ELECTROCHEMICALLY

GENERATED

FREE

RADICALS

COMMUNICATION 3. E P R SPECTRA OF ANION RADICALS OF CERTAIN DERIVATIVES OF AROMATIC CARBOXYLIC ACIDS

A. V. I i ' y a s o v , Yu. M. Kargin, Ya. L. Levin, I. D. Morozova, and N. N. Sotnikova

UDC 541.138.3:547.58'29+541.515+538.113

The study of the hyperfine s t r u c t u r e of the E P R lines of f r e e r a d i c a l s is of undoubted i n t e r e s t , since it yields i n f o r m a t i o n on the distribution of spin density and on the fine details of the s t r u c t u r e of organic compounds. This communication p r e s e n t s the r e s u l t s of an investigation of the E P R s p e c t r a of anion r a d i c a l s of c e r t a i n d e r i v a t i v e s of a r o m a t i c c a r b o x y l i c acids. The p o s s i b i l i t y of the f o r m a t i o n of such anion r a d i c a l s was d e m o n s t r a t e d in our e a r l i e r r e p o r t [1]. EXPERIMENTAL

a

. ~ G / Q R -

-=

~0

f d Fig. 1. E P R s p e c t r a of anion r a d i c a l s of e s t e r s of benzoic acid: methyl (a); t r i d e u t e r o m e t h y l (b); ethyl (c); i s o p r o p y l (d); cyclohexyl (e); and phenyl (f).

The anion r a d i c a l s w e r e produced by e l e c t r o c h e m i c a l reduction in a vacuum cell according to the p r o c e d u r e d e s c r i b e d in [2]. The E P R s p e c t r a w e r e r e c o r d e d on a s t a n d a r d Rl~-1301 r a d i o s p e c t r o m e t e r with f r e q u e n c y 9320 MHz in the t e m p e r a t u r e i n t e r v a l 200-350~ D i m e t h y l f o r m a m i d e and in c e r t a i n c a s e s a c e t o n i t r i l e solutions with concentrations of the d e p o l a r i z e r a n d b a s i c e l e c t r o l y t e (Et4NI o r Me4N1) 1 . 1 0 -3 and 0.08 M, r e s p e c t i v e l y , w e r e studied. T r i d e u t e r o m e t h y l e s t e r s w e r e s y n t h e s i z e d f r o m t r i d e u t e r o m e t h a n o l and the c o r r e s p o n ding acid c h l o r i d e s . The d e g r e e of deuteration, d e t e r m i n e d m a s s s p e c t r o m e t r i c a l l y , was no l e s s than 99.3%.

Table 1 p r e s e n t s the values of the constants of the h y p e r fine s t r u c t u r e , d e t e r m i n e d f r o m the o b s e r v e d s p e c t r a in a c c o r d with [3, 4], and the t h e o r e t i c a l distribution of spin density. The spin density was calculated according to the Hiickel m o l e c u l a r o r b i t a l (HMO) method, as well as in the a p p r o x i m a t i o n of McLachlan [5]. The m e t h y l group was c o n s i d e r e d according to the m o d e l of hyperconjugation. Data on the values of the s e l e c t e d p a r a m e t e r s a r e cited in T a b l e ! ; the p a r a m e t e r 6 r e n d e r s the change in the c o u lombic i n t e g r a l for the h e t e r o a t o m in c o m p a r i s o n with the c a r b o n atom ok = o~C + 6 f l C C , while 7 shows how m a n y t i m e s the r e s o n a n c e i n t e g r a l of the c a r b o n - h e t e r o a t o m bond exceeds the i n t e g r a l of the C - C bond:

Pcx = ~ c c . The E P R s p e c t r u m of the anion r a d i c a l of m e t h y l benzoate (Fig. 1) c o n s i s t s of a b a s i c quintet (1 : 2 : 1.8 : 2 : 1), e a c h of the components of which is split into six lines. Such a l a r g e n u m b e r of o b s e r v a b l e lines cannot be explained by i n t e r a c t i o n of the u n p a i r e d e l e c t r o n only with the t h r e e types of protons of the ring (p-, o - , and m - p r o t o n s ) . It might be a s s u m e d that e i t h e r the protons of the c a r b o m e t h o x y l groups p a r t i c i pate in this splitting, o r inhibition of r o t a t i o n of the c a r b o n y i group around the bond with the ring leads to nonequivalence of the two o - and two m - p r o t o n s , as was found for c e r t a i n a r o m a t i c aldehydes and ketones [6-9]. In o r d e r to r e s o l v e the question of the p a r t i c i p a t i o n of the protons of the m e t h y l g r o u p in the h y p e r fine splitting, we o b s e r v e d the E P R s p e c t r u m of the anion r a d i c a l of t r i d e u t e r o m e t h y l benzoate. A. E. A r b u z o v Institute of Organic and P h y s i c a l C h e m i s t r y , A c a d e m y of Sciences of the USSR. T r a n s lated f r o m I z v e s t i y a A k a d e m i i Nauk SSSR, S e r i y a K h i m i c h e s k a y a , No. 5, pp. 1030-1035, March, 1968. Original a r t i c l e s u b m i t t e d July 17, 1967.

982

TABLE 1 Anion radical

-

,~

~-3,~-

,,

jF

3 ~'-] :2

9L

13

s

,o ~

v

t~13 ~2 H3C--O"

v |i

4

I

.~

3 2,t --O-CFi~

~ ~

j

L '" &,6 3C-~:~ ,6

, a

L

~/'6 .~6

~1 I

,5

j

t

II2 7

'x~c,.U

i

I

'

a

Constant of spin density Ihyperfine Atom 9structure founda calculatedb 1(• oe) 1 0,003 0,005 2 --0,001 3 0,013 4 0,238 5 0,096 6 0,151 7,11 4,{ 0,150 0,t42 8,10 1 0,037 --0,037 9 "0,280. 8,4 0,311 1 0,015 2 15 --0,003 3 14 0,008 4 0,009 5 0,005 5 0,239 7 0,104 8 0,129 9,13 0,157 q, 153 4,5 lO i2 0,044 .--0,042 1,2 II 0,3119 0,270 8,4 l 14 0,002 0,8 01005 2 13 --0,001 3 12 0,009 4 lO 0,128 5 ~1 0,035 6 0,189 1,7 0,063 0,053 t5 5 1,18 2 15 3,14 4,12 5,13 6,11 7,10 8,9 1,13 2.12 3~11 4,9 5,10 6,8 7 14,16 15 1 2,10 3,11 4,9 5,8 6,7

0,9

O,OO5

I

0,041c

3,8 0,9

0,157 c

(1) d

(0,037) 9 0,340 (0,037)

0,005

0,4 2,8'3

0,015 0,102

--0,007 0,001 0,003 0,031 0,090 0,257 --0,059 0,184 0,001 0,000 0,006 0,112 0,045 0,071 --0,033 0,325 --0,085 0,001e 0,135 0,113 0,140 0,021 0,090-

H H IQcH 1 =27OeIQcH31=169Oe.

b T h e c a l c u l a t i o n s w e r e p e r f o r m e d a t 6 0 = 1.9; 7 C _ O = 0.9; 60 = 1.5; YC=O = 1.6; f o r t h e OCH 3 g r o u p :

6H3

=

- 0 . 3 , Y O - C = 0.7; T C _ H 3 = 2;

t h e c o n s t a n t in t h e M c L a c h l a n e q u a t i o n is e q u a l to 1.2. H

o [QcH[= 24.20e.

d T h e c o n s t a n t of t h e h y p e r f i n e s t r u c t u r e p e r t a i n s to t h e p r o t o n e i t h e r in t h e 7 - p o s i t i o n o r in t h e 1 5 - p o s i t i o n . e D i s t r i b u t i o n of t h e s p i n d e n s i t y a c c o r d i n g to Hiickel. T h e d e c r e a s e in t h e n u m b e r of l i n e s in e a c h c o m p o n e n t of the q u i n t e t f r o m s i x to t h r e e , w h i l e p r e s e r v i n g t h e v a l u e s o f t h e s p l i t t i n g s , is c o n v i n c i n g e v i d e n c e in s u p p o r t of t h e f i r s t h y p o t h e s i s . * F i v e b a s i c *As i s w e l l k n o w n , s p l i t t i n g f r o m d e u t e r i u m is n o t o b s e r v e d on a c c o u n t o f t h e low v a l u e of i t s m a g n e t i c moment.

983

e

Fig. 2

b

c

F~. 3

Fig. 2. E P R s p e c t r a of the anion r a d i c a l s of e s t e r s of t e r e p h t h a l i c acid: dimethyl (a); h e x a d e u t e r o d i m e t h y l (b); diethyl (c); diisopropyl (d); d i t e r t - b u t y l (e), and diphenyl (f). Fig. 3. E PR s p e c t r a of the anion r a d i c a l s of phthalic anhydride at 239~ (a) and 240~ (b); t e t r a c h l o r o p h t h a l i c anhydride at 293~ (c). lines a r i s e in the interaction of the unparied e l e c t r o n with the p - p r o t o n and two equivalent o - p r o t o n s a t a_/a~ 44 ~ 2 The two e q u i v a l e n t m - p r o t o n s split the lines of the quintet into t r i p l e t s . In the nondeuterated compounds, the hyp e r f i n e splitting constants of these m - p r o t o n s a r e c o m p a r a b l e with the hyperfine splitting constants of the methyl p r o t o n s , which also leads to the o b s e r v e d splitting of the lines of the quintet into six components. Evidence of the p a r t i c i p a t i o n of the protons of the c a r b a l k o x y l s in the hyperfine splitting is also found in the s u c c e s s i v e d e c r e a s e in the n u m b e r of s u p p l e m e n t a r y splittings of the lines of the quintet as we p a s s f r o m the methyl e s t e r to the ethyl e s t e r and then to the i s o p r o p y l and cyclohexyl e s t e r s (see Fig. 1). The calculated data a r e in sufficiently s a t i s f a c t o r y a g r e e m e n t with the e x p e r i m e n t a l data. The E P R s p e c t r a of the anion r a d i c a l s of t e r e p h t h a l a t e s also contain an unexpectedly l a r g e n u m b e r of hyperfine components (Fig. 2). By analogy with the l i t e r a t u r e data [6, 7], it might have been thought that we w e r e dealing with a s u p e r p o s i t i o n of the s p e c t r a f r o m two f o r m s of r a d i c a l s : c i s - and t r a n s - r o t a m e r s . H o w e v e r , the r e p l a c e m e n t of the m e t h y l protons by d e u t e r i u m in the dimethyl t e r e p h t h a l a t e molecule leads to a d e c r e a s e in the n u m b e r of lines of the hyperfine s t r u c t u r e to five, with an intensity r a t i o of 1 : 4 : 6 : 4 : 1. This m e a s n that the u n p a i r e d e l e c t r o n i n t e r a c t s with the four equivalent protons of the ring. F r o m this it follows that in the s p e c t r u m of the nondeuterated compound, the s u p p l e m e n t a r y splitting of the quintet is due to an i n t e r a c t i o n of the u n p a i r e d e l e c t r o n with the protons of the methyl group. F o r the anion r a d i c a l of diethyl t e r e p h t h a l a t e , 13 lines a r e o b s e r v e d , with an intensity r a t i o close to 1 : 4, 2 : 10, 6 : 23 : 36, 7 : 48 : 36, 7 : 23 : 10.6 : 4.2 : 1, which s a t i s f i e s the t h e o r e t i c a l l y c o n s t r u c t e d s p e c t r u m . In the E P R s p e c t r a of the anion r a d i c a l s of d i i s o p r o p y l , dicyclohexyl, and diphenyl t e r e p h t h a l a t e s , just as in the s p e c t r u m of d e u t e r a t e d d i m e t h y l t e r e p h t h a l a t e , a quintet is o b s e r v e d , the components of which a r e b r o a d e n e d on account of the magnetic m o m e n t s of the protons of the e s t e r g r o u p s (see Fig. 2). F o r the d i t e r t - b u t y l t e r e p h t h a l a t e anion r a d i c a l , a distinct quintet (1 : 4 : 6 : 4 : 1) f r o m the equivalent protons of the ring is o b s e r v e d , the components of which a r e n a r r o w e r than in the p r e c e d i n g c a s e s . The E P R s p e c t r u m of the anion r a d i c a l of phthalic anhydride, consisting of t h r e e b a s i c lines with int e n s i t y r a t i o 1 : 2 : 1, is due to the i n t e r a c t i o n of the u n p a i r e d e l e c t r o n with the two equivalent protons (Fig. 3). A t h e o r e t i c a l calculation of the spin density according to the HMO method p e r m i t s the o b s e r v e d t r i p l e t to be a s c r i b e d to the two equivalent fi-protons. This r e s u l t a g r e e s with the data for the anion r a d i c a l s of o - n i t r o b e n z e n e , phthalodinitrile, and benzocyclobutenedione [10]. When the t e m p e r a t u r e of the solution of the anion r a d i c a l is l o w e r e d , s u p p l e m e n t a r y splitting of the t r i p l e t , due to the r e m a i n i n g two protons of the r i n g , is also o b s e r v e d . J u s t as we should have expected, the E P R s p e c t r u m of the anion r a d i c a l of t e t r a ehlorophthalic anhydride c o n s i s t s of one line (6 H = 0.5 Oe). The s m a l l p e a k s at the edges of the line a r e evidently due to hyperfine i n t e r a c t i o n with the magnetic nucleus C 13.

984

r % z,4oe

Fig. 4, E P R s p e c t r a of anion radicals of e s t e r s of phthalic acid: dimethyl (a); hexadeuterodimethyl (b); diethyl (c). The o b s e r v e d EPR s p e c t r u m of the anion radical of h e x a deuterodimethyl phthalate (Fig. 4) consists of t h r e e basic lines of the hyperfine s t r u c t u r e , arising in the interaction of the unpaired e l e c t r o n with the two equivalent fi-protons. This a s s e r t i o n is based on the data of a calculation of the distribution of spin density (see Table !) and a g r e e s with the aforementioned. The basic t r i p l e t also contains supplementary splitting with a constant of ~ 1 0 e . T h u s , in the t r a n s i t i o n from the anion radicals of phthalic anhydride to the anion r a d i c a l s of phthalates, an appreciable spin denb sity appears on the two equivalent s-protons~ The splitting of the components of the t r i p l e t into seven lines in the s p e c t r a of the anion Fig. 5. E P R s p e c t r a of anion r a d i c a l s of e s t e r s of isophthalic acid: hexadeu- r a d i c a l s of diethyl and dibutyl phthalates (see Fig. 4) is associated with the four protons of the e s t e r r e s i d u e s , the hyperfine splitting t e r o d i m e t h y l (a) and dimethyl (b). constants of which are comparable with the hyperfine splitting constants of the s - p r o t o n s of the ring (in the c a s e of the methyl e s t e r , we should have expected nine supplem e n t a r y lines, but just as for the corresponding anion r a d i c a l of terephthalate, part of them a r e unresolved}. F o r the dimethyl isophthalate anion r a d i c a l , a triplet is o b s e r v e d , due to the interaction of the unpaired e l e c t r o n with the two equivalent protons of the ring (Fig. 5). An analysis of the E P R s p e c t r u m of the anion r a d i c a l of hexadeuterodimethyl isophthalate and a t h e o r e t i c a l calculation of the splitting of the spin density shows that f u r t h e r splitting of the triplet is due to one of the ring protons (7- or ! 5 - a t o m ) and the protons of the methyl groups (see Table 1). Thus, the EPR s p e c t r a of anion radicals of e s t e r s of a r o m a t i c carboxylic acids can be explained without r e s o r t i n g to concepts of r o t a m e r i s m . This means that the rotation frequency of carbalkoxyls around the bond with the ring in these anions radicals is no lower than the frequency of hyperfine splitting of the ring protons (i.e., 1.2. l0 T Hz for benzoates and 4.8.106 Hz for terephthalates). The independence of the nature of the observed EPR spectra from the temperature within the range 200-350~ also confirms the absence of any appreciable hindrances to free rotation of carbalkoxyls. The appearance of spin density on the protons of the carbalkoxyl groups may be explained by several factors, chief among which are the spin polarization and hyperconjugation. In the latter case, a substantial contribution may be made by s t r u c t u r e s of the type

,2---%--C=(~LR ~/

I " The r o l e of each of these effects is o r a t h e r difficult to e s t i m a t e quantitatively, since the spin density on the protons of the methyl groups is v e r y low. Our t h e o r e t i c a l data on the value of the spin density on these protons (see Table t} should not be given absolute significance, since in the calculation we attempted to obtain only a qualitative picture of the d i s t r i bution of spin density o v e r the molecule. The authors are grateful to Yu. Ya. E f r e m o v for the mass s p e c t r o m e t r i c analysis of the deuterated compounds.

985

CONCLUSIONS i. The EPR spectra of electrochemically generated anion radicals of esters of certain aromatic carboxylic acids and phthalie anhydride were obtained. 2. The observed hyperfine structure was explained by the appreciable spin density both on the protons of the benzene ring and on the ~-protons of the ester groups. 3. The data obtained are evidence of rotation of the carbalkoxyl groups in the anion radicals around the bond with the ring with a frequency no lower than 5.106 Hz. 4. The results of a theoretical calculation of the spin density in the investigated anion radicals agree with the experimental data. LITERATURE

1.

2. 3.

4. 5. 6. 7. 8. 9. i0.

986

CITED

A.V. Ii'yasov, Yu. M. Kargin, Ya. L. Levin, and V. Kh. Ivanova, Izv. Akad. Nauk SSSR, Ser. Khim., 583 (1966). A.V. I1'yasov,Yu. M. Kargin, Ya. A. Levin, I. D. Morozova, N. N. Sotnikova, V. Kh. Ivanova, and N. I. Bessolitsyaa, Izv. Akad. NaukSSSR, Ser. Khim., 740 (1968). B.M. Kozyrev,Yu. V. Yablokov,R. O. Matevosyan,M. A. ]krina, Yu. V. ll'yasov, Yu. M. Ryzhmanov, L. I. Stashkov, and L. F. Shatrukov, Optika i Spektroskopiya, 15, 625 (1963); A. E. Arbuzov, F. G. Valitova, A. V. ll'yasov, B. M. Kozyrev,and Yu. V. Yablokov, Dokl. Akad. NaukSSSR, 147, 99 (1962). Atlasof EPR Spectra [in Russian], nNauka~ (1964). A.D. McLachlan,Mol. Phys., 3,233 (1960). A.H. Maki and D. H. Geske, J. Amer. Chem. Soc., 83, 1852 (1961). A.H. Maki, J. Chem. Phys., 35, 761 (1961); E. W. Stone and A. H. Maki, J. Chem. Phys., 38, 1999 (1963). P.H. Rieger and G. K. Fraenkel, J. Chem. Phys., 37, 2811 (1962). N. Steinberger and G. K. Fraenkel, J. Chem. Phys., 40, 723 (1964). D.H. Geske and A. L. Balch, J. Phys. Chem., 68, 3423 (1964).