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Ion—molecule and radical reactions in condensed layers studied by low temperature field ionization mass spectrometry. Part I. Methanol
International Journal o f Mass Spectrometry and Ion Physics, 29 (1979) 125--135 125 © Elsevier Scientific Publishing Company, _hm~terdam -- Printed in The Netherlands
ION--MOLECULE AND RADICAL REACTIONS IN CONDENSED LAYERS STUDIED BY LOW TEMPERATURE FIELD IONIZATION M A S S S P E C T R O M E T R Y . P A R T I. M E T H A N O L
H.H. GIERLICH and F.W. ROLLGEN
Institute o f Physical Chemistry, University o f Bonn, Wegelerstr. 12, 5300 Bonn (W. Germany) (First received 2 March 1978; in revised form 5 June 1978)
ABSTRACT The ion--molecule and radical reactions induced by field ionization (FI) of methanol in a condensed surface layer are investigated at low emitter temperatures (LT) using isot o p e labelling and benzene as radical scavenger. No influence of the emitter on the chem-
ical reactions was detected. The results agree with those obtained by radiation chemistry suggesting that LT-FI can be an interesting tool for the investigation of reactions in liquids, since stable and intermediate reaction products axe supplied directly to mass spectrometric analysis.
INTRODUCTION F i e l d i o n i z a t i o n ( F I ) o f o r g a n i c m o l e c u l e s is u s u a l l y c a r r i e d o u t a t r o o m t e m p e r a t u r e , o r a t h i g h e r t e m p e r a t u r e if t h e e f f e c t o f t e m p e r a t u r e o n t h e rate of ionization, the surface chemistry or the unimolecular decomposition b e h a v i o u r o f i o n s is o f i n t e r e s t . L o w e r i n g t h e e m i t t e r t e m p e r a t u r e a f f e c t s t h e f o r m a t i o n o f p r o t o n a t e d m o l e c u l e s a n d c l u s t e r i o n s d u e t o an i o n i z a t i o n o f m o l e c u l e s in m u l t i - a d s o r p t i o n o r c o n d e n s e d l a y e r s [ 1 - - 4 ] . H o w e v e r , so f a r no detailed investigations of the chemistry of organic molecules induced by F I in c o n d e n s e d l a y e r s h a v e b e e n m a d e . T h e p r e s e n t w o r k w a s c a r r i e d o u t t o e x p l o r e t h e c h e m i c a l r e a c t i o n s foll o w i n g F I p r o c e s s e s in a l i q u i d s u r f a c e l a y e r . T o t h i s e n d a s t a t i o n a r y s u r f a c e layer extending over the minimum ionization distance must be formed. For volatile substances such conditions were found at low emitter temperatures a n d l o w field s t r e n g t h s . Mass a n a l y s i s o f t h e i o n e m i s s i o n f r o m t h e s u r f a c e l a y e r a l l o w e d t h e m e c h a n i s m o f i o n - - m o l e c u l e a n d also r a d i c a l r e a c t i o n s t a k i n g p l a c e in a c o n d e n s e d l a y e r t o be e l u c i d a t e d . T h i s is d e m o n s t r a t e d f o r methanol. No chemical influence of the anode surface on the reaction products was d e t e c t e d .
126 A p p l i c a t i o n o f t h e m e t h o d t o t h e s t u d y o f i o n - - m o l e c u l e a n d radical react i o n s in a c o n d e n s e d phase is o f particular i n t e r e s t since t h e r e a c t i o n p r o d u c t s are supplied directly t o mass s p e c t r o m e t r i c analysis. Using c o n v e n t i o n a l m e t h o d s mass analysis requires e v a p o r a t i o n o f t h e s o l u t i o n t h u s o n l y giving i n f o r m a t i o n o n stable r e a c t i o n p r o d u c t s . EXPERIMENTAL F o r r e c o r d i n g l ow t e m p e r a t u r e (LT)-FI mass s p ectra a 60 ° single focusing m a g n e t i c i n s t r u m e n t a n d a q u a d r u p o l e mass s p e c t r o m e t e r [ 5 , 6 ] , b o t h e q u i p p e d w i t h a c o o l a b l e FI s o u r c e w e r e used. T h e q u a d r u p o l e was also used f o r e n e r g y analysis o f field ions a p p l y i n g a m e t h o d d e s c r i b e d in ref. 6. T h e FI source o f t h e q u a d r u p o l e instmzment is s h o w n s c h e m a t i c a l l y in Fig. 1. T h e cooling o f t h e e m i t t e r is a c h i e v e d b y a c o p p e r b a n d w h i c h c o n n e c t s t h e e m i t t e r s u p p o r t e r t o a liquid n i t r o g e n trap. T h e flexible c o p p e r band allowe d a m e c h a n i c a l a d j u s t m e n t o f t h e p o s i t i o n o f t h e a n o d e to be m a d e by m i c r o m a n i p u l a t o r s since t h e a n o d e was c o u p l e d to t h e r o d o f a conventional FD s o u r c e [ 7 ] . A n y e m i t t e r t e m p e r a t u r e b e t w e e n r o o m t e m p e r a t u r e (RT) a n d a b o u t 145 K c o u l d be selected b y s i m u l t a n e o u s a p p r o p r i a t e heating o f t h e c o p p e r b a n d . T he t e m p e r a t u r e m e a s u r e m e n t s w e r e carried o u t by a c o p p e r c o n s t a n t a n t h e r m o c o u p l e placed o n t h e e m i t t e r h o l d e r . T h e t e m p e r a t u r e o f t h e e m i t t e r surface m a y be a f e w degrees h i g h e r because o f a t e m p e r a t u r e d i f f e r e n c e between the copper block and the emitter. T he gas was supplied t o t h e e m i t t e r b y a m o l e c u l a r b e a m . T h e pressure in
Cold finger
Copper bond
Gas inlet
Resistance heating colt
L, \
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Emitter manipulation /
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Emitter Counter electrode
Ion optics
Fig. 1. Schematic drawing o f the quadrupole mass analyzer combined w i t h a coolable
FI
source.
127
t h e b e a m was e s t i m a t e d t o be a b o u t 0.1 Pa. F o r r e c o r d i n g L T mass spectra t h e a n o d e - - c a t h o d e p o t e n t i a l d i f f e r e n c e d e t e r m i n i n g t h e field strength was set at vanishing d e t e c t o r ion c u r r e n t as m e a s u r e d at RT. Pt tips a n d c o n v e n t i o n a l a c t i va t e d 1 0 / a m W wire e m i t t e r s [7] w e r e used. T h e l a t t e r w e r e e m p l o y e d f o r r e c o r d i n g t h e mass s p ectra s h o w n in Figs. 2, 3, 5 a nd 6. T h e d e g r e e o f d e u t e r a t i o n o f C6D6 and CDsOH (Fa. Merck} was a b o u t 99.5%. RESULTS AND DISCUSSION
1. Field ionization in condensed surface layers A t high field strength, i.e. high i o n i z a t i o n p r o b a b i l i t y , a surface lay er can only e x t e n d u p t o t he critical i o n i z a t i o n d i s t a n c e fo r FI, w h i c h is a b o u t a f e w Angst roms [ 8 ] . FI processes t a k i n g place inside a c o n d e n s e d surface l a y e r r e q u i r e on t h e o n e h a n d l o w i o n i z a t i o n probabilities, i.e. lo w field strengths, and on t h e o t h e r a sufficiently high gas pressure or l o w e m i t t e r t e m p e r a t u r e t o f o r m a t h i c k c o n d e n s e d lay er on th e a n o d e surface. Using a gas supply by a m o l e c u l a r b e a m o n t o t h e field a n o d e such c o n d i t i o n s were f o u n d for a c e t o n e , m e t h a n o l a n d o t h e r alcohols at gas pressures in t h e beam o f a b o u t 0.1 Pa a n d a n o d e t e m p e r a t u r e s n e a r t h e m e l t i n g p o i n t o f t h e substances [9]. U n d e r t he s e c o n d i t i o n s t h e l a y e r th ick n es s is n o t static b u t m a y c h a n g e with t i m e a c c o r d i n g to t h e locally a n d t e m p o r a l l y v ary in g i o n i z a t i o n c o n d i t i o n s , i.e. space charge f l u c t u a t i o n in th e l a y e r has to be a s s u m e d [ 1 0 ] . E v i d e n c e f o r F I in c o n d e n s e d layers is o b t a i n e d f r o m t h e e n e r g y distribut i o n o f t h e d e s o r b e d ions using a Pt tip e m i t t e r . U n d e r n o r m a l ( R T ) c o n d i t i o n s t h e i n t e n s i t y m a x i m u m o f t h e i on e n e r g y d i s t r i b u t i o n is d e t e r m i n e d by t h e m i n i m u m or critical i o n i z a t i o n d i s t a n c e xc. In t h e case o f F I in a c o n d e n s e d l a y e r t h e ions start at t h e phase b o u n d a r y c o n d e n s e d l a y e r ] v a c u u m w h i c h is at a l o w e r p o t e n t i a l t h a n xc, t h u s leading t o a shifting an d b r o a d e n i n g o f ion signals to l o w e r masses, as observed in a single focusing mass s p e c t r o m e t e r d u r i n g e m i t t e r cooling. T h e charge t r a n s p o r t f r o m t h e p o i n t o f origin inside t h e layer t o t h e phase b o u n d a r y can be a s s u m e d t o t a k e place e i t h e r by e l e c t r o n t r a n s f e r processes a n d / o r by ion m i g r a t i o n as f o u n d in e l e c t r o l y t i c solutions. T h e LT mass spectra o f a c e t o n e (m.p. 178 K) an d m e t h a n o l (m.p. 175 K) are s h o w n in Figs. 2 a n d 3 respectively. F o r c o m p a r i s o n t h e R T s p ectra are d ispl a ye d as well. T h e e m i t t e r t e m p e r a t u r e s are c h o s e n to achieve a maxim u m i o n emission, p r e d o m i n a n t l y o f cluster ions. Because o f a limited mass range o f t h e s p e c t r o m e t e r , a c e t o n e cluster ions ~ m / e 3 5 0 c o u l d n o t be d e t e c t e d . A t still l o w e r a n o d e t e m p e r a t u r e s a d r o p in emission was observed, a c c o m p a n i e d b y an increase o f t h e i n t e n s i t y f l u c t u a t i o n s . In spite o f e m i t t e r t e m p e r a t u r e s a few degrees b e l o w t h e m e l t i n g p o i n t o f t h e substances t h e c o n d e n s e d surface l a y e r c a n n o t be c o m p l e t e l y solid b u t m a y be in a liquid o r liquid-like state. T h e i m pi ngi ng m o l e c u l e s supplied f r o m t h e gas p h as e
128 (a)
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Fig. 2. FI ma~s spectra o f a c e t o n e obtained ( a ) at R T and (b) at an emitter temperature o f 1 6 5 K. Emitter: activated 1 0 / ~ n W wire.
cause a heating of the layer according to the higher gas phase temperature m d the transfer of polarization energy to the surface, thus producing a temperature gradient across the layer. This assumption is supported by the ob-
129 100 -
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Fig. 3. FI mass spectra o f m e t h a n o l o b t a i n e d (a) at R T and (b) at an e m i t t e r temperature o f 1 6 3 K. Emitter: activated 1 0 p~n W wlre.
130 served ion c h e m i s t r y discussed b e l o w . T h e observed cessation o f emission at l o w e r t e m p e r a t u r e can be ascribed t o t h e g r o w t h o f a solid l a y e r n o t hind e r e d b y i o n i z a t i o n processes. With t h e alcohols t h e f o r m a t i o n o f (M + H) ÷ ions is always energetically m o r e favourable t h a n t h e f o r m a t i o n o f M-* ions [ 4 ] . H e n c e t h e M.+ peaks in t h e a l c o h o l spectra give e v i d e n c e f o r surface areas o f high i o n i z a t i o n probability in w h i c h t h e r a t e o f FI o f i n c o m i n g m o l e c u l e s c o m p e t e s w i t h t h a t o f their a b s o r p t i o n i n t o t h e c o n d e n s e d l ay er via t h e f o r m a t i o n o f h y d r o g e n bonds. E n e r g y analysis o f M.* ions f o r m e d at a c o o l e d Pt tip s h o w s t h a t th es e ions d o n o t start f r o m t h e i r critical i o n i z a t i o n d i s t a n c e b u t f r o m p o i n t s furt h e r f r o m t h e e m i t t e r surface. This is m o s t p r o b a b l y d u e to FI i n t o e l e c t r o n a c c e p t o r states f o r m e d by ions in t h e layer. In c o n t r a s t to t h e R T mass s p e c t r u m o f a c e t o n e (Fig. 2a) t h e LT spect r u m (Fig. 2b) exhibits n o (M + H) ÷ ion. F u r t h e r m o r e s t r o n g (nM).* ion sigr~!~ are observed w h e r e a s ( n M + H) ÷ ions w i t h n ~> 2 are a b s e n t or o f w e a k intensity. T h e (M + H) ÷ i n t e n s i t y was f o u n d t o d e c r e a s e d u r i n g e m i t t e r cooling a n d was n o m o r e d e t e c t a b l e u n d e r L T c o n d i t i o n s . I t has b e e n suggested t h a t t h e p r o t o n a t i o n o f a c e t o n e t a k e s place b o t h w i t h a n d w i t h o u t surface i n t e r a c t i o n [ 1 1 ] . Since a p r o t o n a t i o n r e a c t i o n is e n e r g e t i c a l l y a n d kinetically f a v o u r e d by a surface i n t e r a c t i o n [4] t h e loss o f an (M + H ) ÷ ion signal in t h e LT mass s p e c t r u m allows t h e c o n c l u s i o n s to be d r a w n : 1, t h a t p r o t o n a t i o n o f a c e t o n e is n o t possible w i t h o u t surface i n t e r a c t i o n an d 2, t h a t u n d e r c o n d i t i o n s o f LT-FI n o c h e m i c a l i n f l u e n c e o f t h e e m i t t e r surface o n t h e ion c h e m i s t r y exists. Obviously at R T (M + H) ÷ a c e t o n e ions are formed by the reaction 2(CH3)2CO ~
*O(C2Hs) + (CI-I3)2COIT
(1)
w h e r e * d e n o t e s a surface b o n d . T h e p r o t o n a t i o n o f a c e t o n e in a c o n d e n s e d l a y e r m a y be h i n d e r e d by an activation e n e r g y , since t h e gas phase r e a c t i o n M ÷ + M -~ (M -- H)" + (M + H) ÷ is a p p r o x i m a t e l y t h e r m o n e u t r a l [ 1 2 ] , T h e ab senc e o f a c h e m i c a l surface i n f l u e n c e o n t h e r e a c t i o n p r o d u c t s at LT is very i m p o r t a n t . This c o n c l u s i o n is also s u p p o r t e d by th e observation t h a t in c o n t r a s t t o t h e R T mass spectra t he LT s p ectra o f t h e alcohols are indep e n d e n t o f t h e e m i t t e r material (Pt, C).
2. R e a c t i o n s o1" m e t h a n o l T h e F I - i n d u c e d surface c h e m i s t r y o f m e t h a n o l at R T has been t r e a t e d in several p u b l i c a t i o n s [ 1 , 4 , 1 3 - - 1 5 ] b u t is so far n o t y e t fully u n d e r s t o o d . Exp e r i m e n t s w i t h i s ot ope labelling [14] a n d e n e r g y analysis o f d i f f e r e n t ioni z a t i o n p r o d u c t s [15] p o i n t t o a s t r o n g surface i n t e r a c t i o n in t h e i o n - f o r m i n g reactions. H o w e v e r , as s h o w n above, such a surface i n f l u e n c e o n t h e mass s p e c t r u m can be n e g l e c t e d at l o w e m i t t e r t e m p e r a t u r e s a n d f o r l o w field strengths. T h e LT mass s p e c t r u m differs f r o m t h e R T s p e c t r u m in t h e rela-
131 tively high intensities o f p r o t o n a t e d species a n d d e h y d r o g e n a t i o n p r o d u c t s in c o m p a r i s o n with t h e M.* i o n i n t e n s i t y . In t h e p r i m a r y process o f FI o f m e t h a n o l in a c o n d e n s e d layer, (M + H ) ÷ ions should be f o r m e d according t o t h e r e a c t i o n 2 CH3OH ~
CH30" + CH3OI-I~2
(2)
Using CD3OH a n d CH~OD it was f o u n d t h a t at RT and high field strengths the m e t h y l h y d r o g e n s are involved in t h e p r o t o n a t i o n reactions to an appreciable e x t e n t (see Fig. 4). H o w e v e r at low field strengths and even at low emitter t e m p e r a t u r e s t h e h y d r o g e n in (M + H) ÷ is mainly t h a t o f t h e hyd r o x y l group, as is to be e x p e c t e d from t h e association o f m e t h a n o l molecules via t h e f o r m a t i o n o f h y d r o g e n bonds. In t h e search for t h e free radical left f r o m reaction (2) b e n z e n e was used as radical scavenger and a d d e d to m e t h a n o l in t h e gas inlet system. The result is shown in Fig. 5. T h e t w o peaks at role 108 and role 109 p r e s e n t in t h e LT s p e c t r u m are n o t observed at RT. These peaks can be formally ascribed to t h e c o m p o s i t i o n s (C6H6 + C H 3 O H - H) ÷ (role 109) and (C6H6 + CH3OH -- 2H) ÷ (m/e 108) respectively. T h e c o r r e s p o n d i n g LT spectmJm o f a m i x t u r e o f C6D6 and CH3OH (Fig. 6a) gives evidence for t h e losses o f benzene h y d r o g e n in role 113 and m e t h a n o l h y d r o g e n in role 115. Finally t h e mass s p e c t r u m o f a m i x t u r e o f C6D6 and CD3OH (Fig. 6b) allows t h e structure o f t h e above ions to be d e t e r m i n e d since role 117 c o r r e s p o n d s t o a loss
[CD3OH" H] "
[I O 30HI "
D3o.Y
L
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o)
Fig. 4. Molecular ion group o f CD3OH obtained by FI at E T o n a Pt tip using a quadrupole mass analyzer. A n o d e - - c a t h o d e potential difference: (a) 7 kV; (b) 10 kV.
132 I'--'1 "r"
100
80. o
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t
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o
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o 109
IOE 20
,
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,
.
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80
m/e Fig. 5. F I mass s p e c t r u m o f a m i x t u r e o f m e t h a n o l i n t e n s i t y o f C 5 H 5 .+ i5 i n d i c a t e d b y a n a r r o w .
ii
_
1 ~0
120
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~--and benzene at 163 H. The higher
o f D and role 115 o f 2D atoms. The h y d r o x y l h y d r o g e n s are retained in b o t h ion structures: H role 10B
H H C H2OH----J+
rNe 109
H
F r o m t h e i s o t o p e distribution in Fig. 6 it follows t h a t (C6D6 + OCD3) ÷ and (Ce~DsOCD3)+ ions also c o n t r i b u t e t o t h e ion signals at role 118 and role 116 respectively. The ions o f mass 109 obviously result from ionization o f intermediates. The additions o f m e t h o x y a n d CH2OH'-radicals t o b e n z e n e have b e e n postulated in t h e radiation c h e m i s t r y o f m e t h a n o l a n d b e n z e n e m i x t u r e s b u t so far have n o t b e e n directly observed [ 1 6 , 1 7 ] . Since reaction (2) leads t o free CH30" radicals whereas CH2OH" radicals are a t t a c h e d t o b e n z e n e t h e follow-
133 -to
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Fig, 6. FI mass spectra at 1 6 3 K: (a) mixhtre o£ CHsOH and C6D6; (b) mixture of CDzOH and C6D6. The higher intensifies of C6D6; are indicated by arrows.
134
ing bimolecular reaction succeeding reaction (2) m u s t be p o s t u l a t e d CHsO" + CH3OH -~ CHsOH + CH2OH" (3) This reaction is well a c c e p t e d in the radiation c h e m i s t r y o f m e t h a n o l [ 1 7 ] . Benzyl alcohol (mass 108) m a y be f o r m e d prior t o FI via t h e reaction C6I-I6CH2OH" + R" -* C6HsCH~OH + RH
(4)
in w h i c h R" is a CH30" o r CH2OH" radical. F o r t h e f o r m a t i o n o f anisole (C~IsOCHs) a similar r e a c t i o n can be f o r m u l a t e d . Benzyl alcohol and anisole are k n o w n t o be f o r m e d in t h e radiation chemistry o f m e t h a n o l - - b e n z e n e mixtures [16]. F o r m a l d e h y d e m a y be p r o d u c e d by reactions o f m e t h o x y radicals CH30" + CH30" -* CH20 + CH3OH (5) This reaction has been suggested t o a c c o u n t in part f o r t h e CH20.* peak o f t h e RT s p e c t ~ i m o f m e t h a n o l [14]. F o r m a l d e h y d e can also be f o r m e d by reactions o f CH2OH" with m e t h o x y radicals or CH2OH', c o m p e t i n g however, with t h e f o r m a t i o n o f semi-acetal and glucol (see below). In addition, t h e field reaction CH2OH" + CH3OH ~
CH20 + CHsOI-I~2
(6)
m a y c o n t r i b u t e to t h e f o r m a t i o n o f neutral f o r m a l d e h y ( l e as well, By successive FI, CH20.* a n d (CH20 + nM + H) ÷ ions axe p r o d u c e d with n ~ 0. Molecular ions o f f o r m a l d e h y d e axe clearly i d e n t i f i e d in t h e LT spectra o f m e t h a n o l . However, t h e latter ions (e.g. for n = 1) c a n n o t be distinguished f r o m t h e p r o d u c t o f t h e reaction (competi.ng with reaction (6)) CH~OH" + CH3OH --~ H O C H ~ H C H ~ (role 63)
(7)
which is e x p e c t e d to o c c u r as well. A d i m e r i z a t i o n o f CH2OH" radicals d e p e n d s on the concentrRtion a n d rate o f diffusion o f these radicals in t h e surface layer and their lifetime with respect t o FI processes. The absence o f a (CD2OH)~ p e a k (role 66) and a (CH2OH)~ p e a k (role 6 2 ) i n t h e spectra o f CD3OH and CH3OH respectively leads t o t h e c o n c l u s i o n t h a t glucol is n o t f o r m e d to a large e x t e n t . However, a small c o n t r i b u t i o n o f p r o t o n a t e d glucol to t h e (2M - - H) ÷ peak (role 63) in t h e s p e c t w m o f Fig. 5 and t o t h e (2M -- HD) + peak (role 67) in Fig. 6b is possible. The FI-induced c h e m i s t r y o f m e t h a n o l in a c o n d e n s e d surface layer is less c o m p l i c a t e d t h a n t h a t o f long chain alcohols. This will be s h o w n in a later publication.
3. Comparison o f LT-FI with Radiation Chemistry The chemical effects o f an ionizing radiation are m u c h m o r e c o m p l i c a t e d t h a n t h e chemistry following FI processes. In LT-FI o n l y i o n - - m o l e c u l e reactions and reactions o f radicals p r o d u c e d by i o n - - m o l e c u l e reaction take
135
place. M o r e o v e r t h e ions are q u i c k l y r e m o v e d f r o m t h e r e a c t i o n z o n e . However, in irradiation o f liquids t h e c h e m i s t r y is n o t o n l y d u e t o primarily f o r m e d ions b u t results also f r o m reactions o f e x c i t e d neutrals, neutr~liT, e d ions, t h e i r f r a g m e n t s a n d in a d d i t i o n f r o m r e a c t i o n s o f negative ions [ 17]. A f u r t h e r d i f f e r e n c e lies in t h e fact t h a t t h e p r o b a b i l i t y o f FI d e p e n d s exponentially o n t h e i o n i z a t i o n p o t e n t i a l ; t h u s u n d e r c o n d i t i o n s o f LT-FI o n l y m o l e c u l e s w i t h 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 will be p r o d u c e d . However, t h e e x a m p l e o f m e t h a n o l shows t h a t LT-FI gives valuable inform a r i o n o n i o n - - m o l e c u l e r e a c t i o n and radical r e a c t i o n s in liquids. By successive FI o f l a y e r m o l e c u l e s left f r o m i o n - - m o l e c u l e r e a c t i o n s it is also possible t o d e t e c t n e u t r a l r e a c t i o n p r o d u c t s a n d even i n t e r m e d i a t e s . T h e analysis of the field-induced reactions of methanol demonstrates that reaction mechanisms difficult t o e l u c i d a t e in r a d i a t i o n c h e m i s t r y are r a t h e r easily derived f r o m LT-FI mass spectra w i t h t h e aid o f i s o t o p e labelling a n d radical scavengers. H o w e v e r , as s h o w n w i t h glucol, difficulties m a y arise w i t h t h e d e t e c t i o n o f p r o d u c t s o f radical r e c o m b i n a t i o n reactions. Bug such r e a c t i o n p r o d u c t s are less difficult t o f i n d by analysis o f liquids after irradiation. ACKNOWLEDGEMENTS
T h e a u t h o r s are i n d e b t e d t o Prof. Dr. H.D. B e c k e y a n d U. G i e s s m a n n for s t i m u l a t i n g discussions. T h e y gratefully a c k n o w l e d g e t h e supporg f r o m t h e W i s s e n s c h a f t s m i n i s t e r i u m des L a n d e s N o r d r h e i n - W e s t f a l e n a n d t h e F o n d s d e r D e u t s c h e n C h e m i s c h e n Industrie. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
M.G. In abram and R. G o m e r , Z. Naturforsch. A, 1O ( 1 9 5 5 ) 863. H.D. Beckey, Z. N a t u r f o r s c h . A, 14 ( 1 9 5 9 ) 712; 15 ( 1 9 6 0 ) 822. W.A. S e h m l d t , O. F r a n k and J.H. Block, Surface Sci., 44 ( 1 9 7 4 ) 185. F.W. Ri~llgen and H.D. B eckey, Z. N a t ur f o rsch. A, 29 ( 1 9 7 4 ) 230. H.H. Gierlich, H.J. H ei nen and H~D. B e c k e y , Biomed. Mass S p e c t r o m . , 2 ( 1 9 7 5 ) 31. H.J. Heinen, H.H. Gierlich and H.D. B e c k e y , J. Phys. E, 8 ( 1 9 7 5 ) 877. H.D. B eck ey and H.-R. S c h u l t e n , Angew. Chem., Int. Ed. Engl., 14 ( 1 9 7 5 ) 403. H.D. B eck ey , Principles o f Field I o n i z a t i o n and Field D esorpt i on Mass S p e c t r o m e t r y , P er g amo n Press, O x f o r d , 1977. A detailed discussion o f F I o f molecules in t he t e m p e r a t u r e / p r e s s u r e range w here a phase t r ~ n ~ t i o n takes place at t he field a n o d e surface is in preparation, R. G o m e r , A c c o u n t s Chem. Res., 5 ( 1 9 7 2 ) 41. F.W. R~llgen and H.D. B e c k e y , Sur f ace Sci., 23 ( 1 9 7 0 ) 69. F.W. R~illgen and H.D. Beckey, Sur f ace Sci., 27 ( 1 9 7 1 ) 312. H.D. B eck ey and P. Schulze, Z. N a t ur f or s c h. A, 21 ( 1 9 6 6 ) 214. I.V. G o l d e n f e l d and I.Z. Korostyshevskij, T eor. Eksp. Khim., 4 ( 1 9 6 8 ) 218. H.J. Heinen, F.W. RSllgen and H.D. B e c k e y , Z. N a t u r f o r s c h . A, 29 ( 1 9 7 4 ) 773; H.J. Hein en an d F.W. R61lgen, Proe. 3rd Int. Conf. Solid Surf., Vienna, 1977. A. Ekst~om and J.L. G r a n e t t , J. Phys. Chem., 70 ( 1 9 6 6 ) 324. R.A. H o l r o y d , in P. Ausloos (Ed.), F u n d a m e n t a l Processes in R a d i a t i o n Chemistry, Interscience, New Y or k, 1968, and references cited therein.
ION--MOLECULE AND RADICAL REACTIONS IN CONDENSED LAYERS STUDIED BY LOW TEMPERATURE FIELD IONIZATION M A S S S P E C T R O M E T R Y . P A R T I. M E T H A N O L
H.H. GIERLICH and F.W. ROLLGEN
Institute o f Physical Chemistry, University o f Bonn, Wegelerstr. 12, 5300 Bonn (W. Germany) (First received 2 March 1978; in revised form 5 June 1978)
ABSTRACT The ion--molecule and radical reactions induced by field ionization (FI) of methanol in a condensed surface layer are investigated at low emitter temperatures (LT) using isot o p e labelling and benzene as radical scavenger. No influence of the emitter on the chem-
ical reactions was detected. The results agree with those obtained by radiation chemistry suggesting that LT-FI can be an interesting tool for the investigation of reactions in liquids, since stable and intermediate reaction products axe supplied directly to mass spectrometric analysis.
INTRODUCTION F i e l d i o n i z a t i o n ( F I ) o f o r g a n i c m o l e c u l e s is u s u a l l y c a r r i e d o u t a t r o o m t e m p e r a t u r e , o r a t h i g h e r t e m p e r a t u r e if t h e e f f e c t o f t e m p e r a t u r e o n t h e rate of ionization, the surface chemistry or the unimolecular decomposition b e h a v i o u r o f i o n s is o f i n t e r e s t . L o w e r i n g t h e e m i t t e r t e m p e r a t u r e a f f e c t s t h e f o r m a t i o n o f p r o t o n a t e d m o l e c u l e s a n d c l u s t e r i o n s d u e t o an i o n i z a t i o n o f m o l e c u l e s in m u l t i - a d s o r p t i o n o r c o n d e n s e d l a y e r s [ 1 - - 4 ] . H o w e v e r , so f a r no detailed investigations of the chemistry of organic molecules induced by F I in c o n d e n s e d l a y e r s h a v e b e e n m a d e . T h e p r e s e n t w o r k w a s c a r r i e d o u t t o e x p l o r e t h e c h e m i c a l r e a c t i o n s foll o w i n g F I p r o c e s s e s in a l i q u i d s u r f a c e l a y e r . T o t h i s e n d a s t a t i o n a r y s u r f a c e layer extending over the minimum ionization distance must be formed. For volatile substances such conditions were found at low emitter temperatures a n d l o w field s t r e n g t h s . Mass a n a l y s i s o f t h e i o n e m i s s i o n f r o m t h e s u r f a c e l a y e r a l l o w e d t h e m e c h a n i s m o f i o n - - m o l e c u l e a n d also r a d i c a l r e a c t i o n s t a k i n g p l a c e in a c o n d e n s e d l a y e r t o be e l u c i d a t e d . T h i s is d e m o n s t r a t e d f o r methanol. No chemical influence of the anode surface on the reaction products was d e t e c t e d .
126 A p p l i c a t i o n o f t h e m e t h o d t o t h e s t u d y o f i o n - - m o l e c u l e a n d radical react i o n s in a c o n d e n s e d phase is o f particular i n t e r e s t since t h e r e a c t i o n p r o d u c t s are supplied directly t o mass s p e c t r o m e t r i c analysis. Using c o n v e n t i o n a l m e t h o d s mass analysis requires e v a p o r a t i o n o f t h e s o l u t i o n t h u s o n l y giving i n f o r m a t i o n o n stable r e a c t i o n p r o d u c t s . EXPERIMENTAL F o r r e c o r d i n g l ow t e m p e r a t u r e (LT)-FI mass s p ectra a 60 ° single focusing m a g n e t i c i n s t r u m e n t a n d a q u a d r u p o l e mass s p e c t r o m e t e r [ 5 , 6 ] , b o t h e q u i p p e d w i t h a c o o l a b l e FI s o u r c e w e r e used. T h e q u a d r u p o l e was also used f o r e n e r g y analysis o f field ions a p p l y i n g a m e t h o d d e s c r i b e d in ref. 6. T h e FI source o f t h e q u a d r u p o l e instmzment is s h o w n s c h e m a t i c a l l y in Fig. 1. T h e cooling o f t h e e m i t t e r is a c h i e v e d b y a c o p p e r b a n d w h i c h c o n n e c t s t h e e m i t t e r s u p p o r t e r t o a liquid n i t r o g e n trap. T h e flexible c o p p e r band allowe d a m e c h a n i c a l a d j u s t m e n t o f t h e p o s i t i o n o f t h e a n o d e to be m a d e by m i c r o m a n i p u l a t o r s since t h e a n o d e was c o u p l e d to t h e r o d o f a conventional FD s o u r c e [ 7 ] . A n y e m i t t e r t e m p e r a t u r e b e t w e e n r o o m t e m p e r a t u r e (RT) a n d a b o u t 145 K c o u l d be selected b y s i m u l t a n e o u s a p p r o p r i a t e heating o f t h e c o p p e r b a n d . T he t e m p e r a t u r e m e a s u r e m e n t s w e r e carried o u t by a c o p p e r c o n s t a n t a n t h e r m o c o u p l e placed o n t h e e m i t t e r h o l d e r . T h e t e m p e r a t u r e o f t h e e m i t t e r surface m a y be a f e w degrees h i g h e r because o f a t e m p e r a t u r e d i f f e r e n c e between the copper block and the emitter. T he gas was supplied t o t h e e m i t t e r b y a m o l e c u l a r b e a m . T h e pressure in
Cold finger
Copper bond
Gas inlet
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..
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\
( ~
/
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Fig. 1. Schematic drawing o f the quadrupole mass analyzer combined w i t h a coolable
FI
source.
127
t h e b e a m was e s t i m a t e d t o be a b o u t 0.1 Pa. F o r r e c o r d i n g L T mass spectra t h e a n o d e - - c a t h o d e p o t e n t i a l d i f f e r e n c e d e t e r m i n i n g t h e field strength was set at vanishing d e t e c t o r ion c u r r e n t as m e a s u r e d at RT. Pt tips a n d c o n v e n t i o n a l a c t i va t e d 1 0 / a m W wire e m i t t e r s [7] w e r e used. T h e l a t t e r w e r e e m p l o y e d f o r r e c o r d i n g t h e mass s p ectra s h o w n in Figs. 2, 3, 5 a nd 6. T h e d e g r e e o f d e u t e r a t i o n o f C6D6 and CDsOH (Fa. Merck} was a b o u t 99.5%. RESULTS AND DISCUSSION
1. Field ionization in condensed surface layers A t high field strength, i.e. high i o n i z a t i o n p r o b a b i l i t y , a surface lay er can only e x t e n d u p t o t he critical i o n i z a t i o n d i s t a n c e fo r FI, w h i c h is a b o u t a f e w Angst roms [ 8 ] . FI processes t a k i n g place inside a c o n d e n s e d surface l a y e r r e q u i r e on t h e o n e h a n d l o w i o n i z a t i o n probabilities, i.e. lo w field strengths, and on t h e o t h e r a sufficiently high gas pressure or l o w e m i t t e r t e m p e r a t u r e t o f o r m a t h i c k c o n d e n s e d lay er on th e a n o d e surface. Using a gas supply by a m o l e c u l a r b e a m o n t o t h e field a n o d e such c o n d i t i o n s were f o u n d for a c e t o n e , m e t h a n o l a n d o t h e r alcohols at gas pressures in t h e beam o f a b o u t 0.1 Pa a n d a n o d e t e m p e r a t u r e s n e a r t h e m e l t i n g p o i n t o f t h e substances [9]. U n d e r t he s e c o n d i t i o n s t h e l a y e r th ick n es s is n o t static b u t m a y c h a n g e with t i m e a c c o r d i n g to t h e locally a n d t e m p o r a l l y v ary in g i o n i z a t i o n c o n d i t i o n s , i.e. space charge f l u c t u a t i o n in th e l a y e r has to be a s s u m e d [ 1 0 ] . E v i d e n c e f o r F I in c o n d e n s e d layers is o b t a i n e d f r o m t h e e n e r g y distribut i o n o f t h e d e s o r b e d ions using a Pt tip e m i t t e r . U n d e r n o r m a l ( R T ) c o n d i t i o n s t h e i n t e n s i t y m a x i m u m o f t h e i on e n e r g y d i s t r i b u t i o n is d e t e r m i n e d by t h e m i n i m u m or critical i o n i z a t i o n d i s t a n c e xc. In t h e case o f F I in a c o n d e n s e d l a y e r t h e ions start at t h e phase b o u n d a r y c o n d e n s e d l a y e r ] v a c u u m w h i c h is at a l o w e r p o t e n t i a l t h a n xc, t h u s leading t o a shifting an d b r o a d e n i n g o f ion signals to l o w e r masses, as observed in a single focusing mass s p e c t r o m e t e r d u r i n g e m i t t e r cooling. T h e charge t r a n s p o r t f r o m t h e p o i n t o f origin inside t h e layer t o t h e phase b o u n d a r y can be a s s u m e d t o t a k e place e i t h e r by e l e c t r o n t r a n s f e r processes a n d / o r by ion m i g r a t i o n as f o u n d in e l e c t r o l y t i c solutions. T h e LT mass spectra o f a c e t o n e (m.p. 178 K) an d m e t h a n o l (m.p. 175 K) are s h o w n in Figs. 2 a n d 3 respectively. F o r c o m p a r i s o n t h e R T s p ectra are d ispl a ye d as well. T h e e m i t t e r t e m p e r a t u r e s are c h o s e n to achieve a maxim u m i o n emission, p r e d o m i n a n t l y o f cluster ions. Because o f a limited mass range o f t h e s p e c t r o m e t e r , a c e t o n e cluster ions ~ m / e 3 5 0 c o u l d n o t be d e t e c t e d . A t still l o w e r a n o d e t e m p e r a t u r e s a d r o p in emission was observed, a c c o m p a n i e d b y an increase o f t h e i n t e n s i t y f l u c t u a t i o n s . In spite o f e m i t t e r t e m p e r a t u r e s a few degrees b e l o w t h e m e l t i n g p o i n t o f t h e substances t h e c o n d e n s e d surface l a y e r c a n n o t be c o m p l e t e l y solid b u t m a y be in a liquid o r liquid-like state. T h e i m pi ngi ng m o l e c u l e s supplied f r o m t h e gas p h as e
128 (a)
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Fig. 2. FI ma~s spectra o f a c e t o n e obtained ( a ) at R T and (b) at an emitter temperature o f 1 6 5 K. Emitter: activated 1 0 / ~ n W wire.
cause a heating of the layer according to the higher gas phase temperature m d the transfer of polarization energy to the surface, thus producing a temperature gradient across the layer. This assumption is supported by the ob-
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-~
Fig. 3. FI mass spectra o f m e t h a n o l o b t a i n e d (a) at R T and (b) at an e m i t t e r temperature o f 1 6 3 K. Emitter: activated 1 0 p~n W wlre.
130 served ion c h e m i s t r y discussed b e l o w . T h e observed cessation o f emission at l o w e r t e m p e r a t u r e can be ascribed t o t h e g r o w t h o f a solid l a y e r n o t hind e r e d b y i o n i z a t i o n processes. With t h e alcohols t h e f o r m a t i o n o f (M + H) ÷ ions is always energetically m o r e favourable t h a n t h e f o r m a t i o n o f M-* ions [ 4 ] . H e n c e t h e M.+ peaks in t h e a l c o h o l spectra give e v i d e n c e f o r surface areas o f high i o n i z a t i o n probability in w h i c h t h e r a t e o f FI o f i n c o m i n g m o l e c u l e s c o m p e t e s w i t h t h a t o f their a b s o r p t i o n i n t o t h e c o n d e n s e d l ay er via t h e f o r m a t i o n o f h y d r o g e n bonds. E n e r g y analysis o f M.* ions f o r m e d at a c o o l e d Pt tip s h o w s t h a t th es e ions d o n o t start f r o m t h e i r critical i o n i z a t i o n d i s t a n c e b u t f r o m p o i n t s furt h e r f r o m t h e e m i t t e r surface. This is m o s t p r o b a b l y d u e to FI i n t o e l e c t r o n a c c e p t o r states f o r m e d by ions in t h e layer. In c o n t r a s t to t h e R T mass s p e c t r u m o f a c e t o n e (Fig. 2a) t h e LT spect r u m (Fig. 2b) exhibits n o (M + H) ÷ ion. F u r t h e r m o r e s t r o n g (nM).* ion sigr~!~ are observed w h e r e a s ( n M + H) ÷ ions w i t h n ~> 2 are a b s e n t or o f w e a k intensity. T h e (M + H) ÷ i n t e n s i t y was f o u n d t o d e c r e a s e d u r i n g e m i t t e r cooling a n d was n o m o r e d e t e c t a b l e u n d e r L T c o n d i t i o n s . I t has b e e n suggested t h a t t h e p r o t o n a t i o n o f a c e t o n e t a k e s place b o t h w i t h a n d w i t h o u t surface i n t e r a c t i o n [ 1 1 ] . Since a p r o t o n a t i o n r e a c t i o n is e n e r g e t i c a l l y a n d kinetically f a v o u r e d by a surface i n t e r a c t i o n [4] t h e loss o f an (M + H ) ÷ ion signal in t h e LT mass s p e c t r u m allows t h e c o n c l u s i o n s to be d r a w n : 1, t h a t p r o t o n a t i o n o f a c e t o n e is n o t possible w i t h o u t surface i n t e r a c t i o n an d 2, t h a t u n d e r c o n d i t i o n s o f LT-FI n o c h e m i c a l i n f l u e n c e o f t h e e m i t t e r surface o n t h e ion c h e m i s t r y exists. Obviously at R T (M + H) ÷ a c e t o n e ions are formed by the reaction 2(CH3)2CO ~
*O(C2Hs) + (CI-I3)2COIT
(1)
w h e r e * d e n o t e s a surface b o n d . T h e p r o t o n a t i o n o f a c e t o n e in a c o n d e n s e d l a y e r m a y be h i n d e r e d by an activation e n e r g y , since t h e gas phase r e a c t i o n M ÷ + M -~ (M -- H)" + (M + H) ÷ is a p p r o x i m a t e l y t h e r m o n e u t r a l [ 1 2 ] , T h e ab senc e o f a c h e m i c a l surface i n f l u e n c e o n t h e r e a c t i o n p r o d u c t s at LT is very i m p o r t a n t . This c o n c l u s i o n is also s u p p o r t e d by th e observation t h a t in c o n t r a s t t o t h e R T mass spectra t he LT s p ectra o f t h e alcohols are indep e n d e n t o f t h e e m i t t e r material (Pt, C).
2. R e a c t i o n s o1" m e t h a n o l T h e F I - i n d u c e d surface c h e m i s t r y o f m e t h a n o l at R T has been t r e a t e d in several p u b l i c a t i o n s [ 1 , 4 , 1 3 - - 1 5 ] b u t is so far n o t y e t fully u n d e r s t o o d . Exp e r i m e n t s w i t h i s ot ope labelling [14] a n d e n e r g y analysis o f d i f f e r e n t ioni z a t i o n p r o d u c t s [15] p o i n t t o a s t r o n g surface i n t e r a c t i o n in t h e i o n - f o r m i n g reactions. H o w e v e r , as s h o w n above, such a surface i n f l u e n c e o n t h e mass s p e c t r u m can be n e g l e c t e d at l o w e m i t t e r t e m p e r a t u r e s a n d f o r l o w field strengths. T h e LT mass s p e c t r u m differs f r o m t h e R T s p e c t r u m in t h e rela-
131 tively high intensities o f p r o t o n a t e d species a n d d e h y d r o g e n a t i o n p r o d u c t s in c o m p a r i s o n with t h e M.* i o n i n t e n s i t y . In t h e p r i m a r y process o f FI o f m e t h a n o l in a c o n d e n s e d layer, (M + H ) ÷ ions should be f o r m e d according t o t h e r e a c t i o n 2 CH3OH ~
CH30" + CH3OI-I~2
(2)
Using CD3OH a n d CH~OD it was f o u n d t h a t at RT and high field strengths the m e t h y l h y d r o g e n s are involved in t h e p r o t o n a t i o n reactions to an appreciable e x t e n t (see Fig. 4). H o w e v e r at low field strengths and even at low emitter t e m p e r a t u r e s t h e h y d r o g e n in (M + H) ÷ is mainly t h a t o f t h e hyd r o x y l group, as is to be e x p e c t e d from t h e association o f m e t h a n o l molecules via t h e f o r m a t i o n o f h y d r o g e n bonds. In t h e search for t h e free radical left f r o m reaction (2) b e n z e n e was used as radical scavenger and a d d e d to m e t h a n o l in t h e gas inlet system. The result is shown in Fig. 5. T h e t w o peaks at role 108 and role 109 p r e s e n t in t h e LT s p e c t r u m are n o t observed at RT. These peaks can be formally ascribed to t h e c o m p o s i t i o n s (C6H6 + C H 3 O H - H) ÷ (role 109) and (C6H6 + CH3OH -- 2H) ÷ (m/e 108) respectively. T h e c o r r e s p o n d i n g LT spectmJm o f a m i x t u r e o f C6D6 and CH3OH (Fig. 6a) gives evidence for t h e losses o f benzene h y d r o g e n in role 113 and m e t h a n o l h y d r o g e n in role 115. Finally t h e mass s p e c t r u m o f a m i x t u r e o f C6D6 and CD3OH (Fig. 6b) allows t h e structure o f t h e above ions to be d e t e r m i n e d since role 117 c o r r e s p o n d s t o a loss
[CD3OH" H] "
[I O 30HI "
D3o.Y
L
E%0.-@
[cD30,.,]. [c%oN-D]"
o)
Fig. 4. Molecular ion group o f CD3OH obtained by FI at E T o n a Pt tip using a quadrupole mass analyzer. A n o d e - - c a t h o d e potential difference: (a) 7 kV; (b) 10 kV.
132 I'--'1 "r"
100
80. o
T
t
60-
o
:o
T--,
40
o 109
IOE 20
,
,
20
I,II
,
.
60
,
,
,
IO0
80
m/e Fig. 5. F I mass s p e c t r u m o f a m i x t u r e o f m e t h a n o l i n t e n s i t y o f C 5 H 5 .+ i5 i n d i c a t e d b y a n a r r o w .
ii
_
1 ~0
120
L I ,11
1fi0
i
~--and benzene at 163 H. The higher
o f D and role 115 o f 2D atoms. The h y d r o x y l h y d r o g e n s are retained in b o t h ion structures: H role 10B
H H C H2OH----J+
rNe 109
H
F r o m t h e i s o t o p e distribution in Fig. 6 it follows t h a t (C6D6 + OCD3) ÷ and (Ce~DsOCD3)+ ions also c o n t r i b u t e t o t h e ion signals at role 118 and role 116 respectively. The ions o f mass 109 obviously result from ionization o f intermediates. The additions o f m e t h o x y a n d CH2OH'-radicals t o b e n z e n e have b e e n postulated in t h e radiation c h e m i s t r y o f m e t h a n o l a n d b e n z e n e m i x t u r e s b u t so far have n o t b e e n directly observed [ 1 6 , 1 7 ] . Since reaction (2) leads t o free CH30" radicals whereas CH2OH" radicals are a t t a c h e d t o b e n z e n e t h e follow-
133 -to
u, 100"
I
la)
8O *,.--, ..r
60.
~o
o
-r
c
!
'=E
I.O
T ,¢=
,_%0 r'~
20
¢J
|,11
,
I,I,
H
,11
rl'l I e
8
100-
ib)
80 T
o -r
"T-
o
60 -ic
o
o
w
L0 O
20
I
I, 2
'
s0
' 80
'
I ~0
i .
. 120 .
.
140
I I ~0
Fig, 6. FI mass spectra at 1 6 3 K: (a) mixhtre o£ CHsOH and C6D6; (b) mixture of CDzOH and C6D6. The higher intensifies of C6D6; are indicated by arrows.
134
ing bimolecular reaction succeeding reaction (2) m u s t be p o s t u l a t e d CHsO" + CH3OH -~ CHsOH + CH2OH" (3) This reaction is well a c c e p t e d in the radiation c h e m i s t r y o f m e t h a n o l [ 1 7 ] . Benzyl alcohol (mass 108) m a y be f o r m e d prior t o FI via t h e reaction C6I-I6CH2OH" + R" -* C6HsCH~OH + RH
(4)
in w h i c h R" is a CH30" o r CH2OH" radical. F o r t h e f o r m a t i o n o f anisole (C~IsOCHs) a similar r e a c t i o n can be f o r m u l a t e d . Benzyl alcohol and anisole are k n o w n t o be f o r m e d in t h e radiation chemistry o f m e t h a n o l - - b e n z e n e mixtures [16]. F o r m a l d e h y d e m a y be p r o d u c e d by reactions o f m e t h o x y radicals CH30" + CH30" -* CH20 + CH3OH (5) This reaction has been suggested t o a c c o u n t in part f o r t h e CH20.* peak o f t h e RT s p e c t ~ i m o f m e t h a n o l [14]. F o r m a l d e h y d e can also be f o r m e d by reactions o f CH2OH" with m e t h o x y radicals or CH2OH', c o m p e t i n g however, with t h e f o r m a t i o n o f semi-acetal and glucol (see below). In addition, t h e field reaction CH2OH" + CH3OH ~
CH20 + CHsOI-I~2
(6)
m a y c o n t r i b u t e to t h e f o r m a t i o n o f neutral f o r m a l d e h y ( l e as well, By successive FI, CH20.* a n d (CH20 + nM + H) ÷ ions axe p r o d u c e d with n ~ 0. Molecular ions o f f o r m a l d e h y d e axe clearly i d e n t i f i e d in t h e LT spectra o f m e t h a n o l . However, t h e latter ions (e.g. for n = 1) c a n n o t be distinguished f r o m t h e p r o d u c t o f t h e reaction (competi.ng with reaction (6)) CH~OH" + CH3OH --~ H O C H ~ H C H ~ (role 63)
(7)
which is e x p e c t e d to o c c u r as well. A d i m e r i z a t i o n o f CH2OH" radicals d e p e n d s on the concentrRtion a n d rate o f diffusion o f these radicals in t h e surface layer and their lifetime with respect t o FI processes. The absence o f a (CD2OH)~ p e a k (role 66) and a (CH2OH)~ p e a k (role 6 2 ) i n t h e spectra o f CD3OH and CH3OH respectively leads t o t h e c o n c l u s i o n t h a t glucol is n o t f o r m e d to a large e x t e n t . However, a small c o n t r i b u t i o n o f p r o t o n a t e d glucol to t h e (2M - - H) ÷ peak (role 63) in t h e s p e c t w m o f Fig. 5 and t o t h e (2M -- HD) + peak (role 67) in Fig. 6b is possible. The FI-induced c h e m i s t r y o f m e t h a n o l in a c o n d e n s e d surface layer is less c o m p l i c a t e d t h a n t h a t o f long chain alcohols. This will be s h o w n in a later publication.
3. Comparison o f LT-FI with Radiation Chemistry The chemical effects o f an ionizing radiation are m u c h m o r e c o m p l i c a t e d t h a n t h e chemistry following FI processes. In LT-FI o n l y i o n - - m o l e c u l e reactions and reactions o f radicals p r o d u c e d by i o n - - m o l e c u l e reaction take
135
place. M o r e o v e r t h e ions are q u i c k l y r e m o v e d f r o m t h e r e a c t i o n z o n e . However, in irradiation o f liquids t h e c h e m i s t r y is n o t o n l y d u e t o primarily f o r m e d ions b u t results also f r o m reactions o f e x c i t e d neutrals, neutr~liT, e d ions, t h e i r f r a g m e n t s a n d in a d d i t i o n f r o m r e a c t i o n s o f negative ions [ 17]. A f u r t h e r d i f f e r e n c e lies in t h e fact t h a t t h e p r o b a b i l i t y o f FI d e p e n d s exponentially o n t h e i o n i z a t i o n p o t e n t i a l ; t h u s u n d e r c o n d i t i o n s o f LT-FI o n l y m o l e c u l e s w i t h 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 will be p r o d u c e d . However, t h e e x a m p l e o f m e t h a n o l shows t h a t LT-FI gives valuable inform a r i o n o n i o n - - m o l e c u l e r e a c t i o n and radical r e a c t i o n s in liquids. By successive FI o f l a y e r m o l e c u l e s left f r o m i o n - - m o l e c u l e r e a c t i o n s it is also possible t o d e t e c t n e u t r a l r e a c t i o n p r o d u c t s a n d even i n t e r m e d i a t e s . T h e analysis of the field-induced reactions of methanol demonstrates that reaction mechanisms difficult t o e l u c i d a t e in r a d i a t i o n c h e m i s t r y are r a t h e r easily derived f r o m LT-FI mass spectra w i t h t h e aid o f i s o t o p e labelling a n d radical scavengers. H o w e v e r , as s h o w n w i t h glucol, difficulties m a y arise w i t h t h e d e t e c t i o n o f p r o d u c t s o f radical r e c o m b i n a t i o n reactions. Bug such r e a c t i o n p r o d u c t s are less difficult t o f i n d by analysis o f liquids after irradiation. ACKNOWLEDGEMENTS
T h e a u t h o r s are i n d e b t e d t o Prof. Dr. H.D. B e c k e y a n d U. G i e s s m a n n for s t i m u l a t i n g discussions. T h e y gratefully a c k n o w l e d g e t h e supporg f r o m t h e W i s s e n s c h a f t s m i n i s t e r i u m des L a n d e s N o r d r h e i n - W e s t f a l e n a n d t h e F o n d s d e r D e u t s c h e n C h e m i s c h e n Industrie. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
M.G. In abram and R. G o m e r , Z. Naturforsch. A, 1O ( 1 9 5 5 ) 863. H.D. Beckey, Z. N a t u r f o r s c h . A, 14 ( 1 9 5 9 ) 712; 15 ( 1 9 6 0 ) 822. W.A. S e h m l d t , O. F r a n k and J.H. Block, Surface Sci., 44 ( 1 9 7 4 ) 185. F.W. Ri~llgen and H.D. B eckey, Z. N a t ur f o rsch. A, 29 ( 1 9 7 4 ) 230. H.H. Gierlich, H.J. H ei nen and H~D. B e c k e y , Biomed. Mass S p e c t r o m . , 2 ( 1 9 7 5 ) 31. H.J. Heinen, H.H. Gierlich and H.D. B e c k e y , J. Phys. E, 8 ( 1 9 7 5 ) 877. H.D. B eck ey and H.-R. S c h u l t e n , Angew. Chem., Int. Ed. Engl., 14 ( 1 9 7 5 ) 403. H.D. B eck ey , Principles o f Field I o n i z a t i o n and Field D esorpt i on Mass S p e c t r o m e t r y , P er g amo n Press, O x f o r d , 1977. A detailed discussion o f F I o f molecules in t he t e m p e r a t u r e / p r e s s u r e range w here a phase t r ~ n ~ t i o n takes place at t he field a n o d e surface is in preparation, R. G o m e r , A c c o u n t s Chem. Res., 5 ( 1 9 7 2 ) 41. F.W. R~llgen and H.D. B e c k e y , Sur f ace Sci., 23 ( 1 9 7 0 ) 69. F.W. R~illgen and H.D. Beckey, Sur f ace Sci., 27 ( 1 9 7 1 ) 312. H.D. B eck ey and P. Schulze, Z. N a t ur f or s c h. A, 21 ( 1 9 6 6 ) 214. I.V. G o l d e n f e l d and I.Z. Korostyshevskij, T eor. Eksp. Khim., 4 ( 1 9 6 8 ) 218. H.J. Heinen, F.W. RSllgen and H.D. B e c k e y , Z. N a t u r f o r s c h . A, 29 ( 1 9 7 4 ) 773; H.J. Hein en an d F.W. R61lgen, Proe. 3rd Int. Conf. Solid Surf., Vienna, 1977. A. Ekst~om and J.L. G r a n e t t , J. Phys. Chem., 70 ( 1 9 6 6 ) 324. R.A. H o l r o y d , in P. Ausloos (Ed.), F u n d a m e n t a l Processes in R a d i a t i o n Chemistry, Interscience, New Y or k, 1968, and references cited therein.