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Raman resonance effect in tetrahedral cobalt(II) complexes
Volume 33, numb;
RAMAN
CHEMICAL
2
RESONANCE
G. CHOTTAXD
EFFECT
PWSICS
IN TETRMIEDRAL
LETTERS
COStiT
CQMIWXES
and J. BOLARD
D6partemerlt des Recherches PA_~~siques, ChiversifC P. 13 4:. Cun‘e, Laborntoire 75,730 Paris Cedex 05 Frame
Received 2.5 hfxch
arsocig 011 CIVRS So. 7i,
1975
coriip!exes, usin,m‘ Llrr excitin: line ~itiin the corr A Ran-~ resonance effect has been obxrxd on trtxhedrti cob&(li) tour of the d-d absorption bands. An interpretation of the spcctrz is proposed in which the Ramzn scztteiing derives its intensity mainly from two excited e!ecironic shtes.
The Raman resonance effect E2.s been observed in compounds showing, in the visib!e region, strong absorption bands, assigned to charge transfer (Mn04 [ I], (delocalised TiI,: [3_1 for examp!e) or z-rr* transitions n electrons in hemoproteins for example [3]). In the present work we extend the previous observations to the case of internal d-d transitions of transition metal ions, an important class of chromophores absorbing in the visible reson. We choose to examine some tetrshedral cobalt(H) complexes in-as-much as iheir d-d transitions are formally ailowed, which is not the case in centrosymmetric complexes. In the present note we report resu!ts on the fcllowing complexes: CoC!z-, CoBrz- , Co (NCO)~- and Co (N(X):-.
2. ExperImentrJ
The complexes have been prepared according to classical methods [4]. They were es2mined in solution in the following so!vents: CH,CN, CH,NO, 2nd (CM,)zCO, in v&rich an excess of ligand ~2s added, to prevent so!vo!ysis. Tne Spectra were obtained on Coderg PIIO 2nd T800 spectrometers, the fol!owing exsiting lines of Art, I&’ and He-Me lasers being used: 488 nm (150 mW), 632.8 nm (50-60 mw), 647.1 urn (500 mW) and 676.4 nm (100 mW). Tine spcctr2.l slit width was 5 cm-l
Peak heights were taken as the intensity of the Raman b2nds. To eva!uate the relative intensities of the vL and u3 bands of CoCl, and Co&, we have used the 360 cm-1 ‘ocnd of CH,CN as an internal standard. These b2nds have frequencies c!ose enoudi so that corrections due to the v4 law for Raman scattering and to reabsorption are negligible, our experimental measurements being accurate within 10%.
3. Results
and &scusslon
The CoX$- tetrahedra possess four fundamental vibrations, al! of which are R2m2t-r active: the stretching vibrations Y! (Al) and r13(Ta) and the bending vibrations ~2 (E) 2nd vG (T.5. We have been abi? to observe the stretching vibrations vL and vj on!y. The v3 frequencies agree well with the values obtained from IR absorption [5]. The “I vibrations have not been observed previously in solutions; for CoCl+ and CoBi<, they agree well with ‘Lheva!ues obserced in the sohd state [6]. The y1 b2nd is po!arised 2nd the ~3 I band is depo!erised, 2s expected. Ou_r recul _ _ts Zie chovin in table i. Our results can be interpreted witbin the framework of Reman intensity theory. Tlie intensity I of 2 Raman line of frequency Y is expressed 2s
309
1 June 1975
CHEhiIC.4L PHYSICS F_XnERs
VoIumc 33, numker 2 Tab!e 1 Fraqcencks
and relarivt: ir.:ensities for the q and VJ bends of CoX4 somplexues. US is the frequency of the solvent band used as inter& stzndzrd. R, ntios zre cqud Zu (f~~~vs)~f~ffif, f being the igtensity of ::r,e brick. at frequency v, MS 2nd Jf being the solvent 2nd solute molarities respectively. The gofarkation is indicated by (P) pol~ksd mnd (DP) deplarized Cone (mole /DI COCIS.
CH3CN
695 (700)
383
lo-*
5
km-‘,
632.8 488
263 (P; 263 (P) _
J
x IO-’
676.4 647.1 632.g 486
x 10-3 1o-2 2.5 X lo-’
6
CH3CN 380
Co& 724 !IlOO>
.y 10-l
Vl
Excitir?g line (nm)
co (NC04) 630 (930)
(CH3)2CO 530
3 3
x 1O-3 x 1o-3
632.8 488
cc (NCS)< 625 (1760)
CH3N$ 480
5
x 1K3 30-l
4aa
632.8
where &no are the terms of the polatizabi!ity tensor. These terms CZT:be given by the following expression r71:
‘1
165 (P)
250 (P) 260
2 VI
vg
R v3
(cm -I)
38 a
290 (DP) 290 (DP)
228
C6 <5 <5 15
225 225 226 (IX’)
373 171 30 c 0.3
< 9-O < 20
350 (DP)
740 < 20
262 1
308 (DP) 308
636 6
2
type. In between, om finds 2T, states, that will noi be considered further, the corresponding 2T, + 4A, transitions being spin forbidden. ‘I”ne Q .>brations that con couple the two 4T1 extired states are such that
charge transfer
w,?cre g is the e!ectrcnic gound state, and e and fare electronic
excited
states, (MO&
is the electric
dipole
transition moment between states g and e in the u direction, $F is the vibrcnic coupling hamiitcnisn connecting states e and jrby the vibration Av, v. and 2s “crethe frequencies d’ the. incident and scattered vibrations respectively, $1,and uJ are the frequencies of the e andfe!ectronic states. Setting e =jand e f fone gets the A and B terms oftibrecht’s theory [S] respectiveiy. me A tern? invo!ves vibrational
interact&x
with a skgle
iiowever, the Ai modes hwe to be discarded, as they do not @e rise to vibronic coupling [9]. Our expetimenial results c!ear!y show that the shift
electronic
excited state and contributes to the intensity of Al modes only,“wherer?s the B term invo!ves vibronic mixing of two electronic excited states and contributes to the intensity of any vibr;lticn whose symmetry ir contained in the direct prodllct of the representations of these -iWOSiateS. The cobalt(H) complexes of tetrahedral symmetry have Zn e!ectronic g:ound state 4 AZ; ce. 1S CEOml-i Ei&er there is 2 “TI (P) :;tate: the trzxition (d-d type] ST1 (P) + “A2 being fonn2.lly 3Howed. A splittiflg of the :T1 state is usuzlly~c~bsewed, due to sDin-orbit coup!ing. Between 2.5 000 2nd $2 000 cm- ‘1 one finds tiother “T, state, the trxxition ‘T, +“A2 being of
Fim. 1. \Wblr absorption: spectrum of CoBs. Vertid lines in&a:e psitio;?s of exci‘tig lines (a) 632.8 nrr;., (b) 6G7.1 r.m and (c) 676.4 nm.
Vo!une
33, number
CHEhfICAL
2
PHYSICS
EETTERS
I June 1975
versus E, the molar extinction coefficient of the complex, or Yersus e2 @yes a straight line Ir. bo& CZSSBS.
1
But 2s the et law is the only oae to extrapoiete to ze:o intensity for E = 0, wz c&k that the 9 I2.w should be favored. We have atiemckd to correlate our resu!is to d-d intensity cdcu!ations iE tekahedral ccmpiexes [IO]. BEt as we were not able to observe the v4 (IT,) mode, we cannot confirm Englman’s results @ving the key role to this vibration in the vibronic couphng. E-Iowevar we think that [he theoretical approach in the present state of the art being not satisfactory, the utility resonance Raman effect is of great potentid for ihc study ofvibronic coup!lng in trmsition m;td complexes.
Ackzowledgemeni We are gxlateful to Dr. Merlin and F. W21Iari lvho allowed us io use their Kri !aser and to Dr. 0. Kahn for 2n helpful discussion. 2
400
500
200
z-1
Fig. 2. Raman spectrum of CoBrc. (a) 438 nm excitation, (b) 676.4 nm excitation.
of the excittig radiatkn from blue to red induces a preferential intensity enhancement of the u3 vibration (except for CO(NCS)~). In the very clear-cut case of CoBr, (see figs. 1 2nd 2) the v1 vibration which is the only one observed in the ordinary Raman spectrum disappears in the resonance spectrum: where the v3 vibration is now the o&one observed. The active vibration in the resonant case, belonging to the T, iepresentation, we conclude that the A terrzl of Lbrecht’s theory does not contribute to the scattered iztensity, which is due to the B temi only. Therefore it is the coupkg of the two “T, stntes by the u3 vibr2tion which is the main ssa:tering process. In the case of CoCI, and Co(NCS), the v1 vibration is enhznceci too but to 2 lesser depee, so that the A term is also irwolved in the scattering mechankm. For CoBr4 ~in ihe limited frequency range explored, a plot of the scattered intensity of the p3 vibration
References [ 11 IV. Kiefer 2nd fi.j. Beinsteti, blol. Phys. 23 (!972) S35. [2] R.J. Clzrk nnd ?.I?. >fi;chell, j. AKI. Chem. Sac. 9.5 (1953) 83SO. [3] T.C. Spiro andT.C. Strekss, J. Am. Chem. Sot. 96 (1974) 338. F.A. Co?:oa, X. Gocjdgnme 2nd 4. Scccc. Inorg. Svnth. 9 (1967) 136;j. Am. Ciiem. Sot. 83 (1961) $157; F.A. Cotton XK! M. Goo@ame, I. Am. Ckm. Sot. S3 (1?62) 1776. D.SI. Adcms, Xktti Iisnd 2nd :&ted vibr~iions (xinokf, London, 1067). KG. Eciwzrds, LA. W’ocd~~id, ?,$.I. Gall and hf.H. Wzx, S~xtrochim. Xctn 26A (1970: 287. D.W. Coi.iir.s, D.B. Fitcfien ant! A. Lev;is. J. Chcrr,. hys. 59 (1953) 57lc. J. Tang 2nd .S_.C..Ubrecht, in: lizman spzctroscop’, Vol. 2, ed. ff.A. Sz>r_m5Xk~ (Plenum Pi2SS NelV VOi!:, 1473) cl!. 2. 0. Win and S.F.A. Kettle, hiol. !31ys. 29 (1975) 61. Z. Jaeger ar,d R. Englman, Ciem. Pkys. Lette.:s k9 (Ia73j 242.
RAMAN
CHEMICAL
2
RESONANCE
G. CHOTTAXD
EFFECT
PWSICS
IN TETRMIEDRAL
LETTERS
COStiT
CQMIWXES
and J. BOLARD
D6partemerlt des Recherches PA_~~siques, ChiversifC P. 13 4:. Cun‘e, Laborntoire 75,730 Paris Cedex 05 Frame
Received 2.5 hfxch
arsocig 011 CIVRS So. 7i,
1975
coriip!exes, usin,m‘ Llrr excitin: line ~itiin the corr A Ran-~ resonance effect has been obxrxd on trtxhedrti cob&(li) tour of the d-d absorption bands. An interpretation of the spcctrz is proposed in which the Ramzn scztteiing derives its intensity mainly from two excited e!ecironic shtes.
The Raman resonance effect E2.s been observed in compounds showing, in the visib!e region, strong absorption bands, assigned to charge transfer (Mn04 [ I], (delocalised TiI,: [3_1 for examp!e) or z-rr* transitions n electrons in hemoproteins for example [3]). In the present work we extend the previous observations to the case of internal d-d transitions of transition metal ions, an important class of chromophores absorbing in the visible reson. We choose to examine some tetrshedral cobalt(H) complexes in-as-much as iheir d-d transitions are formally ailowed, which is not the case in centrosymmetric complexes. In the present note we report resu!ts on the fcllowing complexes: CoC!z-, CoBrz- , Co (NCO)~- and Co (N(X):-.
2. ExperImentrJ
The complexes have been prepared according to classical methods [4]. They were es2mined in solution in the following so!vents: CH,CN, CH,NO, 2nd (CM,)zCO, in v&rich an excess of ligand ~2s added, to prevent so!vo!ysis. Tne Spectra were obtained on Coderg PIIO 2nd T800 spectrometers, the fol!owing exsiting lines of Art, I&’ and He-Me lasers being used: 488 nm (150 mW), 632.8 nm (50-60 mw), 647.1 urn (500 mW) and 676.4 nm (100 mW). Tine spcctr2.l slit width was 5 cm-l
Peak heights were taken as the intensity of the Raman b2nds. To eva!uate the relative intensities of the vL and u3 bands of CoCl, and Co&, we have used the 360 cm-1 ‘ocnd of CH,CN as an internal standard. These b2nds have frequencies c!ose enoudi so that corrections due to the v4 law for Raman scattering and to reabsorption are negligible, our experimental measurements being accurate within 10%.
3. Results
and &scusslon
The CoX$- tetrahedra possess four fundamental vibrations, al! of which are R2m2t-r active: the stretching vibrations Y! (Al) and r13(Ta) and the bending vibrations ~2 (E) 2nd vG (T.5. We have been abi? to observe the stretching vibrations vL and vj on!y. The v3 frequencies agree well with the values obtained from IR absorption [5]. The “I vibrations have not been observed previously in solutions; for CoCl+ and CoBi<, they agree well with ‘Lheva!ues obserced in the sohd state [6]. The y1 b2nd is po!arised 2nd the ~3 I band is depo!erised, 2s expected. Ou_r recul _ _ts Zie chovin in table i. Our results can be interpreted witbin the framework of Reman intensity theory. Tlie intensity I of 2 Raman line of frequency Y is expressed 2s
309
1 June 1975
CHEhiIC.4L PHYSICS F_XnERs
VoIumc 33, numker 2 Tab!e 1 Fraqcencks
and relarivt: ir.:ensities for the q and VJ bends of CoX4 somplexues. US is the frequency of the solvent band used as inter& stzndzrd. R, ntios zre cqud Zu (f~~~vs)~f~ffif, f being the igtensity of ::r,e brick. at frequency v, MS 2nd Jf being the solvent 2nd solute molarities respectively. The gofarkation is indicated by (P) pol~ksd mnd (DP) deplarized Cone (mole /DI COCIS.
CH3CN
695 (700)
383
lo-*
5
km-‘,
632.8 488
263 (P; 263 (P) _
J
x IO-’
676.4 647.1 632.g 486
x 10-3 1o-2 2.5 X lo-’
6
CH3CN 380
Co& 724 !IlOO>
.y 10-l
Vl
Excitir?g line (nm)
co (NC04) 630 (930)
(CH3)2CO 530
3 3
x 1O-3 x 1o-3
632.8 488
cc (NCS)< 625 (1760)
CH3N$ 480
5
x 1K3 30-l
4aa
632.8
where &no are the terms of the polatizabi!ity tensor. These terms CZT:be given by the following expression r71:
‘1
165 (P)
250 (P) 260
2 VI
vg
R v3
(cm -I)
38 a
290 (DP) 290 (DP)
228
C6 <5 <5 15
225 225 226 (IX’)
373 171 30 c 0.3
< 9-O < 20
350 (DP)
740 < 20
262 1
308 (DP) 308
636 6
2
type. In between, om finds 2T, states, that will noi be considered further, the corresponding 2T, + 4A, transitions being spin forbidden. ‘I”ne Q .>brations that con couple the two 4T1 extired states are such that
charge transfer
w,?cre g is the e!ectrcnic gound state, and e and fare electronic
excited
states, (MO&
is the electric
dipole
transition moment between states g and e in the u direction, $F is the vibrcnic coupling hamiitcnisn connecting states e and jrby the vibration Av, v. and 2s “crethe frequencies d’ the. incident and scattered vibrations respectively, $1,and uJ are the frequencies of the e andfe!ectronic states. Setting e =jand e f fone gets the A and B terms oftibrecht’s theory [S] respectiveiy. me A tern? invo!ves vibrational
interact&x
with a skgle
iiowever, the Ai modes hwe to be discarded, as they do not @e rise to vibronic coupling [9]. Our expetimenial results c!ear!y show that the shift
electronic
excited state and contributes to the intensity of Al modes only,“wherer?s the B term invo!ves vibronic mixing of two electronic excited states and contributes to the intensity of any vibr;lticn whose symmetry ir contained in the direct prodllct of the representations of these -iWOSiateS. The cobalt(H) complexes of tetrahedral symmetry have Zn e!ectronic g:ound state 4 AZ; ce. 1S CEOml-i Ei&er there is 2 “TI (P) :;tate: the trzxition (d-d type] ST1 (P) + “A2 being fonn2.lly 3Howed. A splittiflg of the :T1 state is usuzlly~c~bsewed, due to sDin-orbit coup!ing. Between 2.5 000 2nd $2 000 cm- ‘1 one finds tiother “T, state, the trxxition ‘T, +“A2 being of
Fim. 1. \Wblr absorption: spectrum of CoBs. Vertid lines in&a:e psitio;?s of exci‘tig lines (a) 632.8 nrr;., (b) 6G7.1 r.m and (c) 676.4 nm.
Vo!une
33, number
CHEhfICAL
2
PHYSICS
EETTERS
I June 1975
versus E, the molar extinction coefficient of the complex, or Yersus e2 @yes a straight line Ir. bo& CZSSBS.
1
But 2s the et law is the only oae to extrapoiete to ze:o intensity for E = 0, wz c&k that the 9 I2.w should be favored. We have atiemckd to correlate our resu!is to d-d intensity cdcu!ations iE tekahedral ccmpiexes [IO]. BEt as we were not able to observe the v4 (IT,) mode, we cannot confirm Englman’s results @ving the key role to this vibration in the vibronic couphng. E-Iowevar we think that [he theoretical approach in the present state of the art being not satisfactory, the utility resonance Raman effect is of great potentid for ihc study ofvibronic coup!lng in trmsition m;td complexes.
Ackzowledgemeni We are gxlateful to Dr. Merlin and F. W21Iari lvho allowed us io use their Kri !aser and to Dr. 0. Kahn for 2n helpful discussion. 2
400
500
200
z-1
Fig. 2. Raman spectrum of CoBrc. (a) 438 nm excitation, (b) 676.4 nm excitation.
of the excittig radiatkn from blue to red induces a preferential intensity enhancement of the u3 vibration (except for CO(NCS)~). In the very clear-cut case of CoBr, (see figs. 1 2nd 2) the v1 vibration which is the only one observed in the ordinary Raman spectrum disappears in the resonance spectrum: where the v3 vibration is now the o&one observed. The active vibration in the resonant case, belonging to the T, iepresentation, we conclude that the A terrzl of Lbrecht’s theory does not contribute to the scattered iztensity, which is due to the B temi only. Therefore it is the coupkg of the two “T, stntes by the u3 vibr2tion which is the main ssa:tering process. In the case of CoCI, and Co(NCS), the v1 vibration is enhznceci too but to 2 lesser depee, so that the A term is also irwolved in the scattering mechankm. For CoBr4 ~in ihe limited frequency range explored, a plot of the scattered intensity of the p3 vibration
References [ 11 IV. Kiefer 2nd fi.j. Beinsteti, blol. Phys. 23 (!972) S35. [2] R.J. Clzrk nnd ?.I?. >fi;chell, j. AKI. Chem. Sac. 9.5 (1953) 83SO. [3] T.C. Spiro andT.C. Strekss, J. Am. Chem. Sot. 96 (1974) 338. F.A. Co?:oa, X. Gocjdgnme 2nd 4. Scccc. Inorg. Svnth. 9 (1967) 136;j. Am. Ciiem. Sot. 83 (1961) $157; F.A. Cotton XK! M. Goo@ame, I. Am. Ckm. Sot. S3 (1?62) 1776. D.SI. Adcms, Xktti Iisnd 2nd :&ted vibr~iions (xinokf, London, 1067). KG. Eciwzrds, LA. W’ocd~~id, ?,$.I. Gall and hf.H. Wzx, S~xtrochim. Xctn 26A (1970: 287. D.W. Coi.iir.s, D.B. Fitcfien ant! A. Lev;is. J. Chcrr,. hys. 59 (1953) 57lc. J. Tang 2nd .S_.C..Ubrecht, in: lizman spzctroscop’, Vol. 2, ed. ff.A. Sz>r_m5Xk~ (Plenum Pi2SS NelV VOi!:, 1473) cl!. 2. 0. Win and S.F.A. Kettle, hiol. !31ys. 29 (1975) 61. Z. Jaeger ar,d R. Englman, Ciem. Pkys. Lette.:s k9 (Ia73j 242.