- Email: [email protected]
Radical pairs from photolysis of phenoxy dimers
‘_
i, V”‘ume.32; n”c” 3’ ‘. ,_. “.ber .I ... . (, ..:’ : .,’ ~ ;’ ‘., _‘Y’... ‘.‘. .: 1 ~. ., I’ ._ .. ‘, ,’ .. .., ,,. ‘.. : f :; ._‘I. ‘_. ’ RADICAk PAJ& &I$ I
C&r&&q&s _
,,:
.,
.: ‘_ ; :
.. :. ”
.. .‘.
.:
of Rocl+er,
-._. .‘
&ceived 4 &PoIkr 1974 ., ‘,,.. ‘. ..; .Revid r%ukstipt recked 19 Decogber .1974; . ;. : _’
‘. ,:‘-
_. __‘;’
,.;:.,,
:
: : .’ :
A.., :
_.
:
‘k.4:
:
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,,
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:’
I
:
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_Il .‘, ‘.,
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.1 yay 197.5
_,
,...
,..‘,‘. ’ ; ;,;‘. ; ,, ,. : .: ,”
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.. ‘.,
:_,
,;
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r : ‘.
.’
:
: : . . ,,,
_
‘.
.:
-.
:
.. ~~~~~l~t ph~~oIy~~.~f kM samples of the d&& if phm&y radicllls hasbeen fourih to prddicg closely a&&ted
.'
.’
‘.
‘_
.,, ; -::. ::-
R&PQ@.%S ~~~~~~~‘,D~~~~ (’ : .-‘.., . . .
” ‘l)q~rtrr&! ofChetnishy,‘.univet~
..’ .; -. ::.
“.
,’
:.
:,
_ ,:
j&J-&j,, _I.
,:
rdical pairs&I s&ne us@. An elect&n
spinreso~zt~%?‘,iav&iptii6n of m
:
hopped,ra,dic&pak
I.” ‘_
‘- :_ ,.,_ ~,. ..t.. ‘,. 1: ., .I, .. “1 ‘. ,, ::I, ~~~d~~~~~, .‘_. ., .,‘”.’: .’ : ., ‘.. ,., ‘,. ,. : ~ihih~tiin ‘dfhindered phenols (2,6di&kt-but@‘.
:
is reported in ihis note.
., ” ,.
‘~ f!’
Com~u~d
‘.
1
COCYCLOHEXfL
:i’R-$hehols) ie&s to the for&&on of rejatively stabk phenc@, ail&& [ I] LIf ahe R-substituent ii ei$her.a
‘..‘,
:,.r~~c~s.are.g~n~~~y
blllkv’~phatic~oup(~lb~tyl)oranarylgroupthe
SC,.
: cot-WJTYL
:
,.
COCYCLOHEPTYL
~o~~~ri~ ,& both Solution knd a qzbqnyl ‘@oUpis~subs’tit&ted .. ‘at&e ‘para.p&.itiok of’the phenol& ii& the radicak.~~ ;. I$-J. 1, Comp&&investi&te& dimerize through formation 0f.i hand between the:’ . ‘. ... : : .’phenolic oxygen kki the p~a.ca~b~~ atom of a se@ ,I: :-ond &i&: The dimers.are-stable inthe’soiidstate..~ of phenoky dimers wkh bulky substituents but were; &d ck 6e isolated‘& polycrystalhne s’oiids which are ‘. not obseti-ed from dirners with smaller subitituerits. " " '. ~o&kninate~d with small amounts nionomericra~-,....I. '. :, _‘, ‘_ C&.AII ~R~~~~~~=ti~~ of the~tie~iicofthe radi--:-- .L -‘.‘: : . &I the q&dsiate..When
of
reaction [Z] vacated &at 3.: bond~,@&en’thk phwoxy.mon&nerz is relatively
.~~ra~~~~~~ti~~ the
‘,
‘.
._
_‘.
-
.2. ~~~~~~~. :., ,’
.
.-,
., ‘-” ,, 1,
.
.
‘,’
The light source _whiChwas ‘us8 for photolysis. .. ,’ SP+IB.hi&.pressure mercuj arc. The ES& spectra $ere recorded bn _aJ@3LCQ 3 ,BSX BSR .,,, i ‘1’.,. ‘, : sp&ctromr:ter., :- ,:1’.
: tis~I%iUips
[email protected] the.trap~d’radic~,~rs’which.are f0~e;d.b~~ photolysis of-solid samples’of phenoxy dimers. Radi-
. ., : : .. 1,‘. .. ..,:“ 1 ,,’ ‘_.: :(,&~fo&edb$ a&,oci&on of:&&.met& kty]s .. .‘, ‘. . ,_ :,1 [S] &cl by:photodissqci+io~ oftctr&& hydr&&s= : _) 1’. < : .‘_ ,,1: . ._ ‘_ ., been reported. The radical pairs’ “, : :,3.‘kesul~ and ~~0~ ” [b] &aye pre&g$y (.’ ‘. : ‘. 1’ .: :, .. ~e’c&mwtq-ized in each &ise,b~ +e &p&-diptile’, : “’ ,; Tfw phe~oxy ‘~rners,~~~h were ‘@ie&igated aie; ~t~ra~~on between’the ~~~~d:e~ec~on:s~~:G~ ,$I$.:’ .(r&&&s. ~&rinat.ion.of r~dicai pair&from phentjxy;,:. .-i ‘%hown@ :fi& 1. .The bond between’the pi~en_oxy, ; :’ -G&i& &as ~e=~.f~~~~.~o ‘&p&d on.~~:~~~~~ of:: ;:. :1, .: oXygen arid &thepara Carbon ator’n”tifa seconzl raditia!
[
i,the s&s&i.tent bonded to’the’.c,arbofiy~ &oup.,Raje:., .: : t&r+ stable_~~adic~.pilirs’are”prodlrc.edby,~hoto~ys~s _: .,._:_“ ,:. ;; : ., :-. ..< : :. : _;;._.,: rj -.:,_7 _-: : :. .. : ,.: : . _,
-”is relatively wer+k,&d photoIysis of the’dimeh, should: produ~~moaqmeiii::ra~c.~.-ihe : ,, &nomer.s +%ich’i+ fo~~q~‘rea4iy‘.retorn~,~& z’
:,, break t~s.bo~d,~~
., ‘&&&‘& .th&: iadicats _ -., ,, &&l&&c. .:, ,,dim& ::,, ,_,,. . :’ _ : ,: ‘, ,, ,, _ utif&is : ,, .: .l’. .. ‘. $idfr&b,.,:, .~~;;:8o~.FaundafisnFellow; . ,. __. .. ., ., ,. : ,.. ., ‘1 __ ” ” ‘.. : .. ‘.‘. : ,,_’ ; “.) .: .~ $2.;;. ,;,‘:;:‘j-‘ ;.;:;..l:i,: .‘__ $..‘. y.*.__’ q:;._ ,.-:,‘,. ;; : ,, ;;‘y ,y: _j‘:,.:,;;‘.y’:.,.I’.. :-‘ .‘., ; ‘,,.I’ .,:.‘).‘. ‘., ..( __,~ .: ; ., _,, .,, ,..I’ .::.._ :. :. ,, -. _::,;.:,: _ .: _. ‘, ;‘;. ..’ _. ,.: ‘,;.,_, .. ‘: : ,.,,‘. ,./:. ,,.,, __ ..;.,,:, .. .,‘, _’ ; :, ,..:. ‘ .. ‘ .. ,_ __, .; ,: :-. ‘ ._ :,‘ “ ..:_ ..,.( “ ’ : : -.r:. ,, ,_”.,_: .-_‘L.. .’1’.‘, ....‘ .: :: , ,. .’ . ‘,: ,:: _, ,.\ _::, _.._,.-.,.. .; ,,:I,.;‘-.., 1, ‘_‘.y ‘_ : :.:. ~ :s, 1’. .,: .‘(,,. ‘2 ‘, : :’ ‘.’-:. . .:- ,-t:.. :, ,.. -.‘. -. ” ... ... ;. .:.. :. ... .: ,.~, :
CHEMICALPHYSICSLEIXERS
Volume 32, number 3
,100
1
.--:1~~~~&~
G,
steric iriterference between t!nis r-butyl soup
8 Fig. 2. ESR spectrum of the radicz11pti formed on photclysi3 of dimer 4.
diffuse apart and there is a conformational change which prevents recombination. If the monomers tie able to diffuse more than about 10 A apart the dipolar interaction between the spins on the monomers niIl be small and one would expect to observe only a change in the intensity of the g = 2 ESR signal from isolated monomers. If the motion of the monomers is restricted the radicals will be held close to one another and one expects to observe a relatively large dipolar splitting from the spins in the trapped radical pair. The dimers were photolysed in the cavity of the
.ESR spectrometer while the temperature was maintained at.-1 50°C. The ESR spectra of the samples showed a single line at g =‘2 from isolated monoradical impurities before ‘photolysis was initiated. The ESR spectra from samples of dimers in which the R groupwas either .methyl or cycl_ohexyl (compounds 1 .and 2) were ‘unchanged by phstolysis. Radi& Which may have been formed during photolysis of these molecules apparently readily recombine and the steady state concentration of radical pairs is.tdo low ‘. to observe. Phot,olysis of dimers in’which, the R group praseither t-butyldi cycloheptyl (compounds 3 and : 4) gave &R spectra which clearly showed the .dipolar ‘. splittig (fig. 2) from the interaction ‘of the spins on closely spaced monomers in a’radical pair. These spec: &a persisted’after phot,olysis was terminated as long ..’G the temperature & held below about -1 lO”.C..~. ..
._
.;,
1 ,‘,’ ., ,.:
1 May 1975
:
On waking above -110°C the Am = 1 transitions in the.spectra rapidly &appeared. These observations are consistent with tic formation of radical pairs which are unable to recombine because of changes in the relative cdnformation of the two monomers after the bond is brotin. Warming the simpIes allows rotation of the R groups and the monomers in the.radical pair &-I recombine. ‘l&methyl and cyclohexyl groups of dimers 1 and 2 apparently do not have sufflcient.steric bulk to prevent rapid recombination of the monomers. The fbutyl group of dimer 3 has 2 Ixge steric bulk and recombination of the monomers shouId be resfricted by
I
,’
.’
‘.
.._‘.
,_,.. -. ,,
; ,.:,:,,
:.
state.
” ‘An alternate explanation for the difference in be-
havior of the t&o sets of dim& might involve differewes in crystal packing of t!xe polycrystalline solids. The &ti.structures of dimers 1 arid.2 may differ from those of dime; 3 and 4 and prohibit motion of the molecules after the bond is broken. If this were & e &se, one might expectto observe radical pairs frbm riimcrs which occupy sites on the surface of the ‘.solid. Surface radical pairs and bulk radical pairs with different D values were observed in the photolysis of polycrystalline tetraphenil hydrae [4]. ae Belative intensity of the si&ak from the two ‘types of radical’pairs were found to vary with the surface area :
.:
‘.’ ;
.. ;
and the
ortho r-butyl groups on the other phenoxy monomer. The dipolar interaction be.tweeXithe spins (D = 14-4G) in the radical pair formed on photolysis of this dimer is relatively small indicating a large separation of the unpaired spins. If one u&s the point clipoIar approach. [5] to account for the dipoiar interaction the average separation of the spins in this radical pair is found to be 5.8 & The cycloheptyl graup of dimer 4 is sli&Uy larger than the cyclohexyl group of dimer 2 but h5s less steric bulk than the, pbutyl group of dimer 3. The dipolar interaction between spins in the radical pairs formed on photolysis of this dimer is Iarger (0 ~305 G, E = 18 G) than that of the radical pair from dimer 3 indicating that the monomer units are held more closely together. With the point’dipole approximation one determines an average spin separation of 45 A for this radical pair_ The formation of a stable radical pair from dimer 4 whik none is observed from dimer 2 indicates that relatively small changes in strutture lead to stabilization ofradicalpairs in the solid
.’
; ,‘. ._._ ,. . ; -. ...“_. .,.’ ‘. ..’ I’ .‘ .‘, ..‘. ;_ .. . ..
573 _.’ ._,
Pf-iy :. -. ., ‘( ..,,
. ‘.
-
,’ ,.
‘, ,,
i, V”‘ume.32; n”c” 3’ ‘. ,_. “.ber .I ... . (, ..:’ : .,’ ~ ;’ ‘., _‘Y’... ‘.‘. .: 1 ~. ., I’ ._ .. ‘, ,’ .. .., ,,. ‘.. : f :; ._‘I. ‘_. ’ RADICAk PAJ& &I$ I
C&r&&q&s _
,,:
.,
.: ‘_ ; :
.. :. ”
.. .‘.
.:
of Rocl+er,
-._. .‘
&ceived 4 &PoIkr 1974 ., ‘,,.. ‘. ..; .Revid r%ukstipt recked 19 Decogber .1974; . ;. : _’
‘. ,:‘-
_. __‘;’
,.;:.,,
:
: : .’ :
A.., :
_.
:
‘k.4:
:
,. _,
‘,
,,
. ,_ .’ :’ ‘, : ;’
:’
I
:
‘,
:
_Il .‘, ‘.,
.. .
;
.1 yay 197.5
_,
,...
,..‘,‘. ’ ; ;,;‘. ; ,, ,. : .: ,”
:. :.’
Rorhesly~ New &&rf46~7,
.. ‘.,
:_,
,;
‘.‘,_‘. ‘, .. -.,. ., ., : .. .. : .,.,’ ‘. _‘. .,’ 3, -:., -.: :. .’ ;. ‘: ...’ : ” “
r : ‘.
.’
:
: : . . ,,,
_
‘.
.:
-.
:
.. ~~~~~l~t ph~~oIy~~.~f kM samples of the d&& if phm&y radicllls hasbeen fourih to prddicg closely a&&ted
.'
.’
‘.
‘_
.,, ; -::. ::-
R&PQ@.%S ~~~~~~~‘,D~~~~ (’ : .-‘.., . . .
” ‘l)q~rtrr&! ofChetnishy,‘.univet~
..’ .; -. ::.
“.
,’
:.
:,
_ ,:
j&J-&j,, _I.
,:
rdical pairs&I s&ne us@. An elect&n
spinreso~zt~%?‘,iav&iptii6n of m
:
hopped,ra,dic&pak
I.” ‘_
‘- :_ ,.,_ ~,. ..t.. ‘,. 1: ., .I, .. “1 ‘. ,, ::I, ~~~d~~~~~, .‘_. ., .,‘”.’: .’ : ., ‘.. ,., ‘,. ,. : ~ihih~tiin ‘dfhindered phenols (2,6di&kt-but@‘.
:
is reported in ihis note.
., ” ,.
‘~ f!’
Com~u~d
‘.
1
COCYCLOHEXfL
:i’R-$hehols) ie&s to the for&&on of rejatively stabk phenc@, ail&& [ I] LIf ahe R-substituent ii ei$her.a
‘..‘,
:,.r~~c~s.are.g~n~~~y
blllkv’~phatic~oup(~lb~tyl)oranarylgroupthe
SC,.
: cot-WJTYL
:
,.
COCYCLOHEPTYL
~o~~~ri~ ,& both Solution knd a qzbqnyl ‘@oUpis~subs’tit&ted .. ‘at&e ‘para.p&.itiok of’the phenol& ii& the radicak.~~ ;. I$-J. 1, Comp&&investi&te& dimerize through formation 0f.i hand between the:’ . ‘. ... : : .’phenolic oxygen kki the p~a.ca~b~~ atom of a se@ ,I: :-ond &i&: The dimers.are-stable inthe’soiidstate..~ of phenoky dimers wkh bulky substituents but were; &d ck 6e isolated‘& polycrystalhne s’oiids which are ‘. not obseti-ed from dirners with smaller subitituerits. " " '. ~o&kninate~d with small amounts nionomericra~-,....I. '. :, _‘, ‘_ C&.AII ~R~~~~~~=ti~~ of the~tie~iicofthe radi--:-- .L -‘.‘: : . &I the q&dsiate..When
of
reaction [Z] vacated &at 3.: bond~,@&en’thk phwoxy.mon&nerz is relatively
.~~ra~~~~~~ti~~ the
‘,
‘.
._
_‘.
-
.2. ~~~~~~~. :., ,’
.
.-,
., ‘-” ,, 1,
.
.
‘,’
The light source _whiChwas ‘us8 for photolysis. .. ,’ SP+IB.hi&.pressure mercuj arc. The ES& spectra $ere recorded bn _aJ@3LCQ 3 ,BSX BSR .,,, i ‘1’.,. ‘, : sp&ctromr:ter., :- ,:1’.
: tis~I%iUips
[email protected] the.trap~d’radic~,~rs’which.are f0~e;d.b~~ photolysis of-solid samples’of phenoxy dimers. Radi-
. ., : : .. 1,‘. .. ..,:“ 1 ,,’ ‘_.: :(,&~fo&edb$ a&,oci&on of:&&.met& kty]s .. .‘, ‘. . ,_ :,1 [S] &cl by:photodissqci+io~ oftctr&& hydr&&s= : _) 1’. < : .‘_ ,,1: . ._ ‘_ ., been reported. The radical pairs’ “, : :,3.‘kesul~ and ~~0~ ” [b] &aye pre&g$y (.’ ‘. : ‘. 1’ .: :, .. ~e’c&mwtq-ized in each &ise,b~ +e &p&-diptile’, : “’ ,; Tfw phe~oxy ‘~rners,~~~h were ‘@ie&igated aie; ~t~ra~~on between’the ~~~~d:e~ec~on:s~~:G~ ,$I$.:’ .(r&&&s. ~&rinat.ion.of r~dicai pair&from phentjxy;,:. .-i ‘%hown@ :fi& 1. .The bond between’the pi~en_oxy, ; :’ -G&i& &as ~e=~.f~~~~.~o ‘&p&d on.~~:~~~~~ of:: ;:. :1, .: oXygen arid &thepara Carbon ator’n”tifa seconzl raditia!
[
i,the s&s&i.tent bonded to’the’.c,arbofiy~ &oup.,Raje:., .: : t&r+ stable_~~adic~.pilirs’are”prodlrc.edby,~hoto~ys~s _: .,._:_“ ,:. ;; : ., :-. ..< : :. : _;;._.,: rj -.:,_7 _-: : :. .. : ,.: : . _,
-”is relatively wer+k,&d photoIysis of the’dimeh, should: produ~~moaqmeiii::ra~c.~.-ihe : ,, &nomer.s +%ich’i+ fo~~q~‘rea4iy‘.retorn~,~& z’
:,, break t~s.bo~d,~~
., ‘&&&‘& .th&: iadicats _ -., ,, &&l&&c. .:, ,,dim& ::,, ,_,,. . :’ _ : ,: ‘, ,, ,, _ utif&is : ,, .: .l’. .. ‘. $idfr&b,.,:, .~~;;:8o~.FaundafisnFellow; . ,. __. .. ., ., ,. : ,.. ., ‘1 __ ” ” ‘.. : .. ‘.‘. : ,,_’ ; “.) .: .~ $2.;;. ,;,‘:;:‘j-‘ ;.;:;..l:i,: .‘__ $..‘. y.*.__’ q:;._ ,.-:,‘,. ;; : ,, ;;‘y ,y: _j‘:,.:,;;‘.y’:.,.I’.. :-‘ .‘., ; ‘,,.I’ .,:.‘).‘. ‘., ..( __,~ .: ; ., _,, .,, ,..I’ .::.._ :. :. ,, -. _::,;.:,: _ .: _. ‘, ;‘;. ..’ _. ,.: ‘,;.,_, .. ‘: : ,.,,‘. ,./:. ,,.,, __ ..;.,,:, .. .,‘, _’ ; :, ,..:. ‘ .. ‘ .. ,_ __, .; ,: :-. ‘ ._ :,‘ “ ..:_ ..,.( “ ’ : : -.r:. ,, ,_”.,_: .-_‘L.. .’1’.‘, ....‘ .: :: , ,. .’ . ‘,: ,:: _, ,.\ _::, _.._,.-.,.. .; ,,:I,.;‘-.., 1, ‘_‘.y ‘_ : :.:. ~ :s, 1’. .,: .‘(,,. ‘2 ‘, : :’ ‘.’-:. . .:- ,-t:.. :, ,.. -.‘. -. ” ... ... ;. .:.. :. ... .: ,.~, :
CHEMICALPHYSICSLEIXERS
Volume 32, number 3
,100
1
.--:1~~~~&~
G,
steric iriterference between t!nis r-butyl soup
8 Fig. 2. ESR spectrum of the radicz11pti formed on photclysi3 of dimer 4.
diffuse apart and there is a conformational change which prevents recombination. If the monomers tie able to diffuse more than about 10 A apart the dipolar interaction between the spins on the monomers niIl be small and one would expect to observe only a change in the intensity of the g = 2 ESR signal from isolated monomers. If the motion of the monomers is restricted the radicals will be held close to one another and one expects to observe a relatively large dipolar splitting from the spins in the trapped radical pair. The dimers were photolysed in the cavity of the
.ESR spectrometer while the temperature was maintained at.-1 50°C. The ESR spectra of the samples showed a single line at g =‘2 from isolated monoradical impurities before ‘photolysis was initiated. The ESR spectra from samples of dimers in which the R groupwas either .methyl or cycl_ohexyl (compounds 1 .and 2) were ‘unchanged by phstolysis. Radi& Which may have been formed during photolysis of these molecules apparently readily recombine and the steady state concentration of radical pairs is.tdo low ‘. to observe. Phot,olysis of dimers in’which, the R group praseither t-butyldi cycloheptyl (compounds 3 and : 4) gave &R spectra which clearly showed the .dipolar ‘. splittig (fig. 2) from the interaction ‘of the spins on closely spaced monomers in a’radical pair. These spec: &a persisted’after phot,olysis was terminated as long ..’G the temperature & held below about -1 lO”.C..~. ..
._
.;,
1 ,‘,’ ., ,.:
1 May 1975
:
On waking above -110°C the Am = 1 transitions in the.spectra rapidly &appeared. These observations are consistent with tic formation of radical pairs which are unable to recombine because of changes in the relative cdnformation of the two monomers after the bond is brotin. Warming the simpIes allows rotation of the R groups and the monomers in the.radical pair &-I recombine. ‘l&methyl and cyclohexyl groups of dimers 1 and 2 apparently do not have sufflcient.steric bulk to prevent rapid recombination of the monomers. The fbutyl group of dimer 3 has 2 Ixge steric bulk and recombination of the monomers shouId be resfricted by
I
,’
.’
‘.
.._‘.
,_,.. -. ,,
; ,.:,:,,
:.
state.
” ‘An alternate explanation for the difference in be-
havior of the t&o sets of dim& might involve differewes in crystal packing of t!xe polycrystalline solids. The &ti.structures of dimers 1 arid.2 may differ from those of dime; 3 and 4 and prohibit motion of the molecules after the bond is broken. If this were & e &se, one might expectto observe radical pairs frbm riimcrs which occupy sites on the surface of the ‘.solid. Surface radical pairs and bulk radical pairs with different D values were observed in the photolysis of polycrystalline tetraphenil hydrae [4]. ae Belative intensity of the si&ak from the two ‘types of radical’pairs were found to vary with the surface area :
.:
‘.’ ;
.. ;
and the
ortho r-butyl groups on the other phenoxy monomer. The dipolar interaction be.tweeXithe spins (D = 14-4G) in the radical pair formed on photolysis of this dimer is relatively small indicating a large separation of the unpaired spins. If one u&s the point clipoIar approach. [5] to account for the dipoiar interaction the average separation of the spins in this radical pair is found to be 5.8 & The cycloheptyl graup of dimer 4 is sli&Uy larger than the cyclohexyl group of dimer 2 but h5s less steric bulk than the, pbutyl group of dimer 3. The dipolar interaction between spins in the radical pairs formed on photolysis of this dimer is Iarger (0 ~305 G, E = 18 G) than that of the radical pair from dimer 3 indicating that the monomer units are held more closely together. With the point’dipole approximation one determines an average spin separation of 45 A for this radical pair_ The formation of a stable radical pair from dimer 4 whik none is observed from dimer 2 indicates that relatively small changes in strutture lead to stabilization ofradicalpairs in the solid
.’
; ,‘. ._._ ,. . ; -. ...“_. .,.’ ‘. ..’ I’ .‘ .‘, ..‘. ;_ .. . ..
573 _.’ ._,
Pf-iy :. -. ., ‘( ..,,
. ‘.
-
,’ ,.
‘, ,,