ISOMERIC (M-H) + IONS IN THE MASS-SPECTRA OF

MOLECULES FROM THE SURFACE IONIZATION OF

DIMETHYLBENZYLAMINE ON OXIDIZED TUNGSTEN

E. Ya. Zandberg and U. Kh. Rasulev UDC 543.51+543.8+537.58

During the su r face ionizat ion of mo lecu le s of d imethy lbenzy lamine on oxidized tungsten, pos i t ive ions with m a s s m = (M - 1) ainu a r e fo rmed with high p robab i l i ty ; a por t ion of these undergo monomolecu la r decompos i t ion dur ing the t ime of f l ight in the m a s s - s p e c t r o m e t e r to fo rm f ragmenta t ion ions of t ropy l ium with m* = 91 ainu on decomposing in the zone of the a c c e l e r a t i n g f ie ld of the ion source and ions with appa r - ent m a s s m ** = 912/134 = 61.7 ainu on decomposing in the nonfield reg ion of the m a s s - s p e c t r o m e t e r [1]. A hydrogen atom can be sp l i t e i t he r f rom the methyl r a d i c a l or the methylene group of the d imethylbenzyl mo lecu l e s M to fo rm ions of (M-H) + ainu. The p robab i l i t y of fo rming the (M-H) + ions of d i f ferent s t r u c - tu re and a lso the p robab i l i t y of t he i r subsequent d i s soc ia t ion may di f fer in both these c a s e s . In o r d e r to c l a r i fy this p r o b l e m the sur face ioniza t ion of d imethy lbenzy lamine (CH3)2NCD2C6H 5 l abe l l ed with deu te r ium in the methy lene group (M1 = 137 amu) has been studied. A desc r ip t i on of the m a s s - s p e c t r o m e t e r used and the method of m e a s u r e m e n t i s contained in [2].

The m a s s - s p e c t r a f rom the ionizat ion of the vapor of the subs tance with e l ec t rons of energy f rom 70 to 11 eV showed that i t was contamina ted with up to 3% of the molecu le (CH3)2NCHDC~H5 (M2 =- 136 amu) and up to 5% of the molecu le (CH3)2NCH2C6H 5 (M 3 = 135 amu).

The m a s s e s of the p r i m a r y ions of the type (M-H) + and the f r a g m e n t a t i o n i o n s fo rmed with them, the composi t ion of the ions, and the r e l a t i v e i n t ens i t i e s of the l ines for al l the components of the mix tu re of subs tances ionizable on the oxidized tungsten at l120~ a r e given in Table 1. Accord ing to the in tens i ty ca lcu la t ion for the l ine with m = i36 amu a s t r e a m of ions i so topic in r e s p e c t to ions with m = 135 amu has been e l imina ted . It i s evident f rom Table 1 that, dur ing the fo rmat ion of r a d i c a l s of the type (M-H) on the e m i t t e r sur face , a hydrogen a tom (or deu te r ium) can be sp l i t both f rom the methyl group (m = 136 amu) and a lso f rom the methylene (m = 135 ainu). The re a re cons ide rab ly m o r e (M-D) + ions than (M-H) + ions; th is may be a s s o c i a t e d e i the r with a d i f ferent p robab i l i t y of c leavage of the atom f rom the di f ferent groups of the molecu le on the e m i t t e r su r face or a pos s ib l e d i f ference in the ionizat ion po ten t i a l s of the i s o m e r i c r a d i c a l s .

It a l so fol lows f rom Table 1 that the s tab i l i ty of the ions as r e g a r d s monomolecu l a r decompos i t ion is e s s e n t i a l l y dependent on the s t ruc tu r e of the ions. The p r i m a r y ions in which the hydrogen atom has been los t f rom the methyl group (m -- 136 amu) a r e r e a d i l y decomposed to fo rm the f r agmenta t ion ions (CTH5D2)+: the in tens i t i e s of the l ines for the p r i m a r y (m = 136 amu) and f ragmenta t ion ions (m* = 93 amu) a r e compa rab l e in value. The p r i m a r y ions which have los t deu te r ium f rom the methylene group a re s tab le : the s m a l l number of f r agmenta t ion ions (CTH6D) + can be a t t r ibu ted en t i r e ly to the contaminant molecu les M2; in the same way the ions (C7H7) + can be a t t r ibu ted to the contaminant molecu les M3. Moreover , these ions (M-H) + fo rming dur ing the ionizat ion of the contaminant mo lecu le s M2 and M3 and produced by the c leavage of a hydrogen atom f rom the methyl group d i s s o c i a t e .

A. F. Ioffe Phys ico techn ica l Inst i tute , Academy of Sciences of the USSR, Leningrad . T rans l a t ed f rom T e o r e t i c h e s k a y a i t~kspe r imen ta l ' naya Khimiya, Vol. 8, No. 1, pp. 114-117, J a n u a r y - F e b r u a r y , 1972. Or ig ina l a r t i c l e submi t ted October 2, 1970.

�9 1974 Consultants Bureau~ a division of Plenum Publis/ting Corporation, 227 [['est 17t/t Street, .\"ew York, V. Y. 10011. No part of t/tis publication ma)~ be reproduced, stored in a retrieval system, or transmitted, in any form or by an,, means, electronic, mechanical, photocopying, microfilming, recordin~ or ot/terwise, wit~tout written permission of t/te publ}sher. I copy of t/tis article is available front the publisher for $15.00.

95

T A B L E 1

Inten- m, ainu Ions Composition sity, qo

Primary tons

136

�9 135

134

63,6**

92* 62,6**

91" 6!,7"*

(MI--H)+

(M,--D)+ (M2--H) +

(Mz--D) + (M3--H) +

CH3CH~ICD2C6H~

(CH3) 2NCDC~Hs (CHa) ~NCDC6H5 CH~CH2NCHDC6H5

(CH3) 2NCHC~H5 (CH~) ~NCHC6H~ CH3CH2NCH2C6H5

Fragmentation ions

Tropylium ions CTHsD2

Tropylium ions CzH6D

Tr0pylium ions CTH~

3,2

100

1,3

2 N0,15

0,07 traces

0,025 traces

t

log Ifrag/Iprim ~,o r

z 0,5 I

9 - - - I , ~ . i I I ~ I , .

tloo 12oo r~K 41 ~,y 45 5o4o # ~

Fig. 1 Fig. 2

Fig . 1. Dependence of ion ic c u r r e n t s on the t e m p e r a t u r e of the o x i d i z e d t u n g s t e n e m i t t e r a t c o n s t a n t v a p o r p r e s s u r e in the a p p a r a t u s : 1) p r i m a r y ions with m = 135 amu; 2) p r i m a r y ions with m = 136 a inu; 3) f r a g m e n t a - t ion i ons with m * = 93 amu; 4) t o t a l c u r r e n t of p r i m a r y ions wi th m = 136 amu and f r a g m e n t a t i o n ions with m * = 93 amu .

F ig . 2. A r r h e n i u s p l o t s f o r the d e t e r m i n a t i o n of t he e n e r g y of a c t i v a t i o n of the d i s s o c i a t i o n of the N - C bond d u r i n g the g iven t i m e of f l ight of the (CH3CH2NCD2C6Hs) + ions : 1) d i s s o c i a t i o n on the e m i t t e r s u r f a c e (m* = 93 amu) e~* -~ 0.7 eV; 2) d i s s o c i a t i o n in the nonf ie ld zone of the m a s s s p e c t r o m e t e r (m** = 62.6 amu) e~ -~ 0.2 eV.

In compounds of the a l i p h a t i c a m i n e type we a s s u m e d f o r the ions of the type ( M - H ) + a p o s i t i v e c h a r g e a s s o c i a t e d with the h e t e r o n i t r o g e n a t o m . In a r o m a t i c a m i n e s l o c a l i z a t i o n of the c h a r g e on the n i t r o g e n a t o m m a y a l so not o c c u r . H o w e v e r , the s t a b i l i t y of the ( M - H ) + ions of c o m p o s i t i o n [(CH3)2NCHC6Hs] + m a y i n d i c a t e tha t in t h e s e the p o s i t i v e c h a r g e i s a l so l o c a l i z e d on the n i t r o g e n a t o m and tha t the i ons m a y have

the s t r u c t u r e 8~:N+=CHC6H~ . In o r d e r to d i s s o c i a t e such ions to f o r m t r o p y l i u m ions it would be 8 /

n e c e s s a r y to c l e a v e the double bond and s i m u l t a n e o u s l y t r a n s f e r a h y d r o g e n a t o m f r o m the m e t h y l - g r o u p ; t h i s i s e n e r g e t i c a l l y s c a r c e l y p r o b a b l e . Consequen t ly , d i s s o c i a t i o n of ions of t h i s s t r u c t u r e i s not o b s e r v e d .

Eva lua t i on of the i o n i z a t i o n p o t e n t i a l s of the ( M - H ) r a d i c a l s of both i s o m e r i c f o r m s f r o m the d e n s i - t i e s of the m e a s u r a b l e c u r r e n t s [2] g i v e s the v a l u e s V ~ 6.8 V. Since t h e s e v a l u e s f o r V a r e c l o s e to the ionic work func t ion of o x i d i z e d t ungs t en r = 6.8 V, then the ion ic c u r r e n t m u s t be p r a c t i c a l l y i ndependen t of the e m i t t e r t e m p e r a t u r e [2]; t h i s i s c o n f i r m e d by the e x p e r i m e n t . The t e m p e r a t u r e r e l a t i o n s h i p s of the i on i c c u r r e n t s a r e g iven in F i g . 1. The c u r r e n t of the s t a b l e ions (1) i s p r a c t i c a l l y i ndependen t of t he

96

emit ter T; the measurab le cur rent of the unstable ions (2) dec reases with an increase in emit ter T, but the cur rent of the fragmentat ion tropylium ions formed f rom these is increased correspondingly (3). Both of these cur ren t s have the same order of magnitude and their s u m that is , the t rue p r imary current of un- stable ions, is also prac t ica l ly independent of emit ter tempera ture (4).

At ionization potentials V for both i somer ic radicals (M-H) of the order (Pi, the difference in inten- si t ies of the ionic cur ren ts of these rad ica ls (see Table 1) can be explained by a different probabili ty of f o r - mation of i somer ic radica ls on the oxidized tungsten surface. Hence it follows that the probabili ty of r e - moval of a hydrogen atom from the methylene groups of the adsorbed molecules is g rea te r than f rom the methyl radical; this also occurs in the case of ionization of the free molecules by e lect rons [3].

The high probabili ty of cleavage of the N - C bond in (CH3CH2NCH2C6Hs) + indicates that the energy of activation of this p rocess must be low. The Arrhenius plots for the decomposit ions of the (CH3CH2NCD2- CGHs) + ions in the zone of their accelerat ion (for a t ime of flight tt -~ 9 �9 10-7 sec) and in the nonfield region of the m a s s - s p e c t r o m e t e r (for a t ime t2 -~ 6 �9 10-6 sec) [2, 4] a re given in Fig. 2. On plotting these graphs the cur rent of fragmentat ion ions was attributed to the total cur rent represen ted in Fig. I (see 4). The experimental values of the energy of activation of the decomposit ion found f rom the Arrhenius plots are ~T = 0.7 eV for t t and s*2 -- 0.2 eV for t 2. As it should be s~ > ~ [2], but their absolute values are in fact small in compar i son with those found for the decomposit ion of (M-H) + type p r ima ry ions of piperidine [2], diethylamine, and t r ie thylamine [5] at the N - C bond.

The authors thank N. I. Ionova and V. V. Takhistova for discussion and A. S. Polyakova for prepar ing the deuterated benzylamine.

LITERATURE CITED z

I. E. Ya. Zandberg, U. Kh. Rasulev, V. V. Takhistov, and M. R. Sharapudinov, Teor. i Eksperim. Khim., ~, 776 (1970).

2. E. Ya. Zandberg, U. Kh. Rasulev, and M. R. Sharapudinov, Teor. i ~ksperim. Khim., 6_, 328 (1970). 3. J. Collin, in: Advances in Mass-Spect rometry , J. D. Waldron (editor) [Russian translation], IL, Mos-

cow (1963), p. 380. 4. N .D . Potekhina, Teor . i ~ksper im. Khim., 6, 339 (1970). 5. E. Ya . Zandberg, U. 'Kh. Rasulev, and M. R. Sharapudinov, Teor. i Eksper im. Khim., 7_, 363 (1971).

97