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Frequency shifts and intensity changes in the raman spectra of liquid mixtures of TCNE with aromatic electron donors
Volrimc 23, number
1
FREQUENCY
CHEMICAL
SHIFTS
1 Novemb.cc 1973
PHYSICS LE-ITERS
AND INTENSITY
CHANGES
IN THE RAMAN
SPECTRA
OFLIQUIDMlXTURESOFTCNEWITHAROMATICELECTRONDONORS~ K.H. MICHAELIAN, Deparrrnerrrr
K.E. RIECKHOFF
and E.M. VOIGT
oj‘Chcrnistry and Plr)fsics, Sitnorl Fraser University, Burrrob) ’ 2, British CohrnbQ, Canada Received
R;?mnn spectra of liquid mixtures and changes in the intensity ratio of plexation were observed and related Some surprising shifts in donor lines
12 July 1973
of TCNE with aromatic tlecWm donors were studied. Red-shifts up lo 20 cm-’ the C=C and C=N stretching modes TCNE from 0.5 to 1.5-3.0 upon comto an electron transfer of up to 9 % in the Bound state of these complexes. arc reported for complexes of TCNE with methosy-substituted benzencs.
I. Introdxtion
This is to our knowledge the lust successful study of Raman spectra of TT-Relectron donor/acceptor (EDA) complexes in solution’at room temperature. The experimental difficulties are considerable. They will be reported on in detail in an extended study of the phenomena observed and presented here [I]. These phenomena are: (1) Raman bandshifts of the acceptor molecule (TCNE) in donor solvents relative to their correspond-
ing frequencies in “inert” solvents (or in the solid state ofTCNE); (2) striking changes in the relative intensities of the TCNE C=C and C=N stretching modes in all donor solvents; (3) the most surprising observation: certain donors
showed frequency shifts of some of their bands relative to their position in the pure liquid, e~elr rhougIz only I% acceptor is present. These observations are important as they can be used to gain insight into the relative contributions of chemical molecular interactions as distinct from physical molecu!ar interactions* in these solutions. We are
F We are using these terms as defined in ref. (21. “Physical interactions” are essentially long range interactions, while “chemical interactions”are shorl range (valence) interactions.
of
particularly interested in EDA or charge transfer (CT) interaclion whose ground state contribution to the total molecular interactions is very difficult to assess by electronic spectroscopy, the usual experimental method employed. This is especially the case for the weak TT-Tcharge transfer complexes from which we selected a suitable set for this study.
2. Experimental The liquid samples were contained in ordinary absorption cells. Since the complexes showed strong absorptions in the spectral regions of interest the scattered light was observed in a “backscattering” geometry. The TCNE concentrations ranged from 0.1 to I molar in the solutions. In the highly coloured
complexes, the observed total scattering intensities were down compared with uncoloured solutions, presumably largely as a result of self-absorption. Known concentrations of cyclohexane provided an internal standard for relative intensity measurements. Experimental chicks were made to ensure that the effects reported were not the result of chinges_in sample, temperature and that they were independent of.F,he .- -’ strength of excitation. Sources of excitation were the 488.0 and 5 14.5 nm lines of an argon ion laser and the 632.8 nm line of a He-Ne laser. Typical powers of excitation ranged
Volume 23,
CMEMkAL PHYSICS LETIERS
number 1
--
Table 1 TCNB vibrations in non~omplesing
of solvent nD
5.9 x 10-4 2.9 x lo-$
1.4664
TCNE in perfluoror-buIylamine
a)
Average of resolved
index
[TCNE], hi
doubler.
b)
1567
22410)
0.48 + 0.06
- b)
2229
1.4266
1567
2234
1.291
-b)
2252 cl
Masked by solvent band.
SO and 150 mW. light was dispersed by a $m Spex scanning spectrometer. To reduce unwanted Rayleigh scattered background light, the output of the spectrometer was passed through a second monochromator, a Spes Minimate Model 1650. The accuracy of the reported Roman shifts obtained is approximately ~3 CIII-~ with this equipment. The detector employed was a cooled ITT-FW 130 photomultiplier used in conjunction with photoncounting electronics. Our limiting sensitivity For Raman signals at an S/IV ratio of I was about 5 phorons/scc, though ordinarily signals were much larger (of the order of 1000 photons/set).
The scattered
3. Results and discussion shifis [of TCIVE stretchkg
modes)
Both the C=C stretching rrequency and the CsN stretching frequencies occu: at Iower energies in electron donor solutions (table 2) ihan in noncornplexing solvenls (table 1). Choosing cyclohexane as reference solvent, the magnitude of the shifts in the two bands parallel each other and follow closely the known free energies of formation of the n-donors in dichloromethane for the benzene series [3] ; this is also the case for the rnethoxybenzenes [I]. Thus, hcxamethyl benz:ne and the trimethoxybenzene solutions show the largest re&-shifts; they are known to be strong rrdonors. 1,2,3 trimethoxybenzene/TCNE appears to be an exception with irs nearly zero shifts. This, however, is consistent with related data on rhis complex: it has a higher charge transfer energy [4] and a lower 6
ratio
fk’~=C)/f@‘C+)
(20°C)
between
3.1. Frequemy
lnlensity
UC-N(cm-‘)
TCNI (solid) TCNE in Ccl4 TCNE in cyclohexonc
solvents -
Refrnctive
Sample
1 November 1973
UC-C (cm-’ )
c) Uncertainty
in this frequency
0.47 _+0.04
is =!D cm-‘.
free energy of formation
[I] than the two other trimethoxybenzene complexes. This can be attributed to steric hindrance of the three adjacent methoxy substituents which prevents full conjugation of all three methoxy groups with the benzene n-system, as well as interferes with the close approach of TCNE to the n-s;,:tem required for charge transfer interaction. The generally observed Raman red-shift of the TCNE stretches on n-complexation is in complete
agreement with infrared studies of the anion of tetracyanoethylcne [5]. The v,--C in TCNE- is reported at 1370 cm-‘, i.e., %ZOO cm-l to the red of the neutral TCNE C=C stretch. Furthermore, glassy HMEITCNE deposits give an IR absorption at 1550 cm-1 [S] which agrees with our Raman value of 1554cm-’ for HMB-TCNE in cyclohexane solution. The Raman shifts observed by us indicate in the complex ground states a 6-g% electron transfer for the stronger donors and less than 5% for the weaker ones. These figures ignore shifts resulting from solvent polarizabilities which are, however, much less pronounced, as can be seen from table I. 3.2. intensity
changes [in C=C arid EN
stretches)
Referring to tables 1 and 2, it can be seen that whereas the ratio of the Raman intensities of the C=C stretch over the C=N stretch is approximately 0.5 for the uncomplexed TCNE independent of the solvent, it ranges between 1.5 and 3.0 for the complexes with a general tendency to be larger for the stronger donors. Using a cyclohexane band (2853 cm-l, C-H stretch) as internal standard we
notice a large increase in the intensity of both the C=C and the C=N stretch, in the TCNE/HMB com-
Volume 23, number
I
CHEMICAL
PHYSICS LETTERS
1 Noven~bcr
1973
Table 2 TCNE dxations
in the complex
speck
__----Intensity
0.096 a.1 I 0.11 0.083 0.097 0.095 0 a99 0.09R 0.094 0.093 0.092 0.18 0.091 0.094 0.083
1.1 0.19
_
0.21 3.2 0.20 1.1 0.2 1 ___
---
lntensiry
1563 1564 1.559 1560 1547 1552 155x 1557 1554 1556 1550 1.564 1562 1553 1557
2230 c) 1130 -__ 7773 -_711, 2223 22m 2227 -__ 77-lg 2216 2224 2227 2233 2226 2216 2225
1 .a 2.5 2.3 7.6 3.0 2.1 2.n 2.0 1.2 1.9
ratio e)’
____
--
--
------benzene roluene p-xylene mesitylene hcsamerhylbenzene~) IxsamethyIbenzene b) anisole odimethoxybenzene !ndimcthoxybcnzene pdimcthoxybenzencb) 1,2,4-rrimerhoxybenzene 1,2,3-trimcthoxybenzene”) 1,2.3-trimethoxybenzeneb) 1,3.5-trimethosybenzcne”) 1.3,5-trimethosybenzcneb)
ratiod)
1 .h
0.04
I .6 1.9 2.1 2.1 2.1
0.07 0.16
___--___-
___--
b) Solid donor dissolved in cyclohesane. c) Averngr of rcsolvcd doublet. a) Solid donor dissolved in CH2Cl2. e) Normalized IO ‘TCN&=C~/‘C,F] ,tIuC__lj) = 1 for TCNE in cycloheune.
d) Accurncies
of ratios are kO.2 or better.
----Donor
(hexmcthylbenzene anisole rrrdimethoxybenzcnc 1.2.4-trimethosybenzene 1,3,5-trimethoxybenzene a) Taken from infrared
Table 3 Donor bands shifted in the complex spccrra
------.-~--
Shifted (or new)
Probable uncomplcaed
band (cm-‘)
donor band [cm-‘)
065”
1057 3)) 783 764 1085 764 1014 3
815 802 063 795 059 810 data of Mann and Thompson
(6
1
plex. While a detailed interpetation has to be left complex. While a detailed interpretation has to be left for a future account, we note here that the increased electron density in the complexed TCNE as compared with the uncomplexed TCNE makes the observed behaviour plausible, particularly since the polarizability increase will be concentrated in the region of the C=C bond and hence increase the intensity of the C=C stretch drastically compared with that of the CrN stretch. It is suggestive that the dramatic change in the intensity ratio of the C=C to the CEN stretch
provides an extremely sensitive indicator of ground state complexation involving predominantly charge transfer, far more sensitive than the Raman shifts associated with complexation. 3.3. Donor shifts In table 3, we report bands observed in compcexe’s’ which we attribute to shifted donor bands..These were the only changes by more than 10 cm-’ attributable to donors. They are extremely surprising 7
’
Volume
23; number
i
CHEhIICAL
since only abuut 1% acceptor-molecules are present although the presence of strong absorption may -render weak donor bands unobservable unless they are enhanced by complexation. However, this interpretatior. is inconsistent with another oixervation, namely; that upon dilution of the TCNE/m-DMObenzene mixture with Ccl, the shift showed a continuous decrease with increasing concentration of Ccl,. Rather than speculate on the origin of these unexpected observations, we wish to report them at this time as experimental facts which are apparently associated only with TCNE complexes having the methoxybenzenes as donor, c!early a significant observation.
PHYSICS LE.-RS
i November 1973
References [I] K.H. hlich&inn, K.E. Rie&hoff and EM. Voigt, J. Phys. Chcm., to be published; (21 J.O. Hirschfelder, C.F. Curtiss and R.B. Bird. hlolecular theory of gases and liquids (Wiley, Near York. 1954) p. 917. [3] G. Bricgleb. Elckrronen.D3natoriAcceptor Komplexc (Springer. Berlin, I96 I). [J] E.M. Voig, J. Am. Chem. Sac. 86 (1964) 3611. [S] J.C. Moore, D. Smith, Y. Youhne and J.P. Devlin, J. Phys. Chcm. 7.5 (1971) 325. [6] J. Mann a?d H.W. lhompson.Proc. Roy. Sot. A21 I (1953) 168.
1
FREQUENCY
CHEMICAL
SHIFTS
1 Novemb.cc 1973
PHYSICS LE-ITERS
AND INTENSITY
CHANGES
IN THE RAMAN
SPECTRA
OFLIQUIDMlXTURESOFTCNEWITHAROMATICELECTRONDONORS~ K.H. MICHAELIAN, Deparrrnerrrr
K.E. RIECKHOFF
and E.M. VOIGT
oj‘Chcrnistry and Plr)fsics, Sitnorl Fraser University, Burrrob) ’ 2, British CohrnbQ, Canada Received
R;?mnn spectra of liquid mixtures and changes in the intensity ratio of plexation were observed and related Some surprising shifts in donor lines
12 July 1973
of TCNE with aromatic tlecWm donors were studied. Red-shifts up lo 20 cm-’ the C=C and C=N stretching modes TCNE from 0.5 to 1.5-3.0 upon comto an electron transfer of up to 9 % in the Bound state of these complexes. arc reported for complexes of TCNE with methosy-substituted benzencs.
I. Introdxtion
This is to our knowledge the lust successful study of Raman spectra of TT-Relectron donor/acceptor (EDA) complexes in solution’at room temperature. The experimental difficulties are considerable. They will be reported on in detail in an extended study of the phenomena observed and presented here [I]. These phenomena are: (1) Raman bandshifts of the acceptor molecule (TCNE) in donor solvents relative to their correspond-
ing frequencies in “inert” solvents (or in the solid state ofTCNE); (2) striking changes in the relative intensities of the TCNE C=C and C=N stretching modes in all donor solvents; (3) the most surprising observation: certain donors
showed frequency shifts of some of their bands relative to their position in the pure liquid, e~elr rhougIz only I% acceptor is present. These observations are important as they can be used to gain insight into the relative contributions of chemical molecular interactions as distinct from physical molecu!ar interactions* in these solutions. We are
F We are using these terms as defined in ref. (21. “Physical interactions” are essentially long range interactions, while “chemical interactions”are shorl range (valence) interactions.
of
particularly interested in EDA or charge transfer (CT) interaclion whose ground state contribution to the total molecular interactions is very difficult to assess by electronic spectroscopy, the usual experimental method employed. This is especially the case for the weak TT-Tcharge transfer complexes from which we selected a suitable set for this study.
2. Experimental The liquid samples were contained in ordinary absorption cells. Since the complexes showed strong absorptions in the spectral regions of interest the scattered light was observed in a “backscattering” geometry. The TCNE concentrations ranged from 0.1 to I molar in the solutions. In the highly coloured
complexes, the observed total scattering intensities were down compared with uncoloured solutions, presumably largely as a result of self-absorption. Known concentrations of cyclohexane provided an internal standard for relative intensity measurements. Experimental chicks were made to ensure that the effects reported were not the result of chinges_in sample, temperature and that they were independent of.F,he .- -’ strength of excitation. Sources of excitation were the 488.0 and 5 14.5 nm lines of an argon ion laser and the 632.8 nm line of a He-Ne laser. Typical powers of excitation ranged
Volume 23,
CMEMkAL PHYSICS LETIERS
number 1
--
Table 1 TCNB vibrations in non~omplesing
of solvent nD
5.9 x 10-4 2.9 x lo-$
1.4664
TCNE in perfluoror-buIylamine
a)
Average of resolved
index
[TCNE], hi
doubler.
b)
1567
22410)
0.48 + 0.06
- b)
2229
1.4266
1567
2234
1.291
-b)
2252 cl
Masked by solvent band.
SO and 150 mW. light was dispersed by a $m Spex scanning spectrometer. To reduce unwanted Rayleigh scattered background light, the output of the spectrometer was passed through a second monochromator, a Spes Minimate Model 1650. The accuracy of the reported Roman shifts obtained is approximately ~3 CIII-~ with this equipment. The detector employed was a cooled ITT-FW 130 photomultiplier used in conjunction with photoncounting electronics. Our limiting sensitivity For Raman signals at an S/IV ratio of I was about 5 phorons/scc, though ordinarily signals were much larger (of the order of 1000 photons/set).
The scattered
3. Results and discussion shifis [of TCIVE stretchkg
modes)
Both the C=C stretching rrequency and the CsN stretching frequencies occu: at Iower energies in electron donor solutions (table 2) ihan in noncornplexing solvenls (table 1). Choosing cyclohexane as reference solvent, the magnitude of the shifts in the two bands parallel each other and follow closely the known free energies of formation of the n-donors in dichloromethane for the benzene series [3] ; this is also the case for the rnethoxybenzenes [I]. Thus, hcxamethyl benz:ne and the trimethoxybenzene solutions show the largest re&-shifts; they are known to be strong rrdonors. 1,2,3 trimethoxybenzene/TCNE appears to be an exception with irs nearly zero shifts. This, however, is consistent with related data on rhis complex: it has a higher charge transfer energy [4] and a lower 6
ratio
fk’~=C)/f@‘C+)
(20°C)
between
3.1. Frequemy
lnlensity
UC-N(cm-‘)
TCNI (solid) TCNE in Ccl4 TCNE in cyclohexonc
solvents -
Refrnctive
Sample
1 November 1973
UC-C (cm-’ )
c) Uncertainty
in this frequency
0.47 _+0.04
is =!D cm-‘.
free energy of formation
[I] than the two other trimethoxybenzene complexes. This can be attributed to steric hindrance of the three adjacent methoxy substituents which prevents full conjugation of all three methoxy groups with the benzene n-system, as well as interferes with the close approach of TCNE to the n-s;,:tem required for charge transfer interaction. The generally observed Raman red-shift of the TCNE stretches on n-complexation is in complete
agreement with infrared studies of the anion of tetracyanoethylcne [5]. The v,--C in TCNE- is reported at 1370 cm-‘, i.e., %ZOO cm-l to the red of the neutral TCNE C=C stretch. Furthermore, glassy HMEITCNE deposits give an IR absorption at 1550 cm-1 [S] which agrees with our Raman value of 1554cm-’ for HMB-TCNE in cyclohexane solution. The Raman shifts observed by us indicate in the complex ground states a 6-g% electron transfer for the stronger donors and less than 5% for the weaker ones. These figures ignore shifts resulting from solvent polarizabilities which are, however, much less pronounced, as can be seen from table I. 3.2. intensity
changes [in C=C arid EN
stretches)
Referring to tables 1 and 2, it can be seen that whereas the ratio of the Raman intensities of the C=C stretch over the C=N stretch is approximately 0.5 for the uncomplexed TCNE independent of the solvent, it ranges between 1.5 and 3.0 for the complexes with a general tendency to be larger for the stronger donors. Using a cyclohexane band (2853 cm-l, C-H stretch) as internal standard we
notice a large increase in the intensity of both the C=C and the C=N stretch, in the TCNE/HMB com-
Volume 23, number
I
CHEMICAL
PHYSICS LETTERS
1 Noven~bcr
1973
Table 2 TCNE dxations
in the complex
speck
__----Intensity
0.096 a.1 I 0.11 0.083 0.097 0.095 0 a99 0.09R 0.094 0.093 0.092 0.18 0.091 0.094 0.083
1.1 0.19
_
0.21 3.2 0.20 1.1 0.2 1 ___
---
lntensiry
1563 1564 1.559 1560 1547 1552 155x 1557 1554 1556 1550 1.564 1562 1553 1557
2230 c) 1130 -__ 7773 -_711, 2223 22m 2227 -__ 77-lg 2216 2224 2227 2233 2226 2216 2225
1 .a 2.5 2.3 7.6 3.0 2.1 2.n 2.0 1.2 1.9
ratio e)’
____
--
--
------benzene roluene p-xylene mesitylene hcsamerhylbenzene~) IxsamethyIbenzene b) anisole odimethoxybenzene !ndimcthoxybcnzene pdimcthoxybenzencb) 1,2,4-rrimerhoxybenzene 1,2,3-trimcthoxybenzene”) 1,2.3-trimethoxybenzeneb) 1,3.5-trimethosybenzcne”) 1.3,5-trimethosybenzcneb)
ratiod)
1 .h
0.04
I .6 1.9 2.1 2.1 2.1
0.07 0.16
___--___-
___--
b) Solid donor dissolved in cyclohesane. c) Averngr of rcsolvcd doublet. a) Solid donor dissolved in CH2Cl2. e) Normalized IO ‘TCN&=C~/‘C,F] ,tIuC__lj) = 1 for TCNE in cycloheune.
d) Accurncies
of ratios are kO.2 or better.
----Donor
(hexmcthylbenzene anisole rrrdimethoxybenzcnc 1.2.4-trimethosybenzene 1,3,5-trimethoxybenzene a) Taken from infrared
Table 3 Donor bands shifted in the complex spccrra
------.-~--
Shifted (or new)
Probable uncomplcaed
band (cm-‘)
donor band [cm-‘)
065”
1057 3)) 783 764 1085 764 1014 3
815 802 063 795 059 810 data of Mann and Thompson
(6
1
plex. While a detailed interpetation has to be left complex. While a detailed interpretation has to be left for a future account, we note here that the increased electron density in the complexed TCNE as compared with the uncomplexed TCNE makes the observed behaviour plausible, particularly since the polarizability increase will be concentrated in the region of the C=C bond and hence increase the intensity of the C=C stretch drastically compared with that of the CrN stretch. It is suggestive that the dramatic change in the intensity ratio of the C=C to the CEN stretch
provides an extremely sensitive indicator of ground state complexation involving predominantly charge transfer, far more sensitive than the Raman shifts associated with complexation. 3.3. Donor shifts In table 3, we report bands observed in compcexe’s’ which we attribute to shifted donor bands..These were the only changes by more than 10 cm-’ attributable to donors. They are extremely surprising 7
’
Volume
23; number
i
CHEhIICAL
since only abuut 1% acceptor-molecules are present although the presence of strong absorption may -render weak donor bands unobservable unless they are enhanced by complexation. However, this interpretatior. is inconsistent with another oixervation, namely; that upon dilution of the TCNE/m-DMObenzene mixture with Ccl, the shift showed a continuous decrease with increasing concentration of Ccl,. Rather than speculate on the origin of these unexpected observations, we wish to report them at this time as experimental facts which are apparently associated only with TCNE complexes having the methoxybenzenes as donor, c!early a significant observation.
PHYSICS LE.-RS
i November 1973
References [I] K.H. hlich&inn, K.E. Rie&hoff and EM. Voigt, J. Phys. Chcm., to be published; (21 J.O. Hirschfelder, C.F. Curtiss and R.B. Bird. hlolecular theory of gases and liquids (Wiley, Near York. 1954) p. 917. [3] G. Bricgleb. Elckrronen.D3natoriAcceptor Komplexc (Springer. Berlin, I96 I). [J] E.M. Voig, J. Am. Chem. Sac. 86 (1964) 3611. [S] J.C. Moore, D. Smith, Y. Youhne and J.P. Devlin, J. Phys. Chcm. 7.5 (1971) 325. [6] J. Mann a?d H.W. lhompson.Proc. Roy. Sot. A21 I (1953) 168.