- Email: [email protected]
Induced circular dichroism
&me3i,
.
CHEMICAL PHYSIC LEIT’ERS
number-;
April
i975
:
.'.,
: ',
.;
‘15
‘.
.: 'kIDUCEDCRCULAR S.F. MASON’
DICHRdIShl
,’ -’
-,
Chemfstry Department, King’s College, Londotl, WC.27 .XS, UK
I
l&eived 23 December 1974
A recent theory of the circular diclkoism induced in & absorption bands of an echird moIecde (A) in the presence of a chtial molecule (C) is re-esamined. The ICD theory for the case of a .fwed mutual orientation of (A) and (Cl is extended and applied t,o the case of a mutually random orientation of the two molecules. The tmahnent spans the sewed ICD effects of the change in the. CD spectrum of a chiral mok-cule induced by an achiral species. and optical activity changes due to the mutual perturbation of two chiral molecules. The macroscopic ICD effect arising from the differentti left and right circular radiation field in a chkal medium is considered, together with orderof-mzgnitude estimates of the LCD effects.
Craig and co-workers [l] reported recently a theoretical
description
of the optical
activity
induced
in an achiral nolecule,(A) by neighbouring chiral molecules (C), covering induced circular dichroism (ICD) effects [2-71. The theory is restricted to the case where the molecules (A) and (C) have a fmed. mutual orientation, and neither the intermolecular ICD for random mutual orientation nor U¯oscopic ICD due to the differential Lorentz field for -left and right circular. radiation in a chiral medium are considered: The initial [2] and most of the subsequent ~experimental studies of ICD [3-6]’ probably refer to a.preferred mutual orientationbetween the achiral and chiral nolecules,‘since the ICD is markedly temperature dependent in the cases where temperature effects have been studied [5,6]‘. However the.ICD of -the isotropic.octahedralmoLecLle [Mo(C&] in a, cholesteryl, ch.loride/choleste~~l nonanoate solvent is difficult to interpret on a fixed mutual orientation . basis, but not in terms of the. dirTerential Loienfz
axial vector may have common transformation characters. &I electronic transition of the acl-iral molecule from the gholrnd state f.40) to the excited state IA,), higher in energy by E,-,,, has a zero-order ,. electric dipole moment poO and an orthogonal magnetic dipole moment mao- These moments are aug mented in the molecule-pair (AC) by first-order dynamiccoupling [S] contributions,
j#+-
F 2ra,(Ao~,lvlC,C,)~~~~(~~~-~~~) (1)
and ‘: &
T- z
2[mco f ti&R&roc)]
C
.. x (iOA
,v,coc 0
j~o,;cEa,LE;a, C
,
c2j
w-here p,,c and m,o refer respectively to the electric ar+ the niagnetic moment cf the tranrtion with a
radiation field [7]. In the folk.&g dxti&s !m ad&.’ w~venumber~~6e, (EQ#c), cohecttig the ground state lCo) with an exciteti stare lC,)‘of the CM ‘tional ICD term for.ihe fwed inutual orientation case. is indicat,ed, and the intermq~edular ICD for random,.‘, .: molecule. The operator V refers to the intermolecular oricntation.is considered, ti conjunction.with the. ; ; coulombic potentid between (A) and (C). In the .‘m&$scopic; ICD due-to thy different& Lbientz field.. ’ ... dipole approximation. the matrix element of V becctrres, Without loss of gene&x the ach@l molecule (A) ... ,’ ‘- .. : (AoA,I~IcoCc~ ? CC&cr& c,,I(R~~)3 ,, (3). is’tahen’to belong to the group D& S&or any of the ‘. .., are’separated by RAG-‘.C& groups,.in which a polar vector tid an or?.+&& .‘-‘, ‘1’where’&&ntres .,: ,,of (Al, and(C) _.
.‘.
‘_,.,. ,,..
:
. ,.. ‘_ ‘,_ :-, ,_.s ,‘,._. :,,. ‘,, ., y, ,‘.’ ..
,.’
,’
c,, ‘. . ..
..
201
-~Volumi 32, nimbei
2
I
:iqd cc‘, -. IS the angular
‘.
CHEIMKXLPtiYSICSLETTERS
.factor
ray ciytial structuie of [Co(en)3]2[w04]3*PH20 ,indicates, [IO] ti preferred mutual orientation in which the (+)-[Co(en)s-J?+ ,anh [Pd413’ .ions share a com‘mori C3 axis in--the ion-pair, the, structure being sustained by N-2H...O hydrogen-bonding.. A third ICD effect, changes in optical activity due to the interaction of two chiral molecules, is accommodated in cq. (4) by allowing yollinear componerits to &,; anlI mao. As yet this effect has not been studied systema:icaJly, although it is well known that the specific rctation of many chiral molecules is concentration dependent and that the individual rotations of a chiral cation and a chiral anion are ngt generally additive with any precision for the corresponding salt. For a random mutual orientation between the molecules (A) ana (C) the first-order ICD [eq. (4)] Vanishes since the angular factor G,, [eq. (3)] averages to zero: The second-order iCD [eq. (5)] r&mains nonzero, however, as the square of that factor ((G,J2) averages to 2/‘3. In the two-level case where the lowener_gy tiansition of (A) and of(C) are-relatively close’
fpr the dipole-dipole
potential. ..’ ._ The.zer& and
first-order moments give the transitioti connecting !AO) with.iA,j an ICD rotational strength wiih’frrst-order, R&), and se&id+rder, R&!, co.mponents, -R&C
:..
2(~oA,wrcocCj c
.(&-B&j
.-’ X IEo,[~ocR~~.CronX~oc)
+iCroO-mcOl
+q)c.POr’fQl.
ana
(4)
.’
@$) =.r, C
1 2
L(AOAJV&j [,
EooEocRoc
(E&-E20,)
’
(5)
.,where’Roc is.the rotational strength of,the transition b.etween
The
ICo).and
1.5Apiili975'
: .. .,
ICC).
Jwo terms in_squarc brackets on tlte r&s. of
together
and-afe
well-separated
from higher
exoitations,
eq. (4) were obtained previously [l] and they are the second order ICD becomes, yaiid for all achiral systems. The third term is singular to achir&iolecul& of the D2d, Sq, and the C,, $2 = ~=Qc~Oc~O~E0c~Qn (6). groups, and s&h molecules provide favourable cases XRA~>~&-E;,)~ ’ f&ICD studies since the two p-m coupling terms df ‘whereDO,, refer to the dipole strength,,(p0,12. In eq_ (4) are generally of a comparable magnitude. terms of the dissymmetry fact‘dr,g&= 4R,-$/Q,,,,_’ These’two terms are complementary in the steric sense which affords a more dir&t measure of the instrumen-‘ that a pref&red mutual orientation of the molecules fil accessibility ofthe ICD, eq. (6) has the form, (4). and (C) maxi@sing one of the p-m coupling mqdes in general minimises the #her, owing to the ‘gg ; 8g;;;e;3 orfhogonality of IrO= and ma0 2nd the collinearity of [,iy$j2,. ‘. (7) components of g”dc and mcO. The preferred mutual ’ o&t a tion of:tte tiolecules (A) and (C) ill most of. : With current CD instrumentationg-factors of loss the I& .&ystemsstudied to hate is unknown,.being r ‘are generally accessible, with 10m6 as the lower kit dependent,upon such_contingencies as hydrogen-. for a direct sicgle measurement or possibiy a smaller : bonding; iori-pairing, dipole association, and thk like, value !~y sig%d ac&mul,aiion’and averaging. The ,‘. -be’tween molectie-pairs in which +e chiral component. random orientation ICD is expected to be detectable (c) is often’conformationally-mubile &d generally of within these Iknits employing solvents coinposed of low:symFetry.. ,:. smail chiral molecules with a lowest-energy transition _m The compl&rie,nt of the ICD’effeci, the change of .’ of,larpedip$e &en!& &d rotational stren&. With .‘ihe CD :,-ectruti]of a~chir;il’tic$cule (C) induced by ,‘ak intermolecalar separation; R,,; of 5 A,. the data an +iral molecule (A), is covered by intercha@ng [lJ] fdr (%)-51methylene-bicycle-[2.i.l] hept-2& .the.subschpts II and.c in eq. (4) for the fixed mutual s.uggest, for e&rnple, a,g-fact&,g&2,).‘[eq. (7)], of lo-!$$&tion &se. The CD sfiectru’& of (+j-[Co(e&13+. for atiachiral solute with.the en&rgy:separation ‘, ‘&:+rkedly changed &i ion-p&&g wiU~‘[Poi]~-, (Eoc-Eon) of;3000 cm -l or of lO+ for ,an inter& :.tid oth.$ egyanions [!Jj, and ‘for this example the %‘. of lo4 crri-‘I. : : _ ., ., : :,, ,-_;. ,’ .. : 1.. ‘.‘;. .’ y., : .. ‘,. ” ‘,io2., ‘y /
:y_ ‘.:
:
‘.,.
: ‘.
‘,’
I,
.
... ..’ .,
._ .
‘._
:.’
I ., :,
‘.
~volume32, number 2
The @factor of the first-order ICD [eq. (?jj, which refers to a fixed mutual orientation of the molecules (A) and (C), is expected to have an upper value of the :’ order g-factor typical for the single-molecule case of a symmetric ~~ornoph~re located in a dissymmetric molecular environment, e.g., xIW2. More generally the first-order ICD is attenuated by the, incomplete _ locking of the mutual orientation of (A) and (C) on a molecule- and time*average, the coexistence of,two or more preferred orientations, and by the intermolecular &stance factor, (RAG)-2 or (RAC)-3; dependent upon the particuIar term of eq. (4) dominant in the .ICD. In the cases studied [2-6] the ICDg-factor ranges up to =10V3. Eqs. (4)-(7) refer to the ICD af an acb.iraI’mo!ecuI~ irradiated with left .and right circularly polarised light of equal amplitudes. The amplitu.des of left and right circular :adiation;initiaIly equal at.incidence, become unequal in a chiral medium in proportion to the circular birefringence, and the differential Lorentz radiation field contributes macroscopically’an additional ted [7] to theIC!D g-f&or of an achirai solute (A), go&:FJ
= b(9)
15 April 197s
CHEMICALPHYSICSLETTERS
4/@+2)1
(&,o) 2
(8)
where n is the mean refractive index anil (nL-nR) the .circuIar birefringence of the solution at the absorption frequency (ijo&) of the solute. As yet the macroscopic Lorentz-field ICD has been detected only in cholesteric solvents, where the cir- .’ cular birefringence is large [7]. For isotropic chiraf media the macroscopic ICDg-factor [eq. (8)] is estimated to be comparable in magnitude to th.e intermolecular ICDg-factor for random orientation [eq. (7)]. The tyo effects are distinguishable by the expectation that gsn) has the sanie sign as got Fvhether
Eoc
is larger
or smaller thanEoc
[es_ (7)] whereas
go,,(L.F.) changes sign with the circular birefringence, i.e., the opticai rotation, at Foe [eq. (S)] _The two effects are additive when gooc.> Fbn but’are opposed for the converse frequency-inequality. Second-order ICD terms additional to eq. (5) are obtained by considering the static-field perturbation 181 of (A) by 63 in which (A,) is mixed with hi-her states IA,,) under the intermoleculrtr field. Kowever these terms refer to observables which are as yet SeneraLly inaskssible, e.g., the transition connecting IA,) with I&p). S’mce realistic estimates of the magnitudes of these terms are precluded, they are not considered further here.
References
_.
[ J] D-P.Cra&,E-A. Power anndT. Th’Irunynachandran, Chem.
whys.Letters 27 (1974) 149. I21 S.F.Mason and ELI. Normaa. C&n. Commun. (1965) 335. 131 B. Basnich, 3. Am. Chem Sac. S8 (196612606; 89 (1967)
6 143.
[4] E. Ax&ad; G. Barth ani E. Bunnenberg.Tekthedron
Letters (1969) 5031. (51 L.D. Hayward and R.N. Tatty, Can. J. (Shem. 49 (1971) 624. [6] N. Tokura. T. Nagai, S. T&en&a and T. @h&a, J. Cl-rem.Sot. Perkin 11([1974}337. [7] SF. hfason aad R.D. Peacock, f. Ckem. Sac. aem.
Commun. (1973) 712.. IS] E.G. Hiihn and O.E. We&an&Jr.. I. Chem. whys. 48 ‘. (1968) 1127. [9] SF. Masonand BJ; Norma& Proc. Chem. Sot. (1964) 339; I. Chem. Sac. A (1966) 307. [ 1.0] E.N. Dueslerand K.N. &ymond, Inorg. CXem.10 (1971) 1466. Ill] L.S..Forster, A. Moscow&, J.G. Bnger and K. Mislow, J. Am. Chem. Sot. B4 ($962) 4353.
.
CHEMICAL PHYSIC LEIT’ERS
number-;
April
i975
:
.'.,
: ',
.;
‘15
‘.
.: 'kIDUCEDCRCULAR S.F. MASON’
DICHRdIShl
,’ -’
-,
Chemfstry Department, King’s College, Londotl, WC.27 .XS, UK
I
l&eived 23 December 1974
A recent theory of the circular diclkoism induced in & absorption bands of an echird moIecde (A) in the presence of a chtial molecule (C) is re-esamined. The ICD theory for the case of a .fwed mutual orientation of (A) and (Cl is extended and applied t,o the case of a mutually random orientation of the two molecules. The tmahnent spans the sewed ICD effects of the change in the. CD spectrum of a chiral mok-cule induced by an achiral species. and optical activity changes due to the mutual perturbation of two chiral molecules. The macroscopic ICD effect arising from the differentti left and right circular radiation field in a chkal medium is considered, together with orderof-mzgnitude estimates of the LCD effects.
Craig and co-workers [l] reported recently a theoretical
description
of the optical
activity
induced
in an achiral nolecule,(A) by neighbouring chiral molecules (C), covering induced circular dichroism (ICD) effects [2-71. The theory is restricted to the case where the molecules (A) and (C) have a fmed. mutual orientation, and neither the intermolecular ICD for random mutual orientation nor U¯oscopic ICD due to the differential Lorentz field for -left and right circular. radiation in a chiral medium are considered: The initial [2] and most of the subsequent ~experimental studies of ICD [3-6]’ probably refer to a.preferred mutual orientationbetween the achiral and chiral nolecules,‘since the ICD is markedly temperature dependent in the cases where temperature effects have been studied [5,6]‘. However the.ICD of -the isotropic.octahedralmoLecLle [Mo(C&] in a, cholesteryl, ch.loride/choleste~~l nonanoate solvent is difficult to interpret on a fixed mutual orientation . basis, but not in terms of the. dirTerential Loienfz
axial vector may have common transformation characters. &I electronic transition of the acl-iral molecule from the gholrnd state f.40) to the excited state IA,), higher in energy by E,-,,, has a zero-order ,. electric dipole moment poO and an orthogonal magnetic dipole moment mao- These moments are aug mented in the molecule-pair (AC) by first-order dynamiccoupling [S] contributions,
j#+-
F 2ra,(Ao~,lvlC,C,)~~~~(~~~-~~~) (1)
and ‘: &
T- z
2[mco f ti&R&roc)]
C
.. x (iOA
,v,coc 0
j~o,;cEa,LE;a, C
,
c2j
w-here p,,c and m,o refer respectively to the electric ar+ the niagnetic moment cf the tranrtion with a
radiation field [7]. In the folk.&g dxti&s !m ad&.’ w~venumber~~6e, (EQ#c), cohecttig the ground state lCo) with an exciteti stare lC,)‘of the CM ‘tional ICD term for.ihe fwed inutual orientation case. is indicat,ed, and the intermq~edular ICD for random,.‘, .: molecule. The operator V refers to the intermolecular oricntation.is considered, ti conjunction.with the. ; ; coulombic potentid between (A) and (C). In the .‘m&$scopic; ICD due-to thy different& Lbientz field.. ’ ... dipole approximation. the matrix element of V becctrres, Without loss of gene&x the ach@l molecule (A) ... ,’ ‘- .. : (AoA,I~IcoCc~ ? CC&cr& c,,I(R~~)3 ,, (3). is’tahen’to belong to the group D& S&or any of the ‘. .., are’separated by RAG-‘.C& groups,.in which a polar vector tid an or?.+&& .‘-‘, ‘1’where’&&ntres .,: ,,of (Al, and(C) _.
.‘.
‘_,.,. ,,..
:
. ,.. ‘_ ‘,_ :-, ,_.s ,‘,._. :,,. ‘,, ., y, ,‘.’ ..
,.’
,’
c,, ‘. . ..
..
201
-~Volumi 32, nimbei
2
I
:iqd cc‘, -. IS the angular
‘.
CHEIMKXLPtiYSICSLETTERS
.factor
ray ciytial structuie of [Co(en)3]2[w04]3*PH20 ,indicates, [IO] ti preferred mutual orientation in which the (+)-[Co(en)s-J?+ ,anh [Pd413’ .ions share a com‘mori C3 axis in--the ion-pair, the, structure being sustained by N-2H...O hydrogen-bonding.. A third ICD effect, changes in optical activity due to the interaction of two chiral molecules, is accommodated in cq. (4) by allowing yollinear componerits to &,; anlI mao. As yet this effect has not been studied systema:icaJly, although it is well known that the specific rctation of many chiral molecules is concentration dependent and that the individual rotations of a chiral cation and a chiral anion are ngt generally additive with any precision for the corresponding salt. For a random mutual orientation between the molecules (A) ana (C) the first-order ICD [eq. (4)] Vanishes since the angular factor G,, [eq. (3)] averages to zero: The second-order iCD [eq. (5)] r&mains nonzero, however, as the square of that factor ((G,J2) averages to 2/‘3. In the two-level case where the lowener_gy tiansition of (A) and of(C) are-relatively close’
fpr the dipole-dipole
potential. ..’ ._ The.zer& and
first-order moments give the transitioti connecting !AO) with.iA,j an ICD rotational strength wiih’frrst-order, R&), and se&id+rder, R&!, co.mponents, -R&C
:..
2(~oA,wrcocCj c
.(&-B&j
.-’ X IEo,[~ocR~~.CronX~oc)
+iCroO-mcOl
+q)c.POr’fQl.
ana
(4)
.’
@$) =.r, C
1 2
L(AOAJV&j [,
EooEocRoc
(E&-E20,)
’
(5)
.,where’Roc is.the rotational strength of,the transition b.etween
The
ICo).and
1.5Apiili975'
: .. .,
ICC).
Jwo terms in_squarc brackets on tlte r&s. of
together
and-afe
well-separated
from higher
exoitations,
eq. (4) were obtained previously [l] and they are the second order ICD becomes, yaiid for all achiral systems. The third term is singular to achir&iolecul& of the D2d, Sq, and the C,, $2 = ~=Qc~Oc~O~E0c~Qn (6). groups, and s&h molecules provide favourable cases XRA~>~&-E;,)~ ’ f&ICD studies since the two p-m coupling terms df ‘whereDO,, refer to the dipole strength,,(p0,12. In eq_ (4) are generally of a comparable magnitude. terms of the dissymmetry fact‘dr,g&= 4R,-$/Q,,,,_’ These’two terms are complementary in the steric sense which affords a more dir&t measure of the instrumen-‘ that a pref&red mutual orientation of the molecules fil accessibility ofthe ICD, eq. (6) has the form, (4). and (C) maxi@sing one of the p-m coupling mqdes in general minimises the #her, owing to the ‘gg ; 8g;;;e;3 orfhogonality of IrO= and ma0 2nd the collinearity of [,iy$j2,. ‘. (7) components of g”dc and mcO. The preferred mutual ’ o&t a tion of:tte tiolecules (A) and (C) ill most of. : With current CD instrumentationg-factors of loss the I& .&ystemsstudied to hate is unknown,.being r ‘are generally accessible, with 10m6 as the lower kit dependent,upon such_contingencies as hydrogen-. for a direct sicgle measurement or possibiy a smaller : bonding; iori-pairing, dipole association, and thk like, value !~y sig%d ac&mul,aiion’and averaging. The ,‘. -be’tween molectie-pairs in which +e chiral component. random orientation ICD is expected to be detectable (c) is often’conformationally-mubile &d generally of within these Iknits employing solvents coinposed of low:symFetry.. ,:. smail chiral molecules with a lowest-energy transition _m The compl&rie,nt of the ICD’effeci, the change of .’ of,larpedip$e &en!& &d rotational stren&. With .‘ihe CD :,-ectruti]of a~chir;il’tic$cule (C) induced by ,‘ak intermolecalar separation; R,,; of 5 A,. the data an +iral molecule (A), is covered by intercha@ng [lJ] fdr (%)-51methylene-bicycle-[2.i.l] hept-2& .the.subschpts II and.c in eq. (4) for the fixed mutual s.uggest, for e&rnple, a,g-fact&,g&2,).‘[eq. (7)], of lo-!$$&tion &se. The CD sfiectru’& of (+j-[Co(e&13+. for atiachiral solute with.the en&rgy:separation ‘, ‘&:+rkedly changed &i ion-p&&g wiU~‘[Poi]~-, (Eoc-Eon) of;3000 cm -l or of lO+ for ,an inter& :.tid oth.$ egyanions [!Jj, and ‘for this example the %‘. of lo4 crri-‘I. : : _ ., ., : :,, ,-_;. ,’ .. : 1.. ‘.‘;. .’ y., : .. ‘,. ” ‘,io2., ‘y /
:y_ ‘.:
:
‘.,.
: ‘.
‘,’
I,
.
... ..’ .,
._ .
‘._
:.’
I ., :,
‘.
~volume32, number 2
The @factor of the first-order ICD [eq. (?jj, which refers to a fixed mutual orientation of the molecules (A) and (C), is expected to have an upper value of the :’ order g-factor typical for the single-molecule case of a symmetric ~~ornoph~re located in a dissymmetric molecular environment, e.g., xIW2. More generally the first-order ICD is attenuated by the, incomplete _ locking of the mutual orientation of (A) and (C) on a molecule- and time*average, the coexistence of,two or more preferred orientations, and by the intermolecular &stance factor, (RAG)-2 or (RAC)-3; dependent upon the particuIar term of eq. (4) dominant in the .ICD. In the cases studied [2-6] the ICDg-factor ranges up to =10V3. Eqs. (4)-(7) refer to the ICD af an acb.iraI’mo!ecuI~ irradiated with left .and right circularly polarised light of equal amplitudes. The amplitu.des of left and right circular :adiation;initiaIly equal at.incidence, become unequal in a chiral medium in proportion to the circular birefringence, and the differential Lorentz radiation field contributes macroscopically’an additional ted [7] to theIC!D g-f&or of an achirai solute (A), go&:FJ
= b(9)
15 April 197s
CHEMICALPHYSICSLETTERS
4/@+2)1
(&,o) 2
(8)
where n is the mean refractive index anil (nL-nR) the .circuIar birefringence of the solution at the absorption frequency (ijo&) of the solute. As yet the macroscopic Lorentz-field ICD has been detected only in cholesteric solvents, where the cir- .’ cular birefringence is large [7]. For isotropic chiraf media the macroscopic ICDg-factor [eq. (8)] is estimated to be comparable in magnitude to th.e intermolecular ICDg-factor for random orientation [eq. (7)]. The tyo effects are distinguishable by the expectation that gsn) has the sanie sign as got Fvhether
Eoc
is larger
or smaller thanEoc
[es_ (7)] whereas
go,,(L.F.) changes sign with the circular birefringence, i.e., the opticai rotation, at Foe [eq. (S)] _The two effects are additive when gooc.> Fbn but’are opposed for the converse frequency-inequality. Second-order ICD terms additional to eq. (5) are obtained by considering the static-field perturbation 181 of (A) by 63 in which (A,) is mixed with hi-her states IA,,) under the intermoleculrtr field. Kowever these terms refer to observables which are as yet SeneraLly inaskssible, e.g., the transition connecting IA,) with I&p). S’mce realistic estimates of the magnitudes of these terms are precluded, they are not considered further here.
References
_.
[ J] D-P.Cra&,E-A. Power anndT. Th’Irunynachandran, Chem.
whys.Letters 27 (1974) 149. I21 S.F.Mason and ELI. Normaa. C&n. Commun. (1965) 335. 131 B. Basnich, 3. Am. Chem Sac. S8 (196612606; 89 (1967)
6 143.
[4] E. Ax&ad; G. Barth ani E. Bunnenberg.Tekthedron
Letters (1969) 5031. (51 L.D. Hayward and R.N. Tatty, Can. J. (Shem. 49 (1971) 624. [6] N. Tokura. T. Nagai, S. T&en&a and T. @h&a, J. Cl-rem.Sot. Perkin 11([1974}337. [7] SF. hfason aad R.D. Peacock, f. Ckem. Sac. aem.
Commun. (1973) 712.. IS] E.G. Hiihn and O.E. We&an&Jr.. I. Chem. whys. 48 ‘. (1968) 1127. [9] SF. Masonand BJ; Norma& Proc. Chem. Sot. (1964) 339; I. Chem. Sac. A (1966) 307. [ 1.0] E.N. Dueslerand K.N. &ymond, Inorg. CXem.10 (1971) 1466. Ill] L.S..Forster, A. Moscow&, J.G. Bnger and K. Mislow, J. Am. Chem. Sot. B4 ($962) 4353.