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Resonant Raman effect and charge distribution in the TCNEbenzene charge transfer complex
Volume 21, number 2
CHEMICAL PHYSICS LETTERS
I.5 August 1973
RESONANT RAMAN EFFECT AND CHARGE DISTRIBUTION IN THE TCNE-BENZENE CHARGE TRANSFER COMPLEX L.M. FRAAS, J.E. MOORE Inrtituto de Fisica “Gleb Wutughin”, Universidade Estadualde
Campinas,
Campinas, S.P., Brasil
and R.E. BRUNS Institute de Quimica, Universidade Estadual de Chrrpinas, Compinas, S.P., Brasil Received 27 April 1973
The resonant Raman effect due to &radiation in the spectral region of the charge-transfer absorption band of the complex TCNE-benzene is reported. The effect is mainly seen to manifest itself in the + vibrations of TChT.
In electron donor acceptor [l] systems, the charge transfer causes a partial removal of an electron from a donor to an acceptor. The resultant effect is that the bond orders and the bond lengths of the substituents change. This implies a change in the stretching frequencies and the scattering cross sections of the affected modes in the molecules involved in the complex fcrmation. This effect can be examined in Raman experiments by moving the exciting frequency into the,resonant region of the complex. In other words, the partial removal of an electron from the donor, as exists for the ground state of the system, becomes more complete. The resonant Raman effect has been studied often
2 X 1 0m3 molar TCNE in pure benzene is shown along wi,th the relative positions of the three excitation Lines used to take the Raman data of fig. 2. En fig. 2, one is able to see the growth of the TCNE modes at 1577 and 2232 cm-l which correspond to the C=C (%) stretch and the EN (Ag) stretch, respectively. There were also two unidentified modes of TCNE at 1537 and 1526 cm-l which manifested the resonant effect. Since free benzene is in such iarge excess over compiexed benzene, the effect on the allowed benzene modes would be expected to be negligib!e. Taking ratios of the intensities of the allowed benzene modes from 60-Oto 3000 cm-1 and eliminating the possibility of fluorescence progressions this expectation was
ir. inorganic solid state physics [2,3], however, its full
verified, It is then justified to use the Ejg mode of
potential is just now being realized in molecular systems [4]. Good spectroscopic studies of TCNE (tetracyanoethylene, C6N4) complexes have been made by Devlin et al. [5,6] and Moszynska [7]. The purpose of thjs communication is to complement those studies by demonstrating experimentally the resonant Raman effect in TCNE in the molecular complex TCNE-benzene. In fig. 1, the charge-transfer absorption band of
benzene at 1.593 cm-’ to normalize the intensities for the Raman
data t&en
A IaSer lines. This was done
with
5 145,48Kt,
and these
and 4765
data ZIP
pn-
sented in table 1 in the form of per cent change of intensity. The resonant Raman effect from molecuIar vibrations has been investigated by a number of workers [8-131 and more recently a good quantitative treatment for the resonant Raman effect was published by Natie et al. [!4]. A qualitative explanation of the data 357
Fig. 1. Charge-transfer
1.5 August 1973
CHEMICAL; PHYSICS LE-ITERS
Volume 21, number 2
absorption
band of -2 X lOi
molar TCNE in pure benzene.
Also shown are the relative positions
of the
argon laser lines used to take the Raman data in fig. 2.
of table 1 is given by an extension of the Placzek [ I.51 model which states that the Raman effect results via a vkrational -modulation of the electronic molecular polarizabitity. Extended to the resonant Raman case, where the electronic poiar~ab~t~r is determined almost exc!usively by the ground and resonant excited state wavefunctions, and noting that the polarization results from the charge redistribution, intuitively, one can see that, upon laser excitation in the resonant region, more and more charge is pumped into the TCNE molecule involved in the complex formation. This charge is mainly redistributed on the.C=C and CSN
bonds. This manifests itself by increasing the scatter: ing cross section of these modes, whereas the C-C mode is apparently unaffected. These charge redistribution results are supported by CNDO calculations the ;mthors have made on the TCNE molecule. A more complete expe~ental and theoretical study of the resonant Raman effect in the chargetransfer band in TCNE and benzene will lead to a better understanding of the quantum-mechanical resonance between the two molecules in the complex, the details of which are under study now. FREQUENCY
(cm11
Fig. 2. Polarized resonant Raman spectra of the FDA system ‘KNE-benzene.
l[lhe authors are grateful to Professor for helpful comments on this work.
D.A. DOW
Volume 21. number 2
CHEMICAL PHYSICS LETTERS Table 1 phonon intensities
Normalized
Laser lines A
IS August 1973
Phonon frequency (cm-‘)
1526
1537
1577
1593
1612
2232
mode type
TCNE 3
TCNE ?
TCNE C=C (A&r stretch
benzene Ezg
benzene Ezg
TCNE GN (A& stretch
5145
unity
unity
unity
unity
unity
unity
4880
150%
130%
130%
100%
100%
16u%
4765
220%
260%
260%
100%
100%
210%
References [l] R.S. Mulliken and W.B. Pe:son, Molecular complexes (Wiley, New York, 1969). [2] hi-Y. Klein and S.P.S. Porto, Phys. Rev. Letters 22 (1969) 782. [3] P.F. Williams, Ph.D. Dissertation, Univ. of Southern California (1973). [4] T.G. Spiro and T.C. Strekas, Proc. Natl. Acad. Sci. USA 69 (1972) 2622. [S] J. Stanley, D. Smith, B. Latimer and J.P. Devlin, J. Phys. Chem. 70 (1966) 2011. (61 J.C. Moore. D. Smith, Y. Youhne and J.P. Dcvlin, J. Phys. Chem. 75 (1971) 325. [7] B. hioszynska, Bull. Acad. Pol. Sci. Ser. Matt. Phys. 17 (1969) 99. [8] J. Behringer, in: Raman spectroxopy, ed. H.A. Szymanski (Plenum Press, New York, 1967) p. 168.
PI P.P. Shorygin and TM.
Ivanova, Dokt. Akad. Nauk (SSSR) 150 (1963) 533 [English transt. Soviet Ehys. Dokhdy 8 (1963) 4931. T.hl. Ivanova, L.A. Yanovskaya and P.P. Shocpgin, Opt. iSpektroskopiya 18 (1965) 206 [English transt Opt. Spectry. 1 B (1965) 1551. W. Holzer, W.F. Murphy and H-T. Bernstein. J. Chem. Phys. 52 (1969) 399. [I21 W. Kiefer and HJ. Bernstein, Chem. Phys. Letters 8 (1971) 381. [I31 R.J. Gillespie and M.J. Morton, 5. Mol. Spectry. 30 (1969) 178. 1141 L.A. Natie, P. Stein and W.L. Petimhs. Chent. Phys. Letters 12 (1971) 131. [I51 G. Plaeek, Man: Handbuch der Radiologie, Lro’ol.6 (1934). Also available as a translation Rayleigh and Raman Scattering, US At. Energy Comm.. UCRLTRANS-526 (L) (1962).
CHEMICAL PHYSICS LETTERS
I.5 August 1973
RESONANT RAMAN EFFECT AND CHARGE DISTRIBUTION IN THE TCNE-BENZENE CHARGE TRANSFER COMPLEX L.M. FRAAS, J.E. MOORE Inrtituto de Fisica “Gleb Wutughin”, Universidade Estadualde
Campinas,
Campinas, S.P., Brasil
and R.E. BRUNS Institute de Quimica, Universidade Estadual de Chrrpinas, Compinas, S.P., Brasil Received 27 April 1973
The resonant Raman effect due to &radiation in the spectral region of the charge-transfer absorption band of the complex TCNE-benzene is reported. The effect is mainly seen to manifest itself in the + vibrations of TChT.
In electron donor acceptor [l] systems, the charge transfer causes a partial removal of an electron from a donor to an acceptor. The resultant effect is that the bond orders and the bond lengths of the substituents change. This implies a change in the stretching frequencies and the scattering cross sections of the affected modes in the molecules involved in the complex fcrmation. This effect can be examined in Raman experiments by moving the exciting frequency into the,resonant region of the complex. In other words, the partial removal of an electron from the donor, as exists for the ground state of the system, becomes more complete. The resonant Raman effect has been studied often
2 X 1 0m3 molar TCNE in pure benzene is shown along wi,th the relative positions of the three excitation Lines used to take the Raman data of fig. 2. En fig. 2, one is able to see the growth of the TCNE modes at 1577 and 2232 cm-l which correspond to the C=C (%) stretch and the EN (Ag) stretch, respectively. There were also two unidentified modes of TCNE at 1537 and 1526 cm-l which manifested the resonant effect. Since free benzene is in such iarge excess over compiexed benzene, the effect on the allowed benzene modes would be expected to be negligib!e. Taking ratios of the intensities of the allowed benzene modes from 60-Oto 3000 cm-1 and eliminating the possibility of fluorescence progressions this expectation was
ir. inorganic solid state physics [2,3], however, its full
verified, It is then justified to use the Ejg mode of
potential is just now being realized in molecular systems [4]. Good spectroscopic studies of TCNE (tetracyanoethylene, C6N4) complexes have been made by Devlin et al. [5,6] and Moszynska [7]. The purpose of thjs communication is to complement those studies by demonstrating experimentally the resonant Raman effect in TCNE in the molecular complex TCNE-benzene. In fig. 1, the charge-transfer absorption band of
benzene at 1.593 cm-’ to normalize the intensities for the Raman
data t&en
A IaSer lines. This was done
with
5 145,48Kt,
and these
and 4765
data ZIP
pn-
sented in table 1 in the form of per cent change of intensity. The resonant Raman effect from molecuIar vibrations has been investigated by a number of workers [8-131 and more recently a good quantitative treatment for the resonant Raman effect was published by Natie et al. [!4]. A qualitative explanation of the data 357
Fig. 1. Charge-transfer
1.5 August 1973
CHEMICAL; PHYSICS LE-ITERS
Volume 21, number 2
absorption
band of -2 X lOi
molar TCNE in pure benzene.
Also shown are the relative positions
of the
argon laser lines used to take the Raman data in fig. 2.
of table 1 is given by an extension of the Placzek [ I.51 model which states that the Raman effect results via a vkrational -modulation of the electronic molecular polarizabitity. Extended to the resonant Raman case, where the electronic poiar~ab~t~r is determined almost exc!usively by the ground and resonant excited state wavefunctions, and noting that the polarization results from the charge redistribution, intuitively, one can see that, upon laser excitation in the resonant region, more and more charge is pumped into the TCNE molecule involved in the complex formation. This charge is mainly redistributed on the.C=C and CSN
bonds. This manifests itself by increasing the scatter: ing cross section of these modes, whereas the C-C mode is apparently unaffected. These charge redistribution results are supported by CNDO calculations the ;mthors have made on the TCNE molecule. A more complete expe~ental and theoretical study of the resonant Raman effect in the chargetransfer band in TCNE and benzene will lead to a better understanding of the quantum-mechanical resonance between the two molecules in the complex, the details of which are under study now. FREQUENCY
(cm11
Fig. 2. Polarized resonant Raman spectra of the FDA system ‘KNE-benzene.
l[lhe authors are grateful to Professor for helpful comments on this work.
D.A. DOW
Volume 21. number 2
CHEMICAL PHYSICS LETTERS Table 1 phonon intensities
Normalized
Laser lines A
IS August 1973
Phonon frequency (cm-‘)
1526
1537
1577
1593
1612
2232
mode type
TCNE 3
TCNE ?
TCNE C=C (A&r stretch
benzene Ezg
benzene Ezg
TCNE GN (A& stretch
5145
unity
unity
unity
unity
unity
unity
4880
150%
130%
130%
100%
100%
16u%
4765
220%
260%
260%
100%
100%
210%
References [l] R.S. Mulliken and W.B. Pe:son, Molecular complexes (Wiley, New York, 1969). [2] hi-Y. Klein and S.P.S. Porto, Phys. Rev. Letters 22 (1969) 782. [3] P.F. Williams, Ph.D. Dissertation, Univ. of Southern California (1973). [4] T.G. Spiro and T.C. Strekas, Proc. Natl. Acad. Sci. USA 69 (1972) 2622. [S] J. Stanley, D. Smith, B. Latimer and J.P. Devlin, J. Phys. Chem. 70 (1966) 2011. (61 J.C. Moore. D. Smith, Y. Youhne and J.P. Dcvlin, J. Phys. Chem. 75 (1971) 325. [7] B. hioszynska, Bull. Acad. Pol. Sci. Ser. Matt. Phys. 17 (1969) 99. [8] J. Behringer, in: Raman spectroxopy, ed. H.A. Szymanski (Plenum Press, New York, 1967) p. 168.
PI P.P. Shorygin and TM.
Ivanova, Dokt. Akad. Nauk (SSSR) 150 (1963) 533 [English transt. Soviet Ehys. Dokhdy 8 (1963) 4931. T.hl. Ivanova, L.A. Yanovskaya and P.P. Shocpgin, Opt. iSpektroskopiya 18 (1965) 206 [English transt Opt. Spectry. 1 B (1965) 1551. W. Holzer, W.F. Murphy and H-T. Bernstein. J. Chem. Phys. 52 (1969) 399. [I21 W. Kiefer and HJ. Bernstein, Chem. Phys. Letters 8 (1971) 381. [I31 R.J. Gillespie and M.J. Morton, 5. Mol. Spectry. 30 (1969) 178. 1141 L.A. Natie, P. Stein and W.L. Petimhs. Chent. Phys. Letters 12 (1971) 131. [I51 G. Plaeek, Man: Handbuch der Radiologie, Lro’ol.6 (1934). Also available as a translation Rayleigh and Raman Scattering, US At. Energy Comm.. UCRLTRANS-526 (L) (1962).