S O M E G E N E R A L P R I N C I P L E S IN THE M A S S

S P E C T R A O F N I T R O G E N - C O N T A I N I N G

O R G A N I C C O M P O U N D S BY S U R F A C E I O N I Z A T I O N

ON O X I D I Z E D T U N G S T E N

E. Ya . Z a n d b e r g a n d U. Kh . R a s u l e v UDC 543.51 + 543.8 + 537.58

Many nitrogen-containing compounds are capable of being ionized by surface ionization on oxidized tungsten with the formation of positive ions. The composit ion of the ions on ionization of each compound and the cur ren t densities depend essent ial ly on the position of the nitrogen heteroatom in the molecule [1-5]. The surface ionization of complex ni trogen-containing compounds (N,N-heterocyclics) on oxidized tungsten has been investigated in the present work with the aim of establishing general principles of ion- ization with nitrogen heteroatoms.

M a s s S p e c t r a a n d T e m p e r a t u r e D e p e n d e n c e o f C u r r e n t s

We have studied the mass spect ra of eight N,N-heterocycl ic compounds (see Table 1) by surface ion- ization. All the compounds are solid substances with low vapor tensions at room temperature . Conse- quently unlike methods used previously for feeding vapors of substances to the emi t te r f rom the inlet sys - tem [3], ionization of directed s t r eams of molecules traveling f rom a heated quar tz ampule of substance has been car r ied out in the same equipment. To control the composit ion of the vapors was more difficult in this case than on their admission f rom an inlet system. In severa l cases it was possible to form an opinion on the composit ion of vapors and on the absence of decomposition of substance in the ampule f rom the relative change of the mass spect rum on changing the temperature of the vapor izer .

The composit ions of the p r imary ions in the mass spectra, the intensities of lines (in %), and the emi t te r tempera ture when obtaining the mass spec t rum are given in Table 1. In the final column is shown the ion cur ren t density of the chief lines and the evapora tor temperature on recording the mass spectrum. In the mass spect ra of compounds 1, 2, and 8 there are lines which may ar ise f rom contaminating molecules or f rom fragments of molecules of the substances being studied. These are formed f rom it not only on the emi t te r but also within the ampule. For compounds 1 and 2,lines of f ragment ions are separated in Table 1, and molecules of contaminants f rom which they may ar ise (la and 2a) are indicated. In the mass spect rum of allomatrin, in addition to the lines given in Table 1, lines were observed the most intense of which were the following: 209, 206, 171, 168, 148, and 146 amuo

On ionizing vapors of compounds entering f rom the inlet sys tem it is possible to est imate the ioniza- tion potentials V of radicals f rom the value of the ion cur ren t density while calculating the flow of molecules to the emit ter f rom the vapor p ressu re [3]. On ionizing directed s t reams of molecules of unknown density it is only possible to es t imate V from the temperature dependence of cur ren ts I(T) under conditions which correspond with the S a k h - L a n g m u i r formula [3]. Only on ionization of molecules of substances 3 and 6 (see Table 1) was the dependence I(T) of currents of molecular ions exponential. The ionization potentials of the molecules were V 3 -~ 7.2 V and V 6 -~ 7.0 to 7.3 V. However, due to the destruction of the (M + H) complexes (see Table 1) available on the emi t te r and to the possible dissociation of a portion of the mol- ecules into unionized radicals, the values obtained for the ionization potentials must be regarded as ap- proximate [both p rocesses depend on temperature and influence the dependence I(T)].

A. F. Ioffe Physicotechnological Institute, Leningrad. Translated f rom Teoret icheskaya i t ~ s p e r i - mental 'naya Khimiya, Vol. 8, No. 5, pp. 658-664, September-October , 1972. Original ar t icle submiitted July 12, 1971.

�9 1974 Consultants Bureau, a division o f Plenum Publishing Corporation, 227 ~/est 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by an 7 means, electronic, mechanical, photocopying, microfilming, recording or otherwise, witholtt written permission of the publisher. A copy o f this article is available from the publisher for $15.00.

544

TABLE 1

la

2a

Substance

6- (9 ' - Acr idinyl)- l - methyl , 1,2, 3, 4-tetrahyd mqui noline,

M = 324

I

1- Methyl- 1, 2,3,4-tetra-

hydroquinoline; Mz= 147

I

~- (9 ' - Acr id inyl ) - l -e thyl - 1,2, 3,4-tetrabyd mquinoli nei M = 338

I It

I - Ethyl- 1, 2, 3,4- tetrahyd ro-

quinoline, M 1 = 161

I CzHs

4- ( 9 '- Acridinyl) aniline M=270

N~NH~

Composition

32} (M~-H) +

324 M + 322 (M~H) + 321 (M~3H) +

18( ~ + - - H

14~ (Mx-}-H) +

14] Mz + 14{ (/gI~--H) + 14~ (M1--3H) +

33 r (M+H) + 33~ M + 337 (M--H) + 33~ (M--3H) +

322 (M--CHs) +

' ~+--H 18C

16~ (Mr+H) + 161 Mi + 16( (M~--H) + 15~ (M,--3H) +

271 ( M + H ) + 27C M +

.~ l Ic~ent den-

d. wsth e v a p -

100C t ~ 9 0 ~C,

i_~10 - 8

100C t~.~90 ~ C.

i_~5.10 --~

2

0,4 1 , 0 - - 1 ,[

10

15 115( t ,~ 120" C,

100 ],~2,10 - I j

545

T A B L E I (continued)

4

Substance

4- ( 9'- Aeridi nyl) anili he, M = 298

4- ( 9'- Aefidinyl) dimethyl- aniline, M = 326

2-Phenylbenzimidazole, M=194

1,2- Beuzoylenber~irnidazol6 M = 220

II

Alloma~-in, M-.248

,o

299 298

297

327 326 3251

323

3211 31L

1951 194

105

220

249 247 245

2391 237 235

Compo~tion of ion

(M+H) +

M+

(M--H)+

(M+H) + M+

(M_H)+ (~--3H) + (M--5H) +

(M--CH3) +

(M-}-H) + M+

(Mq-H) +

M+

(M+rl) + (M --H) +

(M--3H)+ ~4--5H) +

(M--7H) + (M--9H) +

(M-- I I H) + (M--13H) +

I 100

26 100 50 17

150

2 0 5

100

I00

80

75 8O

I00 60

20

I0 2

Current den- ~ sity j(A/cm 2)

~ with evap- (~

1150 t,~.12~ C

1~__2. I0 -t~

I150 t__ ~ I00~ (]

j,.~ 10 -9

11001 t~__150~ C,

i____lO -1~

i1001 t,~ 150~ C, ],~10 -12

170 t_~25" C,

i~_I0 -8

546

t-'-

Fig . 1. The i n c r e a s e of p r e s s u r e in the m a s s s p e c t r o m e t e r on s u r - f ace i o n i z a t i o n of a l l o m a t r i n on tungs ten ox ide . The a r r o w po in t s to the m o m e n t of r a i s i n g the t e m - p e r a t u r e of the e m i t t e r to l170~

The d e p e n d e n c e I(T) of the c u r r e n t of (M + H) + ions was be l l shaped , and m a x i m a l c u r r e n t s w e r e o b s e r v e d a t t e m p e r a t u r e s l o w e r than f o r o t h e r ions of the s a m e compound , as a l s o o c c u r s on i o n i z a t i o n of m o r e s i m p l e compounds [6]. In Table 1 the p e r c e n t c on t e n t of (M + H) + ions is not g iven when at the e m i t t e r t e m p e r a t u r e m e n t i o n e d the c u r r e n t of (M + H) + ions b e c a m e even l e s s than the c u r r e n t s of ions of o t h e r c h e m i c a l c o m p o s i t i o n .

T h e D e h y d r o g e n a t i o n R e a c t i o n o f A m i n e s o n T u n g s t e n O x i d e s

The c o m p o s i t i o n of ions in the m a s s s p e c t r a on s u r f a c e i o n i z a - t ion s e r v e s as an i n d i c a t i o n of those r e a c t i o n s in the l a y e r a d s o r b e d on the e m i t t e r as a r e s u l t of which ion i zed r a d i c a l s o r m o l e c u l e s (with r e l a t i v e l y low ion i z a t i on p o t e n t i a l s ) a r e f o r m e d . In the m a s s s p e c t r a of many N , N - h e t e r o e y e l i c c o m p o u n d s ( see Table 1) on i o n i z a - t ion on ox id i zed tungs ten and in the m a s s s p e c t r a of m a n y a m i n e s ob-

ra ined p r e v i o u s l y t h e r e a r e l i n e s f o r ( M - n i l ) + ions [3-5] w h e r e the v a l u e s of n depend on the s t r u c t u r e of the m o l e c u l e . The high c o e f f i c i e n t s of the s u r f a c e i o n i z a t i o n of ( M - H ) r a d i c a l s on i o n i z a t i o n of d i e t h y l - and t r i e t h y l a m i n e [5], p i p e r i d i n e [3], and o t h e r a m i n e s ind ica te the high y ie ld of the d e h y d r o g e n a t i o n r e a c - t ion of m o l e c u l e s on the s u r f a c e of hea t ed ox id i zed tungs ten .

A t o m s of h y d r o g e n l i b e r a t e d in d e h y d r o g e n a t i o n r e a c t i o n s m a y r e d u c e tungs ten oxide and a l s o m a y be d e s o r b e d as a m o l e c u l e of h y d r o g e n i n c r e a s i n g the p r e s s u r e in the i n s t r u m e n t . On ion iz ing v a p o r s of c o m - pounds a d m i t t e d into the s p a c e a round the s o u r c e t h e r e is an i n c r e a s e of p r e s s u r e which is d i f f i cu l t to d e - t e c t a g a i n s t the b a c k g r o u n d of the o v e r a l l v a p o r p r e s s u r e . On i o n i z a t i o n of a l l o m a t r i n , a so l id s u b s t a n c e wi th a r e l a t i v e l y low v a p o r t ens ion , which is ab le to lo se up to 13 h y d r o g e n a t o m s , an i n c r e a s e of h y d r o g e n p r e s s u r e was no t i c ed in the i n s t r u m e n t . In F ig . 1 is shown the d e p e n d e n c e of the o v e r a l l p r e s s u r e of r e s i d - ua l gas in the ion s o u r c e of the m a s s s p e c t r o m e t e r on r a p i d he a t i ng to 1170~ of the e m i t t e r whi l e be ing i r r a d i a t e d u n i n t e r r u p t e d l y by a s t r e a m of a l l o m a t r i n m o l e c u l e s . The s t e a d y p r e s s u r e , above the i n i t i a l va lue , is ev iden t . The m a s s s p e c t r a on i o n i z a t i o n of the r e s i d u a l gas by e l e c t r o n s showed that the p r e s s u r e of H 2 and a p p a r e n t l y of CO (m = 28 ainu) was i n c r e a s e d . The l a t t e r m a y be a s s o c i a t e d wi th r e a c t i o n s of a m o r e p ro found d e c o m p o s i t i o n of a p o r t i o n of the m o l e c u l e s of a l l o m a t r i n on the e m i t t e r l e a d i n g to the b r e a k - ing away of c a r b o n a t o m s ; c a r b o n a t o m s m a y a l s o r e d u c e tungs ten oxide, f o r m i n g CO. The r e d u c t i o n of the oxide by h y d r o g e n and c a r b o n to tungs ten m u s t r e d u c e the w o r k i n g output of the e m i t t e r and w e a k e n i o n i z a - t ion. In p r a c t i c e u n d e r the ac t i on of a l l o m a t r i n m o l e c u l e s the e m i t t e r was "po i soned" in t i m e and needed to be r e s t o r e d by r e p e a t e d ox ida t ion .

It should be r e c o r d e d that m a s s s p e c t r a on s u r f a c e i o n i z a t i o n and t e m p e r a t u r e d e p e n d e n c e s of the c u r r e n t s of ( M - n i l ) + type of ions m a y g ive v a l u a b l e i n f o r m a t i o n on the d e h y d r o g e n a t i o n r e a c t i o n s of c o m - p l e x a m i n e s on the s u r f a c e of s o l i d b o d i e s .

T h e P o s i t i o n o f t h e H e t e r o a t o m i n M o l e c u l e s a n d T h e i r M a s s S p e c t r a o n S u r f a c e I o n i z a t i o n

F r o m Table 1 it is s e e n tha t the m a s s s p e c t r a of N , N - h e t e r o e y c l i c compounds c o m p r i s e the s a m e c h a r a c t e r i s t i c s e t of l i ne s as do the m a s s s p e c t r a of s i m p l e r m o l e c u l e s wi th n i t r o g e n h e t e r o a t o m s [1-5] . N i t r o g e n - c o n t a i n i n g r a d i c a l s of the f o r m ( M - H ) , ( M - R ) , and ( M - n i l ) , w h e r e n is an odd n u m b e r , w e r e ion ized wi th the g r e a t e s t e f f i c i e n c y . These r a d i c a l s f o r m s t a b l e i n t e r m e d i a t e ions wi th s a t u r a t e d bonds and with the c h a r g e l o c a l i z e d on the n i t r o g e n a t o m . The i on i z a t i on p o t e n t i a l s of such r a d i c a l s w e r e r e l a - t i ve ly s m a l l , and in the ions f o r m e d f r o m them N + was t e t r a v a l e n t and had sp hyb r id o r b i t a l s .

The c h a r g e d f r a g m e n t s ( M - H ) + with s a t u r a t e d bonds m a y have been ob ta ined by f i s s i o n of a h y d r o g e n a t o m f r o m c a r b o n in the ce p o s i t i o n . F o r e x a m p l e , i t is p o s s i b l e to ob ta in ( M - H ) + ions f r o m d i e t h y l a m i n e m o l e c u l e s which have the f o r m

C~H5 - - N + = CH - - CHa. f H

547

Analogous bond f i s s ions a re poss ib le in adsorbed molecules of p iper id ine , subst i tuted hydraz ines , and in molecu les of subs tances 1, la , 2, 2a, 4, 5, and 8 of Table 1. On ionizat ion all these compounds form (M-H) + ions in p rac t i ce .

Radica l s obtained by s c i s s i o n of a C - C bond in the posi t ion fi to the n i t rogen atom also form n i t rogen- containing ions with sa tu ra t ed bonds to the N + ion. F o r example , the (M-CH3) + ion of d ie thylamine may be r e p r e s e n t e d as

C2H5 - - N + _ CH3.

I H

In compounds with l i nea r a lky l chains s e i s s i o n of T and 6 bonds is a l so probable but the fo rmat ion of s table ions mus t be accompanied by r e a r r a n g e m e n t ; it might the re fo re be expected that the c u r r e n t dens i t ies of the r e spec t ive ions wil l be s m a l l e r .

The ionizat ion of anil ine molecu les conf i rmed the ru les mentioned. Carbon in the posi t ion ~ to the n i t rogen is not bonded to hydrogen in the anil ine molecule , and s c i s s i o n of the benzene r ing at a C - C bond in the fi posi t ion is diff icult and does not lead to the p r e p a r a t i o n of an ion with sa tu ra t ed bonds. Conse- quently on ionization of the aniline molecule the ions (M-H) + and ( M - R ) + a re not observed [2]. The same also occurs on ionizat ion of pyr id ine [3] and 4 - ( 9 ' - a c r i d i ny l ) a n i l i ne (see Table 1). Substitution of hydrogen a toms at n igrogen by methyl r ad i ca l s makes poss ib le the f i ss ion of hydrogen f rom the ~ carbon, and (M-H) + ions a re formed with sa tu ra ted va lenc ies fo r N§

~"CH 2

Substitution of methyl groups by ethyl makes poss ib le the s c i s s ion of a C - C bond in the fi pos i t ion and the fo rmat ion of ions of

with a quadr iv~lent n i t rogen ion. Lines fo r (M-CH3) + became intense in the mass s p e c t r a of 4 - ( 9 ' - a c r i - d inyl)die thylani l ine (see Table 1).

Af te r f i s s ion of the most weakly bonded and mobile hydrogen atom sepa ra t ion of o ther hydrogen atoms is poss ib le . I t was noted that compounds formed by the loss of an odd number of hydrogen a toms were ion- ized more eff ic ient ly than those obtained by sepa ra t ion of an even number of hydrogen a toms. This is con- nected with the fact that in the second case i o n - r a d i c a l s a re formed. In both ca ses dehydrogenat ion r e a c - tions lead to subst i tut ion of a by conjugated bonds: the ionizat ion of r ad i ca l s of amylamine , p iper id ine [3], and a lso the N,N-he te rocyc l i c compounds 1, 2, 4, 5, and 8 in Table 1 may s e r v e as examples .

In the case of cyc l ic amines the e l imina t ion of hydrogen atoms leads to a romat i za t ion of the r ing. Thus in the (M-5H) + ion of p iper id ine the r ing is comple te ly a roma t i zed

(/-%•'+ H). \ = f ' - -

The same phenomenon also takes place on ionizat ion of the N ,N-he te rocyc l i e compounds 1, l a , 2, 2a, and 8. In the case of a l l oma t r in it is poss ib le to fo rm s table ions by remova l of up to 13 hydrogen atoms, and in the m a s s spec t rum all fo rms of (M-n i l ) + ions with odd n up to n = 13 occu r red in p rac t i ce .

It is poss ib le to obtain m e a s u r a b l e c u r r e n t s for m o l e c u l a r ions M + under condit ions when the ion iza - tion potent ia l of the molecule does not exceed the opera t ing output of the e m i t t e r by more than ~ 2 V; in this case molecu les do not undergo comple te convers ion into f ragments of some other chemica l const i tut ion du r - ing the i r t ime in the adsorbed s ta te . However, under these condit ions l ines fo r M + ions are made apparen t more read i ly in the ionization of compounds which do not fo rm (M-H) + ions. The isotope c u r r e n t of these

548

ions interferes with the measurement of the current for M + ions in mass spectrometers of the usual re-

solving power.

In ions of (M + H) complexes the nitrogen ion is also quadrivalent. Ions of this type are formed from those compounds the molecules of which are able to combine with one another on the surface of the emitter by hydrogen bonds and then dissociate, giving (M + H) complexes [6]. The presence of polar bonds is necessary to bring about hydrogen bonding. A hydrogen bond may develop not only between molecules of ionizable compounds (for example, between molecules of primary and secondary but not tertiary amines) but also between molecules of an ionizable compound and adsorbed traces of polar molecules (for example, with water molecules). Thus (M + H) + ions are formed with pyridine [3 and 6], tertiary amines [5], and the majority of N,N-heterocyclic compounds which appear in Table I. However, the intensity of (M + H) +

lines formed with contaminating compounds is not great.

CONCLUSION

From the above-mentioned, it follows that the form of the mass spectra of nitrogen-containing com- pounds on surface ionization on oxidized tungsten may be predicted using the following scheme. If in the molecule carbon in the position c~ to nitrogen is bonded to hydrogen, then (M-H) + ions are always formed. The current density of these ions in series of amines is increased with substitution of hydrogen atoms bonded to nitrogen by alkyl radicals [3 and 5]. Substituents with electron aceeptor properties reduce the current density of (M-H) + ions [4].

If there is a carbon in the position p to nitrogen, then fission of the C a -Cfi bond leads to the appear- ance of (M-R) + ions of nitrogen-containing radicals. These are more probable with longer alkyl chains.

Dehydrogenation of the molecule on the emitter with the replacement of simple by conjugated bonds precedes the ionization of amines with saturated hydrocarbon chains. Ionization of the resulting compounds leads to the appearance of (M-nil) + ions where n is an odd number. M +ions are formed at not too high molecular ionization potentials, and their detection is easier in the absence of large currents from (M-H) + ions. (M + H) + ions with the greatest intensity can be formed on ionization of molecules with polar bondsj, and the current of these ions is more readily detected in the absence of a large current for the molecular ion.

The characteristic form of the mass spectrum of a nitrogen-containing compound is a low number of lines and a large current density for ions of composition (M-H) and (M-R). This makes the use of sur- face ionization convenient for resolving a series of analytical problems for these compounds [7].

The authors thank V. A. Leont'eva, V. V. Rode, and R. A. Khmel'nitskii for giving the N,N-hetero- cyclic compounds.

I. I~. Ya. 2. ~. Ya. 3. ]~. Ya. 4. 1~. Ya.

6, 776 5. ~. Ya. 6. t~o Ya. 7. t~. Ya.

LITERATURE CITED

Zandberg, U. Kh. Rasulev, and B. N. Shustrov, DokL Akad. Nauk SSSR, 172, 885 (1967). Zandberg and U. Kh. Rasulev, Zh. Tekh. Fiz. , 3._~8, 1798 (1968). Zandberg, U. Kh. Rasulev, and M. R. Sharapudinov, Teor. i Fksperim. Khim., 6_ 328 (1970). Zandberg, U. Kh. Rasulev, V. V. Takhistov, and M. R. Sharapudinov, Teor . i ~ksperim. Khim., (1970). Zandberg, U. Kh. Rasulev, and M. R. Sharapudinov, Teor . i Fksperim. Khim., 7, 363 (1971). Zandberg and U. Kh. Rasulev, Dokl. Akad. Nauk SSSR, 1877, 777 (1969). Zandberg and U. Kh. Rasulev, Zh. Analiticheskoi Khim., in p ress .

549