Muonium-substituted organic free radicals in liquids. Isomer distribution and end-of-track radiolytical processes determined from studies of cyclohexadienyl radicals derived from substituted benzenes

Chemical Physics 88 (1984) 143-153 " North-Holland, Amsterdam

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143

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M U O N I U M - S U B S T I T U T E D O R G A N I C F R E E R A D I C A L S IN L I Q U I D S . ISOIMER D I S T R I B U T I O N A N D E N D - O F - T R A C K R A D I O L Y T I C A L P R O C E S S E S DETERMINED FROM STUDIES OF CYCLOHEXADIENYL RADICALS DERIVED FROM SUBSTITUTED BENZENES Emil R O D U N E R Physikalisch-Chemisches Institut der Unir'ersitSt, ;Vinterthurerstrasse 190, CH-8057 Zurich. Switzerland

and Gerard A. B R I N K M A N and Pieter W.F. L O U W R I E R Chemistry Department, National Institute for Nuclear Physics and High Energy Physics (NIKHEF-K), Postbox 4395. 1009 A J Amsterdm*~ The Netherlands Received 5 D e c e m b e r 1983; in final form 15 M a r c h 1984

M u o n i u m - s u b s t i t u t e d free radicals are observed by m u o n spin rotation when positive m u o n s are stopped in liquid substituted benzenes. F r o m m u o n precession frequencies in high external magnetic fields the isotropic m u o n - e l e c t r o n byperfine coupling constants At, are determined. 66 radicals are assigned to ortho-, meta- a n d para-substituted cyclohexadienyl-type radicals. Formally they are produced b y addition o f the light hydrogen isotope m u o n i u m to 24 mono-substituted benzenes. T h e distribution of m u o n s between radicals a n d diamagnetic molecules is suggested to be governed by radiolytical processes n e a r the end-of-track o f the thermalizing muon.

1. Introduction Cyclohexadienyl-type radicals have often been prepared via addition o f radiolytically produced hydrogen atoms to aromatic c o m p o u n d s [1-5]. Here, liquid aromats are irradiated with spinpolarized positive muons. This leads to the formation of cyclohexadienyl-type radicals formally via addition of m u o n i u m (Mu, a b o u n d state between a positive muon and an electron, chemically a hydrogen isotope with a mass equal to one ninth the mass of H [6]). They are detected using the ~ S R technique [7]. M a n y Mu-substituted radicals have been observed in the past few years [7-9] since their discovery [10]. They are identified b y their m u o n - e l e c t r o n hyperfine interaction, A~,. In systematic studies on alkyl- and aUyl-type radicals [8] and on methyl- a n d fluorine-substituted cyclohexadienyl radicals [9] it was shown that A~, is related to the radical structures in the same w a y as

the corresponding p r o t o n - e l e c t r o n couplings, Ap, of the analogous H-substituted radicals. Comparison of A~ with Ap revealed isotope effects which d e p e n d on the radical type a n d w e r e ascribed to the different internal dynamics of isotopic molecules. F o r cyclohexadienyl-type radicals A,#p/ A p # , is in the narrow range of 1.15-1.21. Furthermore it was found that the regioselectivity pattern of M u addition to olefins and dienes [8] and to aromatic c o m p o u n d s [9,11] is qualitatively the same as that of H, but the selectivity o f Mu is less. There is a preference for the more energetically stable radical to be formed [12]. Chemists are familiar with substituent effects on the distribution of electron density in substituted benzenes and think a b o u t it in terms of inductive and mesomerie effects. There are, however, few systematic investigations of substituent effects o n the distribution of spin density in con: j u g a t e d systems. This is d u e to the difficulty to

0 3 0 1 - 0 1 0 4 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B_V. (North-Holland Physics Publishing Division)

144

E. Rodwner et a t / Substituted cvclohexadienvl-~'pe muonic radicals

prepare sufficient numbers of isomers under comparable conditions and to the problem of detecting isomers of m i n o r concentration in complex overlapping ESR spectra. The low regioselectivity of M u addition and the simplicity of I~SR spectra allow the preparation and selective detection of large numbers of substituted cyclohexadienyl radicals. The observation and spectral assignment are the primary subjects of this work. A second topic is related to the signal amplitudes. Only a fraction o f all muons are found in radicals. A further fraction corresponds to m u o n s in possibly different diamagnetic environments which are not distinguishable experimentally. The distribution process of muons between different species is not ~ccessible to direct observation since it occurs at times < 10 ns, but obviously it is a strong function of the chemical environment [13]. Early attempts to explain these findings rested on reactions of hot Mu atoms [14]. but later a model based on radiolytic end-of-track effects of the stopping m u o n was found more appropriate for aqueous environments [15]. Recent results show that radiolytic effects play a major role also in benzene [13]. It is of considerable interest to explore whether these processes are responsible for the muon distribution in non-aqueous media in general [16.17]. The applicability of this model will therefore be discussed for the substituted benzenes under investigation.

2. Experimental and data analysis The compounds were purchased mostly from Fluka. except phenol (Sigfried). benzylalcohol (Fisher). biphenyl (Bender) and benzoic acid (Sigfried). They were used as neat liquids, solid compounds as saturated solutions in ethanol (phenol and benzoic acid) and in diethyl ether (biphenyl). Deuterated compounds were synthesized by preparing the Grignard reagent from the chloroaromat and hydrolyzing it in D20. The liquids were degassed and sealed in spherical glass bulbs of 35 m m diameter. T h e experiments were carried out at the muon beam lines of the Swiss Institute for Nuclear Re-

search (SIN) in Villigen. Thereby, the time evolution of the m u o n spin polarization was followed by counting decay positrons in a given direction as a function o f the time spent in the sample by the corresponding muon. A detailed description of experimental procedures was given previously [7]. In this work transverse magnetic f i d d s of I or 3 kg were used. A total of 6 × 107 good events were accumulated in four histograms (2 K channels with a width of 0.87 ns per channel) corresponding to four positron telescopes. Numerical analysis was performed by Fourier transformation following correction for background and exponential decay due to the m u o n lifetime (2.197 i~s). Subsequently, a theoretical lineshape function was fitted to each spectral line to yield frequency and amplitude as parameters of interest here. This procedure has been justified a n d described elsewhere [18]. The hyperfine coupling constant, A N, is directly the sum of the two corresponding radical frequencies [7,8]. The amplitude A, depends on parameters of the experimental arrangement. Therefore, for discussion purposes the fractional m u o n polarizations P, = PsA,/As are more useful than A i. A s is the amplitude of a standard signal corresponding to the m u o n polarization P~. As in our earlier work, carbon tetrachloride was used as a standard, assuming Ps = 1 [9,12]. Small corrections for m u o n stops in the target glass wall and for target size and density effects h a d to be applied. In addition, the m u o n polarization corresponding to muonium-substituted free radicals had to be corrected for a rather large systematic error that arises from the limited experimental time resolution. Details on these corrections have been given prex~iously [9]. The statistical accuracy of PD and individual PR values is _+0.01. The values describing the fractional distribution of Y~Pp. in table I are thus accurate to 2% for EPR = 0.5, and correspondingly less for weaker radical signals. The errors in E P g values are probably _< 10% and are due to uncertainties in the correction factors. These however are similar for all compounds measured. Eventual errors therefore affect all values in the same way. In the figures Fourier power spectra are displayed. Differences in signal amplitudes thus appear enhanced.

&. Roduner et at / Substitured cychhexadtenyl-type

muontc radtcai?

145

Tabie 1 Muon-dectron hyperfine coupling constants and distribution of muon pohwizatron observed with monosubstituted aromatrc compounds, CsH,X, in a transverse magnetic field of 3 kG X

H CH3 Cf+,+

CH,OH C(CH,), CH,CI CHCi, CCI, CF, *’ F Cl Br :H OCH, 00 OCOCH, SH COOH COGCH, coci + CN NC0 NH,, a) ht “} d’ Cl o

Distnbution of ZIP,(%)

R, (MHz) *’ ortho

!?“a

meta

S14.6

514 6 496.4 b, 498.2 496.0

514 6

0.15

509 3 508.3 508 8

502.4 482.3

508.4 5067 500.9 495.6 510 5 511.8 509.4

025 O-20 0.39 0.26 053 OS9

489.6 b, 490.4 492 0 495.6 487 3 482.3 477.5 5003 48.x? 487 3 490.3

c>

50s 74, 511.8 b, 485 3b’ C) 493.0 491.8 491.0 503.2

508.6 508 0 517.4 505 5 511.6

467-4 453 8 472 6 492 0 468 0 497-r. 499 2

467.8 4694

513 520 514 515

450+.x0 463.9 485.7 4675 4413

46940 422.1 b’ 463 2 460.5 463.4

510.1 515.4 518.2 512.9 523.3

2 8 0 0

067 024 0 19 0.33 046 052 e> 025 022 043 0.76 0.30 0.41

c>

050 0.44 0.35 0.41 027 0.24 0.16 0.42 0.43 028 030 008

5)

027 0 18 036

ortho

Para

meta

48 48 40 47 40 66 e, 64 c’ 37 42 39 58 ‘=’

17 10 24 13 26 c> C)

35 42 36 40 34 34 36

19 14 17

44 44 44 42

c)

100 42 38

043

42 45

16 1s

0.51 023

38 44

24 14

0 15 e)

51 46 46

25 21

38 43 49 29 33

31 Q 46 43 45 4s

29 4 23 25 19 22

40 31 32 36 30

033 C)

_

023 =r 0.32 035 0.42

Ref.

191 [W

WI

All values at room temperature, &OS MHz Assignment supported by deuteration. Ortho and para isomers possibly degenerate. Isomer produced by ipso addmon, with A, = 473 MHz. No absolute vafue, measured m solution. Assignment tentatwe.

3. ExperimentaI rest&s and spectral assignment All experiments performed with an applied magnetic field of 3 kG revealed the presence of a signal at 40.5 MHz corresponding to muons in diamagnetic environments_ In addition to this frequency the 24 monosubstituted benzenes, C,H,X, showed at least one pair of lines corresponding to cyclohexadienyl-type radicals as judged from their high coupling constants, 420 < A, < 525 MHz. The ipso, ortho, meta and para isomeric radicals are derived formally by Mu addition at the substituted carbon atom and in the ortho, meta and para positions, respectively. The

physical Formation process may be more complex and is discussed later. In benzaldehyde and benzopheuone no radical signals were detected. Paramagnetic states observed with styrene (Ar = 213.3 MHz [S]), nitrobenzene (Ar = 38.7 MHZ) and phenylacetylene (A, = 421.2 MHz) are assigned to radicals formed by Mu addition to the substituent. They are not discussed further here. The experimental results of ah the other compounds are collated in table 1. Single substitution of a cyclohexadienyl radical does not lead to major steric interactions. It is therefore expected that the structure of the basic cyclohexadienyl skeleton is to first order unaf-

146

E. R o d u n e r et aL- / S u b s t i t u t e d c~'ciohexadien)'l-t}'pe m u o n i ¢ hzdicals

f e c t e d b y s u b s t i t u t i o n . P r i m a r i l y a s u b s t i t u e n t gives additional room for delocalization of electron spin d e n s i t y a n d leads t h e r e f o r e to a d e c r e a s e o f spirt d e n s i t y at the site o f the m u o n . I n d e e d w e find o n l y t h r e e o u t o f 66 values o f A.~ ( X = O H , C N . N H , _ ) significantly larger t h a n At, o f the u n s u b s t i t u t e d radical. It h a s b e e n n o t e d b e f o r e t h a t m e t a s u b s t i t u t i o n h a s little effect o n A , since the m o l e c u l a r o r b i t a l c o n t a i n i n g the u n p a i r e d e l e c t r o n h a s a n o d e in the m e t a p o s i t i o n [2,9]. W e thus assign f o r e a c h c o m p o u n d the v a l u e o f A N w h i c h is closest to 514.6

MHz, as observed for the unsubstituted radical, to the m e t a isomer. In this w a y the m e t a i s o m e r h a s in all cases also the highest v a l u e o f A , . F o r the d i s t i n c t i o n b e t w e e n the o r t h o a n d p a r a i s o m e r s w e rely o n the a m p l i t u d e s . O r t h o M u a d d i t i o n is f a v o u r e d b y a statistical f a c t o r o f two. F u r t h e r m o r e , it h a s b e e n r e p o r t e d b y B e n n e t t a n d Mile [19] for X = C H 3. i - C 3 H 7. C H 2 O H . b y D i G r e g o r i o et al. [4] for X = F. O H , N H 2. C H 3. b y Brett et al. [20] f o r X = C H 3. O C H 3, F, CI a n d b y P r y o r et al. [21] for X = N H 2 . O H , C H 3. C_,H_~, i - C 3 H 7. C O C H 3. C N , Br. C O O C H 3 a n d N O 2 t h a t a d d i t i o n o f H o r T o c c u r s p r e f e r e n t i a l l y o r at least in the statistical r a t i o in the o r t h o p o s i t i o n . T h e s e a u t h o r s r e p o r t n o s u b s t i t u e n t s t h a t lead to a differe n t b e h a v i o u r . Since it h a s so far b e e n f o u n d [8,9,12] t h a t the p a t t e r n o f regioselectivity is q u a l i t a t i v e l y the s a m e for M u as f o r H w e assign the m o r e a b u n d a n t i s o m e r to the o r t h o a d d u c t a n d the r e m a i n i n g o n e to the p a r a a d d u c t . I n s p e c t i o n o f i n d i v i d u a l s p e c t r a lends s u p p o r t f o r the a s s i g n m e n t g i v e n a b o v e . T h i s is b a s e d e i t h e r o n the g e n e r a l s i m i l a r i t y o f s p e c t r a c o r r e s p o n d i n g to s i m i l a r s u b s t i t u e n t s , o n i s o t o p e e f f e c t s A~p/Ap~a w h i c h are e x p e c t e d to lie in the r a n g e 1.15-1.21 [9], o r o n the s p e c t r a l effect o f selective d e u t e r a t i o n . D s u b s t i t u t i o n h a s a t w o f o l d effect [9]: First, it increases A N b y 3 - 4 M H z if D is b o u n d at the s a m e c a r b o n a t o m as M u . S e c o n d , it leads to the v a n i s h i n g o f line splitting f o r s p e c t r a r e c o r d e d w i t h m a g n e t i c fields o f 1 k G , a g a i n if D a n d M u a r e b o u n d to a c o m m o n c a r b o n a t o m . Figs. 1 - 6 d i s p l a y the 3 k G s p e c t r a o f all 24 c o m p o u n d s in o r d e r to illustrate t h a t c e r t a i n s u b s t i t u e n t s lead to v e r y s i m i l a r s p e c t r a w h e r e a s o t h e r s h a v e d r a s t i c effects o n f r e q u e n c i e s a n d / o r o n a m -

0

~! 1

i

r'

m

P

'~

CH 3

O

,i

I

o,,'

c.~o

t

o!

!°!

le'.." c~o.

i

I i ! i

1o |

0,' r

2'oo

,

~ T

V I

3 0 0 MHz

Fig. I. IxSR spectra obtained with a-substituted toluenes at 3 kG. The broken lines give the position of the radical signals in unsubstituted benzene. The)" visualize the line shifts induced by the substituent in the ortho, meta and para position.

plitudes. B r o k e n fines s h o w t h e p o s i t i o n s o f the t w o r a d i c a l f r e q u e n c i e s f o r the u n s u b s t i t u t e d r a d i cal o b s e r v e d in b e n z e n e . S u b s t i t u t i o n results in e q u a l shifts o f b o t h lines o f o n e radical, usually to l o w e r f r e q u e n c i e s c o r r e s p o n d i n g to a l o w e r v a l u e o f At,. F i n a l l y fig. 7 collects s o m e s p e c t r a o b t a i n e d w i t h d e u t e r a t e d c o m p o u n d s to s u p p o r t the a s s i g n ment. The spectrum obtained with toluene has been d i s c u s s e d in detail e l s e w h e r e [9], l e a v i n g n o r o o m for doubts for the assignment. Almost identical s p e c t r a a r e o b t a i n e d f o r X = CH2~b, C H 2 O H a n d C ( C H 3 ) 3 i n s t e a d o f C H 3 (fig. 1). T h i s suggests a n analogous assignment. ~t-haiogen s u b s t i t u t e d t o l u e n e s give rise to the s p e c t r a s h o w n in fig. 2. I n c r e a s i n g c h l o r i n a t i o n results in a c o n t i n u o u s d e c r e a s e i n A~, w h i c h a m o u n t s to = 4 M H z p e r c h l o r i n e a t o m f o r b o t h t h e o r t h o a n d the m e t a i s o m e r s . F o r the p a r a

E. Roduner e t aL / Substituted cyclohexadienyl-type mu°nic radicals t -,

i

im

_

147 ~

i -

,

[0

Ii

',--

I

t

.j~.,~.~..,.._~__..~.__,.._~:~,., -

1o

_

.~.~

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I !

i

L

I I

_

o 'Ira 0. JL__~_9,

_ ~'~ _

; /~

O -

-

!

,~-

t

.m

i

1°i

,

t

,

.

O ?

I

!U

,

I~l

,

I

o

m r

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:

11!

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R b

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300 M Hz w i t h ~x-halogen-toluenes

1o

I,m

~

I

.

I,"

,

io

.~,,,.~

at 3 k G .

ococ,

3

: d-n

r

SH

~

2"00

Arrows mark the ipso adduct.

o~

-

,

I

p ,:t

Jt.ALJ~

~m

i

"

!1

IoJi

200 F i g . 2. I ~ S R s p e c t r a o b t a i n e d

1°

iI p!l: I~'i

,tti

i j

OCH3

_

I"

L

9"00M ~,',~

Fig. 4. I.tSR s p e c t r a c o r r e s p o n d i n g to c y c l o h e x a d i e n y l radicals

with substituents bound via oxygen or sulfur, at 3 kG.

I

i s o m e r the shift in A N is stronger, b u t this radical is o b s e r v e d o n l y for X = CH2C1. F o r X - - C H C 1 2 a n d CC13 it m a y b e h i d d e n in the noise, a l t e m a -

!

!

of

~t,i' vi ,

Io,L P

t~,m'P .'t

Oi :L_J,~

_

;:

j°'

_

I

p Io ~n I I

I !

, o | |

i ~,

~,

o,

~

[P t, i

s I

,

Im 1/ It-

200

I I

O

300 MHz

i

1~ t

'

-

'

Yn

2bo

'o

~

J ;

lo

-m

c0ci

'

it

3bo MHz

Fig. 5. ~SR spectra obtained with bemzoie add, its methyl ester F i g . 3. F S R s p e c t r a o b s e r v e d w i t h h a l o b e n z e n e s at 3 k G .

a n d its c h l o r i d e at 3 k G .

148

E. Roduner et al. / Substituted c)'dohexadienyl-t)'pe muonic radicals

i0

;i t~/7 7

.p

~0

,

.p

~m t

o:

i

I

t rn

lP

®

I°

,

~p

CN

;~m

IO

,O /2L

~rn

i

p.

NCO

';m i

~O

I

to tm 'P

'P

NH'z

200

u

300 bfHz

Fig_ 6..ttSR spectra corresponding to cyclohexadienyl radicals with strongly conjugating substituents, observed at 3 kG.

olim'p

m.p

J

L__g~L !

t

~,,

J

.~ ;

p-OQCF~

O

i

/Plm

P O(/)O

o!D o,p

1. ::m

I!

m

p-ooc~

O

.......

I

-_-..:.-'.'.__..C__:: 2'oo

.......

7..._ ...... Joo M . .

v -"

Fig. 7. ttSR spectra obtained v,qth para-deuterated a,t~,t~-trifluorobenzene and chlorobeazene at 3 kG (top two), and with para-deuterated chlorobenzene and 4.4"-dideutero-biphenyl at 1 kG (bottom t~o).

tively the ortho a n d para isomers m a y be degenerate for these two compounds as inferred from amplitudes and linewidths (vide infra). Substituent effects o f CF; on A~, are rather small for all three isomers. The assignment given gains support from the spectrum observed with p - D d ? C F3 (see top of fig. 7) which shows an unshifted pair of lines (A~ = 500.3 MHz, ortho adduct) and a second pair of lines ( A . = 510.9 MHz) with increased amplitudes corresponding to a superposition o f the meta and para isomers. The D shift for A~ of the para isomer is + 2.2 M H z and thus somewhat less than what was reported for toluene. (FCF3 is the only c o m p o u n d in this work where a fourth radical is observed (arrows in fig. 2, seen more clearly in fig. 7). It is assigned to the ipso adduct with A~, ----473 MHz. Its protiated analog with t-butyl groups in both meta positions has been observed by Griller et al. [22] with Ap reported to be 119.7 MHz. The ipso-CF3 substituent effect on the hyperfine coupling thus amounts to --8.1% in the Mu-substituted case which compares very favourably with the - 1 0 . 7 % for the H analog with its additional two meta-t-butyl groups. The radical spectra obtained with halobenzenes are shown in fig. 3. The substituent effects for the meta Mu adducts are small but increase continuously in the series F - C I - B r - I . For fluorobenzene it has been shown previously [9] that the meta and para isomers appear degenerate. In chlorobenzene the ortho and para adducts are nearly degenerate. The assignment o f the para isomer is again confirmed using experiments with p-D-chlorobenzene, both at 3 k G and at 1 k G (fig. 7). A~, of the para isomer is shifted due to D by +3.1 MHz, in accord with expectation. At a field of 1 k G the lines corresponding to the ortho and meta isomers split (third spectrum in fig. 7). The para isomer appears unsplit, superimposed with one line from the ortho isomer (intense line). Bromobenzene m a y be a similar case as chlorobenzene, but now the third isomer is not resolved in the spectrum. The lines corresponding to the lower values of A N are broad. Inspection of the distribution o f m u o n polarization in table 1 suggests that the ortho and para isomers appear degenerate (vide infra). Only one radical is observed with iodobenzene. The trend of the meta coupling

~. Roduner et al. / Substituted cyclo_hexadienyl-type muonic radicals

constants suggests that we see the meta isomer only. This situation will be further discussed in section 4. Fig. 4 displays the spectra of radicals with substituents that are connected to the ring via an oxygen or sulfur atom. The first four spectra are basically the same, although a somewhat increased variation of A~ values is apparent. This suggests analogous assignments in these cases. F o r X ----O H the H analog of the ortho isomer has been observed [4]. O u r assignment leads to A ~ , t t p / A p t t ~ = 1.17 and is thus in accord with expectation. F o r X = O C H 3 the assignment is corroborated b y isomer-dependent reaction rates o f the radicals with benzoquinone and duroquinone [11]. F o r X----SH ~ P R is small, with the effect that the third isomer does not exceed the noise level in the spectrum. In view of the general low regioselectivity of M u in the addition to aromats it is likely that the missing species is the para isomer. Radical spectra obtained with benzoic acid, its methyl ester and its chloride are given in fig. 5. T h e characteristic o f the first two spectra is clearly different from previous ones. The most intense lines that are assigned to the ortho isomer are shifted considerably less than the lines for the para adduet. Comparison of substituent effects on A~, is possible with corresponding values for Ap in carboxylated cyclohexadienyl radicals [2]. Unfortunately the mono-substituted isomer has not been measured, b u t for the disubstituted radicals A p o f the methylene protons decreases in the series of the o,rn-, o , o - , r e , p - and o,p-isomers. This order has to be expected for cumulative behaviour of substituent effects (eq. (4) in ref. [9]), providing the effect for para substitution is larger than for ortho a n d negligible for meta positions, in accord with o u r assignment. The I~SR spectr.~m obtained with the ester is very similar to that observed with the acid. This suggests an analogous assignment. The acid chloride on the other hand gives rise to a quite different spectrum with no clear difference in intensities between the ortho and para isomers. T h e assignment here is therefore tentative. M u addition to the carboxyl oxygen might b e expected to take place in analogy with the findings with acetone [10]. It was, however, not observed with these compounds.

149

The s u b s t i t u e n t s - X = ~b, C N , N C O are conj u g a t e d systems. They are therefore ~expected to influence the spin density distribution arid thus Aa in a s i m i l a r w a y as i t - w a s f o u n d for carboxyl groups. Qualitatively the spectra are indeed analogous (see fig. 6), except that substituent effects are considerably larger for X -- qb. The assignment was checked b y measuring 4,4'-D2-biphenyl in a field o f 1 k G (last spectrum in fig. 7). The absence of line splitting for the radical with lowest A~ identifies the para isomer in accord with the assignment given in table 1. The spectrum obtained with aniline ( b o t t o m o f fig. 6) is rather different and reminds one more of the toluene derivatives (fig. 1). T h e large substituent effect must be d u e to conjugative interaction of the lone pair at N with the ring ~ system. The hydrogen analog of the ortho adduct has been observed b y means of E S R [4]. Comparison o f corresponding hyperfine couplings gives A a t t p / A p ~ p . = 1.18, in accord with expectation. In all these radicals the m u o n sits in the methylene position of the cyclohexadienyl system. It has long been recognized [23] that the proton hyperfine constant at these positions is unusually high. T h e spin density distr/bution in cyclohexadienyl radicals and in particular the influence of o and ~r systems on it is still of interest [24]. The present I~SR data with the large variation of substituents, some of which interact predominantly with the "~, others with the o system, m a y serve as a basis for corresponding studies. Conventionally the ~ orbital containing the unpaired electron was assumed to consist o f the five p: orbitals from the sp 2 hybridized carbon atoms, and the C H 2 group was treated as perturbation [23]. This molecular orbital has a node at the two meta positions, and in the simplest a p p r o a c h [25] the squared coefficient of the ortho and para p. orbitals is 1 / 3 each. It is therefore expected and consistent with the present d a t a that further substitution at the meta position has a small effect on this ~r MO. A much larger effect is indeed usually observed for substitution in the ortho or para positions, in particular for substituents which can delocalize spin density via conjugative interaction with the ring. T h e y e t unresolved question is w h y i n some cases the effect is strongest for ortho and in others for para substitu-

150

E. Roduner et a L /

Substituted c)'clohexadienyl-type muonic radicals

tion. W e simply note here that a similar p a t t e r n is expected to be f o u n d for substituent effects on the couplings o f the benzylic p r o t o n s in benzyl radicals, which is isoelectronic with the cyclohexadienyl radical if one includes the two methylene h y d r o g e n atoms o f the latter with the p r o p e r symmetry" in the .,rr system. F o r the benzyl radical A p ( C H 2 ) is 45.8 MI--Iz [1]. T h e average value for these two p r o to n s is reduced by substitution with e.g. C H 3 by 3.2%, 0.9% and 1.8% for the ortho, meta and para isomers [26.27]. T h e effect is thus of the same order as for A~ or CH3-substituted cyclohexadienyl radicals where the cor r es ponding numbers are 4.9%, 1.0% and 3.5%. A theoretical study of substituent effects on the spin density distribution of benzyl and cyclohexadienyl radicals is in progress and will be published separately.

4. Discussion of the muon polarization data

4.1. End-of-track effects on the muon distribution between radicals and diamagnetic species PD represents muons in any diamagnetic environment, i.e. free muons or m uons i n c o r p o r a t e d in molecules with no unpaired electron spins. Chemical shifts are not resolved due to the m u o n life time. Values for PD (table 1) range between 0.15 for unsubstituted benzene and 0.76 for X = SH. In the series o f halobenzenes it increases f r o m 0.19 (X = F) to 0.52 (X = I), and in the series of e~-substituted toluenes from 0.25 (X = C H 3 ) to 0.67 (X ~- C C 1 3 ) with increasing chlorination. T h e formation o f Mu-substituted radicals is in competition with the formation of diamagnetic co m p o u n d s . T h e r e f o r e an increase in PD is naturally correlated with a decrease in EPR- Inspection o f ~ P R values in table 1 confirms this as a general trend. However, PD + ~ P R ranges f r om 0.53 (X = N C O ) to 0.91 (X = SH)- i.e. there is a varying fraction o f lost m u o n polarization, PL -----1 -- PD -Y~PR"

T h e processes o f m u o n distribution a n d depolarization are n o t directly observable since they o c c u r at times < 10 ns. Several models have been introduced to explain the observed variations in PD- Myasishcheva et al. [28,29] pr opos e d M u as

the only a n d c o m m o n precursor o f m uoni c species. It reacts by addition to form radicals, and by H o r halogen abstraction to form diamagnetic compounds. C o r r e s p o n d i n g rate constants were determined from the analysis of PD values for arom at i c c o m p o u n d s and their binary mixtures g i t h different saturated molecules. T h e rate constants for M u addition are not unreasonable (e.g. 3.1 × 109 M - I s - i for benzene, c o m p a r e d with 8.9 X 109 M - 1 s - i from a m ore direct determination [13]). F o r abstraction reactions o n the o th e r hand the rate constants c o m e out orders o f magnitude too high (e.g. 2 . 4 x 1 0 ° M -1 s - I for H abstraction from c-CrH~2, where direct observation o f Mu in this c o m p o u n d has now revealed k _< 1.6 × 105 M - l s - l [30]). T h e model of thermal M u as the general precursor of diamagnetic m uoni c comp o u n d s is thus clearly insufficient. It has been suggested that Mu is form ed at high energies and that it is still hot when it reacts either by addition or by abstraction [14]. This would allow for higher rate constants for the abstraction reaction and may still be com pat i bl e with the observed trend for a correlation of PD values with the strength of the b o n d being broken, in particular v.qth regard to n u m b e r and nat ure of halogen atoms present. However, the rate const ant for M u addition to benzene in CrHr/c-CrH12 mixtures is clearly below the diffusion controlled limit, and in b e n z e n e / d i m e t h y l b u t a d i e n e mixtures addition is quite selective [13] c o n t r a r y to expectation for hot reactions. F u r t h e r m o r e , the influence o f small concentrations o f CCI 4 in benzene on PD and PR is also not com pat i bl e with hot processes [13]. T w o further mechanisms have recently been p r o p o s e d for radical form at i on from a substrate S [16]: C--

I.t~ + S ---} [~S] + --~ MuS, tx+

e - + S --} S - --~ MuS.

(1) (2)

T h e y bot h have m uons in diamagnetic environm ent s as direct m u o n i c precursors and involve free electrons near the end-of-track of the thermalizing m uon. In butadienes it was n o t possible to distinguish between diamagnetic and paramagnetic precursor since the radical f o r m a t i o n m echani s m is

E. Roduner et al. / Substituted cyclohexadienyl-type muonic radicals

too fast to give rise to field-dependent polarizations [12]. M o r e recent results o f experiments with benzene a n d its binary mixtures with cyclohexane, dimethylbutadiene and CCI 4 have been explained in terms o f a different two-step mechanism [13]: C6H6

p.+ + e -

--* M u ~

<__1 p s

10 p s

C6HrMu-

(3)

It involves thermalization of m u o n s as positively charged species and subsequent combination with end-of-track electrons to form M u within __<1 ps. In pure benzene M u then addg within 10 ps to form the cyclohexadienyl radical. The influence o f C C I 4 iS understood as the effect o f inhibition of M u formation b y electron scavenging. Po then represents muons that escape M u formation and possibly M u that has recombined with other paramagnetic species from the radiolytic processes. This model of M u formation has been developed for aqueous systems where PD also increases with the concentration o f a d d e d electron scavengers

[151. W e n o w want to show that the a b o v e end-oftrack processes offer a plausible explanation for the observed variations in I'D and Z~PR values in substituted benzenes. F o r this purpose we distinguish between c o m p o u n d s with high ( > 0.3) a n d low ( < 0.3) PD values. A m o n g the substances with high Po we find all chlorinated compounds, further bromo- and iodobenzene. They all undergo efficient dissociative electron capture [31] and are thus able to inhibit the formation of muonie radicals by irreversibly removing the electron, as it was observed for the solutions of CC14 in benzene. The increase in P o for halobenzenes F < CI < Br < I parallels the efficiency of the reaction of these c o m p o u n d s with electrons [32]. Scavenging of positive m u o n s will also increase PD at the cost o f PR- Substituents which can exchange free m u o n s for protons or solvate ~+ strongly as in aqueous systems thus lead to high P D " This is the case for X -----O H ( P D = 0.38 [14]), C H z O H , S H and N H 3. A 'correlation o f P o with d o n o r number, a measure of the basicity of a molecule, has been observed before with mostly non-aromatic molecules [16]. A m o n g the low PD c o m p o u n d s there are at least two (X = C F 3, C N ) Which react efficiently

151

with electrons [32]. In these cases electron capture is not dissociative. The ~inion formed can c o m b i n e with a m u o n and p r o d u c e a radical with no need for Mu as a physical precursor (mechanism 2). Although it seems clear that end-of-track processes play a major role in governing the distrib u t i o n of m u o n polarization between PD and EPR it will be difficult to find a quantitative correlation. More than just one parameter is required to describe the competing processes involved. Further, PL is not constant. It has been argued [9] that it represents muonic radicals which have partly been depolarized through collisions with other paramagnetic species p r o d u c e d in the radiolysis. It is to be noted that similar trends, e.g. increasing PD values with increasing chlorination, are also found in the vapour phase [17] where they are not associated with end-of-track effects (although there is o f course an ionization track). The gas phase results are more supportive of hot a t o m or even hot ion (~t +) reactions [17]. However, PD values are substantially lower in gases than in corresponding liquids, indicating that the processes involved may be quite different. It is certainly an oversimplification to regard the liquid as j u s t a dense gas. Solids m a y be a somewhat better approximation to liquids than gases. In the solid phase PD values are generally also lower than in the liquid [17]. In the case of solid chlorobenzene PD decreases strongly with decreasing temperature [29]. H o t reactions are generally not expected to be particularly dependent on temperature of the surrounding. In this view a low-temperature limit of P o (which has not yet been measured) would place an u p p e r limit on the contribution of hot reactions to diamagnetic muonie products.

4.2. Regioselectivity in the formation of radicals W e n o w focus on the distribution of m u o n polarization between the ortho, recta, para and ipso isomers. Ipso addition of M u was observed o n l y for X -----CF3, with very low amplitude. It had been seen before with several methyl a n d fluorine polysubstituted benzenes [9]. There, the probability o f addition at a C - X site is reduced b y a factor o f three over addition at a C - H carbon atom. T h e

152

E. Roduner et a L / gubstituted cyclohexadien.vl-type muonic radicals

present observations show that ipso addition is generally disfavoured, although M I N D O / 3 calculations suggested this to be the preferred reaction in the case o f X = C1 [33]. T h e statistical distribution between ortho, m e t a and p a r a isomers is 40, 40 a n d 20%. All three isomers have been observed f o r 19 o u t o f the 24 substituted benzenes (see table 1). In m a n y o f these cases o r t h o addition occurs with a somewhat higher than statistical probability, even in the presence of a bulk), substituent, e.g. X = C ( C H3 ) 3. O nl y for X = COCI o r t h o addition is significantly less than 40%. Appreciably less t h a n 20% o f the p a r a isomer were found for X = C H z # , C ( C H3 ) 3, F and O C O C H 3, whereas almost 30% were observed for X = COCI. Substituents with the ability to enlarge the ,rr system of the ring (last 7 entries in table 1, except COC1) seem to depress the f o r m a t i o n o f meta isomers. It is difficult to find correlations of these deviations from statistics with an y substituent properties. T h e y m ay be related to small energetic differences o f the isomeric radicals formed, as it was f ound for methylsubstituted butadienes [12], f u r t h e r m o r e they may have to do with possibly differing radical formation processes as discussed above. Only the cases where not all three isomers are observed deserve f ur t her discussion. F o r X = CHCI2, CCI 3 and Br the a m o u n t o f m et a isomer is close to what was f o u n d for related substituents (CHzC1 and C1, respectively). T h e a m o u n t of o r t h o isomer, however, seems = 50% too high. At least for b r o m o b e n z e n e this would mean that M u is m o r e selective in its addition than H [11]. This seems rather unlikely. We therefore suggest that the ortho an d p a r a isomers a p p e a r degenerate in the spectrum. F o r X = SH b o t h the o r t h o and meta isomers have rather high relative a b u n d a n cies. In view o f the weak signal (ZPR = 0,15) it is weU possible that the para isomer is hidden in spectral noise. Io d o b en zen e is the only c o m p o u n d where a s i n ~ e radical has been detected. It is again well possible that the para isomer is hi dde n in the noise. T h e course o f A~, values for the o r t h o isomers formed in halobenzenes makes it unlikely that in i o d o b e n z e n e the o r t h o and m e t a isomers a p p e a r degenerate. T h e probability f or the f orm ation o f the o r t h o a dduct must thus be unusually

low. As a possible explanation for this effect it is suggested that M u is substituted for I in i o d o b e n zene. This m a y o c c u r either in a one-step reaction w hen M u comes close to the iodine atom, o r b y I elimination f r o m the o r t h o adduct with c o n c o m itant rearrangem ent to benzene. F o r H instead o f M u the net reaction (~X + H---* e l l +

X

(4)

is exothermic by 190 k J / m o l for X = I, 129 k J / t o o l for X ----Br and less for the o t h e r halogens (calculated using values by Egger and Cocks [34]). T h e c o m p e t i n g addition reaction

+x + H ~ H4X

(5)

is exothermic by 121 k J / m o l for X = H [35] a n d p r o b a b I y o f c o m p a r a b l e exothermicity for X = halogen. Substitution o f H by M u increases the energy on the p r o d u c t sides o f b o t h reactions by similar a m o u n t s due to the increase in zero-point vibrational energies. Mu for X substitution is thus clearly t herm odynam i cal l y favoured for X = I, whereas is seems roughly c o m p a r a b l e with addition for X = Br, and disfavoured for the o t h e r cases. F u r t h e r m o r e , it seems reasonable to assume that the meta and para isomers are not subject to Mu for I exchange since the two atoms are separat ed too m uch in space.

Acknowledgement S u p p o r t by the Swiss N a t i o n a l F o u n d a t i o n fo r Scientific Research and by SIN is gratefully acknowledged. This w ork is also part o f the research p r o g r a m o f the Chem i st ry D e p a r t m e n t o f N I K H E F - K which is s u p p o r t e d by the F o u n d a tion for F u n d a m e n t a l Research on M a t t e r ( F O M ) a n d the N e t h e r l a n d s O r g a n i s a t i o n f o r the A d v a n c e m e n t o f Pure Research (ZWO). We thank Professor H. Fischer for general support o f the project and P. Burkhard and W. Strub for experim ent al assistance.

References [I] H. Fischer and IC-H. Hellwege. eds., Magnetic properties of free radicals, Landold-B0rnstein, New Series, Vol. 2, Part 9b (Springer, Berlin, 1977).

_ E.

Roduner et al.

/

Substituted c)'cJohexadienyl-type muonic radicals.

153

z

[2] K. Eiben and R.H. Schuler, J. Chem. Phys. 62 (1975) 3093. [3] M.B. Yim and D.E. Wood,ft. Am. Chem. So<:. 97 (1975) 1004. • [4] S. DiGregorio, M.B: Yim and D.E. Wood, J. Am. Chem. Soc. 95 (1973) 8455. [5] J. Lichtsch~dl and N. Getoff, Monatshefte Chem. 110 (1979) 1377. [6] D.C. Walkei-, J. Phys. chem_ 85 (1981) 3960. [7] E. Roduner and H. Fischer, Chem. Phys. 54 (1981) 261. [8] E. Roduner, W. Strub, P. Burkhard, J. Hochmann, P.W. Percival, H. Fischer, M. Ramos and B.C. Webster, Chem. Phys. 67 (1982) 275. [9] E. Roduner, G.A. Brinkman and P.W.F. Louwrier, Chem. Phys. 73 (1982) 117. [I0] E. Roduner, P.W. Percival, D.G. Fleming, J. Hochmann and H. Fischer, Chem. Phys. Letters 57 (1978) 37. [11] E. Roduner, G.A. Brinkman and P.W.F. Louwrier, Hyperfine Interactions 17-19 (1984) 797, 803. [12] E. Roduner and B.C. Webster, J. Chem_ Soc. Faraday Trans. I 79 (i983) 1939. [13] E. Roduner, Hyperf'me Interactions 17-19 (1984) 785. [14] D.G. Fleming, D.M. Garner, L.C. Vaz, D.C. Walker, J.H. Brewer and K.M. Crowe, in: Advances in chemistry series Nr. 175, Positronium and muonium chemistry, ed. H.J. Ache (American Chemical Society, Washington, 1979). [15] P.W. Percival, E. Roduner and H. Fischer, Chem. Phys. 32 (1978) 353. [16] A. Hill, G. Allen, G. Stirling and M.C.R. Symons, J. Chem. Soc. Faraday Trans. I 78 (1982) 2959. [17] D.J. Arsenau, D.M. Garner, M. Senba and D.G. Fleming, J. Phys. Chem., to be published. [18] P. Burkhard, E. Roduner, J. Hochmann and H. Fischer, J. Phys. Chem. 88 (1984) 773. [19] J.E. Bennett and B. Mile, J. Chem. $3c. Faraday Trans. I 69 (1973) 1398.

[20] C.L. Brett, V. Gold and G. Perez, J . Chem. S0c. Perkin Trans. II (1973) 1450. [21] W.A. Pt'yor, T.H. Lin, J.P. Stanley and R.W. Henderson, J. Am. Chem. Soc. 95 (1973) 6993. [22] D. Griller, K. Dimroth~ T.M. Fyles and K.U. Ingold, J. Am. Chem. Soc. 97 (1975) 5526. [23] D.H. Whiffen, Mol. Phys. 6 (1963) 223. [24] R.N. Sangster, K.P. Madden and R.H. Schuler, J. Phys. Chem. 87 (1983) 2395. [25] H. Fischer, J. Chem. Phys. 37 (1962) 1094. [26] P. Neta and R.H. Schuler, J. Phys. Chem. 77 (1973) 1368. [27] M.S. Conradi, H. Zeldes and R. Livingston, J. Phys. Chem. 83 (1979) 633. [28] G.G. Myasishcheva, Yu.V. Obukhov, V.S. Roganov and V.G. Firsov, Khim. Vys. Energii 3 (1969) 510 (English transl. High Energy Chem. 3 (1969) 463). [29] G.G. Myasishcheva, Yu.V. Obukhov, V.S. Roganov and V.G. Firsov, Khim. Vys. Energli 4 (1970) 447 (English transl. High Energy Chem. 4 (1970) 398). [30] Y. Ito, B.W. Ng, Y.C. Jean and-D.C. Walker, Can. J. Chem. 58 (1980) 2395. [31] M. Anbar, in: Advances in chemistry series Nr. 50, Solvated electron, ed. E.J. Hart (American Chemical Society, Washington, 1965). [32] M. Anbar and E.J. Hart, J. Am. Chem. Soc. 86 (1964) 5633. [33] R.D. Gandour, Tetrahedron 36 (1980) 1001. [34] K.W. Egger and A.T. Cocks, Heir, Chim. Acta 56 (1973) 1516. [35] J.A. Kerr and M.S. Parsonage, in: Evaluated kinetic data on gas phase addition reactions (Butterworths, London, 1972).