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MCD and absorption spectra of the monovalent anion of tribenzo [12] annulene
Volume 56, number 2
1 June 1978
CHEMICAL PHYSICS LETTERS
MCD AND ABSORPTION SPECTRA OF THE MONOVALENT
ANION OF TRIBENZO 1121 ANNULENE
R-E. KONING* and P.J. ZANDSTRA LVepartmentof Physical Chemistry. Universily of Groningen. Groningen, The Netherlands Received 6 March 1978
The magnetic circular dichrcism (MCD) and absorption spectra of the monovalent anion of tribenzo [ 121 annulene have been measured in the visiib and the near-infrared region. Ihe absence of a temperature dependence of the MCD proves a non-degenerate ground state. An as.sigrunentof the spectra has been given with the aid of a scmi-empiricat ~-eIectron caku&ion (PPP approximation)_ Influence of the counter ion on optical and ESR spectra was tested by the use of different soivents. No divalent anions couId be obtained.
l_ Introduction
Until recently the only known trigonal aromatic hydrocarbon with a nondegenerate first antibonding level was the trinaphthylene molecule [l] . MO calculations on tribenzo 1121 annulene, however, showed that this molecule also might have such a non-degenerate level, leading to a nondegenerate ground state of its monovalent negative ion [2] _Most other trigonal anions have a degenerate or a nearly-degenerate ground state, which leads to ESR spectra with broad hyperfine lines andg-values which deviate from the Stone theory [3]. The ESR spectrum of the tribenzo [ 121 amudene anion does not give an indication for a degenerate ground state [4]. An additional proof of the non-degeneracy might be given by magnetic circular dichroism, which should be temperature independent. We performed an optical absorption and MCD study in order to investigate the ground state degeneracy and also to provide assignments of the spectroscopic states of the anion, which has not been done before. In addition, possible ion-pair effects have been investigated with different solvents and alkali metal ions.
* Present address: The institute for the education of teachers ‘Wbbo Emmius”, Groningen, The Netherlands_
322
2_
Experimental
The preparation of the samples and the measurements were performed in the same standard fashion as described elsewhere ]2,5,6]. As solvents we used tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF), dimetboxyethane (DME) or dierhyleneglycoldimethylether (digl;rme).
3. Results Reduction of the tribenzo [12] annulene in THF or in MTHF with sodium yields its violet coloured monovalent anion, of which the absorption and MCD spectra at room temperature are shown in tig. 1. In the absorption spectrum four distinct bands can be seen; two rather weak bands in the infrared and visible part and two more intense ones in the UV. In the MCD spectrum the number of bands is huger than four. Reasonable intensity also can be seen on places where the optical absorption is very low, for instance between the bands 2 and 3. We tried to get more information about this region by cooling the solution down to about -14$C, but apart from some narrowing of the bands, no temperature dependence was obtained and low-temperature spectra are therefore omitted. The MCD spectrum further shows two sigmoid
Volume 56, number 2
CHEMICAL
r- \
I
00
j
NY1 5-l 10
20
A
25
30
1 June 1978
PHYSICS LETTERS
35 v .lo-3 IO+-‘1-
Fig_1. MCD (above) and absorptionspectra of the monovalent anion of triiazo [ 121 annulenein MTHF at room temperature. The AOD values of the MCD spectrum are valid with a magnetic field strength of about 50 kG-
bands, numbered 1 and 3, both giving a positive sign of the parameter A/D. The other bands are more bellshaped, but in that case the sign of A/D cm easily be deduced from the relative position of the peak-maxima in absorption and MCD [S] . These signs are shown in table 1, together with the relative absorption intensity of the bands. Because of substantial overlap in the spectra we did not perform a moment analysis. The influence of ion-pairing has been studied by the use of a few other solvents. On addition of a small amount of a cyclic poly-ether, 18crown-6, to an anion solution in THF, the spectral bands shifted somewhat towards the red. This is due to a reduced interaction between the aromatic anion and the counter ion, because the latter is solvated more by the crown-ether
[8]. Solutio-ns in DME and iridiglyme coloured more blue than violet on reduction, and the shifts of the spectral bands were larger. For instance, in a mixture of THF and 18-crown-6, band 2 is shifted 250 cm-l towards the red as compared to a solution in MTHF. For DME and diglyme solvent shifts of 500 and 800 cm-l were found. These shifts are consistent with the solvating properties of the ethers [S] _ In solutions of DME and diglyme the ions were unstable and we therefore restricted ourselves to the use of (M)THF. A small influence of solvent and counter ion could be seen in the ESR spectra of the radical anion. The hyperfine coupling constants in THF, after reduction with potassium (1.34 and 0.134 G) are slightly different from those in DME (1.36 and 0.092 G) and the linewidth is somewhat larger by about a factor 1.5. The values in DME (Na-reduction) have been published before [4] and our coupling constants agree with those, within the experimental error of 0.03 G. If sodium is used for the reduction in THF, the lines are much broader, even so that the smallest of the coupling constants could not be determined_ This is probably due to unresolved Na-hyperfme splitting [9]. The spin densities, calculated by the McConnell relation with Q = -23.7 G, are given in table 2, together with some theoretical values. Continued reduction of these solutions led to a decreasing monovalent anion concentration and to formation of a product which absorbs in the near UV (fig. 2). No divalent anions were obtained_ This followed from the fact that, contrary to what might be expected for these ions, reoxidation with air yielded only a small amount of the original hydrocarbon. Also, Table 2 Experimental
Table 1 Experimental values of the hICD and absorptiop parameters
and theoretical
spin densities
K THF, Na
D,ME.K
THF,
0.0039 0.057
0.0056 0.056
0.056
Hiickel
SCF
CI
c1+ perturbation b)
0.0095 0.0381
0.0213 0.0614
0.0218 0.6618
0.0220 0.0624
experimental vabs. (cm-‘) band1
vMCD
12200
band 2
17500
band 3
27000
band4
31300
AID
-I17050 30950
Relative
Pa a)
intensity a)
PB
9
-
13
t
100
-
31
a) In arbitrary units; the values are based on peak-heights and not on peak-areas.
theoretical PO PB
a) Q is the position adjacent to the inner ringb)Counter-ion on position (0, 0,3).
323
Volume 56, number 2
CHEMICAL
1 June 1978
PHYSICS LEITERS
4. Discussion Energies, charge densities and spectral parameters were calculated with a semiempirical nelectron calculation after the methods of Pariser, Parr and Pople [lo]. The calculations were performed with the onecenter integrals I, = 11.22 eV, +yc= 10.84 eV and the two-center electron repulsion integrals according to the Mataga-Nishimoto formula [i I]. The assumptions about the geometry of the compound were the same as in a calculation on the neutral molecule [2], and of the triple bonds only the pz atomic orbitals were taken into account. The resonance integrals of the short triple bonds were taken as 1.4 0 and of the longer bonds as 0.9 /3, also in accordance with the model used for the neutral molecule (p is the resonance integral of the benzene-like bonds, its value being chosen as -2.0 eV). A set of 36 singly and 4 doubly excited configurations, up to an energy of 7 eV above the ground state has been used. In a report in the literature ]4] the spin densities were given as pQ = 0.0169 and pa = 0.0289, following from a Hiickel calculation in which no influences of the triple bonds were taken into account. Table 2 shows that our model, witb three different resonance integrals, gives a better agreement with the experiment, already in the Hiickel calculation_ In the SCF calculation, pg comes close to the experimental value, whereas pa is too high. The conf@ration interaction and also the perturbation by the counter ion have very little influence on these numbers.
17
ICI
L
25
30
Lo
35
Fig. 2. The W absorption spectra of triienzo [ 12 ] annulene and its reduction products in THF, after different reaction times with metal& sodium. (a) Original sohtion. (b) The monovalent anion. (c) After a very long reduction time_
addition
of neutral
molecule
to the solution
of the
possibly dinegative ion gave no indication of a reaction M2- + M + 2 M- . Finally the spectrum of only an absorption band, with no MCD, in the W, looked rather unusual for a divalent ion.
Table 3 Calculated values of the wavenumbel- -, ?hc dipole strengths, the oscillator strengths Transition
324
Y
D (Gebye*)
f
and the MCD parameters A/D
ND
, 2.4
0.178
2.3
0.324
-0.366
24130 25060 28300 31220
0.01 0.08 0.05
0.002 0.026 0.018
0.312 0.158 -0.607
7E"
34380 35020
4.0 2.6
I-494 0.992
0.076 -0.320
3x; 8E”
36240 36270
1.5
0.574
-0.279
1A; --, 1E” 1% r;
6790 12810 20600 12840
2A’; 3E” 4E” 5E” 6E?’
Assignment
(Bohr magn.)
.t
0.721
baud 1 band2
band3 band 4
Volume 56, number 2
CHEMICAL
PHYSICS
In table 3 the calculated transition frequencies and other spectral parameters are given. A comparison of the relative oscillator strengths with the experimental intensities (table 1) and also the signs of A/D lead to the assignment as given here. The agreement is good, only the frequencies of the calculated bands differ somewhat from the experiment; the first two being calculated at too low a frequency, whereas the others are too high. When the interaction with the counter ion is taken into account [S] , all the transitions are calculated at a higher frequency (about 2000 cm-l), which is in the right direction but rather large, compared to experiment. The fact that the MCD spectrum is not temperature dependent proves that the ground state is not orbitally degenerate, unlike most other negative aromatic ions with threefold symmetry.
Acknowledgement We would like to thank Mr. B.H. Kwant for the preparation of the tribenzo [ 121 armulene. The present investigation has been supported by the Netherlands Foundation for Chemical Research (S-0-N.) with
LETTERS
1 June 1978
financial aid from the Nethedands Organization for the Advancement of Pure Research (Z.W.O.).
References J.L. Sommerdijk,E. de Boer, F.W. Pijpers and H. van Willigen, Z. Physik Chem. 63 (1969) 183. c21 RE. Koning and P.J. Zandstra. Chem. Phys. 20 (1977) 53. r31 hi. Szwarc, ed., Ions and ion pairs in organic reactions, Vol. 1 (Wiley, New York, 1972). t41 H. Brt~~er, KH. Hausser, M. Rawitscher and H-k Staab, Tetrahedron Letters (1966) 2775. iSI R.E. Koning, Thesis, University of Groningen(1977). f61 R.E. Koning, RM.E Vliekand P.J. Zandstra, J. Phys. E. 8 (1975) 710. 17] C.N R Rao, V. Kalyanaraman and 1M.V. George, Appl. Spectry. Rev_ 3 (1970) 153; B.J. McClelland, Chem. Rev. 64 (1964) 301. 181 J. Smid, in: Ions and ion pairs in organic reactions, Vol. 1, ed. M. Szwarc (Whey, New York, 1972) ch. 3. r91 J. Howard Sharp and M.C.R. Symons, in: Ions and ion pairs in organic reactions, Vol. 1, ed. M. Szwarc (Wiley, New York, 1972) ch. 5. [l@I R-G- Parr, The quantum theory of molecular electronic structure (Benjamin, New York, 1963). iill N. Mataga and K. Nishimoto, Z. Physrk. Chem. 13 (1957) 140.
[ll
325
1 June 1978
CHEMICAL PHYSICS LETTERS
MCD AND ABSORPTION SPECTRA OF THE MONOVALENT
ANION OF TRIBENZO 1121 ANNULENE
R-E. KONING* and P.J. ZANDSTRA LVepartmentof Physical Chemistry. Universily of Groningen. Groningen, The Netherlands Received 6 March 1978
The magnetic circular dichrcism (MCD) and absorption spectra of the monovalent anion of tribenzo [ 121 annulene have been measured in the visiib and the near-infrared region. Ihe absence of a temperature dependence of the MCD proves a non-degenerate ground state. An as.sigrunentof the spectra has been given with the aid of a scmi-empiricat ~-eIectron caku&ion (PPP approximation)_ Influence of the counter ion on optical and ESR spectra was tested by the use of different soivents. No divalent anions couId be obtained.
l_ Introduction
Until recently the only known trigonal aromatic hydrocarbon with a nondegenerate first antibonding level was the trinaphthylene molecule [l] . MO calculations on tribenzo 1121 annulene, however, showed that this molecule also might have such a non-degenerate level, leading to a nondegenerate ground state of its monovalent negative ion [2] _Most other trigonal anions have a degenerate or a nearly-degenerate ground state, which leads to ESR spectra with broad hyperfine lines andg-values which deviate from the Stone theory [3]. The ESR spectrum of the tribenzo [ 121 amudene anion does not give an indication for a degenerate ground state [4]. An additional proof of the non-degeneracy might be given by magnetic circular dichroism, which should be temperature independent. We performed an optical absorption and MCD study in order to investigate the ground state degeneracy and also to provide assignments of the spectroscopic states of the anion, which has not been done before. In addition, possible ion-pair effects have been investigated with different solvents and alkali metal ions.
* Present address: The institute for the education of teachers ‘Wbbo Emmius”, Groningen, The Netherlands_
322
2_
Experimental
The preparation of the samples and the measurements were performed in the same standard fashion as described elsewhere ]2,5,6]. As solvents we used tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF), dimetboxyethane (DME) or dierhyleneglycoldimethylether (digl;rme).
3. Results Reduction of the tribenzo [12] annulene in THF or in MTHF with sodium yields its violet coloured monovalent anion, of which the absorption and MCD spectra at room temperature are shown in tig. 1. In the absorption spectrum four distinct bands can be seen; two rather weak bands in the infrared and visible part and two more intense ones in the UV. In the MCD spectrum the number of bands is huger than four. Reasonable intensity also can be seen on places where the optical absorption is very low, for instance between the bands 2 and 3. We tried to get more information about this region by cooling the solution down to about -14$C, but apart from some narrowing of the bands, no temperature dependence was obtained and low-temperature spectra are therefore omitted. The MCD spectrum further shows two sigmoid
Volume 56, number 2
CHEMICAL
r- \
I
00
j
NY1 5-l 10
20
A
25
30
1 June 1978
PHYSICS LETTERS
35 v .lo-3 IO+-‘1-
Fig_1. MCD (above) and absorptionspectra of the monovalent anion of triiazo [ 121 annulenein MTHF at room temperature. The AOD values of the MCD spectrum are valid with a magnetic field strength of about 50 kG-
bands, numbered 1 and 3, both giving a positive sign of the parameter A/D. The other bands are more bellshaped, but in that case the sign of A/D cm easily be deduced from the relative position of the peak-maxima in absorption and MCD [S] . These signs are shown in table 1, together with the relative absorption intensity of the bands. Because of substantial overlap in the spectra we did not perform a moment analysis. The influence of ion-pairing has been studied by the use of a few other solvents. On addition of a small amount of a cyclic poly-ether, 18crown-6, to an anion solution in THF, the spectral bands shifted somewhat towards the red. This is due to a reduced interaction between the aromatic anion and the counter ion, because the latter is solvated more by the crown-ether
[8]. Solutio-ns in DME and iridiglyme coloured more blue than violet on reduction, and the shifts of the spectral bands were larger. For instance, in a mixture of THF and 18-crown-6, band 2 is shifted 250 cm-l towards the red as compared to a solution in MTHF. For DME and diglyme solvent shifts of 500 and 800 cm-l were found. These shifts are consistent with the solvating properties of the ethers [S] _ In solutions of DME and diglyme the ions were unstable and we therefore restricted ourselves to the use of (M)THF. A small influence of solvent and counter ion could be seen in the ESR spectra of the radical anion. The hyperfine coupling constants in THF, after reduction with potassium (1.34 and 0.134 G) are slightly different from those in DME (1.36 and 0.092 G) and the linewidth is somewhat larger by about a factor 1.5. The values in DME (Na-reduction) have been published before [4] and our coupling constants agree with those, within the experimental error of 0.03 G. If sodium is used for the reduction in THF, the lines are much broader, even so that the smallest of the coupling constants could not be determined_ This is probably due to unresolved Na-hyperfme splitting [9]. The spin densities, calculated by the McConnell relation with Q = -23.7 G, are given in table 2, together with some theoretical values. Continued reduction of these solutions led to a decreasing monovalent anion concentration and to formation of a product which absorbs in the near UV (fig. 2). No divalent anions were obtained_ This followed from the fact that, contrary to what might be expected for these ions, reoxidation with air yielded only a small amount of the original hydrocarbon. Also, Table 2 Experimental
Table 1 Experimental values of the hICD and absorptiop parameters
and theoretical
spin densities
K THF, Na
D,ME.K
THF,
0.0039 0.057
0.0056 0.056
0.056
Hiickel
SCF
CI
c1+ perturbation b)
0.0095 0.0381
0.0213 0.0614
0.0218 0.6618
0.0220 0.0624
experimental vabs. (cm-‘) band1
vMCD
12200
band 2
17500
band 3
27000
band4
31300
AID
-I17050 30950
Relative
Pa a)
intensity a)
PB
9
-
13
t
100
-
31
a) In arbitrary units; the values are based on peak-heights and not on peak-areas.
theoretical PO PB
a) Q is the position adjacent to the inner ringb)Counter-ion on position (0, 0,3).
323
Volume 56, number 2
CHEMICAL
1 June 1978
PHYSICS LEITERS
4. Discussion Energies, charge densities and spectral parameters were calculated with a semiempirical nelectron calculation after the methods of Pariser, Parr and Pople [lo]. The calculations were performed with the onecenter integrals I, = 11.22 eV, +yc= 10.84 eV and the two-center electron repulsion integrals according to the Mataga-Nishimoto formula [i I]. The assumptions about the geometry of the compound were the same as in a calculation on the neutral molecule [2], and of the triple bonds only the pz atomic orbitals were taken into account. The resonance integrals of the short triple bonds were taken as 1.4 0 and of the longer bonds as 0.9 /3, also in accordance with the model used for the neutral molecule (p is the resonance integral of the benzene-like bonds, its value being chosen as -2.0 eV). A set of 36 singly and 4 doubly excited configurations, up to an energy of 7 eV above the ground state has been used. In a report in the literature ]4] the spin densities were given as pQ = 0.0169 and pa = 0.0289, following from a Hiickel calculation in which no influences of the triple bonds were taken into account. Table 2 shows that our model, witb three different resonance integrals, gives a better agreement with the experiment, already in the Hiickel calculation_ In the SCF calculation, pg comes close to the experimental value, whereas pa is too high. The conf@ration interaction and also the perturbation by the counter ion have very little influence on these numbers.
17
ICI
L
25
30
Lo
35
Fig. 2. The W absorption spectra of triienzo [ 12 ] annulene and its reduction products in THF, after different reaction times with metal& sodium. (a) Original sohtion. (b) The monovalent anion. (c) After a very long reduction time_
addition
of neutral
molecule
to the solution
of the
possibly dinegative ion gave no indication of a reaction M2- + M + 2 M- . Finally the spectrum of only an absorption band, with no MCD, in the W, looked rather unusual for a divalent ion.
Table 3 Calculated values of the wavenumbel- -, ?hc dipole strengths, the oscillator strengths Transition
324
Y
D (Gebye*)
f
and the MCD parameters A/D
ND
, 2.4
0.178
2.3
0.324
-0.366
24130 25060 28300 31220
0.01 0.08 0.05
0.002 0.026 0.018
0.312 0.158 -0.607
7E"
34380 35020
4.0 2.6
I-494 0.992
0.076 -0.320
3x; 8E”
36240 36270
1.5
0.574
-0.279
1A; --, 1E” 1% r;
6790 12810 20600 12840
2A’; 3E” 4E” 5E” 6E?’
Assignment
(Bohr magn.)
.t
0.721
baud 1 band2
band3 band 4
Volume 56, number 2
CHEMICAL
PHYSICS
In table 3 the calculated transition frequencies and other spectral parameters are given. A comparison of the relative oscillator strengths with the experimental intensities (table 1) and also the signs of A/D lead to the assignment as given here. The agreement is good, only the frequencies of the calculated bands differ somewhat from the experiment; the first two being calculated at too low a frequency, whereas the others are too high. When the interaction with the counter ion is taken into account [S] , all the transitions are calculated at a higher frequency (about 2000 cm-l), which is in the right direction but rather large, compared to experiment. The fact that the MCD spectrum is not temperature dependent proves that the ground state is not orbitally degenerate, unlike most other negative aromatic ions with threefold symmetry.
Acknowledgement We would like to thank Mr. B.H. Kwant for the preparation of the tribenzo [ 121 armulene. The present investigation has been supported by the Netherlands Foundation for Chemical Research (S-0-N.) with
LETTERS
1 June 1978
financial aid from the Nethedands Organization for the Advancement of Pure Research (Z.W.O.).
References J.L. Sommerdijk,E. de Boer, F.W. Pijpers and H. van Willigen, Z. Physik Chem. 63 (1969) 183. c21 RE. Koning and P.J. Zandstra. Chem. Phys. 20 (1977) 53. r31 hi. Szwarc, ed., Ions and ion pairs in organic reactions, Vol. 1 (Wiley, New York, 1972). t41 H. Brt~~er, KH. Hausser, M. Rawitscher and H-k Staab, Tetrahedron Letters (1966) 2775. iSI R.E. Koning, Thesis, University of Groningen(1977). f61 R.E. Koning, RM.E Vliekand P.J. Zandstra, J. Phys. E. 8 (1975) 710. 17] C.N R Rao, V. Kalyanaraman and 1M.V. George, Appl. Spectry. Rev_ 3 (1970) 153; B.J. McClelland, Chem. Rev. 64 (1964) 301. 181 J. Smid, in: Ions and ion pairs in organic reactions, Vol. 1, ed. M. Szwarc (Whey, New York, 1972) ch. 3. r91 J. Howard Sharp and M.C.R. Symons, in: Ions and ion pairs in organic reactions, Vol. 1, ed. M. Szwarc (Wiley, New York, 1972) ch. 5. [l@I R-G- Parr, The quantum theory of molecular electronic structure (Benjamin, New York, 1963). iill N. Mataga and K. Nishimoto, Z. Physrk. Chem. 13 (1957) 140.
[ll
325