAITMANN
-- - -
MASS.-
-267
- : - -.-.
. . . .
l n s ~ t u t d e C h i m i e Physi'que, E c o l e P o l y t e c h n i q u e Fdddrale d e L a u s a n n e : C H - 1 0 1 5 Z,a u . ~ n n e ( S w i t z e r l a n d ) ~" --
" - "-"
(Received 6 February 1978)
ABSTRACT S u r f a c e p r o c e s s e s in t h e f i e l d i o n i z a t i o n o f l - h e p t e n e give rise t o t h e d o u b l y c h a r g e d i o n s ( M - - R ) 2÷ a n d ( M M - - R ) 2÷ (M a n d R r e p r e s e n t t h e 1 - h e p t e n e m o l e c u l e a n d a n olefln~e f r a g m e n t , r e s p e c t i v e l y ) . T h e u s e o f 13C- a n d D - l a b e l l i n g a l l o w s i n v e s t i g a t i o n o f t h e s e p h e n o m e n a in detail. T h e mechani-~m.~ p r o p o s e d f o r d i r n e r i z a t i o n a n d s u b s e q u e n t fragmentation are supported by the data of seventeen labelled Compounds. - INTRODUCTION The field ionization (FI) spectm,m of-1-heptene has been recently described [1]. In the context of that study the pattern of formation of monocharged ions resulting from uDimolecular decomposition of molecular ions was reported. The FI spect~,m obtained indicates inter alia the occurrence of surface processes (doubly charged ions). These phenomena have been studied quite extensively in recent years [2--8]. Multiply charged ions are formed via a Lwo-step mechanism: the first involves adsorption of an ion on the emitter surface and the second step is the desorption induced by the second ionization [4]. The field dissociation of the desorbing species may produce doubly charged fragment ions. In the previous surface ionization studies [2--8] the emitters used were generally tips or wires heated up to 1000°C and the fragments o b s e r v e d w e r e m a i n l y o f t y p e s M 2+ o r ( M - - R ) 2 . +. In the present study attention is focussed on the formation of the doubly c h a r g e d i o n s o f t y p e s ( ] V I - - R ) 2+ a n d ( M M - - R ) 2÷ o b s e r v e d i n t h e F I s p e c i . T a of IsC- and D-labelled 1-heptenes using a blade emitter at low tempera°
EXPERIMENTAL
A Du Pont FI/EI source
CEC 21-110B_ mass spectrometer fitted With the conv@ntiom~] was used.-The emitters employed were common uncoated
268 Schick razor blades without any special activation. The source temperature was 80--100°C and the pressure in the source region was typically a few × 10C s torr. The synthesis of the labelled l-heptenes used in t h e p r e s e n t study has been described previously [9].
RESULTS
AND
DISCUSSION
T h e observed ( M - - R ) 2* a n d ( M M - - R ) 2. ions are reported in Tables 1 a n d 2, respectively. Their intensities are generally low. P e ~ b s with sufficient intensity which allow label retention to be followed are indicated as s (strong) or m (medium). The occurrence of pulse trains at certain masses, suggesting the presence of pe~b~ of very small intensity, has been denoted by w (weak). An asterisk (*) in Tables 1 and 2 indicates the presence of a signal of a single charged fragment, thus eventually hiding a doubly charged ion. The translational energy of the doubly charged ions has been measured by using the technique of scanning the emitter potential [10]. This experiment allow-~ the deduction to be made that the doubly charged ions are formed either on or very close to the emitter surface. This conclusion is in agreement with the currently favoured thesis [5] that the formation of mul-
TABLE 1 (M--R) 2 + ions in the field ionization ma~ spectrum of labelled 1-heptenes Compound
M 2+
( M - - C H 2 ) 2+
M/z
(1~[ ---C..2 I-I4 ) 2+
M/z
M/z
50.5
50
49.5
49
43
42.5
42
35.5
35
0 0
0 0
0 s
s m
* *
0 w
* *
0 0
s s
0
0
*
0
s
0
0
s
l-heptene 1 -I 3 C 1-d I 1 2 _-1d3 C
0
s
m
*
0
s
0
0
0
0
s
m
* *
s
w
* *
3 -z 3C
0
0
s
m
*
s
*
s
0
4-13C
0
0
s
m
*
s
*
s
0 0
s
5-13C
0
0
s
m
0
0
s
m
* *
s
6-13C
s
* *
s s
w
7 -z 3C
0
w
s
m
*
s
*
s
0
7-d 3
s
w
0
0
*
1,2-13C2 2,313 C2 1,7-13C2
0 0 0
s s s
m m m
w w w
* * *
s w s
* * *
0 s s
s 0 w
6,7-13C2 5,5-d 2 1 , 2 , 3 J 3 C3 7-cl 1
0 0 s
s s m
w 0
0
* * * *
0 m m s
* * * *
w m s s
w 0
0
m w w s
0 0
*
0 0
M/=
= 36
: s
M'/~r
= 36
. s
2 6 9
-"
. . . .
"-
"a- n- u-
"-
-
-If-
'I"
41-
4(-
.]I-
-M-
-]E,
-X'-
°
o~
o.., -
41-
,K-
-It-
-E-
.:
=,-
"It"
-~
:::
~
~
-X-
41,
"X2 •
.
-~
~
ooOo-~-o
"i-
41~
4I"
4~
-E"
~"
4[-
°.
"
LI~
-
# O
O
O
O
O
O
O
O
O
O
O
O O
~ 0 O O
m
O
-o~
~,'~
~
~o
~
-
~7
O
,o
~.~
-
_
r.~
r..)
r.)
r.D
,'~
270
tiply charged ions involves an adsorbed state at the emitter within the condensed phase at the surface.
or a process
( M - - R ) 2+ i o n s
The most impo~t ( M - - R ) 2+ f r a g m e n t i o n s r e p o r t e d i n t h i s s t u d y r e f e r t o s p e c i e s i n w h i c h 1~ is a n o l e f i n , a l t h o u g h i n s o m e c a s e s f ~ a g T n e n t s w i t h 1~ = a l b y l o r ~ l k e n y l a r e o b s e r v a b l e . C 7 i - l ~ r e p r e s e n t s 1.8~o o f t h e t o t a l i o n i z a t i o n under our experimental conditions. The peak for the non-labelled 1-heptene a p p e a r s a t M / z = 4 9 . F o r m o n o i a b e l l e d c o m p o u n d s i t s p o s i t i o n is s h a f t e d t o M/z = 4 9 . 5 , w h i l e w i t h t h e d i l a b e l l e d 1 - h e p t e n e s a p e a k a t M / z = 5 0 is o b served. These results indicate that CTH~ either originates from a single molecule of 1-heptene or that in the case of bimolecular reaction no inte~molec, J l a r e x c h a n g e is o b s e g l e d . H o w e v e r t h e r e is n o e v i d e n c e t h a t t h i s i o n is t h e result of more complex mechani~mq or the product of decomposition of high molecular weight species. Ce~I~[ is d e t e c t e d w i t h m o n o l a b e l l e d 1 - h e p t e n e a t M / z = 4 2 . 5 p r o v i d e d t h a t t h e l a b e l is n o t i n p o s i t i o n 1 (1-13C, 1 - d l o r l - d 2 ) . T h e s e o b s e r v a t i o n s s h o w t h a t t h e f o r m a t i o n o f C 6 H ~ o r i g i n a t e s f r o m C7H~h v i a a l o s s o f a m e t h y l e n e f r a g m e n t c o m i n g e x c l u s i v e l y f r o m p o s i t i o n 1. C s I - l ~ a p p e a r s a t M / z = 3 5 f o r t h e n o n - l a b e l l e d 1 - h e p t e n e a s w e l l as f o r the compounds l a b e l l e d i n p o s i t i o n I ( I - ' 3 C , l - d , o r l - d o ) o r 2 (2-13C). Labels in other positions result in shifted peaks at M/z = 35.5 in the case of monolabelled compounds or M/z = 36 in the case of complete label retent i o n f o r d i l a b e l l e d s p e c i e s . F r o m t h e s e r e s u l t s i t is d e d u c e d t h a t t h e t w o p o s i t i o n s i n v o l v e d i n t h e l o s s o f C2H4, l e a d i n g t o C s H ~ , o r i g i n a t e f r o m t h e positions 1 and 2 of the molecule. A very weak signal at M/z = 28.5 suggests t h a t a s i m i | s r m e c h s n i ~ m o c c u r s b y t h e l o s s o f C~I-I6 t o f o r m t h e C4H~ ÷ i o n . T h e m o s t n o t a b l e f e a t u r e o f t h e s e o b s e r v a t i o n s lies in t h e s p e c i f i c i t y o f t h e p o s i t i o n s l o s t d u r i n g t h e d e c o m p o s i t i o n s l e a d i n g t o ( M - - R ) 2÷ i o n s . T h i s can he rationalized in terms of the natttre of the first step involved for the formation of the doubly charged ions. The adsorbtion state of the moleculax ion on the surface emitter must represent a specificchemical process, t h e n a t u r e o f w h i c h is n o t d e a r l y u n d e r s t o o d . T h e h i g h l o c a l f i e l d s t r e n g t h experienced by the approaching molecule favours a particular orientation (the rAi~ment of the high electronic density region of the molecule near t h e e m i t t e r ) . I t is q u i t e l i k e l y t h a t t h e u n s a t u r a t e d e n d o f t h e 1 - h e p t e n e m o l e c u l e is p r e f e r e n t S ~ H y a t t a c h e d t o t h e e m i t t e r s u r f a c e . T h e d e s o r p t i o n i n d u c e d b y t h e s e c o n d i o n i z a t i o n is f o l l o w e d b y f i e l d d i s s o c i a t i o n r e s u l t i n g i n t h e f o r m a t i o n o f ( M - - R ) 2+ i o n w i t h t h e n e u t r a l f r a g m e n t R o r i g i n a t i n g from the unsaturated site. (MMmR
) z+ i o n s
U n f o r t ~ m ~ t e l y , i t is n o t p o s s i b l e t o d e t e c t t h e M M 2+ d i m e ~ i o n a s t h e r e s u l t i n g p e a k is s u p e r i m p o s e d u p o n t h e ~ n g l y c h a r g e d m o l e c n l ~ r i o n a t M / z = 9 8 . M o r e o v e r t h e r e is n o p e a k c o r r e s p o n d i n g t o t h e s i n g l y e h a r f f e d
271 d i m e r ( M M ) ÷- i n t h e s ~ t r - m o f t h e n o n - l a b e l l e d c o m p o u n d . Nevertheless the occu_,Tence of a strong peak (1.1% of the total ioniT.ation) at M/z --63 corresponds to a doubly charged rion and implies that species of higher molee-lar weight are formed on the surface or within the condensed phase on the emitter. We have therefore assumed a dimerization of 1-heptene as already proposed by RSllgen and Heinen [4] in the case of ethylene. Similarly to the case of (M ~)2+ ions, the main fragments observed are t h o s e i n w h i c h 1% r e p r e s e n t s a n o l e f i n . A CloI-1~2~ i o n w i l l g i v e a p e a k s u p e r i m p o s e d u p o n t h e s i n g l y c h a r g e d i o n s CsH~0 at M / z = 70. Nevertheless the C,oH]~ ions are detected at M / z = 70.5 i n t h e s p e c t r a o f t h e l a b e l l e d 1 - h e p t e n e s p r o v i d e d t h a t t h e l a b e l is n o t i n position 1, 2, 3 or 4 of the original molecule. This result indicates a specific mechanism, D ~ m e l y , t h a t o n l y t h e C - a t o m s in p o s i t i o n s 1 - - 4 o f t h e s a m e molecule are lost to C4Hs. C~-~ ions appear at M / z = 63 in the non-labelled 1-heptene. The data for the compounds l a b e l l e d i n p o s i t i o n s 1, 2 , 3 , a n d 4 r e v e a l t h a t t h e f o u r Ca t o m s in t h e s e p o s i t i o n s a r e a l w a y s l o 4 t o t h e n e u t r a l C s H 1 0 . T h e o t h e r l a b e l l e d p o s i t i o n s ( 5 - ' 3 C , 6-13C a n d 7 - ' 3 C , s e e a l s o 7 - d , a n d 7 ~ i 3 ) s u p p l y t h e fifth carbon atom with about equal probabilities. A possible interpretation o f t h e s e f i n d i n g s is t h e f o l l o w i n g : A m o l e c u l a r i o n is a d s o r b e d a t t h e s u r f a c e emitter at carbon C(1) and/or C(2). The other end of the radical ion may rearrange via protonated substituted cyclopropane, as already proposed for skeletal rearrangement in allryl ions [11,12]. The formation of the dimer o c c u r s w h e n a s e c o n d m o l e c u l e o f 1 - h e p t e n e is b o u n d e d w i t h t h e a d s o r b e d ion. The attachment can occur by bond formation with the terminal carbon atom of the adsorbed ion, which can be C(5), C(6) or C(7) after the cyclic rearrangement (Scheme 1). Subsequently during the desorption of the dou-
.-~4-
s-
6-k
[÷
_.
S c h e m e 1. F o r m a t i o n o f a d i m e r b y a t t a c h m e n t o f a n o r i e n t e d m o l e c u l e t o a n a d s o r b e d ion. T h e e q u i v a l e n c e o f t h e p o s i t i o n s 5 , 6 a n d 7 a f t e r t h e r e a r r a n g e m e n t is i n d | c a t e d b y A.
272 b l y c h a r g e d d i m e r , t h e f i e l d d i s s o c i a t i o n p r o d u c e s t h e CsH10 f r ~ e n t cont a i n i n g C ( 1 ) t o C ( 4 ) a n d o n e C a t o m f r o m p o s i t i o n s 5, 6 o r 7. T h e d a t a f o r t h e 5,5-d2 1 - h e p t e n e are in a g r e e m e n t w i t h tbi~ s k e l e t a l r e a r r a n g e m e n t . CsH~ ~ i o n s y i e l d a p e a k s u p e r i m p o s e d u p o n t h e s i n g l y c h a r g e d i o n s C4H~a" a t ~ 4 / z = 5 6 . N e v e r t h e l e s s t h e y a r e d e t e c t e d a t M / z = 5 6 . 5 f f C ( 6 ) o r C ( 7 ) is a l a b e l l e d C - a t o m . T h i s m e a n s t h a t p o s i t i o n s 1 - - 5 a r e l o s t t o t h e n e u t r a l t o g e t h e r w i t h a C - a t o m f r o m e i t h e r p o s i t i o n 6 o r 7. H o w e v e r n o i n t e r m o l e c u l a r e x c h a n g e is s e e n . Ion C~3H~ (corresponding to a loss of CH2 atM/z = 91 in the non-13belled 1-heptene) gives a peak of smaller intensity than the ions discussed above. T h i s is t h e r e a s o n w h y t h e r e s u l t s a r e n o t g i v e n f o r a l l t h e l a b e l l e d s ~ r n p l e s . Nevertheless the data obtained seem to indicate a specific loss of the C(1) to t h e f r a g m e n t CH2I o n C 1 2 H ~ ( ~ o r m e d v i a l o s s o f C2H4 f r o m t h e d i n n e r a n d o b s e r v e d a t M / z = 8 4 . 5 i n t h e s p e c t r a o f 2-13C 1 - h e p t e n e ) a n d i o n C ~ x H ~ ( f o r m e d v i a l o s s o f C3H6 f r o m t h e d i m e r a n d o b s e r v e d a t M / z -- 7 7 i n t h e s p e c t r a o f t h e n o n labelled 1-heptene) are of very low intensity and therefore not reported in T a b l e s I a n d 2. H o w e v e r i t is l i k e l y t h a t t h e sArne t y p e o f m e c h a n i s m s as p r e v i o u s l y s u g g e s t e d f o r t h e ( M - - R ) 2+ i o n s o c c u r s d u r i n g t h e l o s s o f C2H4 a n d C3H6 f r o m t h e d i m e r . CONCLUSION
T h e f o x ~ a t i o n o f d o u b l y c h a r g e d i o n s ( M - - R ) 2÷ a n d ( M M - - R ) 2÷ a f t e r f i e l d ionization of 1-heptene may be rationalized in terms of a pattern involving the field dissociation of desorbing ions. The results discussed in this work s h o w t h a t t h e a d s o r p t i o n is a s p e c i f i c p r o c e s s , t h i s s p e c i f i c i t y m o s t l i k e l y b e ing caused by the high electric field and the geometry of the adsorbed state. A skeletal rearrangement of the adsorbed ion occurs at the emitter-distant side (the region where the positive charge has a high probability of being l o c a t e d ) w h i l e t h e i o n r e m a i n s a t t a c h e d t o t h e e m i t t e r . T h e b i n d i n g o f a seco n d o r i e n t e d m o l e c u l e t o fo~uL a d i m e r o c c u r s t h e r e f o r e w i t h o n e C a t o m C(5)--C(7) of the adsorbed ion, the second molecule aligning itself along the first in a head-to-tail fashion. T h e r e is n o e v i d e n c e o f i n t e r m o l e c u l a r r e a r r a n g e m e n t d u r i n g t h e f o . . . . ation of doubly charged ions. AC KNOWLEDGEiVIENT T h e financial support of the Swiss National Science F o u n d a t i o n is gratefully acknowledged. REFERENCES
1 P. T e c o n , D . S t a h l a n d T . G ~ u m a n n , I n t . J. M a s s S p e c t r o m . I o n P h y s . , 2 7 ( 1 9 7 8 ) 8 3 . 2 V_A_ N a z a r e n k o , I . V . G o l d e n f e l d a n d P . S . D i b r o v a , I n t . J. 1V[=~ S p e c t x o m . I o n P h y s . , 2 (1969) 92.
2731 3 H.D. Beckey, M.D. Migahed and F.W. R6llgen, Int. J. M*~ Spectrom. Ion Phys., 10 (1972/1973) 471. 4 F.W. R~Hgen and H.J. Heinen, Int. J. Mass Spectrom. Ion Phys., 17 (1975) 92. 5 F.W. RSHgen and H.J. Heinen, Z. Naturforsch. A, 30 (1975) 918. 6 F . W . R S H g e n , H . J . H e i n e n a n d I~. L e v s e n , O r g . 1 ~ Spectrom., 11 (1976) 780. 7 P . J . D e t E i c k , O r g . A~=~-~S p e c t r o m . , 1 0 ( 1 9 7 5 ) 1 1 7 1 . 8 M.D. Migahed and S.H. Hallam, OrE. M~ Spectrom., 12 (1977) 695. 9 A . H e u s l e r , P . G~,~= a n d T . G ~ ' u m a n n , J . L a b e l . C o m p o u n d s , 1 1 ( 1 9 7 5 ) 3 7 . 1 0 T h i s t e c h n i q u e is u s e d t o r e c o r d t h e F I X c u r v e s : s e e f o r e x a m p l e A . M . F a l i c k , P . J . Derrick and AT.. Burlingame, Int. J. Mass Spectrom. Ion Phys., 12 (1973) 101. 11 D. Stahl and T. G~um~nn, OrE. M~ Spectrom., 12 (1977) 761. 12 D. Stahl and T. G~umznn, Advances in Mass Spectrometry, Vol. 7, Heyden, Londe~n, 1 9 7 7 , p . 1 1 9 0 .
I~TJBLY CEL~_RGED IoNS IN THE SPECTRUM OF 1-iIET~rENE
FIELD
IONIZATION
PIERRE TECON, DANIEL STAHL and TINO GAITMANN
-- - -
MASS.-
-267
- : - -.-.
. . . .
l n s ~ t u t d e C h i m i e Physi'que, E c o l e P o l y t e c h n i q u e Fdddrale d e L a u s a n n e : C H - 1 0 1 5 Z,a u . ~ n n e ( S w i t z e r l a n d ) ~" --
" - "-"
(Received 6 February 1978)
ABSTRACT S u r f a c e p r o c e s s e s in t h e f i e l d i o n i z a t i o n o f l - h e p t e n e give rise t o t h e d o u b l y c h a r g e d i o n s ( M - - R ) 2÷ a n d ( M M - - R ) 2÷ (M a n d R r e p r e s e n t t h e 1 - h e p t e n e m o l e c u l e a n d a n olefln~e f r a g m e n t , r e s p e c t i v e l y ) . T h e u s e o f 13C- a n d D - l a b e l l i n g a l l o w s i n v e s t i g a t i o n o f t h e s e p h e n o m e n a in detail. T h e mechani-~m.~ p r o p o s e d f o r d i r n e r i z a t i o n a n d s u b s e q u e n t fragmentation are supported by the data of seventeen labelled Compounds. - INTRODUCTION The field ionization (FI) spectm,m of-1-heptene has been recently described [1]. In the context of that study the pattern of formation of monocharged ions resulting from uDimolecular decomposition of molecular ions was reported. The FI spect~,m obtained indicates inter alia the occurrence of surface processes (doubly charged ions). These phenomena have been studied quite extensively in recent years [2--8]. Multiply charged ions are formed via a Lwo-step mechanism: the first involves adsorption of an ion on the emitter surface and the second step is the desorption induced by the second ionization [4]. The field dissociation of the desorbing species may produce doubly charged fragment ions. In the previous surface ionization studies [2--8] the emitters used were generally tips or wires heated up to 1000°C and the fragments o b s e r v e d w e r e m a i n l y o f t y p e s M 2+ o r ( M - - R ) 2 . +. In the present study attention is focussed on the formation of the doubly c h a r g e d i o n s o f t y p e s ( ] V I - - R ) 2+ a n d ( M M - - R ) 2÷ o b s e r v e d i n t h e F I s p e c i . T a of IsC- and D-labelled 1-heptenes using a blade emitter at low tempera°
EXPERIMENTAL
A Du Pont FI/EI source
CEC 21-110B_ mass spectrometer fitted With the conv@ntiom~] was used.-The emitters employed were common uncoated
268 Schick razor blades without any special activation. The source temperature was 80--100°C and the pressure in the source region was typically a few × 10C s torr. The synthesis of the labelled l-heptenes used in t h e p r e s e n t study has been described previously [9].
RESULTS
AND
DISCUSSION
T h e observed ( M - - R ) 2* a n d ( M M - - R ) 2. ions are reported in Tables 1 a n d 2, respectively. Their intensities are generally low. P e ~ b s with sufficient intensity which allow label retention to be followed are indicated as s (strong) or m (medium). The occurrence of pulse trains at certain masses, suggesting the presence of pe~b~ of very small intensity, has been denoted by w (weak). An asterisk (*) in Tables 1 and 2 indicates the presence of a signal of a single charged fragment, thus eventually hiding a doubly charged ion. The translational energy of the doubly charged ions has been measured by using the technique of scanning the emitter potential [10]. This experiment allow-~ the deduction to be made that the doubly charged ions are formed either on or very close to the emitter surface. This conclusion is in agreement with the currently favoured thesis [5] that the formation of mul-
TABLE 1 (M--R) 2 + ions in the field ionization ma~ spectrum of labelled 1-heptenes Compound
M 2+
( M - - C H 2 ) 2+
M/z
(1~[ ---C..2 I-I4 ) 2+
M/z
M/z
50.5
50
49.5
49
43
42.5
42
35.5
35
0 0
0 0
0 s
s m
* *
0 w
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0 0
s s
0
0
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0
s
0
0
s
l-heptene 1 -I 3 C 1-d I 1 2 _-1d3 C
0
s
m
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0
s
0
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0
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s
m
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s
w
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3 -z 3C
0
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4-13C
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5-13C
0
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s
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s
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1,2-13C2 2,313 C2 1,7-13C2
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m m m
w w w
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s w s
* * *
0 s s
s 0 w
6,7-13C2 5,5-d 2 1 , 2 , 3 J 3 C3 7-cl 1
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w 0
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* * * *
w m s s
w 0
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0 0
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M/=
= 36
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2 6 9
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270
tiply charged ions involves an adsorbed state at the emitter within the condensed phase at the surface.
or a process
( M - - R ) 2+ i o n s
The most impo~t ( M - - R ) 2+ f r a g m e n t i o n s r e p o r t e d i n t h i s s t u d y r e f e r t o s p e c i e s i n w h i c h 1~ is a n o l e f i n , a l t h o u g h i n s o m e c a s e s f ~ a g T n e n t s w i t h 1~ = a l b y l o r ~ l k e n y l a r e o b s e r v a b l e . C 7 i - l ~ r e p r e s e n t s 1.8~o o f t h e t o t a l i o n i z a t i o n under our experimental conditions. The peak for the non-labelled 1-heptene a p p e a r s a t M / z = 4 9 . F o r m o n o i a b e l l e d c o m p o u n d s i t s p o s i t i o n is s h a f t e d t o M/z = 4 9 . 5 , w h i l e w i t h t h e d i l a b e l l e d 1 - h e p t e n e s a p e a k a t M / z = 5 0 is o b served. These results indicate that CTH~ either originates from a single molecule of 1-heptene or that in the case of bimolecular reaction no inte~molec, J l a r e x c h a n g e is o b s e g l e d . H o w e v e r t h e r e is n o e v i d e n c e t h a t t h i s i o n is t h e result of more complex mechani~mq or the product of decomposition of high molecular weight species. Ce~I~[ is d e t e c t e d w i t h m o n o l a b e l l e d 1 - h e p t e n e a t M / z = 4 2 . 5 p r o v i d e d t h a t t h e l a b e l is n o t i n p o s i t i o n 1 (1-13C, 1 - d l o r l - d 2 ) . T h e s e o b s e r v a t i o n s s h o w t h a t t h e f o r m a t i o n o f C 6 H ~ o r i g i n a t e s f r o m C7H~h v i a a l o s s o f a m e t h y l e n e f r a g m e n t c o m i n g e x c l u s i v e l y f r o m p o s i t i o n 1. C s I - l ~ a p p e a r s a t M / z = 3 5 f o r t h e n o n - l a b e l l e d 1 - h e p t e n e a s w e l l as f o r the compounds l a b e l l e d i n p o s i t i o n I ( I - ' 3 C , l - d , o r l - d o ) o r 2 (2-13C). Labels in other positions result in shifted peaks at M/z = 35.5 in the case of monolabelled compounds or M/z = 36 in the case of complete label retent i o n f o r d i l a b e l l e d s p e c i e s . F r o m t h e s e r e s u l t s i t is d e d u c e d t h a t t h e t w o p o s i t i o n s i n v o l v e d i n t h e l o s s o f C2H4, l e a d i n g t o C s H ~ , o r i g i n a t e f r o m t h e positions 1 and 2 of the molecule. A very weak signal at M/z = 28.5 suggests t h a t a s i m i | s r m e c h s n i ~ m o c c u r s b y t h e l o s s o f C~I-I6 t o f o r m t h e C4H~ ÷ i o n . T h e m o s t n o t a b l e f e a t u r e o f t h e s e o b s e r v a t i o n s lies in t h e s p e c i f i c i t y o f t h e p o s i t i o n s l o s t d u r i n g t h e d e c o m p o s i t i o n s l e a d i n g t o ( M - - R ) 2÷ i o n s . T h i s can he rationalized in terms of the natttre of the first step involved for the formation of the doubly charged ions. The adsorbtion state of the moleculax ion on the surface emitter must represent a specificchemical process, t h e n a t u r e o f w h i c h is n o t d e a r l y u n d e r s t o o d . T h e h i g h l o c a l f i e l d s t r e n g t h experienced by the approaching molecule favours a particular orientation (the rAi~ment of the high electronic density region of the molecule near t h e e m i t t e r ) . I t is q u i t e l i k e l y t h a t t h e u n s a t u r a t e d e n d o f t h e 1 - h e p t e n e m o l e c u l e is p r e f e r e n t S ~ H y a t t a c h e d t o t h e e m i t t e r s u r f a c e . T h e d e s o r p t i o n i n d u c e d b y t h e s e c o n d i o n i z a t i o n is f o l l o w e d b y f i e l d d i s s o c i a t i o n r e s u l t i n g i n t h e f o r m a t i o n o f ( M - - R ) 2+ i o n w i t h t h e n e u t r a l f r a g m e n t R o r i g i n a t i n g from the unsaturated site. (MMmR
) z+ i o n s
U n f o r t ~ m ~ t e l y , i t is n o t p o s s i b l e t o d e t e c t t h e M M 2+ d i m e ~ i o n a s t h e r e s u l t i n g p e a k is s u p e r i m p o s e d u p o n t h e ~ n g l y c h a r g e d m o l e c n l ~ r i o n a t M / z = 9 8 . M o r e o v e r t h e r e is n o p e a k c o r r e s p o n d i n g t o t h e s i n g l y e h a r f f e d
271 d i m e r ( M M ) ÷- i n t h e s ~ t r - m o f t h e n o n - l a b e l l e d c o m p o u n d . Nevertheless the occu_,Tence of a strong peak (1.1% of the total ioniT.ation) at M/z --63 corresponds to a doubly charged rion and implies that species of higher molee-lar weight are formed on the surface or within the condensed phase on the emitter. We have therefore assumed a dimerization of 1-heptene as already proposed by RSllgen and Heinen [4] in the case of ethylene. Similarly to the case of (M ~)2+ ions, the main fragments observed are t h o s e i n w h i c h 1% r e p r e s e n t s a n o l e f i n . A CloI-1~2~ i o n w i l l g i v e a p e a k s u p e r i m p o s e d u p o n t h e s i n g l y c h a r g e d i o n s CsH~0 at M / z = 70. Nevertheless the C,oH]~ ions are detected at M / z = 70.5 i n t h e s p e c t r a o f t h e l a b e l l e d 1 - h e p t e n e s p r o v i d e d t h a t t h e l a b e l is n o t i n position 1, 2, 3 or 4 of the original molecule. This result indicates a specific mechanism, D ~ m e l y , t h a t o n l y t h e C - a t o m s in p o s i t i o n s 1 - - 4 o f t h e s a m e molecule are lost to C4Hs. C~-~ ions appear at M / z = 63 in the non-labelled 1-heptene. The data for the compounds l a b e l l e d i n p o s i t i o n s 1, 2 , 3 , a n d 4 r e v e a l t h a t t h e f o u r Ca t o m s in t h e s e p o s i t i o n s a r e a l w a y s l o 4 t o t h e n e u t r a l C s H 1 0 . T h e o t h e r l a b e l l e d p o s i t i o n s ( 5 - ' 3 C , 6-13C a n d 7 - ' 3 C , s e e a l s o 7 - d , a n d 7 ~ i 3 ) s u p p l y t h e fifth carbon atom with about equal probabilities. A possible interpretation o f t h e s e f i n d i n g s is t h e f o l l o w i n g : A m o l e c u l a r i o n is a d s o r b e d a t t h e s u r f a c e emitter at carbon C(1) and/or C(2). The other end of the radical ion may rearrange via protonated substituted cyclopropane, as already proposed for skeletal rearrangement in allryl ions [11,12]. The formation of the dimer o c c u r s w h e n a s e c o n d m o l e c u l e o f 1 - h e p t e n e is b o u n d e d w i t h t h e a d s o r b e d ion. The attachment can occur by bond formation with the terminal carbon atom of the adsorbed ion, which can be C(5), C(6) or C(7) after the cyclic rearrangement (Scheme 1). Subsequently during the desorption of the dou-
.-~4-
s-
6-k
[÷
_.
S c h e m e 1. F o r m a t i o n o f a d i m e r b y a t t a c h m e n t o f a n o r i e n t e d m o l e c u l e t o a n a d s o r b e d ion. T h e e q u i v a l e n c e o f t h e p o s i t i o n s 5 , 6 a n d 7 a f t e r t h e r e a r r a n g e m e n t is i n d | c a t e d b y A.
272 b l y c h a r g e d d i m e r , t h e f i e l d d i s s o c i a t i o n p r o d u c e s t h e CsH10 f r ~ e n t cont a i n i n g C ( 1 ) t o C ( 4 ) a n d o n e C a t o m f r o m p o s i t i o n s 5, 6 o r 7. T h e d a t a f o r t h e 5,5-d2 1 - h e p t e n e are in a g r e e m e n t w i t h tbi~ s k e l e t a l r e a r r a n g e m e n t . CsH~ ~ i o n s y i e l d a p e a k s u p e r i m p o s e d u p o n t h e s i n g l y c h a r g e d i o n s C4H~a" a t ~ 4 / z = 5 6 . N e v e r t h e l e s s t h e y a r e d e t e c t e d a t M / z = 5 6 . 5 f f C ( 6 ) o r C ( 7 ) is a l a b e l l e d C - a t o m . T h i s m e a n s t h a t p o s i t i o n s 1 - - 5 a r e l o s t t o t h e n e u t r a l t o g e t h e r w i t h a C - a t o m f r o m e i t h e r p o s i t i o n 6 o r 7. H o w e v e r n o i n t e r m o l e c u l a r e x c h a n g e is s e e n . Ion C~3H~ (corresponding to a loss of CH2 atM/z = 91 in the non-13belled 1-heptene) gives a peak of smaller intensity than the ions discussed above. T h i s is t h e r e a s o n w h y t h e r e s u l t s a r e n o t g i v e n f o r a l l t h e l a b e l l e d s ~ r n p l e s . Nevertheless the data obtained seem to indicate a specific loss of the C(1) to t h e f r a g m e n t CH2I o n C 1 2 H ~ ( ~ o r m e d v i a l o s s o f C2H4 f r o m t h e d i n n e r a n d o b s e r v e d a t M / z = 8 4 . 5 i n t h e s p e c t r a o f 2-13C 1 - h e p t e n e ) a n d i o n C ~ x H ~ ( f o r m e d v i a l o s s o f C3H6 f r o m t h e d i m e r a n d o b s e r v e d a t M / z -- 7 7 i n t h e s p e c t r a o f t h e n o n labelled 1-heptene) are of very low intensity and therefore not reported in T a b l e s I a n d 2. H o w e v e r i t is l i k e l y t h a t t h e sArne t y p e o f m e c h a n i s m s as p r e v i o u s l y s u g g e s t e d f o r t h e ( M - - R ) 2+ i o n s o c c u r s d u r i n g t h e l o s s o f C2H4 a n d C3H6 f r o m t h e d i m e r . CONCLUSION
T h e f o x ~ a t i o n o f d o u b l y c h a r g e d i o n s ( M - - R ) 2÷ a n d ( M M - - R ) 2÷ a f t e r f i e l d ionization of 1-heptene may be rationalized in terms of a pattern involving the field dissociation of desorbing ions. The results discussed in this work s h o w t h a t t h e a d s o r p t i o n is a s p e c i f i c p r o c e s s , t h i s s p e c i f i c i t y m o s t l i k e l y b e ing caused by the high electric field and the geometry of the adsorbed state. A skeletal rearrangement of the adsorbed ion occurs at the emitter-distant side (the region where the positive charge has a high probability of being l o c a t e d ) w h i l e t h e i o n r e m a i n s a t t a c h e d t o t h e e m i t t e r . T h e b i n d i n g o f a seco n d o r i e n t e d m o l e c u l e t o fo~uL a d i m e r o c c u r s t h e r e f o r e w i t h o n e C a t o m C(5)--C(7) of the adsorbed ion, the second molecule aligning itself along the first in a head-to-tail fashion. T h e r e is n o e v i d e n c e o f i n t e r m o l e c u l a r r e a r r a n g e m e n t d u r i n g t h e f o . . . . ation of doubly charged ions. AC KNOWLEDGEiVIENT T h e financial support of the Swiss National Science F o u n d a t i o n is gratefully acknowledged. REFERENCES
1 P. T e c o n , D . S t a h l a n d T . G ~ u m a n n , I n t . J. M a s s S p e c t r o m . I o n P h y s . , 2 7 ( 1 9 7 8 ) 8 3 . 2 V_A_ N a z a r e n k o , I . V . G o l d e n f e l d a n d P . S . D i b r o v a , I n t . J. 1V[=~ S p e c t x o m . I o n P h y s . , 2 (1969) 92.
2731 3 H.D. Beckey, M.D. Migahed and F.W. R6llgen, Int. J. M*~ Spectrom. Ion Phys., 10 (1972/1973) 471. 4 F.W. R~Hgen and H.J. Heinen, Int. J. Mass Spectrom. Ion Phys., 17 (1975) 92. 5 F.W. RSHgen and H.J. Heinen, Z. Naturforsch. A, 30 (1975) 918. 6 F . W . R S H g e n , H . J . H e i n e n a n d I~. L e v s e n , O r g . 1 ~ Spectrom., 11 (1976) 780. 7 P . J . D e t E i c k , O r g . A~=~-~S p e c t r o m . , 1 0 ( 1 9 7 5 ) 1 1 7 1 . 8 M.D. Migahed and S.H. Hallam, OrE. M~ Spectrom., 12 (1977) 695. 9 A . H e u s l e r , P . G~,~= a n d T . G ~ ' u m a n n , J . L a b e l . C o m p o u n d s , 1 1 ( 1 9 7 5 ) 3 7 . 1 0 T h i s t e c h n i q u e is u s e d t o r e c o r d t h e F I X c u r v e s : s e e f o r e x a m p l e A . M . F a l i c k , P . J . Derrick and AT.. Burlingame, Int. J. Mass Spectrom. Ion Phys., 12 (1973) 101. 11 D. Stahl and T. G~um~nn, OrE. M~ Spectrom., 12 (1977) 761. 12 D. Stahl and T. G~umznn, Advances in Mass Spectrometry, Vol. 7, Heyden, Londe~n, 1 9 7 7 , p . 1 1 9 0 .