Field Ionization Kinetic Studies of Gas-Phase Ion Chemistry

5 Field Ionization Kinetic Studies of Gas-Phase Ion

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Chemistry N. M. M. NIBBERING Laboratory for Organic Chemistry, University of Amsterdam, Amsterdam, The Netherlands

In the last few years the method of Field Ionization Kinetics (FIK) has been developed rapidly by several groups (1). The technique has proved to be successful in providing detailed information on the mechanisms of decompositions of ions in the gas phase (1,2). This information has been d i f f i c u l t or even impossible to obtain from conventional electron impact (EI) studies. Although the underlying theory of the FIK method has been described extensively i n detail (1,3), i t w i l l be repeated here i n a very simple way. If, for example, a 10 μm conditioned wire emitter (4) at potential V (- 8 kV) is positioned approximately 1.5 mm from a grounded and slotted cathode, gas phase molecules may be ionized in the strong e l e c t r i c f i e l d at a very narrow region close to the emitter. To a good approximation, the potential is equal to the emitter potential V at this point (Figure 1). Suppose that the ions, formed by either direct ionization or unimolecular decomposition close to the emitter, are stable for ≥ 1 0 sec. They w i l l acquire a kinetic energy eV (Turing acceleration from the emitter to the cathode and w i l l pass the e l e c t r i c sector of a double focussing mass spectrometer, set to transmit ions with a kinetic energy eV . Consequently, they w i l l be mass analyzed at their correct m/e-values. In this way a f i e l d ionization (FI) spectrum is obtained, containing peaks corresponding to the ions generated very near to the emitter, i . e . , wijhin approximately 1 0 sec. Fragment ions, m , generated by expulsion of (M-m) from M ions between the emitter and cathode at potent i a l V (Figure 1) are not transmitted through the e l e c t r i c sector, because they have insufficient kinetic 0

0

-5

0

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- 1 1

+

+

x

©0-8412-0422-5/78/47-070-080$05.00/0

Gross; High Performance Mass Spectrometry: Chemical Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

5.

81

Field Ionization Kinetic Studies

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energy (em/M ( V Q - V ^ ) + eVy) t o be f o c u s s e d by t h i s s e c t o r . However, t h e s e i o n s can be t r a n s m i t t e d t h r o u g h the e l e c t r i c s e c t o r by i n c r e a s i n g the e m i t t e r p o t e n t i a l , V Q , by an amount A V , such t h a t the k i n e t i c energy of the fragment i o n s , m , i s g i v e n by e q u a t i o n (1) (3a). +

e V

0

=

ΤΓ

( V

0

+

Δ Υ

* V

+

e V

X

Thus i t i s p o s s i b l e t o m o n i t o r a t a f i x e d e l e c t r i c s e c t o r v o l t a g e the abundance o f m i o n s as a f u n c t i o n o f the e m i t t e r p o t e n t i a l V Q + A V ( 3 a ) . A scan o f the e m i t t e r p o t e n t i a l , a c h i e v e d by i n c r e a s i n g A V , a l l o w s one t o v i e w the d e c o m p o s i t i o n s o f i o n s M o c c u r r i n g a t e v e r - i n c r e a s i n g d i s t a n c e s from the e m i t t e r . Of c o u r s e , these i n c r e a s e d d i s t a n c e s correspond to longer l i f e t i m e s (5) o f the p a r e n t i o n , M . E x p r e s s i n g the abun dance o f m i o n s due t o l o s s o f (M-m) as a f u n c t i o n o f time f o l l o w i n g FI o f M, the FIK c u r v e o f m i s ob t a i n e d ( F i g u r e 2 ) . The time r a n g e , c o v e r e d c o n t i n u o u s l y by the FIK method, i s a p p r o x i m a t e l y 1 0 " to 10" s e c . A d d i t i o n a l p o i n t s a t lçnger t i m e s can be o b t a i n e d from d e c o m p o s i t i o n s o f M i o n s i n the f i r s t and second f i e l d f r e e r e g i o n s , which a r e a l s o a c c e s s i b l e by the c o n v e n t i o n a l EI t e c h n i q u e ( F i g u r e 1 ) . The s t r e n g t h o f the FIK method i s shown t o f u l l advantage, i f i t i s used i n c o m b i n a t i o n w i t h a double f o c u s s i n g mass s p e c t r o m e t e r and w i t h i s o t o p i c l a b e l l i n g , f o r the e l u c i d a t i o n o f mechanisms o f gas-phase ion decomposition processes at 1 0 " t o 10' sec. (3a). The f i r s t and a l r e a d y c l a s s i c a l example o f such mecFP a n i s t i c s t u d i e s has been the FIK s t u d y on c y c l o h e x e n e 3,3,6,6-dit ( 6 ) . S i n c e then many m e c h a n i s t i c FIK s t u d i e s Kave "appeared i n the l i t e r a t u r e , t o w h i c h the reader i s r e f e r r e d ( l c , 2 , 7 ) . In our l a b o r a t o r y , tïïe FIK method i n c o m b i n a t i o n w i t h d e u t e r i u m l a b e l l i n g has been a p p l i e d t o gas phase d e c o m p o s i t i o n s o f the m o l e c u l a r i o n s o f 2-phenoxyethyl c h l o r i d e and o f 3 - p h e n y l p r o p a n a l , as w i l l be d i s c u s s e d below. In t h i s way, we i n t e n d t o show the a p p l i c a t i o n of t h i s p o w e r f u l t e c h n i q u e f o r i n v e s t i g a t i o n o f gasphase i o n i c d e c o m p o s i t i o n s . +

+

+

1 1

9

1 1

7

FIK Study on 2-Phenoxyethyl

Chloride (8).

A p r e v i o u s EI s t u d y o f 2-phenoxyethyl

chloride,

C 6 H 5 O C H 2 C H 2 C I , i n combination w i t h deuterium l a b e l l i n g has shown, t h a t the m o l e c u l a r i o n s l o s e i n a p p r o x i 1

m a t e l y e q u a l amounts «C^Cl and

2

•CH C1 ( 9 ) . 2

T h i s must


82

HIGH PERFORMANCE MASS SPECTROMETRY

slotted cathode emitter

/

J

electric sector set to transmit ions with a kinetic energy eV magnetic sector

v

x

Q

1 field free region

,'

detector

st

2 field free region nd

Figure 1. Double-focusing field ionization mass spectrometer (not drawn to scale)

Figure 2. FIK curve of ions m*

(right) *Advances in Mass Spectrometry Figure 3. FI spectra of 2-phenoxyethyl chloride and of its 1,1-d«- and 2,2,-d analogs. The left ordinate scale refers to the intensities of ions with m/e < 130; the right ordinate scale to those of ions with m/e > 130. f


5.

83


NiBBERiNG

94

63

.O-CH2-CH2-CI

COT

1

?

6

ri» 80 60

107 107

Lo

121

121 20

15 20

I lift 60

40

I ill.J

100

80

120

160

15 20

0-CD,-CH,-CI

65

158

I·/·

HOO 0.84 80 0.64

M 158

94

—95

60

0.44

M 158

—•'94 109

40

123 109

0.24

123

I-20 15 20

liiiji 40

60

-lilt— 80

100

ϊ ί ! 0 13θ/J\lI50


160

84

HIGH P E R F O R M A N C E MASS

SPECTROMETRY

a r i s e from a n e a r l y c o m p l e t e l y e q u i l i b r a t e d p o s i t i o n a l i n t e r c h a n g e o f t h e phenoxy group and t h e c h l o r i n e atom i n t h e m o l e c u l a r i o n s p r i o r t o l o s s o f a CH C1 r a d i c a l . We have a d d r e s s e d o u r s e l v e s t o t h e q u e s t i o n o f whether FIK c o u l d show t h e time dependence o f t h i s p o s i t i o n a l i n t e r c h a n g e . Of c o u r s e , normal e l e c t r o n - i m p a c t ( E I ) mass s p e c t r o m e t e r s p r e s e n t an i n t e g r a t e d view o f i o n c h e m i s t r y from t h e time o f f o r m a t i o n t o ^ 1 0 " s e c , and, t h e r e f o r e , t h i s i n f o r m a t i o n i s n o t o b t a i n a b l e on t h e s e instruments. The F I s p e c t r a o f 2-phenoxyethyl c h l o r i d e and o f the 1,1-d - and 2,2,-d analogues a r e g i v e n i n F i g u r e 3 without c o r r e c t i o n f o r c o n t r i b u t i o n s of n a t u r a l isotopes and o f i s o t o p i c i m p u r i t i e s . They show t h a t a t 2

6

2

2

1 1

10 s e c , t h e o r i g i n a l - C r ^ C l group i s e l i m i n a t e d from the m o l e c u l a r i o n s w i t h o u t s u b s t a n t i a l i n t e r f e r e n c e by the p o s i t i o n a l i n t e r c h a n g e p r o c e s s . To f o l l o w t h e p o s i t i o n a l i n t e r c h a n g e i n t h e m o l e c u l a r i o n s as a f u n c t i o n o f t j m e , F I K measurements were p e r f o r m e d on the (M-CH C1) and (M-CD C1) i o n s g e n e r a t e d from t h e 1 , l - d - and 2,2-d m o l e c u l a r i o n s . Thç measured abundances o f t h e (M-CH C1) and (M-CD C1) i o n s a r e e x p r e s s e d i n p e r c e n t a g e s o f t h e i r sum as a f u n c t i o n o f time and t h e s e r e s u l t s a r e g i v e n i n F i g u r e s 4a and 4b f o r t h e 1 , l - d - and 2,2-d compounds, r e s p e c t i v e l y . I t i s v e r y c l e a r from these f i g u r e s , t h a t a t s h o r t e r times t h e p o s i t i o n a l i n t e r c h a n g e o f t h e phenoxy group and t h e c h l o r i n e atom cannot compete w i t h t h e e l i m i n a t i o n o f -CD C1 (-CH C1) from t h e 1,1d2 - ( 2 , 2 - d ) m o l e c u l a r i o n s , b u t i t competes much more e f f e c t i v e l y a t l o n g e r times (~ 1 0 " sec) c o r r e s p o n d i n g t o m o l e c u l a r i o n s o f lower i n t e r n a l energy. This observation i n d i c a t e s that the p o s i t i o n a l interchange o c c u r s i n t h e gas phase m o l e c u l a r i o n s o f 2-phenoxye t h y l c h l o r i d e , and t h a t i t i s a p r o c e s s h a v i n g a lower f r e q u e n c y f a c t o r and lower a c t i v a t i o n energy than t h e e l i m i n a t i o n o f -CH C1 v i a a s i m p l e c l e a v a g e . Note a l s o from F i g u r e 4 t h a t t h e t i m e - r e s o l v e d p o s i t i o n a l i n t e r change reaches a t l e a s t a h i g h e r p e r c e n t a g e o f e q u i l i b r a t i o n (87-931) than can be deduced from p r e v i o u s E I r e s u l t s (851) ( 9 ) , w h i c h r e p r e s e n t o n l y an i n t e g r a t e d v a l u e o v e r t h e time range up t o ~ 10 s e c . 2

2

2

2

2

2

2

2

2

2

2

1 0

2

6

FIK Study on 3 - P h e n y l p r o p a n a l The

(10).

e l i m i n a t i o n s o f C H 0 and o f C r U 0 from t h e 3 2 1 m o l e c u l a r i o n s o f 3 - p h e n y l p r o p a n a l , C H CH CH CHO upon e l e c t r o n impact have been s t u d i e d p r e v i o u s l y by D- and C-13 l a b e l i n g (11,12). In s p i t e o f the extensive 2

2

3

6

5

2

2


NiBBERiNG


mVe 107

n

-11.0

-11.0

/el09

-10.5

-10.0

-9.5

logt

-10.5

logt Advances in Mass Spectrometry

Figure 4. Intensities of m/e 107 and m/e 109 expressed in percentage of their sum as a function of time for the l,l-d - and 2,2-d -2-phenoxyethyl chlorides [(a) and (b), respectively]. The dashed lines refer to the percentage of equilibrated positional interchange of the phenoxy group and the chlorine atom. 2


2

86


hydrogen r a n d o m i z a t i o n i n t h e m o l e c u l a r i o n s p r i o r t o o r d u r i n g f r a g m e n t a t i o n , i t has been s u g g e s t e d t h a t a hydrogen atom from p o s i t i o n 2 m i g r a t e s t o t h e p h e n y l r i n g d u r i n g t h e e l i m i n a t i o n s o f C H 0 and o f C 3 H 4 O ( 1 1 ) . T h i s h y p o t h e s i s c a n be t e s t e d w i t h t h e F I K method. I n F i g u r e 5 t h e E I spectrum and t h e F I s p e c t r a o f 3 - p h e n y l p r o p a n a l a t an unheated and h e a t e d e m i t t e r have been r e p r o d u c e d . I t i s s e e n , t h a t t h e i o n s m/e 92 and m/e 78, g e n e r a t e d by e l i m i n a t i o n s o f C H 0 and CaHitO from t h e m o l e c u l a r i o n s r e s p e c t i v e l y , a r e p r e s e n t i n t h e F I s p e c t r a . The F I s p e c t r a o f 3phenylpropanals, l a b e l l e d with deuterium i n the 1 p o s i t i o n ( 1 - d i ) , the 2 p o s i t i o n , (2,2-d ), the 3 p o s i t i o n ( 3 , 3 - d ) and i n t h e p h e n y l r i n g (0-ds) show t h a t t h e i o n s m/e 92 and m/e 78 r e t a i n t h e hydrogen atom from p o s i t i o n 1 t o tïïe e x t e n t o f a p p r o x i m a t e l y 90% (10.)· C o n t r a r y t o c o n c l u s i o n s from p r e v i o u s E I r e s u l t s (11) we now c o n c l u d e t h a t a hydrogen atom from p o s i t i o n 1 and n o t from p o s i t i o n 2 m i g r a t e s t o the p h e n y l r i n g d u r i n g t h e e l i m i n a t i o n s o f C H 0 and o f C r U 0 , a t l e a s t w i t h i n 1 0 " s e c . The e l i m i n a t i o n o f C H 0 o c c u r s by a 1,5 hydrogen m i g r a t i o n , i^.e. , t h e w e l l - k n o w n M c L a f f e r t y r e a r r a n g e m e n t , whereas the second e l i m i n a t i o n might p r o c e e d v i a a 1,5and/or a 1,4-hydrogen s h i f t (Scheme 1 ) . 2

2

2

2

2

2

2

2

1 1

3

2

2

Scheme 1

0

0 The e l i m i n a t i o n o f C3rU0 g i v e s an i s o l a t e d peak a t m/e 78 i n t h e F I spectrum ( F i g u r e 5 ) , so t h a t t h i s r e a c t i o n c a n be r e a d i l y s t u d i e d by F I K making u s e o f the v a r i o u s d e u t e r a t e d 3 - p h e n y l p r o p a n a l s . This i s not t r u e f o r t h e e l i m i n a t i o n o f C H 0 because t h i s r e a c t i o n i s s u b j e c t t o i n t e r f e r e n c e by t h e s i m p l e c l e a v a g e o f t h e C - C bond i n t h e d e u t e r a t e d 3phenylpropanals which occurs w i t h randomization o f hydrogen atoms (cf_. peaks a t m/e 91 and 92 i n t h e F I spectra, given i n Figure 5). F I K c u r v e s have been measured f o r t h e [M-C3H D 0 (x+y=4)] i o n s from t h e v a r i o u s d e u t e r a t e d ^ 2

2

2

3

+


5.

NiBBERiNG

87


3 - p h e n y l p r o p a n a l s . E x p r e s s i n g t h e measured abundances o f t h e s e i o n s from d e u t e r a t e d 3 - p h e n y l p r o p a n a l i n p e r c e n t a g e s o f t h e i r sum as a f u n c t i o n o f time i n t h e g r a p h s , p r e s e n t e d i n F i g u r e 6, t h e f o l l o w i n g i n t e r e s t i n g r e s u l t s are obtained. i ) The hydrogen atom o r i g i n a l l y a t p o s i t i o n 1 i s p r e d o m i n a n t l y t r a n s f e r r e d t o g i v e CerU** ions at short i o n l i f e t i m e s . i i ) T r a n s f e r o f t h e hydrogen atoms from p o s i t i o n 3 t o y i e l d t h e C H * i o n s becomes more i m p o r t a n t a t l o n g e r t i m e s (note t h e i n c r e a s e o f m/e 80, i _ . e. C n^O * , with increasing time). 6

6

+

s

2

i i i ) The hydrogen atoms from p o s i t i o n 2 a r e n o t i n v o l v e d i n the p r o d u c t i o n o f C H ions i n t h e measured time range. + e

6

6

i v ) One o f t h e o r i g i n a l p h e n y l r i n g hydrogen atoms i s i n c r e a s i n g l y r e t a i n e d i n t h e n e u t r a l e l i m i n a t e d CatUO m o i e t y a t l o n g e r t i m e s , a l b e i t to a small extent. The f o l l o w i n g e x p l a n a t i o n i s proposed f o r t h e s e results. I n t h e measured time range o f 1 0 " to 1 0 " * s e c , two s p e c i f i c hydrogen exchange p r o c e s s e s i n the molecular ions o f 3-phenylpropanal occur p r i o r t o o r d u r i n g t h e f o r m a t i o n o f t h e C H * i o n s . The most i m p o r t a n t hydrogen exchange p r o c e s s t a k e s p l a c e between t h e hydrogen atoms from p o s i t i o n s 1 and 3, as d e p i c t e d i n Scheme 2. 1 0 , 3

9

5

+

6

6

Scheme 2

b

a

d

c

c KrÇ 6

etc.


HIGH PERFORMANCE MASS V. too

y. 100

PhCH CH CH0 2

SPECTROMETRY

2

El 78 (a)

50

50

65 0

Ί

1

Ί

115 _iL.

Τ

133

ο too

PhCH CH CHO

1.25

2

2

Fi (b)

OJ75

50 025

67 0

I

I

I

Γ

Ί

2

too

2

133

I high emitter temperature! (c)

3.0

1.0

0

Γ

syo PhCH CH CHO FI

50

615·(134*921

0

—ι )

I 110

100

90

70

130 m/e

120

Journal of the American Chemical Society

Figure 5. EI and FI spectra of 3-phenylpropanal [(b) unheated emitter, (c) heated emitter]. In the FI spectra the left ordinate scale refers to the intensities of ions with m/e < 134; the right ordinate scale to those of ions with m/e > 134. %

.

100

»

1

1

1

1

1

• 1 _

80

60

£3~ 2CH CDO CH

Q-CD CH2CHO

2

2

40

m/e 7$^ 20 — 1

1 I I

I

I

'

m/f80^-«-

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100 m/#78

"

~*~—*—--*^£n/f83

80

60

{3* 2 °2CH0 CH

C

^-CH CH CHO 2

2

AO m/e 82

20

-

m/e 79 • 103

, 1 , , , < 1 10.0 9.5 10.3

,1 10.0

9.5


Figure 6. Intensities of the (M-C H D O fx + y = 4)Υ' ions expressed in percentage of their sum as a function of time for 3-phenylpropanals, deuterated in the 1, 2, and 3 positions ana in the phenyl ring as indicated s

x

y


5.

NiBBERiNG


89

I t s h o u l d be n o t e d t h a t the hydrogen atom a b s t r a c t i o n from p o s i t i o n 3 by t h e r a d i c a l s i t e a t the oxygen atom i n t h e r e a c t i o n a b ( o r the e q u i v a l e n t p r o t o n a b s t r a c t i o n from p o s i t i o n 3 by the l o n e p a i r o f e l e c t r o n s a t the oxygen atom, i f the charge r e s i d e s i n i t i a l l y i n the p h e n y l r i n g ) has been o b s e r v e d p r e v i o u s l y i n the m o l e c u l a r i o n s o f e t h y l 3-phenylpropionate (13). The o t h e r hydrogen exchange p r o c e s s a t s h o r t t i m e s i s b e s t e x p l a i n e d by an exchange between the hydrogen atoms from the p h e n y l r i n g and p o s i t i o n 1, as r a t i o n a l i z e d i n Scheme 3. Scheme 3

A s i m i l a r exchange p r o c e s s , as p r e s e n t e d i n Scheme 3, has been o b s e r v e d i n the m o l e c u l a r i o n s o f 3-phenylp r o p y l bromide ( 1 4 ) . I t f u r t h e r s u p p o r t s the i d e a t h a t the e l i m i n a t i o n o f C2H2O v i a a M c L a f f e r t y r e a r rangement ( v i d e supra) o c c u r s s t e p w i s e , w h i c h i s now w e l l a c c e p t e d (15). I n any e v e n t , the o p e r a t i o n o f b o t h schemes 2 and 3 a c c o u n t s f o r the i n c o r p o r a t i o n of the two hydrogen atoms from p o s i t i o n 3 i n the ΟβΗβ i o n s a t l o n g e r t i m e s ( v i d e s u p r a ) . T h i s FIK s t u d y does n o t show any a c t i v e p a r t i c i p a t i o n o f the hydrogen atoms from p o s i t i o n 2 i n the hydrogen exchange p r o c e s s e s p r i o r t o o r d u r i n g the f o r m a t i o n o f the 0βΗ ' i o n s . I t i s o f i n t e r e s t t o note t h a t such exchanges have been o b s e r v e d f o r m e t a s t a b l e decompo s i t i o n s i n the f i r s t f i e l d f r e e r e g i o n f o r i o n s g e n e r a t e d by E I (11_). T h i s s u g g e s t s t h a t an exchange between the hydrogen atoms a t p o s i t i o n 2 and, f o r example, t h e p h e n y l r i n g can o n l y compete w i t h the exchange p r o c e s s e s g i v e n i n schemes 2 and 3 f o r m o l e c u l a r i o n s o f l o w e r i n t e r n a l energy, i - e . t h o s e decomposing i n the f i r s t f i e l d f r e e r e g i o n . U n f o r t u n a t e l y , t h i s p r o p o s a l c o u l d not be t e s t e d because of t h e absence o f a m e t a s t a b l e peak f o r the e l i m i n a t i o n o f C3Hi»0 from the m o l e c u l a r i o n s o f 3p h e n y l p r o p a n a l under F I c o n d i t i o n s . The mechanism f o r the C H« 0 e l i m i n a t i o n r e q u i r e s f u r t h e r comment. The C H * i o n s , r e s u l t i n g from t h i s +

6

+

6

3

+

6


90


e l i m i n a t i o n , appear t o o r i g i n a t e from the m o l e c u l a r i o n s and from the (M-28) * i o n s , as shown by appro p r i a t e m e t a s t a b l e d e c o m p o s i t i o n s i n the f i r s t f i e l d f r e e r e g i o n under EI c o n d i t i o n s . T h e r e f o r e , i t may be c o n c l u d e d t h a t t h e C H * i o n s are g e n e r a t e d by suc c e s s i v e hydrogen m i g r a t i o n from p o s i t i o n 1 t o the p h e n y l r i n g , c a r b o n monoxide l o s s i n a slow and r a t e d e t e r m i n i n g s t e p and a f a s t e l i m i n a t i o n o f e t h y l e n e , as i n d i c a t e d i n scheme 1. F i n a l l y , some comments s h o u l d be made on c i n n a m y l a l c o h o l , which has been s t u d i e d e x t e n s i v e l y by EI i n c o m b i n a t i o n w i t h D- and C - l a b e l l i n g (16>,17). This compound, an isomer o f 3 - p h e n y l p r o p a n a l , behaves q u i t e d i f f e r e n t l y upon E I , a l t h o u g h i t s m o l e c u l a r i o n s e l i m i n a t e C H 0 and C 3 H 4 O as w e l l (16^,17). FI s p e c t r a o f c i n n a m y l a l c o h o l a t an unheated ancPheated e m i t t e r a r e g i v e n i n F i g u r e 7. C l e a r l y these s p e c t r a are d i f f e r e n t from the FI s p e c t r a o f 3 - p h e n y l p r o p a n a l ( F i g u r e 5 ) . However, peaks a t m/e 92 and m/e 78 a r e o b s e r v e d , e s p e c i a l l y a t h i g h e r emTtter t e m p e r a t u r e , a l t h o u g h t h e i r abundances were too low t o o b t a i n r e l i a b l e FIK results. N e v e r t h e l e s s , i t may be c o n c l u d e d t h a t s m a l l f r a c t i o n o f the m o l e c u l a r i o n s o f c i n n a m y l a l c o h o l may have i s o m e r i s e d t o 3 - p h e n y l p r o p a n a l , p o s s i b l y as follows : +

6

6

1 3

2

2

C H CH=CHCH OH 6

5

2

ί

, +

*

+

C H CH-CH CH=OH*>C H CH CH CHo' * 6

5

2

£

6

5

2

2

h

Note, t h a t i o n g i s i d e n t i c a l t o i o n b i n Scheme 2. I t i s i n t e r e s t i n g Tn t h i s r e s p e c t t h a t tEe r a t i o o f i n t e n s i t i e s o f the m e t a s t a b l e peaks i n the f i r s t f i e l d f r e e r e g i o n f o r t h e l o s s o f C 2 H 2 O and C3Hi*0 upon EI a r e ^150 f o r c i n n a m y l a l c o h o l and ^200 f o r 3 - p h e n y l p r o p a n a l . T h i s would mean t h a t i n c i n n a m y l a l c o h o l t h e e l i m i n a t i o n o f C 3 H 4 O w h i c h r e q u i r e s a h i g h e r a c t i v a t i o n energy, can compete more e f f e c t i v e l y w i t h the l o s s o f C 2 H 2 O than i n 3-phenylpropanal, i f t h e i r decompositions i n t h e same t i m e window are c o n s i d e r e d . Ions h seem, t h e r e f o r e t o be s l i g h t l y more e x c i t e d t h a n t h o s e d i r e c t l y g e n e r a t e d from 3 - p h e n y l p r o p a n a l . T h i s can be e x p l a i n e d , i f the i s o m e r i z a t i o n of cinnamyl a l c o h o l p r o c e e d s t h r o u g h a s l o w s t e p , presumably f -> to e v e n t u a l l y g i v e a r e l a t i v e l y h i g h e r e x c i t e d 3-phenyl p r o p a n a l i o n . The e l i m i n a t i o n s o f C 2 H 2 O and o f CaH^O can t h e n compete more e f f e c t i v e l y w i t h each o t h e r i n the f i r s t f i e l d f r e e r e g i o n . A s i m i l a r phenomenon has been d e s c r i b e d r e c e n t l y . Fast unimolecular d i s -


5.

NiBBERiNG

91


" PhCH=CHCH OH 2

FI

- ία)

I

L

l

l

1

I

1

ι

I

1

- PhCH=CHCH OH 2

_

133

FI (high emitter temperature)

(b) 1I,

" . . . . ι . , 60

70

.ii 80

90

, .1 , 1 100

110

LJ 120

ι

130

I m/e


Figure 7. FI spectra of cinnamylalcohol at an unheated (a) and heated (h) emitter. The left ordinate scale refers to the intensities of ions with m/e < 134; the right ordinate scale to those of ions with m/e > 134.


92


s o c i a t i o n s o f the (M-methyl) i o n from m e t h y l i s o p r o p y l e t h e r i n t h e m e t a s t a b l e r e g i o n are a t t r i b u t e d t o a r a t e d e t e r m i n i n g i s o m e r i z a t i o n o f t h i s i o n t o the (M-methyl) s t r u c t u r e from d i e t h y l e t h e r (18J). Conclusions. The r e s u l t s , o b t a i n e d from FIK f o r the systems d e s c r i b e d , show t h a t t h i s method i s e x t r e m e l y v a l u a b l e f o r o b t a i n i n g a deeper i n s i g h t i n t r u e gas-phase i o n d e c o m p o s i t i o n s and i n the l o n g - s t a n d i n g p r o b l e m o f s c r a m b l i n g o f hydrogen atoms. T h i s l a t t e r phenomenon appears t o c o n s i s t o f a s e r i e s o f s p e c i f i c hydrogen exchange p r o c e s s e s , as n o t e d p r e v i o u s l y by o t h e r a u t h o r s (2a). Experimental. E x p e r i m e n t a l d e t a i l s w i l l be d e s c r i b e d f u l l y e l s e where (8,10) and the most i m p o r t a n t c o n d i t i o n s are o n l y given here. The FIK e x p e r i m e n t s were p e r f o r m e d on a V a r i a n MAT 711 double f o c u s s i n g mass s p e c t r o m e t e r (Mattauch-Herzog geometry) equipped w i t h a c o m m e r c i a l l y combined EI/FI/FD s o u r c e . Data a c q u i s i t i o n and d a t a p r o c e s s i n g were a c h i e v e d w i t h the V a r i a n S p e c t r o System 100 on l i n e and s p e c i a l computer programmes, w r i t t e n f o r t h e FIK e x p e r i m e n t s , were used ( 1 9 ) . F u r t h e r exp e r i m e n t a l c o n d i t i o n s f o r 2-phenoxyetEyl c h l o r i d e were: I n l e t system: r e f e r e n c e i n l e t system a t 130°C Ion s o u r c e t e m p e r a t u r e : 100°C Emitter Current: S mA Analyzer pressure: 6x10" Torr Source p r e s s u r e : 5 x 1 0 " T o r r Mass r e s o l u t i o n (10% v a l l e y d i f i n i t i o n ) : 1200 Energy r e s o l u t i o n ( a t an a c c e l e r a t i n g v o l t a g e o f 8453 Volts): 1.4* V o l t a g e a t f o c u s s i n g e l e c t r o d e : ±6 kV. The e x p e r i m e n t a l c o n d i t i o n s employed f o r t h e 3p h e n y l p r o p a n a l s t u d i e s were: I n l e t system: c o o l e d d i r e c t i n s e r t i o n probe at -5 t o -10°C Ion s o u r c e t e m p e r a t u r e : 98°C E m i t t e r c u r r e n t : 0 mA Analyzer pressure: 6x10" Torr Source p r e s s u r e : 7x10" Torr Mass r e s o l u t i o n (101 v a l l e y d e f i n i t i o n ) : 500 Energy r e s o l u t i o n ( a t an a c c e l e r a t i n g v o l t a g e o f 8400 Volts): II V o l t a g e a t f o c u s s i n g e l e c t r o d e : ±6 kV. 9

6

9

6


5.

NiBBERiNG


93

For b o t h s t u d i e s a c t i v a t e d t u n g s t e n w i r e e m i t t e r s o f 10 \xm w i t h an average n e e d l e l e n g t h o f 10 ym were used and p o s i t i o n e d 1.45 mm from t h e grounded s l o t t e d c a t h o d e . Acknowledgement. The a u t h o r i s e x t r e m e l y g r a t e f u l t o Drs. J. v a n der G r e e f and Mr. F. A. P i n k s e , who, w i t h g r e a t enthu s i a s m , have d e v e l o p e d t h e F I K method i n h i s l a b o r a t o r y . He f u r t h e r acknowledges t h e c o n t r i b u t i o n s o f Drs. C. B. T h e i s s l i n g and Dr. P. W o l k o f f C h r i s t e n s e n i n t h e c h e m i c a l f i e l d and t h e c o n t r i b u t i o n o f Dr. C. W. F. K o r t who d e v e l o p e d t h e s o f t w a r e f o r FIK. F i n a l l y , he w o u l d l i k e t o thank P r o f . H. D. Beckey and h i s group o f the U n i v e r s i t y o f Bonn f o r s t i m u l a t i n g d i s c u s s i o n s and the g i f t o f some w e l l - a c t i v a t e d e m i t t e r s .

Abstract. Field Ionization Kinetics has provided a clear i n sight into the positional interchange of the phenoxy group and halogen atom in the molecular ions of 2-phen oxyethyl chloride and into the various hydrogen ex change processes i n the molecular ions of 3-phenyl propanal prior to or during fragmentation. This information could not be obtained from previous elec tron impact studies. Literature Cited. 1.

2.

a. Beckey, H. D . , Field Ionisation Mass Spectro etry, (1971), Akademie-Verlag, Berlin and Pergamon Press, Oxford, b. Robertson, A. J. B . , i n "Mass Spectrometry", Maccoll, A. (Ed.), Int. Rev. Sci., Physical Chemistry Ser. One, V o l . 5, Butterworths, London and University Park Press, Baltimore, (1972), p. 103. c) Derrick, P. J., i n "Mass Spectrometry", Maccoll, A. (Ed.), Int. Rev. Sci., Physical Chemistry Ser. Two, V o l . 5, Butterworths, London and Boston, Mass., (1975), p. 1. a. Derrick, P. J. and Burlingame, A. L., Acc. Chem. Res., (1974) 7, 328. b. McMaster, Β. N., Johnstone, R. A. W. (Ed.), i n Mass Spectrometry (Specialist Periodical Reports), The Chemical Society, London, (1975), V o l . 3, p. 33. c. Burlingame, A. L., Kimble, B. J., and Derrick, P. J., Anal. Chem., (1976) 48, 368R.


94

HIGH PERFORMANCE MASS

SPECTROMETRY

3.

a. Falick, Α. Μ., Derrick, P. J., and Burlingame, A. L., Int. J . Mass Spectrom. Ion Phys., (1973) 12, 101. b. Beckey, H. D. Hey, Η., Levsen, K. and Tenschert, G., Int. J . Mass Spectrom. Ion Phys., (1969) 2, 101. 4. Schulten, H.-R. and Beckey, H. D., Org. Mass Spectrom., (1972), 6, 885. 5. Falick, Α. Μ., Int. J . Mass Spectrom. Ion Phys., (1974) 14, 313. 6. Derrick, P. J., Falick, A. M. and Burlingame, A. L., J . Am. Chem. Soc., (1972) 94, 6794. 7. Borchers, F . , Levsen, K. and Beckey, H. D., Int. J. Mass Spectrom. Ion Phys., (1976) 21, 125. 8. Greef, J . van der, Theissling, C. B . , and Nibbering, Ν. Μ. Μ., 7th International Mass Spectrometry Conference, Florence, I t a l y , 1976, Paper No. 168; will appear in Advan. Mass Spectrom. 7 Daly, N. (Ed.). 9. Theissling, C. Β . , Nibbering, Ν. Μ. Μ., and Boer, Th.J. de, Advan. Mass Spectrom., (1971) 5, 642. 10. Christensen, P. Wolkoff, Greef, J . van der, and Nibbering, N. M. M. J. Am. Chem. Soc. (submitted) 11. Venema, Α., Nibbering, Ν. M. M . , and Boer, Th.J. de, Org. Mass Spectrom., (1970) 3, 583. 12. Venema, A. and Nibbering, Ν. Μ. Μ., Org. Mass Spectrom., (1974) 9, 628. 13. Resink, J . J., Venema, Α., and Nibbering, Ν. M. M. Org. Mass Spectrom., (1974) 9, 1055. 14. Nibbering, Ν. M. M. and Boer, Th.J. de, Tetrahedron, (1968) 24, 1427. 15. Kingston, D. G. I., Bursey, J. T., and Bursey, M. M . , Chem. Rev., (1974) 74, 215. 16. Schwarz, H . , Köppel, C. and Bohlmann, F . , Org. Mass Spectrom., (1973) 7, 881. 17. Köppel, C. and Schwarz, Η., Org. Mass Spectrom., (1976) 11, 101. 18. Hvistendahl, G. and Williams, D. H . , J. Am. Chem. Soc., (1975) 97, 3097. 19. Greef, J . van der, Pinkse, F. Α., Kort, C. W. F. and Nibbering, Ν. Μ. Μ., Int. J . Mass Spectrom. Ion Phys., i n press. RECEIVED December 30, 1977


Field Ionization Kinetic Studies of Gas-Phase Ion Chemistry

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