14. 15.

H . V . Serheew, V. J. Gurman, V. U. Papisova, and E. J. Jacovenko, Fifth Intern. Symposium on Free Radicals, Upsala, Sweden (1961). R . V . Kucher, A. A. Turovs'kii, and N. V. Pzumedzei, Dokl. Akad. Nauk Ukr. RSR, Ser. B, 533 (1973).

QUANTUM-MECHANICAL PROPFRTIFS ACETALS

INVESTIGATION OF

IN

A

THE

MOLECULES

STRONG

V. V. Lobanov, Yu. and M. M. Aleksankin

AN]2

ELECTRIC A.

OF IONS

THE OF

FIFLD

UDC 539.19.08+539.193.196+543.51

Kruglyak,

The theory of molecular systems in external electric fields is a comparatively underdeveloped field in quantum chemistry. At the same time, the study of the effect of electric fields on the electronic structure of molecules and molecular ions is of both theoretical and practical importance for several fields of investigation in physics and chemistry, primarily for field mass spectrometry. It should be recalled that in the experimental study of field mass spectra, besides the electric field, adsorption on the surface of the emitter, and in some cases the selection of the material of the emitter and the state of its surface have a great effect on the nature and the kinetics of the decomposition of positive molecular ions. Therefore, from the entire complex set of real conditions for an experimental study of the behavior of molecules and ions in electric fields both on the theoretical level and with the purpose of a more thorough interpretation of the experimental data it would be useful to single out those effects which are due only to the presence of the field. The modern semiempirical methods of calculating the electronic structure of molecular systems allow us to do this in a fairly reliable ma nne r.

One of the first theoretical studies in the field of the quantum-mechanical investigation of the properties of molecules and ions in a constant homogeneous external electric field was c a r r i e d out by Hiskes [1]. The methods of perturbation theory with the application of the most exact presently available wave functions of small molecules calculated without consideration of a field were used to evaluate the important physical properties of molecules in a field (the dissociation energy, the populations of the vibrational levels, the probabilities of ionization and dissociation, etc.) and to establish their dependence on the field intensity. In [2] the method of equivalent molecular orbitals was used to calculate a s e r i e s of singly ionized paraffin molecules in a homogeneous external electric field oriented in various directions for the purpose of ascertaining the possibility of an a priori evaluation of the bonds most likely to undergo cleavage in molecular ions. The most complete investigation of the changes in the electronic structure of the HF, CH3CI, and H20 molecules and several ions under the effect both of uniform and nonuniform electric fields has been c a r r i e d out by Drossbach and Schmittinger [3-5] in the ab initio approximation. In the present work we have considered the electronic configuration of several acetal molecules and their positive molecular ions in the presence and absence of a field in the CNDO/2 approximation [6] with explicit inclusion of the uniform external electric field [7, 8]. RESULTS

AND D I S C U S S I O N

In calculating the electronic structure of the dimethoxymethane molecule (1) (see Table 1), the bond lengths and the angles between them were selected on the basis of the experimental data in [9]. The methyl groups were arranged symmetrically relative to the plane of the O-- C - - O atoms. The calculations showed that the most stable confirmation of the molecule has a dipole moment equal to 0.87 D, which is in satisfactory agreement with the experimental value of 0.74 D,[10]. Since there are no experimental data on the spatial L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of the Ukrainian SSR. Institute of Theoretical Physics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i t~ksperimental'naya Khimiya, Vol. 12, No. 1, pp. 48-55, J a n u a r y - F e b r u a r y , 1976. Original article submitted January 28, 1975. This material is protected by copyright registered in the name of Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 100I 1. No part I ]of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, [ [micro~ming, recording or othe~vise, without written permisSion o f the publisher. A copy of this article i.s available from the publisher for $ 7.50. ]

36

o,p 1,0-

Ofu)

~J

0,"

[ ....

/,0

aR,a

O

,

Fig. i

,

L

[

,,

,

~,o~R/c,-OD

Fig. 2

F i g . 1. V a r i a t i o n in the c h a r g e Q(H) of a h y d r o g e n a t o m in ion I with d i f f e r e n t v a t u e s of the f i e l d s t r e n g t h F: 1, 2) 0.02; 3) 0.01; 4) 0 as a f u n c t i o n of the l e n g t h e n i n g of the C - - H bond ( c u r v e s 1, 3, and 4) o r i t s z p r o j e c t i o n ( c u r v e 2). F i g . 2. D e p e n d e n c e of the c h a r g e Q (curve 1) and the s p i n d e n s i t y p (curve 2) of the CH3OCH 2 f r a g m e n t of ion I on the l e n g t h e n i n g of the C1 -- 0 bond with the field s t r e n g t h F = 0.02. s t r u c t u r e of the o t h e r m o l e c u l e s i n v e s t i g a t e d , f o r t h e m the a r r a n g e m e n t of the C - O - - C - O - - C g r o u p of a t o m s was a s s u m e d to be the s a m e a s in m o l e c u l e I, and the l e n g t h s of the C -- C and C -- H b o n d s and the b o n d a n g l e s < CCC and < CCH w e r e s e t e q u a l to the m o s t f r e q u e n t l y e n c o u n t e r e d v a l u e s in c o m p o u n d s with s i m i l a r s t r u c t u r e s [11]. The a p p l i c a t i o n of an e x t e r n a l e l e c t r i c f i e l d r e s u l t s not only in the o r i e n t a t i o n of the m o l e c u l e in a d e f i n i t e m a n n e r with r e s p e c t to the f i e l d v e c t o r , but a l s o in c h a n g e s in the e l e c t r o n i c s t r u c t u r e , the s p a t i m s t r u c t u r e , and p o s s i b l y the c o n f o r m a t i o n . As has b e e n shown in [8], the d i s t o r t i o n s in the g e o m e t r i c s t r u c t u r e a r e i n s i g n i f i c a n t and m a y be n e g l e c t e d . C o n f o r m a t i o n a l c h a n g e s a r e a l s o u n l i k e l y , s i n c e the b a r r i e r s to r o t a t i o n of the h y d r o c a r b o n g r o u p s a r o u n d the s i m p l e b o n d s a r e c o n s i d e r a b l y h i g h e r due to the r e o r i e n t a t i o n of the m o l e c u l e in the field. We u s e d the m e t h o d p r e s e n t e d in [7, 8] to e s t a b l i s h that all the m o l e c u l e s we i n v e s t i g a t e d a r e o r i e n t e d in the f i e l d with the oxygen a t o m s t o w a r d the s u r f a c e of the e m i t t e r and the h y d r o c a r b o n g r o u p s m o r e d i s t a n t f r o m the s o u r c e of the p o s i t i v e c h a r g e . P o s i t i v e m o l e c u l a r ions f o r m e d as a r e s u l t of i o n i z a t i o n have the s a m e o r i e n t a t i o n . In c a l c u l a t i n g t h e i r e l e c t r o n i c s t r u c t u r e , the t h r e e - d i m e n s i o n a l s t r u c t u r e of t h e s e ions was a s s u m e d to be the s a m e as in the o r i g i n a l m o l e c u l e s . A f t e r i o n i z a t i o n , all the b o n d s in the ion f o r m e d w e r e g e n e r a l l y weakened, but to d i f f e r e n t d e g r e e s . In the p r e s e n t s t u d y the c r i t e r i o n f o r e v a l u a t i n g the d e g r e e of w e a k e n i n g of a bond was the d i f f e r e n c e b e t w e e n the t w o - c e n t e r c o n t r i b u t i o n s of t h i s b o n d EAB to the total e n e r g y of the m o l e c u l e and the ion, i . e . , ion the q u a n t i t y AAB = E ~ ol - - E A B . In the CNDO/2 a p p r o x i m a t i o n the total e n e r g y of a m o l e c u l e o r ion m a y be r e p r e s e n t e d as a s u m of o n e - c e n t e r E A a n d t w o - c e n t e r E A B c o n t r i b u t i o n s [6], i . e . , T A B L E I.

M a s s S p e c t r a of S p l i n t e r Ions of A e e t a l s * CH3CH(OCH3)2

I-I,C(OCHs)~ (I) I

,n/e type of ion 75 74* 6l 45

2 1

.,'v/~7 6

M--1 M--2 M--15 M--31

intensity

(11)

CH3CHaCH(OCHs)~(lid

CH~CI-12CH2CH(OCHa)~

3

4

2

1

3

2

(IV)

1

CH3CH(OCH~CHa)2 '2

(V)

l

M~90

I

rn/e

type of intensity ion

,,~/~

type of intensity ion

I]Z/e

type of ~'ntemity ion r

75 59

M--15 M--31

89 73 75

M--15 M--31 M--29

0 28,.t 24,2

Ion 87 89 75

M--15 M--a1 M--29 M--43

31 29 15

M--73 M--75 M--89

3,6 36.0 7,8

31 29 15

m/e

type of .ntensity mn

0,4

32,9 0 53,8

22,8

74,6

31

M--45

I1,6

31

M--59

2,6

15

M--61

1,3

15

M--75

0

0,4 16.8 0 15,9

M--87 6,2 M--89 15,9 M - - 1 0 3 37,2

I03

M--15

6,6

89

,,VI--29 ]

0

73 45 29 15

M--45 I

t3,0 1,9 M--89 i7,1 M--103 [ 61,4

M--73

I

*The intensities of the lines are given as percentages of the total current of splinter ions. The numbers of the atoms in the main chain of the hydrocarbon skeleton are given under the formula~ of the acetals. 37

Ogc/tO /,g ...................

~;(CHA. . . . . .

l,O .

1 Y O,5

2 1

2

0

I.

/,a

o

~R,A

zo

Fig. 3

dR(C, G)a

Fig. 4

F i g . 3. D e p e n d e n c e of the c h a r g e Q o n t h e C H a g r o u p : 1) in ion I; 3) in ion II on the l e n g t h e n i n g of the O - - CH 3 bond; 2) in ion II on the l e n g t h e n i n g of the C 1 - C 2 bond. The f i e l d s t r e n g t h is F = 0.02. F i g . 4o V a r i a t i o n in the c h a r g e Q of the ethyl g r o u p C2H 5 in ion III a s a f u n c t i o n of the l e n g t h e n i n g of the C 1 -C 2 bond in f i e l d s of d i f f e r e n t s t r e n g t h s : 1) F = 0; 2) F = 0.01; 3) F = 0.02

E=Z A+ZZ AO A

A~B

w h e r e the s u m m a t i o n is c a r r i e d out o v e r the a t o m s of the m o l e c u l e . c l o u d the c o n t r i b u t i o n s c a n be w r i t t e n in the f o r m 6, =

E A p..u..

1 + T

Ix

Ean = x A 2 B {2Pwfl~,., - IX

AEA

F o r a s y s t e m with an open e l e c t r o n

-

- (PL)q VAA.

it

[(P~v)=+ (P~v)=]7AB} +

ZAZBRA~ - - PaaVA. - - Pe.Ve~ + PAAPsBrAs,

v

w h e r e the s u m m a t i o n with r e s p e c t to # is c a r r i e d out o v e r all the o r b i t a l s with c e n t e r s on a t o m A, and the s u m m a t i o n with r e s p e c t to v is c a r r i e d out o v e r all the o r b i t a l s with c e n t e r s on a t o m B; P ~ v and l~p v a r e e l e m e n t s of the m a t r i c e s of the b o n d o r d e r s of the e l e c t r o n s with s p i n a or/3; TAA and TAB a r e the o n e a n d t w o - c e n t e r C o u l o m b i n t e g r a l s of e l e c t r o n - - e l e c t r o n r e p u l s i o n ; P A A = 2 P/#~ is the p o p u l a t i o n of a t o m IX

A; /3pv is an o f f - d i a g o n a l e l e m e n t , and Up# is a d i a g o n a l e l e m e n t of the H a m i l t o n i a n of the skeleton; RAB is the d i s t a n c e b e t w e e n the a t o m s A and I3. The p a r a m e t e r VAB, which d e s c r i b e s the i n t e r a c t i o n b e t w e e n the v a l e n c e e l e c t r o n on a t o m A a n d the s k e l e t o n of a t o m B, is c a l c u l a t e d f r o m the r e l a t i o n s h i p VAB =

ZBYAB

T a b l e 1 p r e s e n t s the m a s s n u m b e r s , t y p e s of i o n s , and e x p e r i m e n t a l i n t e n s i t i e s of the s p l i n t e r ions f o r m e d as a r e s u l t of the f r a g m e n t a t i o n of the m o l e c u l a r ions. F r o m the t a b l e it is s e e n that the field m a s s T A B L E 1. C h a n g e s in the T w o - C e n t e r C o n t r i b u t i o n s * AA/3 in the T r a n s i t i o n f r o m the A c e t a l M o l e c u l e s to T h e i r P o s i t i v e Ions in the P r e s e n c e and A b s e n c e of a n E l e c t r i c Field]" AAB

Bond

Dime~oxymethane (I) F~0

(C1--H) (C1--O) (Cl__C2) (C~--Cr (ca--Q) (o--c) (C__CHa)

[1,1-Dimethoxyethane(II) 1,1-Dirnethoxypropane (III]l.l-Dimethoxybutane(IV) t,l-Diethoxyethane (V)

F~002

--0,0691 --0,0751 --0,0354 --0,0394 --. . . . . . --0,0323 --0,0355 . . .

F=0

--0,0204 --0,0312 --0,0731 . . . --0,0383 .

F~O,02

--0,0232 --0,0386 --0,0802 . . --0,0397 .

F=0

--0,0201 --0,0683 --0,1283 0,0350 . --0,0300 .

F=0.02

--0,0073 --0,0382 --0,1697 --0,0388 --0,0308

[

F~0

F~0,02

F=0

F=0,02

--0,0098 --0,0434 --0,1203 --0,0526 --0,0887 --0,0300

--0,0114 --0,0525 --0,1655 --0,0542 --0,0913 --0,0306

--0,0181 --0,0318 --0,0758

--0,0203 --0,0405 --0,0756

-0,0504 --0,0398

--0,0503 --0,0501

*The values of the differences AB are given in atomic units, i.e., I at. unit= 27.21 eV. SThe values of the field strength F are given in atomic units, i.e., 1 at. unit= 5.1436 • 10 9 V / c m . 38

. . . . . . . . . . . . . . . . .

a

b a

Fig. 1

0

/,0~ . . . . . . . . . . . .

~

f

5

,

- -

~

Fig. 2

F i g . 5. D e p e n d e n c e of the c h a r g e Q ( c u r v e s a and a') and the spin d e n s i t y p ( c u r v e s b and b ') of the CH a g r o u p in ion III on t h e l e n g t h e n i n g of the C 2 - - C 3 bond: a, b) in the p r e s e n c e of an e l e c t r i c f i e l d (F = 0.02); a ' , b') in the a b s e n c e of a f i e l d (F = 0). F i g . 6. the CH 3 F=0.02 function 0.02 (a)

D e p e n d e n c e of the c h a r g e Q in ion IV: 1) on g r o u p a s a f u n c t i o n of the C a -- C 4 bond with (a) a n d F = 0 (b); 2) on t h e C a H 7 g r o u p a s a of the l e n g t h e n i n g of the C l - - C 2 bond with F = and F = 0 (b).

spectra of the acetals contain ions formed as a result of the cleavage of all the C -- C and C -- O bonds and some of the C - - H bonds. The current of splinter ions is approximately 3/4 of the total ionic current. Table 2 presents of the values of the changes in the two-center contributions in the transition from the molecule to its positive ion with and without the application of the field. In ion I the bond which is weakened to the greatest extent is the Ct -- H bond, whose cleavage can result either in the formation of the ion [CHaOCHOCHa] + and a hydrogen atom or in the formation of a neutral radical with the same composition and a proton. From the distribution of the charge in the molecular ion it is impossible to draw any unambiguous conclusions concerning the nature of the cleavage of a particular bond. However, if we c a r r y out a series of calculations with gradual lengthening of the Ct --H bond, then, as we see from Fig. 1, the charge on the hydrogen atom tends to zero, i . e . , the cleavage of this bond in the presence and absence of the field must involve the formation of a hydrogen atom and the [CH3OCHOCH3] + with a mass number of 75 amu, r e gardless of whether we remove the hydrogen atom from ion I along the direction of the field (Fig. 1, curve 2) or along the bond under consideration (curve 1). The low intensity of the line of the splinter ion (M -- 1)+ with a mass number of 75 ainu may be attributed to the fact that, besides the rapid cleavage of the hydrogen atom from ion I, which occurs immediately after the ionization of the molecule, there is "slow" cleavage of the Ci --H bond, which results in the appearance in the spectrum of a line of a metastable ion with an apparent mass number of 74 ainu. Such cleavage of a bond occurs both in the region of the action of the field and in a field-free space, in both cases involving the cleavage of a hydrogen atom (Fig. 1, curves 3 and 4). Since the particle cleaved (the hydrogen atom) is located on the more distant side of the bond being cleaved from the source of the positive charge, in this case the field hinders dissociation. As a result, the intensity of the line of the ion obtained as a result of such cleavage is reduced, while in the case of decomposition in a field-free space the effect of the field is a b s e n t . This causes the high intensity of the line corresponding to an ion with an apparent mass number of 74 ainu. In the field mass spectra of the remaining acetals there are no lines corresponding to the cleavage of hydrogen atoms from a molecular ion. This is consistent with the theoretical data (see Table 2), which indicate that the value of A(CI -- H) for these compounds is much smaller than in the case of dimethoxymethane and reaches values on the order of 0.0200-0.0100. For carbon-- hydrogen bonds in alkyl groups of all the compounds investigated this quantity equals 0.0030-0.0080, indicating that the probability of their cleavage under the conditions of field mass spectrometry is low. When the molecule of I undergoes ionization, the CI -- O and O -- CH3 bonds are weakened almost to the same extent; therefore, the probabilities of the decomposition of the molecular ion at these bonds should be of approximately the same order. The method dissociating the groups of atoms from the case of

39

#

JY

F i g . 7. C h a n g e s in the c h a r g e Q in ionVwithF = 0 . 0 2 : 1) on t h e C 2 H 5 g r o u p a s a f u n c t i o n of the l e n g t h e n ing of t h e O - - C bond; 2) on t h e CH 3 g r o u p a s a f u n c t i o n of the l e n g t h e n ing of t h e C 1 - C 2b0nd; 3) on the CH 3 g r o u p a s a f u n c t i o n of the l e n g t h e n i n g of the C - CH 3 bond; 4) on the OCH2CH 3 g r o u p a s a f u n c t i o n of the l e n g t h e n i n g of the C 1 - - O bond.

the c l e a v a g e of the C l - - O b o n d d o e s not m a k e it p o s s i b l e to u n i q u e l y d e t e r m i n e t h e l o c a t i o n of the p o s i t i v e c h a r g e , s i n c e the s o l u t i o n d i v e r g e s when the b o n d i s l e n g t h e n e d b y 0.8 ~ . The c h a r g e Q on the CH3OCH 2 g r o u p r e a c h e s a v a l u e of +0.60 a s it i s v a r i e d ( F i g . 2). T h i s s u g g e s t s t h a t the c l e a v a g e of t h i s bond, u n l i k e t h a t in the p r e c e d i n g c a s e , m a y i n v o l v e the f o r m a t i o n e i t h e r of the [OCH3] + ion with a m a s s n u m b e r of 31 ainu o r an ion with a m a s s n u m b e r of 45 ainu with s o m e p r e d o m i n a n c e of the l a t t e r . T h i s is c o n s i s t e n t with the e x p e r i m e n t a l d a t a (see T a b l e 1). Such a l a w , i . e . , t h e p r e d o m i n a n c e of the l i n e of the (M - - 31) s p l i n t e r o v e r the l i n e of the [OCH3] + ion in the f i e l d m a s s s p e c t r a is o b s e r v e d f o r a l l the c o m p o u n d s we i n v e s t i g a t e d with the e x c e p t i o n of 1 , 1 - d i e t h o x y e t h a n e (V), f o r w h i c h the f o r m a t i o n of a m e t h o x y p a r t i c l e is i m p o s s i b l e a s a r e s u l t of the s i m p l e c l e a v a g e of b o n d s w i t h o u t r e a r r a n g e m e n t p r o c e s s e s . As the c a l c u l a t i o n s h a v e shown, f o l l o w i n g the c l e a v a g e of the O - - CHa b o n d in ion I the p o s i t i v e c h a r g e i s l o c a l i z e d c o m p l e t e l y on the m e t h y l g r o u p ( F i g . 3, c u r v e 1), r e s u l t i n g in the f o r m a t i o n of a CH + ion With a m a s s n u m b e r of 15 a m u , w h o s e l i n e i s p r e s e n t in the e x p e r i m e n t a l m a s s s p e c t r u m of d i m e t h o x y m e t h a n e . In the p o s i t i v e m o l e c u l a r i o n s of 1, 1 - d i m e t h o x y e t h a n e (II), 1 , 1 - d i m e t h o x y p r o p a n e (III), and 1 , 1 - d i m e t h o x y b u t a n e (IV) the d i s s o c i a t i o n p r o d u c t s of t h i s b o n d a r e t h e n e u t r a l CH 3 r a d i c a l and a c h a r g e d (M - - 15) ion (see, f o r e x a m p l e , F i g , 3, c u r v e 2). T h e m a i n c h a i n s a r e a n o t h e r s o u r c e of m e t h y l p a r t i c l e s f o r s u c h m o l e c u l a r i o n s . In the c a s e of ion II, we h a v e A(C l - - C 2) = - - 0 . 0 8 0 2 , w h i c h p o i n t s out the p o s s i b i l i t y of the c l e a v a g e of t h i s bond. A s we s e e f r o m F i g . 3, c u r v e 3, i t s d i s s o c i a t i o n a l s o r e s u l t s in t h e f o r m a t i o n of a n e u t r a l m e t h y l r a d i c a l and t h e [CH3OCHOCH3] + ion, w h o s e i n t e n s i t y in the e x p e r i m e n t a l m a s s s p e c t r u m r e a c h e s 22.8% of t h e t o t a l ionic c u r r e n t of the s p l i n t e r s . T h e d e c o m p o s i t i o n of m o l e c u l a r i o n s III a n d IV, r e s u l t s in the f o r m a t i o n of C2H 5and C3H7 p a r t i c l e s , a s w e l l a s (M - - 29) and (M - - 43) s p l i n t e r s , a s a r e s u l t of the c l e a v a g e of b o n d s in the m a i n c h a i n , in a d d i t i o n to the f r a g m e n t s l i s t e d in t h e f o r e g o i n g . In the c a s e of 1 , 1 - d i m e t h o x y p r o p a n e , the C 1 - - C 2 b o n d (see T a b l e 2), w h o s e c l e a v a g e r e s u l t s in p r a c t i c a l l y i d e n t i c a l p r o b a b i l i t i e s f o r the l o c a l i z a t i o n of the p o s i t i v e c h a r g e on the C2H 5 and CH(OCH3) 2 f r a g m e n t s f o r m e d , a s i n d i c a t e d b y the e x p e r i m e n t a l m a s s s p e c t r u m , is w e a k e n e d to the g r e a t e s t e x t e n t . The c a l c u l a t i o n , h o w e v e r , s h o w s (Fig. 4) t h a t the d i s s o c i a t i o n p r o d u c t s in t h i s c a s e d e p e n d on the a p p l i c a t i o n of the e x t e r n a l f i e l d . In the z e r o f i e l d the b o n d is c l e a v e d with the p r e d o m i n a n t f o r m a t i o n of t h e [CH(OCH3)2] + ion. The a p p l i c a t i o n of the f i e l d r e s u l t s in a s i t u a t i o n in which the p o s i t i v e c h a r g e is l o c a l i z e d on a n o t h e r f r a g m e n t , w h i c h is l o c a t e d f u r t h e r f r o m the s o u r c e of the p o s i t i v e e l e c t r i c i t y , i. e . , on the C2H 5 g r o u p , a n d in the f i e l d m a s s s p e c t r u m of 1 , 1 - d i m e t h o x y p r o p a n e the l i n e s of the ion with the m a s s n u m b e r of 29 a m u b e c o m e d o m i n a n t . T h i s e x a m p l e g r a p h i c a l l y s h o w s t h a t b e s i d e s i t s m a i n f u n c t i o n , w h i c h c o n s i s t s of the i o n i z a t i o n of m o l e c u l e s , the e x t e r n a l e l e c t r i c f i e l d h a s a s i g n i f i c a n t i n f l u e n c e on the f o r m a t i o n of the f i e l d m a s s s p e c t r u m . The C i - C 3 b o n d in ion III i s a l s o a p p r e c i a b l y w e a k e n e d a s a r e s u l t of i o n i z a t i o n . F i g u r e 5, which p r e s e n t s the d e p e n d e n c e of the c h a r g e Q a n d the s p i n d e n s i t y p of the m e t h y l g r o u p on the l e n g t h e n i n g of the b o n d u n d e r c o n s i d e r a t i o n , s h o w s t h a t , r e g a r d l e s s of the f i e l d s t r e n g t h , i t s c l e a v a g e r e s u l t s in the f o r m a t i o n of the s a m e p r o d u c t s , m o r e s p e c i f i c a l l y , the n e u t r a l CH2CH(OCH3) 2 r a d i c a l and the CH + ion, the i n t e n s i t y of w h o s e l i n e s r e a c h e s 7.8% in the e x p e r i m e n t a l m a s s s p e c t r u m . T h e a b s e n c e of a l i n e f o r the (M - - 15) ion, w h i c h would b e f o r m e d a s a r e s u l t of t h e c l e a v a g e of t h e O - - C bond, in the s p e c t r u m of c o m pound III is a p p a r e n t l y due to the r e l a t i v e l y s m a l l e x t e n t of w e a k e n i n g of t h i s b o n d d u r i n g the i o n i z a t i o n of the m o l e c u l e . T h e s p e c t r u m of 1 , 1 - d i m e t h o x y b u t a n e is the r i c h e s t in l i n e s of s p l i n t e r i o n s . The m o s t i n t e n s e is the l i n e of the ion with a m a s s n u m b e r of 15 a m u . T h i s ion f o r m s a s a r e s u l t of the Cleavage o f t h e C ~ - C4bond in the m a i n c h a i n . F o l l o w i n g i t s c l e a v a g e , the p o s i t i v e c h a r g e is l o c a l i z e d , a s in the p r e c e d i n g c a s e , on the CH 3 g r o u p , r e g a r d l e s s of t h e p r e s e n c e o r a b s e n c e of the e l e c t r i c f i e l d (Fig. 6). A s s e e n in T a b l e 2, the C 1 - - C 2 b o n d i s w e a k e n e d to the g r e a t e s t e x t e n t in ion IV. T h i s is not in t o t a l a g r e e m e n t with the e x p e r i m e n t a l

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data. The calculation shows that when this bond is lengthened, the dissociation products of the ion v a r y as a function of the field strength (see Fig. 6). This suggests that the probability of the positive c h a r g e being localized on any of the f r a g m e n t s is a function of the field, and since in the e x p e r i m e n t it is v e r y difficult to achieve a definite value of the field, the m a s s s p e c t r u m g e n e r a l l y shows lines of ions which fit both p o s sible f r a g m e n t s . The dissociation of the C 2 - C 3 bond involves the formation of the C2H~ ion, r e g a r d l e s s of the field strength. This ion c o r r e s p o n d s to a line with a m a s s number of 29 amu in the experimental m a s s spectrum. The fragmentation of the positive ion of V differs slightly f r o m the p r e v i o u s l y c o n s i d e r e d c a s e s . This is due to complicating of the s t r u c t u r e of the side chains. Unlike 1,1-dimethoxyethane, ionV d i s s o c i a t e s at the C1 -- C2 bond, which is weakened to the g r e a t e s t extent upon ionization into the CH~ ion and a neutral CH(OCH2CH3) 2 f r a g m e n t (Fig. 7). As we see in this figure, the cleavage of the methyl group f r o m the side chains r e s u l t s in the formation of an (M -- 15) + ion. After the decomposition of the m o l e c u l a r ion of 1,1diethoxyethane at the O - - C bond, the positive c h a r g e is c o m p l e t e l y localized on the C2H 5 group. This r e sults in the appearance of a line with a m a s s number of 29 amu in the field m a s s spectrum. Unlike the c a s e of the dimethoxyalkanes, here the lengthening of the Ci -- O bond allows us to draw an unambiguous conclusion that its cleavage r e s u l t s in the formation of the [CH3CH2OCHCH3] + and the neutral particle OCH2CH3. On the basis of the r e s u l t s of the p r e s e n t investigation and [7, 8] it m a y be concluded that the approach we have proposed is applicable only for a qualitative interpretation of the part of the field m a s s s p e c t r u m which is due to the splinter ions. Consideration of the quantitative dependences in field m a s s s p e c t r a r e q u i r e s us to call upon finer fragmentation m e c h a n i s m s , in p a r t i c u l a r , the t h e o r y of vibrational excitations in m o l e c u l a r ions [12, 13]. By starting only f r o m the distribution of the positive c h a r g e in the m o l e c u l a r ion formed, as has been suggested, for example, in [2], it is impossible to c o r r e c t l y predict the dissociation products for any p a r t i c u l a r bond in this ion. It is n e c e s s a r y to investigate the behavior of the c h a r g e of the group under consideration as the bond being cleaved is lengthened. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

CITED

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