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How far can electrons be transferred?
Voluh28,
ntimber 4
CHEMIC!& PHYSICS LETTERS
15 October 1974
.:
,_
.
HO,W
FAR CAN ELECTRONS BE TRANSFERRED? ..
Dep&wnt
of
K. SHItiADAand
fllemishy,
SUNY
M. SZWARC
College of
Environmex!ai Syracme,.New York lZ.?IO, USA
‘-
..
Science and Forestry,
Received 22 April 1374 Rwiz-cd manuscript remived 21 June 1974
cyclahesanc in HMPA was ctudied over 3 rmgc of temperatures N.CH2+C6Hlo-CHaV
~‘:N.CH2-~-~6HIo-~~2N.
Although the inspection of the model indicates a separation by 9 A between transfer is 0.9 X 10’s_’ at 15°C.
Following his associates
the pioneering work of Weissman and i1-3 J, many investigators b.ave studied the kinetics of electron transfer between radical anions and their parent molecules. The rate constant of the transfer is deterrnined.from.the broadening of the ESR hyperfme lines in the “s!ow exchange” [l-3] or from the degree of narrol?:ing of the collapsed spectrum in the “fast exchange” [4]. A procedure, applicable at any rate of exchange, is aiso available [5,6]. For radical ions present as ion pairs, the electron transfer has to proceed simultaneously with the transfer of an associated.counterion, and this hinders the reaction. Hs?ever, exchange involving free ions appear to be diffusion controlled [3,7], i.e., ineach encounter its probability is 0.5. The above statement raises the question of what ,constitutes an encounter. The probability of transfer is a function of their mutual orientation and of the distance, f, separating the partners. We may ask, *therefore, at what distance, q, the transfer is still measurable. Studies of intramolecular electron exchange may provide the required information. The problem ofintramolecular electron transfer has been discussed by,several workers. McConnell [8] treated the iansfer betwe:en ttio phenyl groups link-ia by .a chain of n CH2 units on the assumption that -:. 540
.Y ,.
;‘
: -
:.
:
the naphthyl groups the frequency
of
the interaction between the aromatic systems is facilitated through an overlap of their.7 orbitals with the antibonding orbit& of the aliphatic hydrocarbon chain. Such a process is classified as a transfer through a chain. McConnell’s calculations led to the conclusion that the transfer is fast for n = 1 and 2 - its frequency betilg greater than b (in cps), where a denotes the couphn~ constants characterizing the pertinent ESR spectm. According to his calculations insertion of each additional CH, unit should reduce the rate of exchange b!f a factor of 10, and hence for n > 3 the electron transfer would be too slow to be revealed by the ESR spectrum of the pertinent radical anion. McConnell’s calculations justified the experimental data available at that time 19,101 which implied a complete delocalization of an unpaired electron throughout the framework of bibenzyl radical anion. However, the more reliable studies of Gerson and Martin [ll] conclusively proved that the unpaired electron of this radical anion is localized on one phenyl moie?y only;.the frequency of the intramolecular transfer in this species is, therefore; lower than l@ cps. Since Gerson and Martin recorded the spectra of potassium salts of radical ions iri DME sobrtion kept at -70°C it is probable that they represent those of bibenzyF,K”ion pairs.and ihe localization is the con-
CHEMICAL
Volume 28, number 4’
PHYSICS LETTERS
I.5
October 1974
.
sequence of a relatively slow cation tr&fer. Unfortunately, our attempts to produce bibenzyl radical ions in hexamethylphosphorictriamide (HMPA), a solvent dissociating ion pairs [12], failed - no radical ions of bibenzyl were observed. The results of Gerson and Martin confirmed the previously reported findings of Harriman and M&i [13] who studied the intramolecular electron transfer between two p-nitrophenyl groups linked through X, X = CHZ, CHZ.CH~, 0 or S. The radical anions were formed by electrolytic reduction in a’cetonitrile or DMSO with tetra-n-propyfamnlonium acting as the counterions. For bis~-~trophenyl)nle~~e the exchange was very fast p 108 cps) but, interestingly,
a direct overlap of n orbit& of the aromatic moieties, which are permanentIy kept in contact, leads not onIy to the exchange but also to an enhancement of eIectron : affmity of these hydrocarbons [ 111. It is ~teresting to inquire what is ‘he range of innteraction that stiII leads to an effective electron transfer. With this goal in mind, we prepared frarrs-I ,6bis(a.naphthyl-methyl)-cycle-hexane (N.CH2-~-C6H10CH,.N) by Wurtz-,Fittig reaction ofcr-bromonaphthalene with Imns-1,4-bisfbromo-methyl)-cyclehexane: The latter compound was synthesized accord-
slower for the respective ether and sulphide (in the
and sublimed in vacua. It melts sharply at I61 2 0.5 “C and its 100 MHz NMR spectrum in CDCI, consists of an aromatic multiplet S = 7.18-8.06 ppm, benzylic doublet 6 = 2.93 and 2.87 ppm, J= 6cfs, and of two broad multiplets at F x 1.7 ppm for the equatorial and at G=1 ppm for the axial protons. The observed lines have the expected intensities of about 14:4:4:6. The mass spectrum revealed the molecular ion, m/e = 364 (21%), and hvo intense peaks at nz/e of I41 (51%) and 142 (8%), the abund~~es being corrected for isqtopes. These data unequivocal& identify the compound and establish its purity. About 10% or less of hydrocarbon was reduced with metallic potassium in TKF, the solvent distilled off in vacuum and the residue dissolved in HMPA. The ESR spectrum of.abok l@ M solution of radical ions (* 10m3 M of the unreduked hydrocarbon) was recorded at temperatures ranging from -IS”C’.to t45 “C and the results displayed in fig. 1. It was established that any intermolecular electron transfer is neg.& gibie undei these conditions. To elucidate the signiAcance of these jlectra monorr-naphthyl-me~yl-cyclo-here (c-C~H11-CH2N) was prepared by Wurtz-Fittig reaction from cu-bromonaphthalene and bromomethyl-cycle-hexane. The oily product was distilled in high vacuum and fmally purified by VPC. Its 100 ~z.~R spectrum in CDC13shows an aromatic multiplet at 6-z 7.22-8.0 ppm, benzylic doublet at 6 = 2.96 and 2.89 ppm; J= 7c/s, and two broad multiplets centered around 6 = 1.68 and 1.l ppm, all having the espected intendties.
range 106-107 cps). Moreover, the exchange was slow for the radical anion of dinitrobibenzyl (= 2 X 106 cps in DMSO and less than IO6 cps in ACN), its ESR spectrum indicating a virtual localization of the unpaired electron on one aromatic moiety. The relatively high dielectric constant of these solvents makes it probable that the investigated radical anions are not coupled with counterions. However, a subst~~i~ degree of salvation of the nitro group, that bears a large fraction of the negative charge, could lead to electron trapping and account for the slowness of exchange. Nevertheless, all these results suggest that an intramolecular transfer via chain is slow when n 2 2, even for a free ion. In view of these findings it may appear sfrange that we have observed i@ramolecuIar electron transfer [14-171 in a series of radical anions having the strut-’ ture N-(CH&r-N: (N d enoting an cYnaphthy1 moiety) with 92ranging from 3 to 20. Our results have been accotited for in terms of ~tr~olecular coIlisions between the two naphthyl end groups, a process basically identical with a bimolecular exchange, e.g., CH,(CH,),-N:
+ N-(CH2)3 CH3 + exchange ,
and not by involking a mechanism of transfer via chain.
The ~te~olecul~ transfer requires a direct interaction between a donor and an acceptor, e.g., a perturbation arising from the poIarization of an acceptor by a charged donor may lead to an exchange. Such interactions could lead also to intramolecular electron transfer provided that the tie of exchange the partners are properIy oriented, and sufflcien~y close ,to each other. In an extreme case, as in paracyclopha- ..
at
nes [I I, 18,191 and in diphenyl methane,
ing to +e method of Haggins and Owen [20]. The resulting hydrocarbon was recrystallized from petether
541 .
.
: .‘
,:.
.‘_
:
:‘
Fig. 2. The ?SR spectnzm ofK&~-CH$F
in Hh&A. (a) The reccrded spectrum; (b) the computer simulkted spec trum using the coupling konstants listed in ‘.table ._ i and I/?‘, =
0.25 c.
Fig. 1. The ESR spectrum of h’.kHZ-E.C&rj-CH$? (~10~ hl) in HiWA at various temperatures. The radical anian is present,as a free ion and undergoes intra.mqlecular.elec-’ tron transfer, the rate of khich increases’with fempcrature,
_
I.
The mass spectrtim rweds the molecular ion n;/e = 224 (18%), and Tao intense peaks at m/e df 142 (32%) ana 142 (35%j. Hence; the identity of tbfs ‘. compo~d is also verified: The~ESR&pectrum of cJZ6H1+$NF in HMPA ,.’ (“: 10-4Mj iS shown in Q. 2. ~ts’~alysis Ied to the couphng constant listed in table i and the computer, simulated spectrum is displayed in the ssme figure be- ‘low the expizimentai one. The’ ESR spectra arising from inter- and inframokc&r excharigtiare ~~~~yidentic~~ the slow exchange brnit, provided the rate of exchange is +e sake in both systems. However, their shapes aregreatly different in ,&e’fast exchan& Iirnit. A clear iihrstra’ tion of t&se p~ci~les_,~~as provided by out past : work [14) Hence, the approach developed’in thoS ‘studies [I+173 has been applied &r&e presentwo&_:‘,‘ .’ .:‘:
,.‘^,
.Q?.., .I_ ,_‘. ,’ ,;, ’ .‘..’
.:
:, : ., ” ,; ..
-,.
.
._.’ :
” ..“,I‘I. ‘,
,. :
...I ‘, .. .;. ,T_,
.’
In tie pkence of 2.5 X iO;2 M of the unreduced hydrocarbon the s$&urn broadens as shown in fig. 3. Although in shape at iS0C is not entirely identical with *at of N-~~-c-~~H~~-~H~~~ recorded at the same temperature, the resemblance is suffcicntly close * lo indicate that the rate of ~tr~~~~c~~r transfer,
‘.
’ ,- ~N~~~-~C~H~~-~H~.N’,
N.CH,-c+~H,,LCH,.N
is.about the same as that of the inte~olecular transfer at 2.5 X 10~~ M concentration of the parent hydrocarb,on. Qre rate of intermolecular transfer was determined by camput& simulation of the spectra using the procedure described pretiously~[15~~7] as well as’ ‘, $ie method developed by Norris [5]. In Ihe slow er.-, change limit both approaches lead to iden tical results;’ the ex&nple; of computer’~m~2ted sped&a are shown : .. * It was ~~*~~~~e io,qactly
match the &&a resultirig from the intra- and intermoieculk excbaxge, It seems thkt the c&qling constants of the N.C!H~-cC~H~~--C&.NS radical ion are, slightly different from tbase of the ‘. _’ .,. I c-C&l ~.-C&.NS radical ion.
: ,.
.’ ,_( ;, :,
‘,
,: ,‘,
_. .; ._’ :
,’
,, ,’ .:. : .:‘I
‘. .
,_
” :’
.”
Coup&::
Table 1 of IP-NsH: in HMPk at 15’C
constants
15 October 1974
CHEMICAL PHYSICS LETTERS
Volume 28, number 4
[cr.NcH’l
c-t4ex-CHeNT
+ c-Hex-CHZN
=+
exchange
= (2-3) x. lo-’ M
Position
‘Q (G) in n.butyl-c ~pllth~lene radical ion a)
b CC)
2 3
4
1.53 1.68 4.46
1.53 1.68 4.46
5
5.09
5.09
6. 7 8.
1.62 2.02 4.89
1’
2.40
I-62 2.02 4.89 2.78
2’
0.30
0.130 l/T;! = 0.25 G
a) Note the similarity betrveen the coupling constants of
these two Iadicat ions.
Fig. 3. The ESR spectra Of C-C6HiI_CH*N:
in fig. 4. The agreement between the simulated and observed spectra implies that the spectrum computed for P = 0.5 G (se& fig. 4) corresponds to that obtained for’the intermolecular exchange at 15°C when the concentration, of the parent molecule is.2.5 X 10d2 .’ M (see fig. 3). The bimolecular rate constant of the exchange is therefore 3.5 X 108 M-l s-l at lS”C, whereas the analogous rate’constant previously reported [lS-17,211 for the exchange of n-BuW with ~-butyl~-naph~y~ is 4.9 X lO* M-l s-l at that temperature. The agreement is gratifying. The similarity of the ESR spectra recorded at 15 OC for the intramoleklar and intermolecular exchange (see figs. 1 and 3) implies that the frequency of intramolecular electron transfer, N.C~z-c-C,H10,CH2.~’
~~.CII~-C’C~H~~~H~.N’
&I HMPA atlS”C
is about 0.9 X IO7 s-l. Thk value correspori& to slow’cxcha.+ge but is su~~s~n~y high in view’of the.: niagnitude of the distance,‘separatig
., ,.
,.‘..(
” :.‘
resulting from the intramolecular perature shown in-fig. 1.
exchange at the %krnetem-
the two naphthyl moieties when they are most favorably oriented for the transfer. The model shows that the naphthyl moieties oriented parallel with respect to each other are about 9 A apart in such a conformaIion. six CH, groups separate the two naphthyl moieties in N.CH,-c-C6HIo-CH,.N: and N-(CK,),--W radicaI ions. However, the frequency of the intramolecular electron transfer in zhe open chain radical is 4.4 X IO7 s-I at 1S°C 1171, i.e., five times geater than iq tic rigid cyclohexane radical~Akhough the relative number, of conformations in which the transfer is *possible is greater in the opeg cha$ species than in the rigid one, t-he benefit arisingfrom its flex-. ibiiity v&ich &lows fir a closer approach of the naphc.’ ,thyl groups, leads to a higher rate of exchange. Clear.‘
-.
:
,’ :
(0 10”; hi) in
HhfPA in the Presence of 2.5 X IO-* hEofc-C&~~-CH2N at various femperatur~s. Note tfle dmitvity offhe spectrum resuiting from the intermolecuk exchange at LS’Cwith that
.
.‘ .:
;
543
., I” : . . . .’ :: .- ‘:, ;.. ‘. ‘. c-ijexGH2N”~ .+ c-Hex-Ctj$J
C~~~IC~PHYSK~S
~:Volume 38, &u’nbef 4
.’ _.
_‘.
:
,.‘ ,‘,
LETTERS
,.
: .’
,’‘
. .
.‘
15 October
1974
.‘
.“’ “..’
”
exchange
: :,
The finakial &pp,orl: of these studies by the Nat: :io+i S&u% Foun+ttian Bnd by the'Petroleum.Re: search Fund admikitered by the American Chemical Sctiety is gktefnlly ac~owie~g~d. ” _._ ,) .. “
:
drawnattention to the basic diiferkce between the effect of irk- and intermolecular electron transfer on thk shape’of the respective PSR spectra. In es-’ sence, for intermolecular transfer &erJ, unpaired electrbn will sooner or later visit-a different nuclear spin
Fig_ 4. The comptitkr simulated ESR spaxra calcukt~d for various siates of intermolplar exchange using the procedure described inref&,[S,6].Note thd similarity of the computed spectruin ‘ for F= 015 G with,‘$at recorded at 15°C for the in; 1~tioIecu~ exchange and shown in fig. 3.
.’
-.
ly the probab~t~ of transfer considerably’~cre3ses with decre@ug separation bf the partners. Finally, s few words should be devgted to the observations,of Gerson and Martin fl l] and of Weiss-, man [IS] who,found the electron in 4,4-pk+cyclo; phane rad?ml ioni viti_ually locelizeh although the distance sepa.rating.the phfkyl groups in this species is shorter than 9 A. Two factors may account for these results..Eithe; tbc ndica3 ions were in the formlof ion 1: p&s and _r.hepresence, of coutiterionshind&ed the ex-. change, or the’lowdr pa~a~~b~ty of the phenyl moiety, as compared with naphthil, reduces the intera& tion so much that ,Jhe rate of transfer is too slow to. ‘.be observed. ,: L., ,_ ,: ,.: :. ,‘,
544 :-
.,
.‘
.:‘ : :.‘. ._. ,‘,.‘ _.
.,
,, .:.
Fig. 5, The computer sknulated ESR spectrum for various rate5 of inirYnolecti exchange ,using the p’rocedure.de_. S@b,Hin ret?.[ 14 7171. Note.the simi@ity b.etwe& t.he-spe& tium sirmilatcd forP=O.El G and that shownin fig.4 2nd s&mrated for tlrc kermokcuiar kxchange but for the same value ofP. H&wevei,‘the structure is los?.at higher, rates of intermcr lecular-ekhchakg’e (s,e$, eg., ref. 115)). ‘..
:
,/._
_I
,Y: ‘
_’
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‘.
.
-
.
.‘,.,‘~.
;:
., .’
:
.. ~.I.
.
,,:,_ ‘
.
_.
-Volume 28, number 4
state.from that irwhich it started. However, for intra~~ q-m 1morecular transfer there ar’e some unpaired eiectrons which never visit a different nuclear spin state. This distinction greatly affects the shape of an _ESR spectrum
provided
that the unpaired
electron
is
coupled to a few protons only and the degeneracy of some lines is large relative to the total number of soin states. The example given in Athcrton’s paper ilktrates weii such a situation. The ESR spectra composed of many lines of low degree of degeneracy are less affected by this difference in the mode of electron transfer, provided the exchange is not too rupid. For examp!e, figs. 4 and 5 show the computer simulated spectra of c-C&l11-CH2N: radical broadened L
by intermolecular
and intrqolecular
exchange,
respec-
tively. Both are identical at .the rate of exchange P,= 0.8 G. In conclusion, the inter-molecularly and intramo. lecularly broadenedESR
1.5 October
CHEMICAL PHYSICS LETTERS
spectra are identical
at a not
too fast rate of exchange, provided the number of lines is large and their degeneracy relatively low.
References [l] R.L. Ward and S.L Weis,mul, J. Am. Chem. Sot 79 (1957) 2086. [2] S.I. Weissman, Z. Elektrochem. 64 (1960) 47.
[3] P.J. Zandstm and S.L Weisrmn,
1974
J. Am. Chcm. Sot 84
[4] R. Chang and CS. Johnson, J; Am. Chem. Sot. 88 (1966) 2338. [5] J.R. Norris, Chem. Phys. Letters l (1967) 333. [$I R.F. Adams and N.hi. Atherton, Chem. Phys. Letters 1 : (1967)351. [7] N. Hirota, R. Carraway and W. Schook, J. Am. Chem. Sot 90 (1968) 3611. 181 M. McConne!!; J, Chem, __.,_ Phv% 35 961) SOB. -- II ,____, -__. ,~~ H ._-. [9] V.V. Voevcdskii, S.P. Solodovnikov and V.hf. Chibrikin, Dokl. Akad. Nauk SSSR 129 (F?SSj 1082. [lOi S.P. Solodotiikov. Zh. Strukt. R&-II. 2,(1961) 282. 1111 F. Gcrson and W.B. hi&n, I. Am. Chem. Soi 91 (1969) 1883. [12] A Cserhegyi, J. Jagur-Grodzinski and M. Szwuc, 5. Am. C%em. See 91 (1969) 1892. [13] J.E..Harrimanand A.H. Mnki, I. C%em. Fhys 39 (1963)
778. [ 141 K. Shimada, G. hlo’shuk, H.D. Connor, P. C&we
and M.
Szwarc,Chem’. Phys Letters 14 (L972) 396. [15] M Saw~rc, 22nd Nobel Symposium (klmqvist and Wiksells, Uppsala, 1973) p. 291.
[ 161 H.D. Connor, K. Shimadi: and M. cules 5 (1972) 801. [17] M. Szwarc and K. Shim&
SZWUC,
!vIacromole-
1. Polymer ScL (19741, to
be published [ 181 S.I. Weissman, J. iun. Chenr. Sac 80 (1’358) 6462. [19] A. Ishitani and S. Nagtira, hfol. Phys. 12 (1967) I. [20] G.A. Hagins and LN. Owen, I. Chcm. Sot (1953) 404. (211 H.D. COMOI, K. !%ima& 2nd hf. Szwuc, Chem. Phys. Letters 14 (1972) 402 [22] N.M. Atherton, Chem. Phys. Letters 23 (1973) 454.
ntimber 4
CHEMIC!& PHYSICS LETTERS
15 October 1974
.:
,_
.
HO,W
FAR CAN ELECTRONS BE TRANSFERRED? ..
Dep&wnt
of
K. SHItiADAand
fllemishy,
SUNY
M. SZWARC
College of
Environmex!ai Syracme,.New York lZ.?IO, USA
‘-
..
Science and Forestry,
Received 22 April 1374 Rwiz-cd manuscript remived 21 June 1974
cyclahesanc in HMPA was ctudied over 3 rmgc of temperatures N.CH2+C6Hlo-CHaV
~‘:N.CH2-~-~6HIo-~~2N.
Although the inspection of the model indicates a separation by 9 A between transfer is 0.9 X 10’s_’ at 15°C.
Following his associates
the pioneering work of Weissman and i1-3 J, many investigators b.ave studied the kinetics of electron transfer between radical anions and their parent molecules. The rate constant of the transfer is deterrnined.from.the broadening of the ESR hyperfme lines in the “s!ow exchange” [l-3] or from the degree of narrol?:ing of the collapsed spectrum in the “fast exchange” [4]. A procedure, applicable at any rate of exchange, is aiso available [5,6]. For radical ions present as ion pairs, the electron transfer has to proceed simultaneously with the transfer of an associated.counterion, and this hinders the reaction. Hs?ever, exchange involving free ions appear to be diffusion controlled [3,7], i.e., ineach encounter its probability is 0.5. The above statement raises the question of what ,constitutes an encounter. The probability of transfer is a function of their mutual orientation and of the distance, f, separating the partners. We may ask, *therefore, at what distance, q, the transfer is still measurable. Studies of intramolecular electron exchange may provide the required information. The problem ofintramolecular electron transfer has been discussed by,several workers. McConnell [8] treated the iansfer betwe:en ttio phenyl groups link-ia by .a chain of n CH2 units on the assumption that -:. 540
.Y ,.
;‘
: -
:.
:
the naphthyl groups the frequency
of
the interaction between the aromatic systems is facilitated through an overlap of their.7 orbitals with the antibonding orbit& of the aliphatic hydrocarbon chain. Such a process is classified as a transfer through a chain. McConnell’s calculations led to the conclusion that the transfer is fast for n = 1 and 2 - its frequency betilg greater than b (in cps), where a denotes the couphn~ constants characterizing the pertinent ESR spectm. According to his calculations insertion of each additional CH, unit should reduce the rate of exchange b!f a factor of 10, and hence for n > 3 the electron transfer would be too slow to be revealed by the ESR spectrum of the pertinent radical anion. McConnell’s calculations justified the experimental data available at that time 19,101 which implied a complete delocalization of an unpaired electron throughout the framework of bibenzyl radical anion. However, the more reliable studies of Gerson and Martin [ll] conclusively proved that the unpaired electron of this radical anion is localized on one phenyl moie?y only;.the frequency of the intramolecular transfer in this species is, therefore; lower than l@ cps. Since Gerson and Martin recorded the spectra of potassium salts of radical ions iri DME sobrtion kept at -70°C it is probable that they represent those of bibenzyF,K”ion pairs.and ihe localization is the con-
CHEMICAL
Volume 28, number 4’
PHYSICS LETTERS
I.5
October 1974
.
sequence of a relatively slow cation tr&fer. Unfortunately, our attempts to produce bibenzyl radical ions in hexamethylphosphorictriamide (HMPA), a solvent dissociating ion pairs [12], failed - no radical ions of bibenzyl were observed. The results of Gerson and Martin confirmed the previously reported findings of Harriman and M&i [13] who studied the intramolecular electron transfer between two p-nitrophenyl groups linked through X, X = CHZ, CHZ.CH~, 0 or S. The radical anions were formed by electrolytic reduction in a’cetonitrile or DMSO with tetra-n-propyfamnlonium acting as the counterions. For bis~-~trophenyl)nle~~e the exchange was very fast p 108 cps) but, interestingly,
a direct overlap of n orbit& of the aromatic moieties, which are permanentIy kept in contact, leads not onIy to the exchange but also to an enhancement of eIectron : affmity of these hydrocarbons [ 111. It is ~teresting to inquire what is ‘he range of innteraction that stiII leads to an effective electron transfer. With this goal in mind, we prepared frarrs-I ,6bis(a.naphthyl-methyl)-cycle-hexane (N.CH2-~-C6H10CH,.N) by Wurtz-,Fittig reaction ofcr-bromonaphthalene with Imns-1,4-bisfbromo-methyl)-cyclehexane: The latter compound was synthesized accord-
slower for the respective ether and sulphide (in the
and sublimed in vacua. It melts sharply at I61 2 0.5 “C and its 100 MHz NMR spectrum in CDCI, consists of an aromatic multiplet S = 7.18-8.06 ppm, benzylic doublet 6 = 2.93 and 2.87 ppm, J= 6cfs, and of two broad multiplets at F x 1.7 ppm for the equatorial and at G=1 ppm for the axial protons. The observed lines have the expected intensities of about 14:4:4:6. The mass spectrum revealed the molecular ion, m/e = 364 (21%), and hvo intense peaks at nz/e of I41 (51%) and 142 (8%), the abund~~es being corrected for isqtopes. These data unequivocal& identify the compound and establish its purity. About 10% or less of hydrocarbon was reduced with metallic potassium in TKF, the solvent distilled off in vacuum and the residue dissolved in HMPA. The ESR spectrum of.abok l@ M solution of radical ions (* 10m3 M of the unreduked hydrocarbon) was recorded at temperatures ranging from -IS”C’.to t45 “C and the results displayed in fig. 1. It was established that any intermolecular electron transfer is neg.& gibie undei these conditions. To elucidate the signiAcance of these jlectra monorr-naphthyl-me~yl-cyclo-here (c-C~H11-CH2N) was prepared by Wurtz-Fittig reaction from cu-bromonaphthalene and bromomethyl-cycle-hexane. The oily product was distilled in high vacuum and fmally purified by VPC. Its 100 ~z.~R spectrum in CDC13shows an aromatic multiplet at 6-z 7.22-8.0 ppm, benzylic doublet at 6 = 2.96 and 2.89 ppm; J= 7c/s, and two broad multiplets centered around 6 = 1.68 and 1.l ppm, all having the espected intendties.
range 106-107 cps). Moreover, the exchange was slow for the radical anion of dinitrobibenzyl (= 2 X 106 cps in DMSO and less than IO6 cps in ACN), its ESR spectrum indicating a virtual localization of the unpaired electron on one aromatic moiety. The relatively high dielectric constant of these solvents makes it probable that the investigated radical anions are not coupled with counterions. However, a subst~~i~ degree of salvation of the nitro group, that bears a large fraction of the negative charge, could lead to electron trapping and account for the slowness of exchange. Nevertheless, all these results suggest that an intramolecular transfer via chain is slow when n 2 2, even for a free ion. In view of these findings it may appear sfrange that we have observed i@ramolecuIar electron transfer [14-171 in a series of radical anions having the strut-’ ture N-(CH&r-N: (N d enoting an cYnaphthy1 moiety) with 92ranging from 3 to 20. Our results have been accotited for in terms of ~tr~olecular coIlisions between the two naphthyl end groups, a process basically identical with a bimolecular exchange, e.g., CH,(CH,),-N:
+ N-(CH2)3 CH3 + exchange ,
and not by involking a mechanism of transfer via chain.
The ~te~olecul~ transfer requires a direct interaction between a donor and an acceptor, e.g., a perturbation arising from the poIarization of an acceptor by a charged donor may lead to an exchange. Such interactions could lead also to intramolecular electron transfer provided that the tie of exchange the partners are properIy oriented, and sufflcien~y close ,to each other. In an extreme case, as in paracyclopha- ..
at
nes [I I, 18,191 and in diphenyl methane,
ing to +e method of Haggins and Owen [20]. The resulting hydrocarbon was recrystallized from petether
541 .
.
: .‘
,:.
.‘_
:
:‘
Fig. 2. The ?SR spectnzm ofK&~-CH$F
in Hh&A. (a) The reccrded spectrum; (b) the computer simulkted spec trum using the coupling konstants listed in ‘.table ._ i and I/?‘, =
0.25 c.
Fig. 1. The ESR spectrum of h’.kHZ-E.C&rj-CH$? (~10~ hl) in HiWA at various temperatures. The radical anian is present,as a free ion and undergoes intra.mqlecular.elec-’ tron transfer, the rate of khich increases’with fempcrature,
_
I.
The mass spectrtim rweds the molecular ion n;/e = 224 (18%), and Tao intense peaks at m/e df 142 (32%) ana 142 (35%j. Hence; the identity of tbfs ‘. compo~d is also verified: The~ESR&pectrum of cJZ6H1+$NF in HMPA ,.’ (“: 10-4Mj iS shown in Q. 2. ~ts’~alysis Ied to the couphng constant listed in table i and the computer, simulated spectrum is displayed in the ssme figure be- ‘low the expizimentai one. The’ ESR spectra arising from inter- and inframokc&r excharigtiare ~~~~yidentic~~ the slow exchange brnit, provided the rate of exchange is +e sake in both systems. However, their shapes aregreatly different in ,&e’fast exchan& Iirnit. A clear iihrstra’ tion of t&se p~ci~les_,~~as provided by out past : work [14) Hence, the approach developed’in thoS ‘studies [I+173 has been applied &r&e presentwo&_:‘,‘ .’ .:‘:
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In tie pkence of 2.5 X iO;2 M of the unreduced hydrocarbon the s$&urn broadens as shown in fig. 3. Although in shape at iS0C is not entirely identical with *at of N-~~-c-~~H~~-~H~~~ recorded at the same temperature, the resemblance is suffcicntly close * lo indicate that the rate of ~tr~~~~c~~r transfer,
‘.
’ ,- ~N~~~-~C~H~~-~H~.N’,
N.CH,-c+~H,,LCH,.N
is.about the same as that of the inte~olecular transfer at 2.5 X 10~~ M concentration of the parent hydrocarb,on. Qre rate of intermolecular transfer was determined by camput& simulation of the spectra using the procedure described pretiously~[15~~7] as well as’ ‘, $ie method developed by Norris [5]. In Ihe slow er.-, change limit both approaches lead to iden tical results;’ the ex&nple; of computer’~m~2ted sped&a are shown : .. * It was ~~*~~~~e io,qactly
match the &&a resultirig from the intra- and intermoieculk excbaxge, It seems thkt the c&qling constants of the N.C!H~-cC~H~~--C&.NS radical ion are, slightly different from tbase of the ‘. _’ .,. I c-C&l ~.-C&.NS radical ion.
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Table 1 of IP-NsH: in HMPk at 15’C
constants
15 October 1974
CHEMICAL PHYSICS LETTERS
Volume 28, number 4
[cr.NcH’l
c-t4ex-CHeNT
+ c-Hex-CHZN
=+
exchange
= (2-3) x. lo-’ M
Position
‘Q (G) in n.butyl-c ~pllth~lene radical ion a)
b CC)
2 3
4
1.53 1.68 4.46
1.53 1.68 4.46
5
5.09
5.09
6. 7 8.
1.62 2.02 4.89
1’
2.40
I-62 2.02 4.89 2.78
2’
0.30
0.130 l/T;! = 0.25 G
a) Note the similarity betrveen the coupling constants of
these two Iadicat ions.
Fig. 3. The ESR spectra Of C-C6HiI_CH*N:
in fig. 4. The agreement between the simulated and observed spectra implies that the spectrum computed for P = 0.5 G (se& fig. 4) corresponds to that obtained for’the intermolecular exchange at 15°C when the concentration, of the parent molecule is.2.5 X 10d2 .’ M (see fig. 3). The bimolecular rate constant of the exchange is therefore 3.5 X 108 M-l s-l at lS”C, whereas the analogous rate’constant previously reported [lS-17,211 for the exchange of n-BuW with ~-butyl~-naph~y~ is 4.9 X lO* M-l s-l at that temperature. The agreement is gratifying. The similarity of the ESR spectra recorded at 15 OC for the intramoleklar and intermolecular exchange (see figs. 1 and 3) implies that the frequency of intramolecular electron transfer, N.C~z-c-C,H10,CH2.~’
~~.CII~-C’C~H~~~H~.N’
&I HMPA atlS”C
is about 0.9 X IO7 s-l. Thk value correspori& to slow’cxcha.+ge but is su~~s~n~y high in view’of the.: niagnitude of the distance,‘separatig
., ,.
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” :.‘
resulting from the intramolecular perature shown in-fig. 1.
exchange at the %krnetem-
the two naphthyl moieties when they are most favorably oriented for the transfer. The model shows that the naphthyl moieties oriented parallel with respect to each other are about 9 A apart in such a conformaIion. six CH, groups separate the two naphthyl moieties in N.CH,-c-C6HIo-CH,.N: and N-(CK,),--W radicaI ions. However, the frequency of the intramolecular electron transfer in zhe open chain radical is 4.4 X IO7 s-I at 1S°C 1171, i.e., five times geater than iq tic rigid cyclohexane radical~Akhough the relative number, of conformations in which the transfer is *possible is greater in the opeg cha$ species than in the rigid one, t-he benefit arisingfrom its flex-. ibiiity v&ich &lows fir a closer approach of the naphc.’ ,thyl groups, leads to a higher rate of exchange. Clear.‘
-.
:
,’ :
(0 10”; hi) in
HhfPA in the Presence of 2.5 X IO-* hEofc-C&~~-CH2N at various femperatur~s. Note tfle dmitvity offhe spectrum resuiting from the intermolecuk exchange at LS’Cwith that
.
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C~~~IC~PHYSK~S
~:Volume 38, &u’nbef 4
.’ _.
_‘.
:
,.‘ ,‘,
LETTERS
,.
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15 October
1974
.‘
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”
exchange
: :,
The finakial &pp,orl: of these studies by the Nat: :io+i S&u% Foun+ttian Bnd by the'Petroleum.Re: search Fund admikitered by the American Chemical Sctiety is gktefnlly ac~owie~g~d. ” _._ ,) .. “
:
drawnattention to the basic diiferkce between the effect of irk- and intermolecular electron transfer on thk shape’of the respective PSR spectra. In es-’ sence, for intermolecular transfer &erJ, unpaired electrbn will sooner or later visit-a different nuclear spin
Fig_ 4. The comptitkr simulated ESR spaxra calcukt~d for various siates of intermolplar exchange using the procedure described inref&,[S,6].Note thd similarity of the computed spectruin ‘ for F= 015 G with,‘$at recorded at 15°C for the in; 1~tioIecu~ exchange and shown in fig. 3.
.’
-.
ly the probab~t~ of transfer considerably’~cre3ses with decre@ug separation bf the partners. Finally, s few words should be devgted to the observations,of Gerson and Martin fl l] and of Weiss-, man [IS] who,found the electron in 4,4-pk+cyclo; phane rad?ml ioni viti_ually locelizeh although the distance sepa.rating.the phfkyl groups in this species is shorter than 9 A. Two factors may account for these results..Eithe; tbc ndica3 ions were in the formlof ion 1: p&s and _r.hepresence, of coutiterionshind&ed the ex-. change, or the’lowdr pa~a~~b~ty of the phenyl moiety, as compared with naphthil, reduces the intera& tion so much that ,Jhe rate of transfer is too slow to. ‘.be observed. ,: L., ,_ ,: ,.: :. ,‘,
544 :-
.,
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.,
,, .:.
Fig. 5, The computer sknulated ESR spectrum for various rate5 of inirYnolecti exchange ,using the p’rocedure.de_. S@b,Hin ret?.[ 14 7171. Note.the simi@ity b.etwe& t.he-spe& tium sirmilatcd forP=O.El G and that shownin fig.4 2nd s&mrated for tlrc kermokcuiar kxchange but for the same value ofP. H&wevei,‘the structure is los?.at higher, rates of intermcr lecular-ekhchakg’e (s,e$, eg., ref. 115)). ‘..
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-Volume 28, number 4
state.from that irwhich it started. However, for intra~~ q-m 1morecular transfer there ar’e some unpaired eiectrons which never visit a different nuclear spin state. This distinction greatly affects the shape of an _ESR spectrum
provided
that the unpaired
electron
is
coupled to a few protons only and the degeneracy of some lines is large relative to the total number of soin states. The example given in Athcrton’s paper ilktrates weii such a situation. The ESR spectra composed of many lines of low degree of degeneracy are less affected by this difference in the mode of electron transfer, provided the exchange is not too rupid. For examp!e, figs. 4 and 5 show the computer simulated spectra of c-C&l11-CH2N: radical broadened L
by intermolecular
and intrqolecular
exchange,
respec-
tively. Both are identical at .the rate of exchange P,= 0.8 G. In conclusion, the inter-molecularly and intramo. lecularly broadenedESR
1.5 October
CHEMICAL PHYSICS LETTERS
spectra are identical
at a not
too fast rate of exchange, provided the number of lines is large and their degeneracy relatively low.
References [l] R.L. Ward and S.L Weis,mul, J. Am. Chem. Sot 79 (1957) 2086. [2] S.I. Weissman, Z. Elektrochem. 64 (1960) 47.
[3] P.J. Zandstm and S.L Weisrmn,
1974
J. Am. Chcm. Sot 84
[4] R. Chang and CS. Johnson, J; Am. Chem. Sot. 88 (1966) 2338. [5] J.R. Norris, Chem. Phys. Letters l (1967) 333. [$I R.F. Adams and N.hi. Atherton, Chem. Phys. Letters 1 : (1967)351. [7] N. Hirota, R. Carraway and W. Schook, J. Am. Chem. Sot 90 (1968) 3611. 181 M. McConne!!; J, Chem, __.,_ Phv% 35 961) SOB. -- II ,____, -__. ,~~ H ._-. [9] V.V. Voevcdskii, S.P. Solodovnikov and V.hf. Chibrikin, Dokl. Akad. Nauk SSSR 129 (F?SSj 1082. [lOi S.P. Solodotiikov. Zh. Strukt. R&-II. 2,(1961) 282. 1111 F. Gcrson and W.B. hi&n, I. Am. Chem. Soi 91 (1969) 1883. [12] A Cserhegyi, J. Jagur-Grodzinski and M. Szwuc, 5. Am. C%em. See 91 (1969) 1892. [13] J.E..Harrimanand A.H. Mnki, I. C%em. Fhys 39 (1963)
778. [ 141 K. Shimada, G. hlo’shuk, H.D. Connor, P. C&we
and M.
Szwarc,Chem’. Phys Letters 14 (L972) 396. [15] M Saw~rc, 22nd Nobel Symposium (klmqvist and Wiksells, Uppsala, 1973) p. 291.
[ 161 H.D. Connor, K. Shimadi: and M. cules 5 (1972) 801. [17] M. Szwarc and K. Shim&
SZWUC,
!vIacromole-
1. Polymer ScL (19741, to
be published [ 181 S.I. Weissman, J. iun. Chenr. Sac 80 (1’358) 6462. [19] A. Ishitani and S. Nagtira, hfol. Phys. 12 (1967) I. [20] G.A. Hagins and LN. Owen, I. Chcm. Sot (1953) 404. (211 H.D. COMOI, K. !%ima& 2nd hf. Szwuc, Chem. Phys. Letters 14 (1972) 402 [22] N.M. Atherton, Chem. Phys. Letters 23 (1973) 454.