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He(I) photoelectron spectra of various alkylamines
Journal of Electron Spectroscopy and Related Phenomena, 37 (1985) 215-290 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Data bank
He(I) PHOTOELECTRON SPECTRA OF VARIOUS ALKYLAMINES
MASAO TAKAHASHI, Department
IWAO WATANABE and SHIGERO IKEDA
of Chemistry,
Faculty
of Science,
Osaka University,
Toyonaka,
Osaka 560
(Jopa (Received 15 July 1985)
The shape of the first band in the UPS corresponding to the ionization of the HOMO electron provides information on the chemical reactivity of the molecule. We performed accurate measurements of He(I) photoelectron spectra for a series of alkylamines paying special attention to the first bands. We found that the bandwidth correlates with the symmetry factor for the electrochemical oxidation reaction in solution [l]. We have also carried out ab initio MO calculations for neutral and cation molecules of several alkylamines and have investigated the relation between the shape of the first band and the potential energy surfaces of neutral and cation molecules [2]. The photoelectron spectra for a number of amines have been measured by many workers; they are, however, inappropriate for discussion of the bandshape because they were measured under different conditions (temperature, spectral resolution, etc.). We present here all the spectra obtained under the same conditions.
EXPERIMENTAL
The He(I) photoelectron spectrometer used a hemispherical analyzer (127 mm radius, constant pass energy of -1 eV) and was controlled by a mini-computer. The ionization energy scale was calibrated with argon or xenon used as an internal reference (IP = 15.759 eV for argon, 12.130 eV for xenon). The computer acquisition provides accurately calibrated photoelectron spectra using a real-time energy calibration technique. It works as follows: (1) The computer measures the narrow region of the reference peak and calibrates the center of the peak. (2) It runs the IP region of interest for the sample gas in 5-20 min. (3) It measures the reference peak again. 0368-2048/85/$03.30
0 1985 Elsevier Science Publishers B.V.
276 TABLE 1 VERTICAL IP, AND THRESHOLD IPti (OR PHOTOELECTRON SPECTROSCOPIC ADIABATIC IP,) IONIZATION POTENTIALS, WIDTH 6 (FWHM), AND VIBRATIONAL FREQUENCIES, v, FOR THE FIRST BANDS Fig.
Compound
IP,(eV)
IPth(eV)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
Methylamine Ethylamine Propylamine i-Propylamine Butylamine i-Butylamine s-Butylamine &Butylamine Amylamine Dimethylamine Diethylamine Dipropylamine Di-i-propylamine Dibutylamine Di-i-butylamine Diamylamine Trimethylamine Triethylamine Tripropylamine Tributylamine Tri-i-butylamine Triamylamine Dimethylethylamine
10.90 9.64 9.46 9.34 9.36 9.29 9.28 9.27 9.25 9.30 8.94 8.67 8.55 8.42 8.49 8.47 8.45 8.51 8.09 7.94 7.88 7.98 7.85 8.39
10.08 8.95 8.78 8.68 8.66 8.59 8.63 8.60 8.56 8.61 8.18 7.99 7.80 7.75 7.74 7.73 7.68 7.79 7.31 7.18 7.15 7.14 7.11 7.61
s(eV)
V
0.95
0.90 0.87 0.85 0.85 0.84 0.83 0.81 0.82 0.81 0.79 0.78 0.77 0.75 0.79 0.79 0.78 0.70 0.72 0.72 0.72 0.70 0.72 0.73
(-
((((-
(cm-‘) 900 880 850 770 790 810) 770 740 800 780) 520) 310) 600)
(- 400) (” 280) (- 320)
The following data are ionization potentials determined by other workers. IP,s are in parentheses. Ammonia; (10.16) [3], 10.85 (10.16) [4], 10.85 [5], 10.88 (10.15) [6], (10.25) [7], (10.13) [8], (10.073) [9], 10.8 (10.073) [lo], (10.175) [ll]. Methylamine; (9.18) [3], 9.45 (8.80) [12], 9.67 (8.89) [4], 9.58 [13,14], 9.64 [15,16], 9.66 (9.08) [17], 9.7 [18], 9.65 [19]. Ethylamine; (9.19) [3], 9.44 [20], 9.47 (8.76) [4], 9.50 [14,15, 211. Propylamine; 9.41 (8.55) [4], 9.44 [15], 9.37 [22]. iPropylamine; (8.86) [3], 9.32 (8.63) [4], 9.31 [15]. Butylamine; (8.79) 133, 9.40 [15]. i-Butylamine; 9.31 (8.50) [4]. s-Butylamine; 9.31 (8.46) [4]. t-Butylamine; (8.83) [3], 9.26 (8.46) [4]. Dimethylamine; (8.36) [3], 8.93 (8.25) [12], 8.96 (8.07) [23], 8.93 (8.16) [4], 8.85 1241, 8.94 (8.30) [17], 8.97 [16], 8.95 [5], 8.9 [18]. Diethylamine; (8.51) [3], 8.63 (7.85) [4], 8.68 [24]; Dipropylamine; 8.54 (7.77) [4], 8.59 [22]. Di-i-propylamine; 8.40 (7.59) [4]. Trimethylamine; (8.12) [3], 8.54 (7.83) [12], 8.45 [5,25], 8.53 (7.77) [4], 8.47 [13,26], 8.55 [27], 8.54 [28], 8.50 (7.88) [17], 8.44 [16], 8.5 [18]. Triethylamine; (7.84) [3], 8.19 [20], 8.08 (7.11) [4], 8.08 [29]. Tripropylamine; 7.92 (7.03) [4], 8.04 [22], 7.94 [29]. Tributylamine; 7.90 (6.98) [4], 7.80 [29].
277
(4) From the two values of the reference peak position, the IP region for the sample can be perfectly corrected and the corrected spectrum is stored in a core memory. This cycle is automatically repeated and the corrected spectra obtained are accumulated. Thus the effect of any energy drift of the spectrometer during the measurement was compensated for. Spectral resolution defined by the full width at half-maximum (FWHM) of the 2P,,2 argon or xenon peak was held at 20-30 meV. Data points were taken every 10meV for full scan spectra. The spectrometer was kept at about 300 K but the temperature of the ionization chamber was higher by 20 K because of the heat of the helium discharge lamp. In the case of triamylamine, having a low vapor pressure, the spectrometer was heated to 307 K to obtain sufficient vapor pressure. Heating of the whole spectrometer gave a stronger signal without degradation of resolution. In addition to the measurements of full spectra, we made narrow scan measurements for the first ionization bands. The narrow scan spectra were measured with data points every 5meV except ammonia which was measured every 3meV. All samples were commercial products. The samples obtained in the form of aqueous solutions (ammonia, methylamine, dimethylamine, and trimethylamine) were evaporated from the solution by adding sodium hydroxide. The first ionization potentials, bandwidths (FWHM), and vibrational frequencies observed in the first bands are listed in Table 1. For bands without vibrational progressions, vertical ionization potentials were read at the maximum of the band, and threshold ionization potentials were read as follows: We take as the starting point the intensity being 1% of the band maximum around the threshold region. The threshold ionization potential, IPth, was obtained by adding 0.15 eV to the starting point. 0.15 eV was used because it was the average distance between the starting point and the first vibrational peak of the band for those molecules which display vibrational progressions. It is possible that IP,, measured in this study corresponds to the thermodynamic adiabatic ionization energy, but, in general, this is not a requirement. The accuracy of ionization potentials and bandwidths depends on the bandshape and we believe that it is better than 0.02 eV in this data. The spectra corresponding to the data of Table 1 are collected in Figs. l31; Figures l-24 show the full spectra; Figs. 25-31 give expanded spectra of the first bands.
278
ammonia
32000
Fig. 1.
20.0 lP/.V
Fig.
2. l-
! < “. .t:
l-yethylamin
c
.-.
I b.
,
:
i
ci
10.0
IP/*V
Fig. 3.
L
L
279
10000 :: :: i
I? 5 5000
e
IP/*V
Fig. 4.
32000
ti
,f ,
:: ?
~16000
2 .
5
z
? 0 20.0
f
i ; 1 \ \.
. 10.0
T .P/eV
Fig. 5.
T
butylamlne
IP/eV
Fig. 6.
280
IP/eV
Fig. 7.
s-htylamine
16.0
IP/*V
1
15000
E ,10000 ii
Li 2 c_l
;I :!!
D
5000
ou
20.0
L
0.0
15.0
IP/eV
Fig. 9.
281
IP/*V
11 ylamin
Ii1 .._ 1 ‘i .O.C
,T Tc
dlethylamine
r
I1
0 20.0
Fig. 12.
16.0
IP/*V
q
COlJNTS/IOSEC COUNTS/iOSEC
crl
6
(c
COUNTWZOSEC E
283 T
IP/*V
Fig. 16.
Fig. 17.
10000
is
.z
r
5000
5 =:
0
20.0
Fig. 18.
0
IP/*V
-I
I-. 9
COUNTWlOSEC cn
COUNTWZOSEC
r
F
$i
COUNTS/lOSEC
285
* %
L
IP/*V
Fig. 22.
20000
tnamytammc
I
E
F2 =;10000 c
2 "
t
I I I I I I
20.0
Fig. 23. -r dlmethytethylamme
Ar
IP/*V
Fig. 24.
0
Fig. 26.
Fig. 27.
1.2.0
i (
i ;
( ;
i . [
10.0
IP/eV Fig. 28.
288
289
i t
~imethy(ethylamine
Fig. 31.
i If’
290 REFERENCES M. Takahashi, I. Watanabe and S. Ikeda, J. Phys. Chem., 87 (1983) 5059. M. Takahashi, I. Watanabe and S. Ikeda, Bull. Chem. Sot. Jpn., submitted. M.I. Al-Joboury and D.W. Turner, J. Chem. Sot., (1964) 4434. D.H. Aue, H.M. Webb and M.T. Bowers, J. Am. Chem. Sot., 98 (1976) 311. H. Daamen and A. Oskam, Inorg. Chim. Acta, 26 (1978) 81. A.W. Potts and W.C. Price, Proc. R. Sot. London, Ser. A, 326 (1972) 181. K. Watanabe, J. Chem. Phys., 22 (1954) 1564.
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
E.C.Y. Inn, Phys. Rev., 91 (1953) 1194. J.W. Rabalais, L. Karlsson, L.O. Werme, T. Bergmark and K. Siegbahn, J. Chem. Phys., 58 (1973) 3370. G. Bieri, L. Asbrink and W.V. Niessen, J. Electron Spectrosc. Relat. Phenom., 27 (1982) 129, M.J. Weiss and G.M. Lawrence, J. Chem. Phys., 53 (1970) 214. J.P. Maier and D.W. Turner, J. Chem. Sot., Faraday Trans. 2, (1973) 521. T. Kobayashi, Phys. Lett. A, 69 (1978) 105. C. Utsunomiya, T. Kobayashi and S. Nagakura, Bull. Chem. Sot. Jpn., 53 (1980) 1216. S. Katsumata, T. Iwai and K. Kimura, Bull. Chem. Sot. Jpn., 46 (1973) 3391. K. Kimura and K. Osafune, Mol. Phys., 29 (1975) 1073. V.I. Vovna and F.I. Vilesov, Opt. Spektrosk., 36 (1974) 436. A.B. Cornford, D.C. Frost, F.G. Herring and C.A. McDowell, Can. J. Chem., 49 (1971) 1135. H. Ogata, H. Onizuka, Y. Nihei and H. Kamada, Chem. Lett., (1972) 895. S. Leavell, J. Steichen and J.L. Franklin, J. Chem. Phys., 59 (1973) 4343. H. Ogata, H. Onizuka, Y. Nihei and H. Kamada, Bull. Chem. Sot. Jpn., 46 (1973) 3036. J.B. Peel and G.D. Willett, Aust. J. Chem., 30 (1977) 2571. W.R. Cullen, D.C. Frost and W.R. Leeder, J. Fluorine Chem., 1 (1971/1972) 227. S.G. Gibbins, M.F. Lappert, J.B. Pedley and G.J. Sharp. J. Chem. Sot., Dalton Trans., (1975) 72. D.R. Lloyd and N. Lynaugh, J. Chem. Sot., Faraday Trans. 2, (1972) 947. R.F. Lake, Spectrochim. Acta, Part A, 27 (1971) 1220. M.E. Akopyan and Y.V. Loginov, Opt. Spektrosk., 37 (1974) 442. S. Elbel, H. Bergmann and W. En@in, J. Chem. Sot., Faraday Trans. 2, (1974) 555. C. Utsunomiya, T. Kobayashi and S. Nagakura, Chem. Phys. Lett., 39 (1976) 245.
Data bank
He(I) PHOTOELECTRON SPECTRA OF VARIOUS ALKYLAMINES
MASAO TAKAHASHI, Department
IWAO WATANABE and SHIGERO IKEDA
of Chemistry,
Faculty
of Science,
Osaka University,
Toyonaka,
Osaka 560
(Jopa (Received 15 July 1985)
The shape of the first band in the UPS corresponding to the ionization of the HOMO electron provides information on the chemical reactivity of the molecule. We performed accurate measurements of He(I) photoelectron spectra for a series of alkylamines paying special attention to the first bands. We found that the bandwidth correlates with the symmetry factor for the electrochemical oxidation reaction in solution [l]. We have also carried out ab initio MO calculations for neutral and cation molecules of several alkylamines and have investigated the relation between the shape of the first band and the potential energy surfaces of neutral and cation molecules [2]. The photoelectron spectra for a number of amines have been measured by many workers; they are, however, inappropriate for discussion of the bandshape because they were measured under different conditions (temperature, spectral resolution, etc.). We present here all the spectra obtained under the same conditions.
EXPERIMENTAL
The He(I) photoelectron spectrometer used a hemispherical analyzer (127 mm radius, constant pass energy of -1 eV) and was controlled by a mini-computer. The ionization energy scale was calibrated with argon or xenon used as an internal reference (IP = 15.759 eV for argon, 12.130 eV for xenon). The computer acquisition provides accurately calibrated photoelectron spectra using a real-time energy calibration technique. It works as follows: (1) The computer measures the narrow region of the reference peak and calibrates the center of the peak. (2) It runs the IP region of interest for the sample gas in 5-20 min. (3) It measures the reference peak again. 0368-2048/85/$03.30
0 1985 Elsevier Science Publishers B.V.
276 TABLE 1 VERTICAL IP, AND THRESHOLD IPti (OR PHOTOELECTRON SPECTROSCOPIC ADIABATIC IP,) IONIZATION POTENTIALS, WIDTH 6 (FWHM), AND VIBRATIONAL FREQUENCIES, v, FOR THE FIRST BANDS Fig.
Compound
IP,(eV)
IPth(eV)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
Methylamine Ethylamine Propylamine i-Propylamine Butylamine i-Butylamine s-Butylamine &Butylamine Amylamine Dimethylamine Diethylamine Dipropylamine Di-i-propylamine Dibutylamine Di-i-butylamine Diamylamine Trimethylamine Triethylamine Tripropylamine Tributylamine Tri-i-butylamine Triamylamine Dimethylethylamine
10.90 9.64 9.46 9.34 9.36 9.29 9.28 9.27 9.25 9.30 8.94 8.67 8.55 8.42 8.49 8.47 8.45 8.51 8.09 7.94 7.88 7.98 7.85 8.39
10.08 8.95 8.78 8.68 8.66 8.59 8.63 8.60 8.56 8.61 8.18 7.99 7.80 7.75 7.74 7.73 7.68 7.79 7.31 7.18 7.15 7.14 7.11 7.61
s(eV)
V
0.95
0.90 0.87 0.85 0.85 0.84 0.83 0.81 0.82 0.81 0.79 0.78 0.77 0.75 0.79 0.79 0.78 0.70 0.72 0.72 0.72 0.70 0.72 0.73
(-
((((-
(cm-‘) 900 880 850 770 790 810) 770 740 800 780) 520) 310) 600)
(- 400) (” 280) (- 320)
The following data are ionization potentials determined by other workers. IP,s are in parentheses. Ammonia; (10.16) [3], 10.85 (10.16) [4], 10.85 [5], 10.88 (10.15) [6], (10.25) [7], (10.13) [8], (10.073) [9], 10.8 (10.073) [lo], (10.175) [ll]. Methylamine; (9.18) [3], 9.45 (8.80) [12], 9.67 (8.89) [4], 9.58 [13,14], 9.64 [15,16], 9.66 (9.08) [17], 9.7 [18], 9.65 [19]. Ethylamine; (9.19) [3], 9.44 [20], 9.47 (8.76) [4], 9.50 [14,15, 211. Propylamine; 9.41 (8.55) [4], 9.44 [15], 9.37 [22]. iPropylamine; (8.86) [3], 9.32 (8.63) [4], 9.31 [15]. Butylamine; (8.79) 133, 9.40 [15]. i-Butylamine; 9.31 (8.50) [4]. s-Butylamine; 9.31 (8.46) [4]. t-Butylamine; (8.83) [3], 9.26 (8.46) [4]. Dimethylamine; (8.36) [3], 8.93 (8.25) [12], 8.96 (8.07) [23], 8.93 (8.16) [4], 8.85 1241, 8.94 (8.30) [17], 8.97 [16], 8.95 [5], 8.9 [18]. Diethylamine; (8.51) [3], 8.63 (7.85) [4], 8.68 [24]; Dipropylamine; 8.54 (7.77) [4], 8.59 [22]. Di-i-propylamine; 8.40 (7.59) [4]. Trimethylamine; (8.12) [3], 8.54 (7.83) [12], 8.45 [5,25], 8.53 (7.77) [4], 8.47 [13,26], 8.55 [27], 8.54 [28], 8.50 (7.88) [17], 8.44 [16], 8.5 [18]. Triethylamine; (7.84) [3], 8.19 [20], 8.08 (7.11) [4], 8.08 [29]. Tripropylamine; 7.92 (7.03) [4], 8.04 [22], 7.94 [29]. Tributylamine; 7.90 (6.98) [4], 7.80 [29].
277
(4) From the two values of the reference peak position, the IP region for the sample can be perfectly corrected and the corrected spectrum is stored in a core memory. This cycle is automatically repeated and the corrected spectra obtained are accumulated. Thus the effect of any energy drift of the spectrometer during the measurement was compensated for. Spectral resolution defined by the full width at half-maximum (FWHM) of the 2P,,2 argon or xenon peak was held at 20-30 meV. Data points were taken every 10meV for full scan spectra. The spectrometer was kept at about 300 K but the temperature of the ionization chamber was higher by 20 K because of the heat of the helium discharge lamp. In the case of triamylamine, having a low vapor pressure, the spectrometer was heated to 307 K to obtain sufficient vapor pressure. Heating of the whole spectrometer gave a stronger signal without degradation of resolution. In addition to the measurements of full spectra, we made narrow scan measurements for the first ionization bands. The narrow scan spectra were measured with data points every 5meV except ammonia which was measured every 3meV. All samples were commercial products. The samples obtained in the form of aqueous solutions (ammonia, methylamine, dimethylamine, and trimethylamine) were evaporated from the solution by adding sodium hydroxide. The first ionization potentials, bandwidths (FWHM), and vibrational frequencies observed in the first bands are listed in Table 1. For bands without vibrational progressions, vertical ionization potentials were read at the maximum of the band, and threshold ionization potentials were read as follows: We take as the starting point the intensity being 1% of the band maximum around the threshold region. The threshold ionization potential, IPth, was obtained by adding 0.15 eV to the starting point. 0.15 eV was used because it was the average distance between the starting point and the first vibrational peak of the band for those molecules which display vibrational progressions. It is possible that IP,, measured in this study corresponds to the thermodynamic adiabatic ionization energy, but, in general, this is not a requirement. The accuracy of ionization potentials and bandwidths depends on the bandshape and we believe that it is better than 0.02 eV in this data. The spectra corresponding to the data of Table 1 are collected in Figs. l31; Figures l-24 show the full spectra; Figs. 25-31 give expanded spectra of the first bands.
278
ammonia
32000
Fig. 1.
20.0 lP/.V
Fig.
2. l-
! < “. .t:
l-yethylamin
c
.-.
I b.
,
:
i
ci
10.0
IP/*V
Fig. 3.
L
L
279
10000 :: :: i
I? 5 5000
e
IP/*V
Fig. 4.
32000
ti
,f ,
:: ?
~16000
2 .
5
z
? 0 20.0
f
i ; 1 \ \.
. 10.0
T .P/eV
Fig. 5.
T
butylamlne
IP/eV
Fig. 6.
280
IP/eV
Fig. 7.
s-htylamine
16.0
IP/*V
1
15000
E ,10000 ii
Li 2 c_l
;I :!!
D
5000
ou
20.0
L
0.0
15.0
IP/eV
Fig. 9.
281
IP/*V
11 ylamin
Ii1 .._ 1 ‘i .O.C
,T Tc
dlethylamine
r
I1
0 20.0
Fig. 12.
16.0
IP/*V
q
COlJNTS/IOSEC COUNTS/iOSEC
crl
6
(c
COUNTWZOSEC E
283 T
IP/*V
Fig. 16.
Fig. 17.
10000
is
.z
r
5000
5 =:
0
20.0
Fig. 18.
0
IP/*V
-I
I-. 9
COUNTWlOSEC cn
COUNTWZOSEC
r
F
$i
COUNTS/lOSEC
285
* %
L
IP/*V
Fig. 22.
20000
tnamytammc
I
E
F2 =;10000 c
2 "
t
I I I I I I
20.0
Fig. 23. -r dlmethytethylamme
Ar
IP/*V
Fig. 24.
0
Fig. 26.
Fig. 27.
1.2.0
i (
i ;
( ;
i . [
10.0
IP/eV Fig. 28.
288
289
i t
~imethy(ethylamine
Fig. 31.
i If’
290 REFERENCES M. Takahashi, I. Watanabe and S. Ikeda, J. Phys. Chem., 87 (1983) 5059. M. Takahashi, I. Watanabe and S. Ikeda, Bull. Chem. Sot. Jpn., submitted. M.I. Al-Joboury and D.W. Turner, J. Chem. Sot., (1964) 4434. D.H. Aue, H.M. Webb and M.T. Bowers, J. Am. Chem. Sot., 98 (1976) 311. H. Daamen and A. Oskam, Inorg. Chim. Acta, 26 (1978) 81. A.W. Potts and W.C. Price, Proc. R. Sot. London, Ser. A, 326 (1972) 181. K. Watanabe, J. Chem. Phys., 22 (1954) 1564.
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
E.C.Y. Inn, Phys. Rev., 91 (1953) 1194. J.W. Rabalais, L. Karlsson, L.O. Werme, T. Bergmark and K. Siegbahn, J. Chem. Phys., 58 (1973) 3370. G. Bieri, L. Asbrink and W.V. Niessen, J. Electron Spectrosc. Relat. Phenom., 27 (1982) 129, M.J. Weiss and G.M. Lawrence, J. Chem. Phys., 53 (1970) 214. J.P. Maier and D.W. Turner, J. Chem. Sot., Faraday Trans. 2, (1973) 521. T. Kobayashi, Phys. Lett. A, 69 (1978) 105. C. Utsunomiya, T. Kobayashi and S. Nagakura, Bull. Chem. Sot. Jpn., 53 (1980) 1216. S. Katsumata, T. Iwai and K. Kimura, Bull. Chem. Sot. Jpn., 46 (1973) 3391. K. Kimura and K. Osafune, Mol. Phys., 29 (1975) 1073. V.I. Vovna and F.I. Vilesov, Opt. Spektrosk., 36 (1974) 436. A.B. Cornford, D.C. Frost, F.G. Herring and C.A. McDowell, Can. J. Chem., 49 (1971) 1135. H. Ogata, H. Onizuka, Y. Nihei and H. Kamada, Chem. Lett., (1972) 895. S. Leavell, J. Steichen and J.L. Franklin, J. Chem. Phys., 59 (1973) 4343. H. Ogata, H. Onizuka, Y. Nihei and H. Kamada, Bull. Chem. Sot. Jpn., 46 (1973) 3036. J.B. Peel and G.D. Willett, Aust. J. Chem., 30 (1977) 2571. W.R. Cullen, D.C. Frost and W.R. Leeder, J. Fluorine Chem., 1 (1971/1972) 227. S.G. Gibbins, M.F. Lappert, J.B. Pedley and G.J. Sharp. J. Chem. Sot., Dalton Trans., (1975) 72. D.R. Lloyd and N. Lynaugh, J. Chem. Sot., Faraday Trans. 2, (1972) 947. R.F. Lake, Spectrochim. Acta, Part A, 27 (1971) 1220. M.E. Akopyan and Y.V. Loginov, Opt. Spektrosk., 37 (1974) 442. S. Elbel, H. Bergmann and W. En@in, J. Chem. Sot., Faraday Trans. 2, (1974) 555. C. Utsunomiya, T. Kobayashi and S. Nagakura, Chem. Phys. Lett., 39 (1976) 245.