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The triplet-singlet emission spectra of benzene in carbon tetrachloride and dioxane matrices at 90°K
Spectrochimica Acta,1961, vol. 17,pp.$ to13. PergamonPress Ltd. Printed inNorthern Ireland
The triplet~~glet emission spectra of benzene in carbon tetrachloride and dioxane matrices at 90°K YOSHIYA KANDA and RYOICHI SHIMADA Departmentof Chemistry,Faculty of Science,Kyushu University,Fukuoka, Japan (Received 7 July
1960)
Ab&&---The phosphorescence spectrum of benzene has been studied in carbon tetraehloride and dioxane matrices at 90”K, and vibrational analyses are discussed. The intensity of the 0,0-band of the spectrum in carbon tetrachloride is fairly strong and no bzs vibration is found, It is concluded that the molecular shape is while an e,, vibration of 404 cm-l is obtained. probably of C,, symmetry because of deformation of the molecule due t,o lattice forces of the matris. On the other hand, the spectrum in dioxane is similar to, but sharper than, that in EPA. An e18 vibration (849 cm-“) is found in the spectrum. The occurrence of this vibration is also ascribed t,o the deformation of benzene in dioxane matrix. The reduced symmetry is probably Dw
Introduction THE phosphores~enee
spectrum of benzene was studied by SHULL in EPA at i’i’OK and a detailed analysis was given in 1949 [I]. Independently DIHWN and SVESHNIKOV analysed the spectrum of benzene in ethanol at 90°K and the same result as that by SHULL was reported [2]. The lifetime measurement was made by MCCLURE in 1949 and a value of 7.0 set was given [S]. SVESHNIKOV and PETROV studied the influence of media on the phosphorescence decay of benzene and found 3.3 set in ethanol, O-95 set in water, and 0.66 set in carbon tetrachloride in 1950 [4]. They also gave 4.1 see in ethanol in 1951 151. PESTEIL and his collaborators studied the phosphorescence spectrum of benzene crystal at 20°K in 1955 [6]. SPONER et ab. [7] reported the specLrum of crystal benzene at 4.2”K in 1956. They also studied the triplet-singlet emission spectrum of benzene in cyclohexane matrix at 4.2”K and 77°K. It consisted of many sharp line-like bands. We studied the triplet-singlet emission spectra of benzene in matrices of carbon tetrachloride and dioxane at 90°K and found that these spectra had different characteristics from those in EPA, in ethanol, and in cyclohexane matrix. Vibrational analyses will be discussed.
Experimental ~ommeraial
benzene
[ 11 H. SHULL, J. Chem. Phys.
of chemically
pure grade was shaken with concentrated
17, 295 (1949).
[2] P. P. DXP’ON and B. Y. SVESRNIKOV, Zhw. Eksp. i Teoret. Fiz. 19,lOOO (1949); Doklady Akad. Nauk. S.S.S.R. 65, 637(1949). [3] D. S. MCCLURE, J. Chem. Phys. 17, 905 (1949). [4] B. Y. SVESHNIKOV and A. A. PETROV, Doklady Akad. Nauk S.S.S.R. 71, 46 (1950). [5] P. P. Dmux, A. A. PETROV Ltnd B. Y. S~ESE~IKOV, Zhw. Eksp. i Teoret. F&z. 21, I50 (1951). [Sl P. PESTEIL and L. PESTEIL, Corn@. rend. 240, 1987 (1955); P. PESTEIL, A. Z~~LI and L. PESTEIL, &m@. rend. 241, 29 (1955). [i] H. SPONER, L. A. BL_QXWELL and Y. KANDA, Paper presented at the Symposium on Molecular Structure and Spectroscopy, Ohio State University, Columbus, Ohio (1956); H. SPONER, Y. KANDA and L. A, BLACKWELL, Spectrochim. Acla 10, 1135(1960).
7
YO~RXYA KAXDA
and RYOICHI QHIMADA
sulphuric acid until no colour change was detected, and then washed with water, a 30% sodium hydroxide solution, a potassium permanganate solution containing sodium hydroxide, and water in order. It was dried with calcium chloride. Twothirds of the sample was frozen and separated from the remaining liquid part. It was again dried with phosphorus pentoxide, passed through a column of freshly activated silica gel and finally distilled over sodium. The middle fraction was used. The purification of carbon tetrachloride was the same as that in a previous paper
w
Commercial
dioxane of chemically pure grade was boiled under reflux with acid of 10 per cent of the total volume for 7 hr, and then air bubbles were introduced into the solution through a condenser to remove aldehyde compounds formed. Potassium hydroxide was added to the dioxane after distillation and the aqueous layer was removed. It was dried with calcium chloride and passed twice through a column of freshly activated silica gel. It was refluxed for 4 hr and then carefully fractionated over sodium. A fraction of the distillate boiling at IOI-102°C was collected. The purified benzene was dissolved in the carbon tetrachloride and dioxane at 1O-3mole/l. A sample solution was cooled at liquid-air temperature and excited with an unfiltered 400 W high-pressure mercury arc lamp of Toshiba type HL-LOOP. The optical set-up and other apparatus used were essentially the same as those reported previously 191. A quartz spectrograph of medium dispersion was used throughout the work. Exposure time ranged from 5 to 36 hr for the spectra in carbon tetrachloride and from 4 to 20 hr for the spectra in dioxane, with a slit width of 100 ,U and Fuji special test plates for very low luminosity. The sample solution was replaced with a fresh one every 3 hr. Wavelengths and intensities of the spectral bands were measured with a recording microphotometer which we constructed [S]. The nlaximum precision was 15 cm-l for the bands in the spectra.
1 W hydrochloric
Results and discussion The spectrum in carbon tetrachloride The triplet-singlet emission spectrum of benzene was studied at liquid-air temperature in a carbon tetra~hloride solution and the microphotometer tracing curve is shown in Fig. 1. Spectral data and the analysis are given in Table 1. It emitted a blue-green phosphorescence, extremely weak in intensity and of about 1 see in its lifetime by visual estimate. These characteristics are in remarkable contrast to those in the EPA spectrum. Actually benzene in EPA emits a violet phosphorescence, strong in intensity and of 7 set in its lifetime according to MCCLURE [3]. The spectral appearance of benzene in carbon tetrachloride was somewhat similar to that in EPA but the bands in the spectrum in Ccl, were seen to shade to the shorter wavelengths. The spectrum was sharper than that in EPA, although it was not so sharp as that in cyclohexane matrix. It is believed that benzene was trapped in the matrix of carbon tetrachloride in such a way as to give sharpness to the spectral structure. The band at 39,350cm-l was assigned to the [Sly. KANDA [9]Y. KANDA
and R. S~~~~~,S~ectrochim.Acta and R. SHIMADA, Xpectrochim.Acta
17,L (1961). 15,211 (1959). 8
The triplet-singlet
emission spectra of benzene in carbon tetrachloride
.
5..
,
.,
241xX)
.,
.,
26000
Fig. 1. Phosphorescence Table
1. Phosphorescence
Wavenumber
Rel.
spectrum int.
- ~-
of benzene
.,
and dioxane matrices
.,
2&ooo
_
at 90°K
,
cm”
spectra of benzene. in carbon
tetrachloride
matrix
at 90°K
Analysis
AG ____
-
29,350 28,955 28,750 28,560 28,355 28,175 27,955 27,760 27,365 27,195 26,985 26,765 26,575 26,385
6 1 2 0.5 7 6 0.5 10 6 7 3 10 5 6
0 395 600 790 995 1175 1395 1590 1985 2155 2365 2585 2775 2965
26,175
8
3175
25,995
5
3355
25,790
9
3560
25,620 25,410
7 5
3730 3940
25,195
7
4155
25,020 24,800 24,610 24,450 24,205 24,045 23,825 23,630 23,455 23,245 23,055 22,665
7 7 7 5 4 5 3 4 2 1 2 1
4330 4550 4740 4900 5145 5305 5525 5720 5895 6105 6295 6685
9
0s’ O-404 O-606 o-304 :< 2 O-992 (J-1178 O-404-992 O-1596 (1585/1606) o-992 x 2 o-1 178-992 U-1178 x 2 O-1596-992 O-1596-1178 o-992 )c 3 o-1596 i 2 o-1178-992 x 2 o-1178 x 2-992 (O-1596-992 x 2 I o-1178 x 3 O-1596-1178-992 O-1596-1178 x 2 1 O-1596 x 22992 \o-1178-992 x 3 o-1596 x 2-1178 O-1596-992 x 3 O-1596 x 3 O-1596-1178 x 22992 (J-1178-992 Y 4 O-1596 x 2-1178-992 O-1596-992 x 4 O-1596 Y 3-992 O-1596-1178 x 2-992 O-1178-992 x 5 o-1596 x 2-1178-992 @1596 Y 3-992 x 2
,v 2 x 2
YOSRIYA
KANDA~~~RYOICRI
SHIMADA
O,O-band of the system. It was fairly strong in intensity, while the O,O-band of the EPA spectrum was very weak in intensity. Frequencies of 395, 600, 790, 995, 1175, 1395 and 1590 cm-l were found and attributed to vibrations 404 (e,,), 606 (e,,), 404 x 2 (%,f, 992 (al,), 1178 (e,,), 404 + 992 (e,,), and 1585~1606 (e,,), respectively. The intensity of the band at 28,355 cm-l (O-995) was fairly strong and stronger than that of the band at 28,175 cm-l (O-1175). These two bands were clearly separated from each other as can be seen in Fig. 1, and this spectral feature was entirely different from that in EPA. According to SHTJLL, the intensity of the band 1178 cm-1 was strong and a slight shoulder on its shorter wavelength side was taken as the band 1010 cm-l, which was assigned to a vibration 985 (b,,) or 992 (ai,). The shoulder was so slight that DIKUN and SVESHNIKOV failed to discriminate it from the strong band 1179 cm- l. The frequency 995 cm-l observed in this experiment was seen to form several progressions. The first and the second overtone bands of the frequency from the 0,0-band were found and the progression starting from the band at 27,760 cm-l (o-1590) was the most remarkable, followed by the one beginning at the band at 28,175 cm-l (O-1175). Overtone frequencies of 1178 and 1596 cm-l and a combination frequency 1178 + 1596 cm-l were found. These three were not found in the EPA spectrum. The second overtones of 1178 and 1596 cm-l and combinations of 1178 x 2 + 1596 and 1178 + 1596 x 2cm -l were also found, and the 992 cm-r progressions were seen to combine also with those frequencies. The frequency 404 cm-l, its overtone and the combination frequency 404 + 992 cm-l were found in this spectrum which were observed neither in the EPA spectrum nor in the spectrum in cyclohexane. The frequency 606 cm-l was observed here which was not found in the EPA spectrum but was obtained in cyclohexane. No b,, vibration was found in the spectrum. The frequency 995 cm-l found here cannot be ascribed to the vibration 985 cm-l (b,,) but to the vibration 992 cm-l (al,,) because of the strong intensity of the band at 28,355 cm-i and the regular appearance of the progression of 995 cm-l beginning at the O,O-band. This progression was observed for the first time in this spectrum and never found in the spectra in EPA and in cyclohexane. The symmetry of the lowest triplet was concluded by SHULL to be B,, due to the occurrence of b,, vibrations in the EPA spectrum. MCCLURE [IO] and MIZUSHINA and KOIDE [ll] gave theoretical discussions favourable to SEIULL. DIKUN and SVESWNIKOV [2] reported that it was difficult to determine whether the triplet state was of B,, or B,, symmetry. PARISER [12] assigned the B,, symmetry to the triplet state from his detailed calculation, which is the most accurate so far. Thus This transition is the transition is 3Blu -+ ‘Al, if benzene retains Dsh symmetry. forbidden by symmetry and spin multiplicity~ On the other hand, the spectrum in carbon tetrachloride matrix had no b,, vibration. This makes it impossible to discuss whether the symmetry of the lowest triplet state was of B,, or B,,. Moreover, the spectrum had the strong O,O-band and the fairly intense band at 28,355 cm-i (O-995). This suggests that the transition of the triplet-singlet emission in earbon tetrachloride should be one allowed by symmetry although forbidden by [IO] D. R.McCm~~,J.Chern. Phys. 17, 655 (1949). [11]M. MIZUSRIMA and 8. KOIDE. J. Chem. Php. 20, 765 (1952). [12]R. PARISER, J. Chem Phy8. 24, 250 (1956).
10
The triplet-singlet emission spectra of benzene in carbon tetrachlorlde and dioxane lnatrices at 90°K
spin multiplicity. The symmetry of benzene in the triplet state may no longer be of D,, but reduced to a lower symmetry. Finally, since the vibration of 404 cm-l was found in the spectrum, the lower symmetry should be point group C,,, otherwise the occurrence of the u-vibration cannot be justified. The shape of the benzene molecule is probably that of a shallow boat. It is possible that benzene is deformed by lattice forces of the matrix at low temperature. A question arose whether benzene was reduced to a lower symmetry than D,, even in the ground state in carbon tetrachloride matrix before excitation. We studied the absorption spectra of benzene in the crystal and in solutions in carbon tetrachloride at various concentrations at 77°K and at room temperature, 15°C. The forbidden O,O-band, which was first observed by KRONENBERGER [13] and interpreted by SPONER et al. [la] as the crystal O,O-band, was seen to appear strongly in rigid solutions at 5, 10, 25 and 50 wt. per cent concentrations at 77°K. Recently ZMERLI et al. [ 151, and BROUDE et al. [ 161 studied the spectrum of benzene crystal at 20°K. The “crystalline” and “molecular” band systems were seen to shift to longer wavelengths with decreasing concentrations, and the magnitude of the shift of the “crystalline” system was greater than that of the “molecular” system. The intensity ratio (in area) of the 0,0-band to the O.l-band was approximately 1.3: 1 in a spectrum of a 5% solution, while the ratio was nearly 1: 5 in the spectrum of benzene crystal. Therefore the solution spectrum was entirely different from the spectrum of pure benzene crystal at 77°K. It is concluded that benzene in the matrix is also reduced to a lower symmetry in the ground state since the solution spectrum has the appearance of an allowed transition. A detailed report about this result will soon be published. The reduced symmetry in the ground state is probably C,,. Although a molecule of reduced symmetry such as D,, or D, can give rise to an allowed transition whose transition moment lies along the direction perpendicular to the molecular plane, the intensity of the 0,0-band may not be so strong. Since the strong O,O-bands were observed in the absorptionspectra, benzene in the ground state is also probably of C,, symmetry in carbon tetrachloride matrix at 77°K. The spectrum in dioxane The triplet-singlet emission spectrum of benzene was also studied in a dioxane solution at liquid-air temperature, and the microphotometer tracing is shown in Fig. 1. Spectral data and the analysis are given in Table 2. Benzene emitted a blue-violet phosphorescence in dioxane with a lifetime of about 5 set by visual estimate. The intensity of the phosphorescence was not so weak as that in carbon tetrachloride. These characteristics and the spectral appearance were very similar to those in the EPA spectrum. Thirty bands were observed in dioxane, while the EPA spectrum had nineteen bands. It is believed that benzene is trapped and oriented in the dioxane matrix in such a way as to give sharp bands. The band at 29,481 cm-l was assigned to the O,O-band of the system. It was weak but not so [13] A. r141H. ‘[15j A. [16] V.
KRONENBERQER,2. Physik 6& 494 (1930). SPONER, G. NORDHEIM, A. L. SKLAR and E. TELLER, J. Ckem. Phua. 7. 207 (1939). ZMERLI, H. POULET and P. PESTEIL, Coopt. rend. M, 517 (1957).. . L. BROUDE, V. S. MEDVEDEV and A. F. PRIRHOT’EO, Op&a i Spektroakopiya 2, 317 (1957).
11
YOSHIYA KANDA and RYOICHI SHIYADA
weak as the O,O-band in the EPA spectrum. Frequencies of 604, 704, 844, 995, 1181 and 1595 cm-r were found and attributed to vibrations 606 (e,,), 703 (bar), 849 (e,,), 992 (a,,), 1178 (e,,) and 1585/1606 (e,,), respectively. The intensity of the band at 28,486 cm-r (O-995) was somewhat strong but weaker than that of the band at 28,300 cm-l (O-1181). These two bands were clearly separated from Table 2. Phosphorescence spectrum of benzene in dioxane matrix at 90°K Wavenumber 29,481 28,877 28,777 28,637 28,486 28,300 27,886 27,624 27,487 27,307 26,889 26,497 26,315 26,130 25,899 25,713 25,529 25,322 25,131 24,912 24,709 24,545 24,330 24,130 23,934 23,718 23,545 23,347 23,131 22,731
I
/
Analysis
Rel. int. 1 0.5 0.5 0.2 2 6 10 1 2 7 10 3 6 3 Y 4 5 5 7 7 6 4 3 5 4 5 3 2 4 3
1
0 604 704 844 995 1181 1595 1857 1994 2174 2592 2984 3166 3351 3582 3768 3952 4159 4350 4569 4772 4936 5151 5351 5547 5763 5936 6134 6350 6750
090 O-606 O-703 o-849 o-992 O-1178 o-1596 (158511606) o-849-992 o-992 x 2 O-1178-992 C&1596-992 o-992 x 3 O-1178-992 x 2 O-1178 x 2-992 o-1596-992 x 2 o-1596-1178-992 O-1596-1178 x 2 O-1178-992 x 3 o-1596 x 2-1178 O-1596-992 x 3 O-1596 x 3 O-1596-1178 x 2-992 O-1178-992 x 4 O-1596 x 2-1178-992 o-159&992 X 4 o-1596 x 3-992 (k-1596-1178 x 2-992 o-1178-992 x 5 O-1596 x 2-1178-992 O-1596 x 3-992 x 2
x 2 x 2
each other, as they were in the spectrum in Ccl,, although the intensity relation The spacing 995 cm-l could alternatively be ascribed to a b,, was different. vibration of 985 cm-l, because the spectrum in dioxane is very similar to the spectra in EPA and in cyclohexane in which the vibration 985 cm-l was found. However, we do not assign it to the b,, vibration. We will come back to this point at the end of this section. The frequency 606 cm-l was found in this spectrum as it was in the spectra in Ccl, and in cyclohexane, while it was not found in the EPA spectrum. The frequency 763 cm-l was obtained in this spectrum as in the spectra in EPA and in cyclohexane, while it was not found in the Ccl, spectrum. The frequencies 1178 and 1596 cm-l were very prominent in the spectrum. The 992 progression started 12
The triplet-singlet
emission spectra
of benzene in carbon tetrachloride
and dioxane matrices at 90°K
in combination with vibrations of those frequencies. It was, however, strange that a combination band of 1178 x 2 + 992 cm-l was found while a band of 1178 x 2 cm-l was not found. Similarly the combination 1178 + 1596 cm-l was not found, while that of 1178 + 1596 + 992 cm-l was. Neither a band of 1596 x 2 cm-1 nor a band of 1596 x 2 + 992 cm-l was observed. Combination bands of 1178 x 2 + 1596 and 1178 + 1596 x 2 cm-l and the second overtone of 1596 cm-i were found. The 992 progressions were seen to start from these bands. The most instructive frequency found in the spectrum in dioxane was 844 cm-l. It might be interpreted as an overtone frequency of 404 cm-l (e,,). However, it was ascribed to an e,, vibration (849 cm-l, Raman value). It is obviously impossible for the vibration to appear in the phosphorescence spectrum of benzene so far as benzene retains D,, symmetry. If we reduce the symmetry of benzene to point group D,,, the transition 3Bl,, + lAls of D,, goes over to 3Blu + lA,. This transition is allowed by symmetry although forbidden by spin multiplicity. Then the erg vibration goes over to a vibration of b,, or b,, species, and a vibration of b,, is A b,, vibration of benzene of allowed by symmetry in the 3Blu + lA, transition. DGh symmetry is to appear as a vibration of b,, species of D,, symmetry in the Therefore, the spacing 995 cm-l obtained in the spectrum could be transition. attributed to the b,, vibration 985 cm-l, being reduced here to b,, species. However, the magnitude of error in measurements of wavelengths was 15 cm-l in this region and the value of 995 cm--l seemed a little too large to be ascribed to the b,, vibration. The bands at 27,487 and 26,497 cm-r were interpreted as involving the first and the second overtones (of 992), respectively. If they were assigned to combination bands 985 + 992 and 985 + 992 x 2 cm-l, differences between observed and calculated frequencies would be larger than those in the case of assignments to the first and the second overtones. The ultra-violet absorption spectrum of benzene in dioxane was studied at 77°K and room temperature (15°C) at concentrations of 5, 10, 25, 50, 75 and 90 wt. per cent. The “crystalline” system appeared in the spectrum as a series of very broad bands at 77°K while the “molecular” system remained sharp. Positions of bands in both systems were the same as those of the spectrum of benzene crystal. The intensity ratio (in area) of the O,O- to 0,1-bands of a spectrum of a 5% solution was approximately 1: 3, while that of the crystal spectrum was approximately 1: 5. In other words, the “crystalline” system was intensified in the spectrum. The spectrum appeared to be due to an allowed transition, but the intensity of the O,O-band was not so strong as that in the Ccl, spectrum. Benzene was probably deformed to point group D,, in the ground state but not to the extent of C,,. Ack~towledgemerzis--‘rho writers wish to express their hearty thanks to Professor H. SPONER of Duke University for her kind suggestions and valuable discussion. They are also indebted to the Fuji Photo-Film Company for the generous offer of the highly sensitive plates. Their gratitude is also due to the Ministry of Education for grants-in-aid out of the Scientific Research Expenditure.
13
The triplet~~glet emission spectra of benzene in carbon tetrachloride and dioxane matrices at 90°K YOSHIYA KANDA and RYOICHI SHIMADA Departmentof Chemistry,Faculty of Science,Kyushu University,Fukuoka, Japan (Received 7 July
1960)
Ab&&---The phosphorescence spectrum of benzene has been studied in carbon tetraehloride and dioxane matrices at 90”K, and vibrational analyses are discussed. The intensity of the 0,0-band of the spectrum in carbon tetrachloride is fairly strong and no bzs vibration is found, It is concluded that the molecular shape is while an e,, vibration of 404 cm-l is obtained. probably of C,, symmetry because of deformation of the molecule due t,o lattice forces of the matris. On the other hand, the spectrum in dioxane is similar to, but sharper than, that in EPA. An e18 vibration (849 cm-“) is found in the spectrum. The occurrence of this vibration is also ascribed t,o the deformation of benzene in dioxane matrix. The reduced symmetry is probably Dw
Introduction THE phosphores~enee
spectrum of benzene was studied by SHULL in EPA at i’i’OK and a detailed analysis was given in 1949 [I]. Independently DIHWN and SVESHNIKOV analysed the spectrum of benzene in ethanol at 90°K and the same result as that by SHULL was reported [2]. The lifetime measurement was made by MCCLURE in 1949 and a value of 7.0 set was given [S]. SVESHNIKOV and PETROV studied the influence of media on the phosphorescence decay of benzene and found 3.3 set in ethanol, O-95 set in water, and 0.66 set in carbon tetrachloride in 1950 [4]. They also gave 4.1 see in ethanol in 1951 151. PESTEIL and his collaborators studied the phosphorescence spectrum of benzene crystal at 20°K in 1955 [6]. SPONER et ab. [7] reported the specLrum of crystal benzene at 4.2”K in 1956. They also studied the triplet-singlet emission spectrum of benzene in cyclohexane matrix at 4.2”K and 77°K. It consisted of many sharp line-like bands. We studied the triplet-singlet emission spectra of benzene in matrices of carbon tetrachloride and dioxane at 90°K and found that these spectra had different characteristics from those in EPA, in ethanol, and in cyclohexane matrix. Vibrational analyses will be discussed.
Experimental ~ommeraial
benzene
[ 11 H. SHULL, J. Chem. Phys.
of chemically
pure grade was shaken with concentrated
17, 295 (1949).
[2] P. P. DXP’ON and B. Y. SVESRNIKOV, Zhw. Eksp. i Teoret. Fiz. 19,lOOO (1949); Doklady Akad. Nauk. S.S.S.R. 65, 637(1949). [3] D. S. MCCLURE, J. Chem. Phys. 17, 905 (1949). [4] B. Y. SVESHNIKOV and A. A. PETROV, Doklady Akad. Nauk S.S.S.R. 71, 46 (1950). [5] P. P. Dmux, A. A. PETROV Ltnd B. Y. S~ESE~IKOV, Zhw. Eksp. i Teoret. F&z. 21, I50 (1951). [Sl P. PESTEIL and L. PESTEIL, Corn@. rend. 240, 1987 (1955); P. PESTEIL, A. Z~~LI and L. PESTEIL, &m@. rend. 241, 29 (1955). [i] H. SPONER, L. A. BL_QXWELL and Y. KANDA, Paper presented at the Symposium on Molecular Structure and Spectroscopy, Ohio State University, Columbus, Ohio (1956); H. SPONER, Y. KANDA and L. A, BLACKWELL, Spectrochim. Acla 10, 1135(1960).
7
YO~RXYA KAXDA
and RYOICHI QHIMADA
sulphuric acid until no colour change was detected, and then washed with water, a 30% sodium hydroxide solution, a potassium permanganate solution containing sodium hydroxide, and water in order. It was dried with calcium chloride. Twothirds of the sample was frozen and separated from the remaining liquid part. It was again dried with phosphorus pentoxide, passed through a column of freshly activated silica gel and finally distilled over sodium. The middle fraction was used. The purification of carbon tetrachloride was the same as that in a previous paper
w
Commercial
dioxane of chemically pure grade was boiled under reflux with acid of 10 per cent of the total volume for 7 hr, and then air bubbles were introduced into the solution through a condenser to remove aldehyde compounds formed. Potassium hydroxide was added to the dioxane after distillation and the aqueous layer was removed. It was dried with calcium chloride and passed twice through a column of freshly activated silica gel. It was refluxed for 4 hr and then carefully fractionated over sodium. A fraction of the distillate boiling at IOI-102°C was collected. The purified benzene was dissolved in the carbon tetrachloride and dioxane at 1O-3mole/l. A sample solution was cooled at liquid-air temperature and excited with an unfiltered 400 W high-pressure mercury arc lamp of Toshiba type HL-LOOP. The optical set-up and other apparatus used were essentially the same as those reported previously 191. A quartz spectrograph of medium dispersion was used throughout the work. Exposure time ranged from 5 to 36 hr for the spectra in carbon tetrachloride and from 4 to 20 hr for the spectra in dioxane, with a slit width of 100 ,U and Fuji special test plates for very low luminosity. The sample solution was replaced with a fresh one every 3 hr. Wavelengths and intensities of the spectral bands were measured with a recording microphotometer which we constructed [S]. The nlaximum precision was 15 cm-l for the bands in the spectra.
1 W hydrochloric
Results and discussion The spectrum in carbon tetrachloride The triplet-singlet emission spectrum of benzene was studied at liquid-air temperature in a carbon tetra~hloride solution and the microphotometer tracing curve is shown in Fig. 1. Spectral data and the analysis are given in Table 1. It emitted a blue-green phosphorescence, extremely weak in intensity and of about 1 see in its lifetime by visual estimate. These characteristics are in remarkable contrast to those in the EPA spectrum. Actually benzene in EPA emits a violet phosphorescence, strong in intensity and of 7 set in its lifetime according to MCCLURE [3]. The spectral appearance of benzene in carbon tetrachloride was somewhat similar to that in EPA but the bands in the spectrum in Ccl, were seen to shade to the shorter wavelengths. The spectrum was sharper than that in EPA, although it was not so sharp as that in cyclohexane matrix. It is believed that benzene was trapped in the matrix of carbon tetrachloride in such a way as to give sharpness to the spectral structure. The band at 39,350cm-l was assigned to the [Sly. KANDA [9]Y. KANDA
and R. S~~~~~,S~ectrochim.Acta and R. SHIMADA, Xpectrochim.Acta
17,L (1961). 15,211 (1959). 8
The triplet-singlet
emission spectra of benzene in carbon tetrachloride
.
5..
,
.,
241xX)
.,
.,
26000
Fig. 1. Phosphorescence Table
1. Phosphorescence
Wavenumber
Rel.
spectrum int.
- ~-
of benzene
.,
and dioxane matrices
.,
2&ooo
_
at 90°K
,
cm”
spectra of benzene. in carbon
tetrachloride
matrix
at 90°K
Analysis
AG ____
-
29,350 28,955 28,750 28,560 28,355 28,175 27,955 27,760 27,365 27,195 26,985 26,765 26,575 26,385
6 1 2 0.5 7 6 0.5 10 6 7 3 10 5 6
0 395 600 790 995 1175 1395 1590 1985 2155 2365 2585 2775 2965
26,175
8
3175
25,995
5
3355
25,790
9
3560
25,620 25,410
7 5
3730 3940
25,195
7
4155
25,020 24,800 24,610 24,450 24,205 24,045 23,825 23,630 23,455 23,245 23,055 22,665
7 7 7 5 4 5 3 4 2 1 2 1
4330 4550 4740 4900 5145 5305 5525 5720 5895 6105 6295 6685
9
0s’ O-404 O-606 o-304 :< 2 O-992 (J-1178 O-404-992 O-1596 (1585/1606) o-992 x 2 o-1 178-992 U-1178 x 2 O-1596-992 O-1596-1178 o-992 )c 3 o-1596 i 2 o-1178-992 x 2 o-1178 x 2-992 (O-1596-992 x 2 I o-1178 x 3 O-1596-1178-992 O-1596-1178 x 2 1 O-1596 x 22992 \o-1178-992 x 3 o-1596 x 2-1178 O-1596-992 x 3 O-1596 x 3 O-1596-1178 x 22992 (J-1178-992 Y 4 O-1596 x 2-1178-992 O-1596-992 x 4 O-1596 Y 3-992 O-1596-1178 x 2-992 O-1178-992 x 5 o-1596 x 2-1178-992 @1596 Y 3-992 x 2
,v 2 x 2
YOSRIYA
KANDA~~~RYOICRI
SHIMADA
O,O-band of the system. It was fairly strong in intensity, while the O,O-band of the EPA spectrum was very weak in intensity. Frequencies of 395, 600, 790, 995, 1175, 1395 and 1590 cm-l were found and attributed to vibrations 404 (e,,), 606 (e,,), 404 x 2 (%,f, 992 (al,), 1178 (e,,), 404 + 992 (e,,), and 1585~1606 (e,,), respectively. The intensity of the band at 28,355 cm-l (O-995) was fairly strong and stronger than that of the band at 28,175 cm-l (O-1175). These two bands were clearly separated from each other as can be seen in Fig. 1, and this spectral feature was entirely different from that in EPA. According to SHTJLL, the intensity of the band 1178 cm-1 was strong and a slight shoulder on its shorter wavelength side was taken as the band 1010 cm-l, which was assigned to a vibration 985 (b,,) or 992 (ai,). The shoulder was so slight that DIKUN and SVESHNIKOV failed to discriminate it from the strong band 1179 cm- l. The frequency 995 cm-l observed in this experiment was seen to form several progressions. The first and the second overtone bands of the frequency from the 0,0-band were found and the progression starting from the band at 27,760 cm-l (o-1590) was the most remarkable, followed by the one beginning at the band at 28,175 cm-l (O-1175). Overtone frequencies of 1178 and 1596 cm-l and a combination frequency 1178 + 1596 cm-l were found. These three were not found in the EPA spectrum. The second overtones of 1178 and 1596 cm-l and combinations of 1178 x 2 + 1596 and 1178 + 1596 x 2cm -l were also found, and the 992 cm-r progressions were seen to combine also with those frequencies. The frequency 404 cm-l, its overtone and the combination frequency 404 + 992 cm-l were found in this spectrum which were observed neither in the EPA spectrum nor in the spectrum in cyclohexane. The frequency 606 cm-l was observed here which was not found in the EPA spectrum but was obtained in cyclohexane. No b,, vibration was found in the spectrum. The frequency 995 cm-l found here cannot be ascribed to the vibration 985 cm-l (b,,) but to the vibration 992 cm-l (al,,) because of the strong intensity of the band at 28,355 cm-i and the regular appearance of the progression of 995 cm-l beginning at the O,O-band. This progression was observed for the first time in this spectrum and never found in the spectra in EPA and in cyclohexane. The symmetry of the lowest triplet was concluded by SHULL to be B,, due to the occurrence of b,, vibrations in the EPA spectrum. MCCLURE [IO] and MIZUSHINA and KOIDE [ll] gave theoretical discussions favourable to SEIULL. DIKUN and SVESWNIKOV [2] reported that it was difficult to determine whether the triplet state was of B,, or B,, symmetry. PARISER [12] assigned the B,, symmetry to the triplet state from his detailed calculation, which is the most accurate so far. Thus This transition is the transition is 3Blu -+ ‘Al, if benzene retains Dsh symmetry. forbidden by symmetry and spin multiplicity~ On the other hand, the spectrum in carbon tetrachloride matrix had no b,, vibration. This makes it impossible to discuss whether the symmetry of the lowest triplet state was of B,, or B,,. Moreover, the spectrum had the strong O,O-band and the fairly intense band at 28,355 cm-i (O-995). This suggests that the transition of the triplet-singlet emission in earbon tetrachloride should be one allowed by symmetry although forbidden by [IO] D. R.McCm~~,J.Chern. Phys. 17, 655 (1949). [11]M. MIZUSRIMA and 8. KOIDE. J. Chem. Php. 20, 765 (1952). [12]R. PARISER, J. Chem Phy8. 24, 250 (1956).
10
The triplet-singlet emission spectra of benzene in carbon tetrachlorlde and dioxane lnatrices at 90°K
spin multiplicity. The symmetry of benzene in the triplet state may no longer be of D,, but reduced to a lower symmetry. Finally, since the vibration of 404 cm-l was found in the spectrum, the lower symmetry should be point group C,,, otherwise the occurrence of the u-vibration cannot be justified. The shape of the benzene molecule is probably that of a shallow boat. It is possible that benzene is deformed by lattice forces of the matrix at low temperature. A question arose whether benzene was reduced to a lower symmetry than D,, even in the ground state in carbon tetrachloride matrix before excitation. We studied the absorption spectra of benzene in the crystal and in solutions in carbon tetrachloride at various concentrations at 77°K and at room temperature, 15°C. The forbidden O,O-band, which was first observed by KRONENBERGER [13] and interpreted by SPONER et al. [la] as the crystal O,O-band, was seen to appear strongly in rigid solutions at 5, 10, 25 and 50 wt. per cent concentrations at 77°K. Recently ZMERLI et al. [ 151, and BROUDE et al. [ 161 studied the spectrum of benzene crystal at 20°K. The “crystalline” and “molecular” band systems were seen to shift to longer wavelengths with decreasing concentrations, and the magnitude of the shift of the “crystalline” system was greater than that of the “molecular” system. The intensity ratio (in area) of the 0,0-band to the O.l-band was approximately 1.3: 1 in a spectrum of a 5% solution, while the ratio was nearly 1: 5 in the spectrum of benzene crystal. Therefore the solution spectrum was entirely different from the spectrum of pure benzene crystal at 77°K. It is concluded that benzene in the matrix is also reduced to a lower symmetry in the ground state since the solution spectrum has the appearance of an allowed transition. A detailed report about this result will soon be published. The reduced symmetry in the ground state is probably C,,. Although a molecule of reduced symmetry such as D,, or D, can give rise to an allowed transition whose transition moment lies along the direction perpendicular to the molecular plane, the intensity of the 0,0-band may not be so strong. Since the strong O,O-bands were observed in the absorptionspectra, benzene in the ground state is also probably of C,, symmetry in carbon tetrachloride matrix at 77°K. The spectrum in dioxane The triplet-singlet emission spectrum of benzene was also studied in a dioxane solution at liquid-air temperature, and the microphotometer tracing is shown in Fig. 1. Spectral data and the analysis are given in Table 2. Benzene emitted a blue-violet phosphorescence in dioxane with a lifetime of about 5 set by visual estimate. The intensity of the phosphorescence was not so weak as that in carbon tetrachloride. These characteristics and the spectral appearance were very similar to those in the EPA spectrum. Thirty bands were observed in dioxane, while the EPA spectrum had nineteen bands. It is believed that benzene is trapped and oriented in the dioxane matrix in such a way as to give sharp bands. The band at 29,481 cm-l was assigned to the O,O-band of the system. It was weak but not so [13] A. r141H. ‘[15j A. [16] V.
KRONENBERQER,2. Physik 6& 494 (1930). SPONER, G. NORDHEIM, A. L. SKLAR and E. TELLER, J. Ckem. Phua. 7. 207 (1939). ZMERLI, H. POULET and P. PESTEIL, Coopt. rend. M, 517 (1957).. . L. BROUDE, V. S. MEDVEDEV and A. F. PRIRHOT’EO, Op&a i Spektroakopiya 2, 317 (1957).
11
YOSHIYA KANDA and RYOICHI SHIYADA
weak as the O,O-band in the EPA spectrum. Frequencies of 604, 704, 844, 995, 1181 and 1595 cm-r were found and attributed to vibrations 606 (e,,), 703 (bar), 849 (e,,), 992 (a,,), 1178 (e,,) and 1585/1606 (e,,), respectively. The intensity of the band at 28,486 cm-r (O-995) was somewhat strong but weaker than that of the band at 28,300 cm-l (O-1181). These two bands were clearly separated from Table 2. Phosphorescence spectrum of benzene in dioxane matrix at 90°K Wavenumber 29,481 28,877 28,777 28,637 28,486 28,300 27,886 27,624 27,487 27,307 26,889 26,497 26,315 26,130 25,899 25,713 25,529 25,322 25,131 24,912 24,709 24,545 24,330 24,130 23,934 23,718 23,545 23,347 23,131 22,731
I
/
Analysis
Rel. int. 1 0.5 0.5 0.2 2 6 10 1 2 7 10 3 6 3 Y 4 5 5 7 7 6 4 3 5 4 5 3 2 4 3
1
0 604 704 844 995 1181 1595 1857 1994 2174 2592 2984 3166 3351 3582 3768 3952 4159 4350 4569 4772 4936 5151 5351 5547 5763 5936 6134 6350 6750
090 O-606 O-703 o-849 o-992 O-1178 o-1596 (158511606) o-849-992 o-992 x 2 O-1178-992 C&1596-992 o-992 x 3 O-1178-992 x 2 O-1178 x 2-992 o-1596-992 x 2 o-1596-1178-992 O-1596-1178 x 2 O-1178-992 x 3 o-1596 x 2-1178 O-1596-992 x 3 O-1596 x 3 O-1596-1178 x 2-992 O-1178-992 x 4 O-1596 x 2-1178-992 o-159&992 X 4 o-1596 x 3-992 (k-1596-1178 x 2-992 o-1178-992 x 5 O-1596 x 2-1178-992 O-1596 x 3-992 x 2
x 2 x 2
each other, as they were in the spectrum in Ccl,, although the intensity relation The spacing 995 cm-l could alternatively be ascribed to a b,, was different. vibration of 985 cm-l, because the spectrum in dioxane is very similar to the spectra in EPA and in cyclohexane in which the vibration 985 cm-l was found. However, we do not assign it to the b,, vibration. We will come back to this point at the end of this section. The frequency 606 cm-l was found in this spectrum as it was in the spectra in Ccl, and in cyclohexane, while it was not found in the EPA spectrum. The frequency 763 cm-l was obtained in this spectrum as in the spectra in EPA and in cyclohexane, while it was not found in the Ccl, spectrum. The frequencies 1178 and 1596 cm-l were very prominent in the spectrum. The 992 progression started 12
The triplet-singlet
emission spectra
of benzene in carbon tetrachloride
and dioxane matrices at 90°K
in combination with vibrations of those frequencies. It was, however, strange that a combination band of 1178 x 2 + 992 cm-l was found while a band of 1178 x 2 cm-l was not found. Similarly the combination 1178 + 1596 cm-l was not found, while that of 1178 + 1596 + 992 cm-l was. Neither a band of 1596 x 2 cm-1 nor a band of 1596 x 2 + 992 cm-l was observed. Combination bands of 1178 x 2 + 1596 and 1178 + 1596 x 2 cm-l and the second overtone of 1596 cm-i were found. The 992 progressions were seen to start from these bands. The most instructive frequency found in the spectrum in dioxane was 844 cm-l. It might be interpreted as an overtone frequency of 404 cm-l (e,,). However, it was ascribed to an e,, vibration (849 cm-l, Raman value). It is obviously impossible for the vibration to appear in the phosphorescence spectrum of benzene so far as benzene retains D,, symmetry. If we reduce the symmetry of benzene to point group D,,, the transition 3Bl,, + lAls of D,, goes over to 3Blu + lA,. This transition is allowed by symmetry although forbidden by spin multiplicity. Then the erg vibration goes over to a vibration of b,, or b,, species, and a vibration of b,, is A b,, vibration of benzene of allowed by symmetry in the 3Blu + lA, transition. DGh symmetry is to appear as a vibration of b,, species of D,, symmetry in the Therefore, the spacing 995 cm-l obtained in the spectrum could be transition. attributed to the b,, vibration 985 cm-l, being reduced here to b,, species. However, the magnitude of error in measurements of wavelengths was 15 cm-l in this region and the value of 995 cm--l seemed a little too large to be ascribed to the b,, vibration. The bands at 27,487 and 26,497 cm-r were interpreted as involving the first and the second overtones (of 992), respectively. If they were assigned to combination bands 985 + 992 and 985 + 992 x 2 cm-l, differences between observed and calculated frequencies would be larger than those in the case of assignments to the first and the second overtones. The ultra-violet absorption spectrum of benzene in dioxane was studied at 77°K and room temperature (15°C) at concentrations of 5, 10, 25, 50, 75 and 90 wt. per cent. The “crystalline” system appeared in the spectrum as a series of very broad bands at 77°K while the “molecular” system remained sharp. Positions of bands in both systems were the same as those of the spectrum of benzene crystal. The intensity ratio (in area) of the O,O- to 0,1-bands of a spectrum of a 5% solution was approximately 1: 3, while that of the crystal spectrum was approximately 1: 5. In other words, the “crystalline” system was intensified in the spectrum. The spectrum appeared to be due to an allowed transition, but the intensity of the O,O-band was not so strong as that in the Ccl, spectrum. Benzene was probably deformed to point group D,, in the ground state but not to the extent of C,,. Ack~towledgemerzis--‘rho writers wish to express their hearty thanks to Professor H. SPONER of Duke University for her kind suggestions and valuable discussion. They are also indebted to the Fuji Photo-Film Company for the generous offer of the highly sensitive plates. Their gratitude is also due to the Ministry of Education for grants-in-aid out of the Scientific Research Expenditure.
13