Investigations of Selective Reagent Ions in Chemical Ionization Mass

9 Investigations of Selective Reagent Ions in Chemical Ionization Mass Spectrometry K. R. JENNINGS

Downloaded by CORNELL UNIV on May 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0070.ch009

Department of Molecular Sciences, University of Warwick, Coventry, United Kingdom

The great majority of reagent ions used in chemical ionization mass spectrometry (1) are either charge exchange reagents such as A r and N or ions which react as gas phase acids or bases such as H and C H or H and CH O . For each group of reagent ions, the amount of fragmentation depends primarily on the exothermicity of the electron or proton transfer reaction. There are, however, other types of reagent ions which undergo different reactions with specific classes of compounds and which, therefore, may give more detailed information about the presence or location of particular functional groups. within the molecule. This paper gives a brief account of work recently carried out in our laboratories in which the reactions of the vinyl methyl ether molecular ion, CHOCH=CH and the atomic oxygen negative ion, O' with a number of substrates were investigated. +

+

2

+

3

-

+

5

-

3

+

3

2

Reactions of the Vinyl Methyl Ether Molecular Ion. Previous work using ion cyclotron resonance (ICR) spectrometry on the reactions of olefinic molecular ions with various fluoro-olefins (2,3) showed that in each system, a new olefinic ion was Formed. Deuterium-labelling experiments suggested that these reactions occur by the formation of a substituted cyclobutane ion as an intermediate which fragments with the retention of the structural identity of the substituted methylene groups. A similar reaction was observed (4) in the vinyl methyl ether system in which the two secondary ions of m/e = 88 and 84 correspond to the elimination of ethylene and methanol, respectively. In each system, therefore, an olefinic ion attacks a neutral olefin molecule to yield an ion of even mass by the elimination of an olefin, and in the ©0-8412-0422-5/78/47-070-179$05.00/0

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

180

HIGH

PERFORMANCE

SPECTROMETRY

v i n y l m e t h y l e t h e r system a f u r t h e r i o n o f even mass i s formed by the l o s s o f methanol from the r e a c t i o n complex. The r e a c t i o n s o f i o n s o f t h i s t y p e w i t h o t h e r u n s a t u r a t e d s p e c i e s were t h e r e f o r e i n v e s t i g a t e d w i t h a v i e w t o u s i n g t h i s t y p e o f r e a c t i o n to l o c a t e the p o s i t i o n o f d o u b l e bonds i n m o l e c u l e s . No u s e f u l r e s u l t s were o b t a i n e d u s i n g C H C F * and C Fi»* as r e agent i o n s s i n c e t h e y r e a c t e d m a i n l y by charge t r a n s fer. But i n a number o f s y s t e m s , r e a c t i o n s o f the CH 0CHCH * i o n p r o v i d e d i n f o r m a t i o n w h i c h e n a b l e s one t o l o c a t e the p o s i t i o n o f a d o u b l e bond ( 5 ) . In s t u d i e s i n t h e h i g h p r e s s u r e s o u r c e o f the d o u b l e f o c u s s i n g MS50 mass s p e c t r o m e t e r , i t was f o u n d t h a t a 9:1 m i x t u r e o f C 0 and CH 0CHCH gave a good y i e l d o f CH 0CHCH * and s u b s t a n t i a l l y r e d u c e d the r e a c t i o n o f t h i s i o n w i t h i t s parent molecule. The r e a c t i o n s o f i n t e r e s t a r e o f the g e n e r a l t y p e Η Η γ CH 0CH=CH t + X-CH=CH-Y 2

3


MASS

2

2

2

2

3

2

3

2

X

3

2

H XCH=CH

+

2

0CH

3

Η YCH=CHOCH Î 3

i n w h i c h , i n g e n e r a l , the s i m p l e r o f the two p o s s i b l e o l e f i n s i s e l i m i n a t e d as a n e u t r a l and a s u b s t i t u e n t c o n t a i n i n g a h e t e r o a t o m i s r e t a i n e d a l o n g w i t h the m e t h o x y l group as p a r t o f the o l e f i n i c i o n . Hence, f o r example, a l l t e r m i n a l o l e f i n s r e a c t by the f o r m a t i o n o f a complex f r o m w h i c h e t h y l e n e i s e l i m i n a t e d . The e x p e c t e d even-mass s e c o n d a r y i o n s were obs e r v e d i n the mass s p e c t r a g i v e n by the r e a c t i o n s o f CH 0CHCH * i o n s w i t h a number o f s i m p l e h y d r o c a r b o n olefins. In each c a s e , a f u r t h e r even-mass i o n was o b s e r v e d due t o the l o s s o f CH 0H from the r e a c t i o n complex, t o g e t h e r w i t h odd mass i o n s w h i c h a r i s e from the l o s s o f atoms o r r a d i c a l s from the complex. These r e a c t i o n s r e a d i l y a l l o w one t o d i s t i n g u i s h between i s o m e r i c s p e c i e s such as 1-butene and 2-butene. In F i g u r e I t h e v e r y d i f f e r e n t s p e c t r a g i v e n by 1-octene and t r a n s - 4 - o c t e n e a r e i l l u s t r a t e d . B o t h CH =CHCH CH= CH and CeH CH CH=CH r e a c t e d as t e r m i n a l m o n o - o l e f i n s and gave a c h a r a c t e r i s t i c peak due t o the l o s s o f C Hi» from the r e a c t i o n complex. S i m i l a r r e a c t i o n s were a l s o o b s e r v e d w i t h a l l y l f o r m a t e and w i t h CH =CH-CH C00H. When o l e i c a c i d was r u n as an "unknown , the p o s i t i o n o f i t s d o u b l e bond was c o r r e c t l y l o c a t e d by means o f the r e a c t i o n 3

2

3

2

2

5

2

2

2

2

2

2

11


9.

Selective Reagent Ions

JENNINGS

CH (CH ) CH=CH(CH )7COOH 3

2

7

+

2

181 CH 0CH=CH Î 3

2

+

CH (CH )7CH-CH-(CH )7C00H t 3

2

2

CH CH-0CH 2

CH (CH ) CH=CH 3

2

7

2

+

3

CH OCH=CH-(CH ) COOH ί 3

2

7


m/e = 200 The i m p o r t a n c e o f t h e r e a c t i o n i n w h i c h methanol i s e l i m i n a t e d i n c r e a s e s as t h e s i z e o f the m o l e c u l e i n c r e a s e s and i s t h e o n l y r e a c t i o n y i e l d i n g an even mass i o n w i t h u n s a t u r a t e d c y c l i c compounds and w i t h n o n - c o n j u g a t e d d i e n e s such as 1,4 and 2 , 6 - o c t a d i e n e s . U n s a t u r a t e d n i t r i l e s a r e s i m p l y p r o t o n a t e d . The con j u g a t e d s p e c i e s 1,3-butadiene undergoes a D i e l s - A l d e r r e a c t i o n i n w h i c h methanol i s e l i m i n a t e d . I t i s p l a n n e d t o e x t e n d t h i s s t u d y t o more c o m p l i c a t e d u n s a t u r a t e d systems. I n v i e w o f t h e q u i t e gen e r a l n a t u r e o f t h e r e a c t i o n , i t i s hoped t h a t o t h e r o l e f i n i c i o n s w i l l be i d e n t i f i e d as u s e f u l r e a g e n t ions. I t s h o u l d t h e n be p o s s i b l e t o l o c a t e the po s i t i o n s o f c a r b o n - c a r b o n double bonds i n a wide v a r i e t y of compounds w i t h o u t p r i o r d e r i v a t i z a t i o n and i n f a v o r a b l e c a s e s , as i n t h e 1 - f l u o r o p r o p e n e s ( 6 ) , i t may be p o s s i b l e t o d i s t i n g u i s h c i s - and t r a n s - i s o m e r s . Reactions o f the

A t o m i c Oxygen N e g a t i v e

Ion,

In r e c e n t y e a r s n e g a t i v e c h e m i c a l i o n i z a t i o n mass s p e c t r o m e t r y (7) has become i n c r e a s i n g l y i m p o r t a n t . I n c e r t a i n a p p l i c a t i o n s , t h e t y p e o f spectrum produced and t h e s e n s i t i v i t y o f the method make i t s u p e r i o r t o p o s i t i v e c h e m i c a l i o n i z a t i o n , as w i l l be d e s c r i b e d e l s e w h e r e i n t h i s volume. Most n e g a t i v e r e a g e n t i o n s r e a c t p r i m a r i l y as s t r o n g gas phase bases b u t t h e r a d i c a l a n i o n , 0 · , has been found t o have some u n i q u e r e a c t i o n s which y i e l d i n t e r e s t i n g s t r u c t u r a l i n f o r m a t i o n i n a number o f c a s e s . Most o f t h e e a r l y s t u d i e s (8_,9) were c a r r i e d o u t at l o w p r e s s u r e s i n a ICR s p e c t r o m e t e r . N 0 was found to be a c o n v e n i e n t s o u r c e o f 0· i o n s a l t h o u g h t h e f a c t t h a t t h e y a r e formed w i t h 0.38 eV o f t r a n s l a t i o n a l energy makes N 0 l e s s s a t i s f a c t o r y as a s o u r c e o f 0· i o n s f o r d e t e r m i n a t i o n o f r a t e c o n s t a n t s . I n t h e com b i n e d E I / C I s o u r c e o f the MS50, f u r t h e r r e a c t i o n s i n v o l v i n g N 0 may o c c u r , b u t t h e s e were s u p p r e s s e d by u s i n g a 9:1 m i x t u r e o f N and N 0 a t a t o t a l p r e s s u r e 2

2

2

2

2


182

HIGH

PERFORMANCE

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SPECTROMETRY

o f 0.5 T o r r . T h i s g a v e a h i g h y i e l d o f 0· i o n s w i t h o n l y a t r a c e o f N 0 ~ i o n s . The D a l y d e t e c t o r o f the MS50 was r e p l a c e d by an e l e c t r o n m u l t i p l i e r f o r n e g a t i v e i o n work, but m o d i f i c a t i o n s c u r r e n t l y b e i n g c a r r i e d out to the D a l y d e t e c t o r w i l l e n a b l e i t to o p e r a t e i n the n e g a t i v e i o n mode. E a r l y work (8) on the r e a c t i o n of 0· w i t h CH CD had shown t h a t o n l y CH C> and C D C i o n s are formed, i n d i c a t i n g t h a t the r e a c t i o n o c c u r s by H abstraction from a s i n g l e c a r b o n atom. T h i s was l a t e r shown (9) to bç a r e a c t i o n c h a r a c t e r i s t i c o f many 1 - o l e f i n s a l t K o u g h H a b s t r a c t i o n from a l k y l groups p r e d o m i n a t e u n l e s s the t e r m i n a l =CH group i s a c t i v a t e d by the p r e s e n c e o f an e l e c t r o n w i t h d r a w i n g group i n the m o l e c u l e , e.g., F o r CF . Other c l a s s e s o f compounds were found to undergo the H a b s t r a c t i o n r e a c t i o n . In the case o f a l i p h a t i c n i t r i l e s , the f a c t t h a t the r e a c t i o n o c c u r s w i t h nC 3 H 7 C N but not w i t h (CH ) CHCN p r o v i d e s s t r o n g e v i d e n c e that H a b s t r a c t i o n t a k e s p l a c e a t the α-C atom._ In an e x t e n s i v e s t u d y o f the r e a c t i o n s of 0· i o n s w i t h c a r b o n y l compounds ( 1 0 ) , the major r e a c t i o n s were f o u n d t o be o f the general~~type 2

2

:

2

2

+

2


2

3

+

2

3

2

+

2

(M-l)"

+

•OH

(1)

(M-2)·

+

H0

(2)

->·

(M+0-R)"

+

R-

(3)

->

(M-2-R)"

+

R- +

(M-l)·

+

OH"

2

H0

(4)

2

(5)

F u r t h e r r e a c t i o n o f OH" l e a d s to the p r o d u c t i o n o f (Μ Ι ) " so t h a t the i n t e n s i t y r a t i o (M-l) /(Μ-2)· r i s e s as the p r e s s u r e r i s e s . In the h i g h p r e s s u r e s o u r c e , the t h r e e i s o m e r i c pentanones g i v e q u i t e d i f f e r e n t s p e c t r a as shown i n F i g u r e 2, each o f w h i c h can be r a t i o n a l i z e d i n terms o f the above r e a c t i o n s . The v a r y i n g y i e l d s o f (Μ-2)· i o n s a r e a t t r i b u t e d to the r e l a t i v e ease o f a b s t r a c t i o n of H from the c a r b o n atoms a d j a c e n t to the c a r b o n y l group s i n c e d e u t e r i u m - l a b e l l i n g e x p e r i ments i n d i c a t e d t h a t H a b s t r a c t i o n o c c u r r e d o n l y from t h e s e p o s i t i o n s . I n the case o f ( C H ) C H C 0 C H ( C H ) 2 , no (Μ-2)· i o n s are formed, c o n s i s t e n t w i t h t h i s i n t e r p r e t a t i o n ^ Hence i n compounds o f t h i s type^, the r e a c t i o n s o f 0· i o n s can be used to d e t e c t the p r e s e n c e o f a c t i v a t e d CH groups. W i t h a r o m a t i c compounds (11) , r e a c t i o n s analogous to ( l ) - ( 3 ) o c c u r but a f u r t h e r r e a c t i o n c h a r a c t e r i s t i c +

2

+

2

3

2

2


3

9.


JENNINGS

183

o f a r o m a t i c compounds i s : M

+

0·

+

(M-H+0)-

+

Η·

(6)

I n c e r t a i n c a s e s , l o w i n t e n s i t y peaks were o b s e r v e d due to t h e occurrence o f the r e a c t i o n Μ

+

0·

(M-H+O-HX)"

+

Η·

+

HX (7)

Benzene g i v e s a v e r y s i m p l e s p e c t r u m c o n s i s t i n g o f a p p r o x i m a t e l y e q u a l i n t e n s i t y peaks a t (Μ-2)· and (MH+0) o n l y . W i t h 1 , 3 , 5 - b e n z e n e - d 3 , the major low mass peak i s (Μ-3)· s u g g e s t i n g 1,2 e l i m i n a t i o n o f H a l though 1,4 e l i m i n a t i o n cannot be r u l e d o u t . The two h i g h e r mass peaks were o f v e r y s i m i l a r i n t e n s i t i e s , i n d i c a t i n g a n e g l i g i b l e H/D i s o t o p e e f f e c t . Pyridine y i e l d s a s p e c t r u m c l o s e l y r e s e m b l i n g t h a t o f benzene, but t h e r e i s i n a d d i t i o n a low i n t e n s i t y (M-l)"" peak. S i n c e t h e hydrogen atoms a r e no l o n g e r e q u i v a l e n t , i t is of interest to investigate positional s e l e c t i v i t y i n the displacement r e a c t i o n . I n t h e case o f 4-deuterop y r i d i n e , t h e peak a r i s i n g from d e u t e r i u m d i s p l a c e m e n t i s o n l y s l i g h t l y l e s s i n t e n s e t h a n t h a t due t o hydrogen d i s p l a c e m e n t a l t h o u g h s t a t i s t i c a l l y , an i n t e n s i t y r a t i o o f 1:4 would be p r e d i c t e d . T h i s i n d i c a t e s a marked p r e f e r e n c e f o r t h e d i s p l a c e m e n t o f the hydrogen atom i n the 4 - p o s i t i o n . The r e s u l t s o b t a i n e d w i t h 1,3,5benzene-d.3 show t h a t t h i s cannot be a t t r i b u t e d t o an isotope effect. However, i t was w i t h t h e a l k y l a r o m a t i c compounds t h a t t h e most i n t e r e s t i n g s p e c t r a were o b t a i n e d . Ob s e r v a t i o n s on s p e c t r a g i v e n by d e u t e r o - t o l u e n e s showed t h a t t h e two major peaks a r i s e from t h e a b s t r a c t i o n o f H from the m e t h y l group t o g i v e t h e ( M - l ) " i o n and t h e d i s p l a c e m e n t o f a r i n g hydrogen atom t o form the (MH+0)~ i o n s . The t h r e e i s o m e r i c x y l e n e s g i v e t h e spec t r a shown i n F i g u r e 3 and i t i s seen t h a t the metaisomer g i v e s a spectrum q u i t e d i f f e r e n t from t h o s e o f the o t h e r two i s o m e r s . The prominence o f t h e (Μ-2)· i o n appears t o a r i s e from the p r e s e n c e o f t h e two m e t h y l groups meta t o each o t h e r s i n c e when one o f t h e m e t h y l groups i s f u l l y d e u t e r a t e d , t h e (Μ-3)· i s formed by l o s s o f HOD, i n d i c a t i n g t h a t one hydrogen atom i s removed from each m e t h y l group i n f o r m i n g t h e (Μ-2)· ion. This i s also consistent with the observation that the (Μ-2)· i o n i s t h e base peak i n t h e s p e c t r u m g i v e n by m e s i t y l e n e . In t h e s p e c t r a o f a l l t h e a l k y l benzenes d i s c u s s e d above, t h e r a t i o I ( M - H + 0 ) " / I ( M - l ) " l i e s i n t h e range 0.07-0.19. However, i n t h e case o f t - b u t y l b e n z e n e ,


2

+


HIGH

PERFORMANCE

MASS

SPECTROMETRY

ΛΛΛ/ (b) / S / V ^

[MW-OCK,]*

[MM'-OCKJ]* 155

142138

ΊΪ2

ÏOÔ~~ "Ve

155

p

138

112

100

Organic Mass Spectrometry


Figure 1. Spectra given by 1-octene and trans-4-octene on reaction with CHsOCHCH,*

(a)

(b)

es, «Ι ι 15 Figure 2. Ο ·" NCI spectra of three isomeric pentanones, (a) C H COC H (b) CH CO-n-C H (c) CH COCH(CH ) 2

2

5>

s

s

7>

5

3

s 2

(a)

ι. 104

.21

151 I

105 (b) 1 .

121 1

Τ

105

Μ Figure 3. O" NCI spectra of the three isomeric xylenes, (a) o-xylene, (b) m-xylene, (c) p-xylene

•

1.

121 1


151

1

9.

185


JENNINGS


+

t h i s r a t i o r i s e s t o about 5, s u g g e s t i n g t h a t t h e H a b s t r a c t i o n t o form t h e ( M - l ) ~ i o n i s l a r g e l y from t h e c a r b o n atom α t o t h e a r o m a t i c r i n g . The a b i l i t y t o d i s t i n g u i s h p o s i t i o n a l isomers i l l u s t r a t e d by t h e s p e c t r a o f t h e x y l e n e s i s a l s o i l l u s t r a t e d by t h e s p e c t r a g i v e n by t h e t h r e e m e t h y l p y r i d i n e s . The r e l a t i v e i n t e n s i t i e s o f t h e major peaks a r e g i v e n i n t h e T a b l e I . I t i s seen t h a t t h e base peak o f t h e 2 - m e t h y l p y r i d i n e i s t h e (M-H+O)" i o n where as when t h e 4 - p o s i t i o n i s b l o c k e d , as i n 4-methylp y r i d i n e , t h i s i s a peak o f low i n t e n s i t y , c o n s i s t e n t w i t h t h e o b s e r v a t i o n s made on 4 - d e u t e r o p y r i d i n e . This was n o t always t h e c a s e , however, and t h e s p e c t r a g i v e n by t h e t h r e e f l u o r o t o l u e n e s on r e a c t i o n w i t h 0· a r e very s i m i l a r .

Table I .

R e l a t i v e I n t e n s i t i e s o f t h e M a j o r Peaks i n the 0" NCI S p e c t r a o f M e t h y l p y r i d i n e s

Ion m/e

2-Methylpyridine

(M-H+O)" 108

3-Methylpyridine

4-Methylpyridine

100

77

26

(M-3H+0)" 106

13

40

15

(M-CH^+0)" 94^

13

13

9

(M-H)~ 92

68

100

100

95

89

57

10

36

9

(M-2H) 91

7

(M-3H)" 90


186

high performance mass spectrometry

Conclusion. This brief account of the reactions of CH OCHCH * and 0 · indicates the possibilities of the use of ionmolecule reactions in probing structural features of certain classes of compounds. Undoubtedly, other ions w i l l be found which w i l l undergo different types of reactions and one can look forward to the establishing of a series of selective reagent ions which w i l l allow one greatly to extend the type of structural informa tion obtainable. 3

2


Experimental. The work was carried out in part using a Varian V5900 ion cyclotron resonance (ICR) spectrometer for preliminary work at low pressure and in part in an MS50 double focussing mass spectrometer (ΑΕΙ Scientific Apparatus Ltd) equipped with a dual purpose electronimpact/chemical ionization (EI/CI) source. The source pressure was measured by means of an MKS "Baratron" (Model 77H-10) the output of which fed the control unit of a Granville-Phillips automatic pressure regulator (series 216) which controlled the flow rate of the major component in the high pressure source. The instrument was modified to allow i t to operate in the negative ion mode. Literature Cited. 1. 2. 3. 4. 5. 6. 7. 8.

Field, F . H . , MTP Int. Rev. of S i c . , Series One, Physical Chemistry, Vol. 5 (Ed. A. Maccoll), Ch. 5, Butterworths, London, 1972. Ferrer-Correia, A.J.V. and Jennings, K.R., Int. J. Mass Spectrom Ion Phys., (1973), 11, 111. Drewery, C.J., Goode, G. C., and Jennings, K.R., Int. J. Mass Spectrom Ion Phys., (1976), 22, 211. Drewery, C.J. and Jennings, K.R., Int. J. Mass Spectrom. Ion Phys., (1976), 19, 287. Ferrer-Correia, A . J . V . , Sen Sharma, D.K. and Jennings, K.R., Org. Mass Spectrom., (1976), 11, 867. Drewery, C.J., Goode, G . C . , and Jennings, K.R., Int. J. Mass Spectrom Ion Phys., (1976), 20, 403. Jennings, K.R., Mass Spectrometry, Vol. 4, Specialist Periodical Report, Chemical Society, London, Ed. R.A.W. Johnston, 1977, p. 209. Goode, G.C. and Jennings, K.R., Adv. in Mass Spectrom., (1974), 6, 797.


9.

JENNINGS


Dawson, J.H.J., and Jennings, K.R., J. Chem. Soc. Faraday Trans. II, (1976), 72, 700. 10. Harrison, A.G. and Jennings, K.R., J. Chem. Soc. Faraday Trans. I, (1976), 77, 1601. 11. Bruins, A . P . , Ferer-Correia, A . J . V . , Harrison, A . G . , Jennings, K.R. and Mitchum, R.K., Adv. in Mass Spectrom. 7, (in press). Received December 30, 1977


9.


Investigations of Selective Reagent Ions in Chemical Ionization Mass

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