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Electron reactions in the pulse radiolysis of metal carbonyls dissolved in 2,2,4-trimethylpentane
Radiat. Phys. Chem, Vol. 20, No. 3, pp. 237-240, 1982 Printed in Great Britain.
0146-5724/82/090237-04503.00/0 Pergamon Press Ltd.
ELECTRON REACTIONS IN THE PULSE RADIOLYSIS OF METAL CARBONYLS DISSOLVED IN 2,2,4-TRIMETHYLPENTANEt Y. S. K A N G * and R. A. H O L R O Y D Chemistry Department, Brookhaven National Laboratory, Upton, N Y I1973, U.S.A. (Received 8 October 1981) Abstract--The role of electron reactions in the radiolysis of solutions of chromium, molybdenum and tungsten carbonyls in 2,2,4-trimethylpentane was investigated. These compounds are very reactive with the electron and observed rate constants for attachment are: 7, 8 and 5 x 10t3 M- I sec- I for Cr, Mo and W hexacarbonyls, respectively. The spectra of the metal carbonyl anions were studied by both pulse radiolysis and laser photodetachment methods. The data indicate.that there is a maximum in the absorption spectra of Mo(CO)~- at 412 nm and that the anions of the other hexacarbonyls also absorb in this region. Photodetachment from M(CO)6- occurs throughout the visible with thresholds in the IR. The electron affinities of the metal carbonyls are obtained from 3/2's power law plots of the cross section data. INTRODUCTION A RECENT STUDYtl~ of the pulse radiolysis of c y c l o h e x a n e solutions of chromium,- m o l y b d e n u m and tungsten carbonyls indicated that a t t a c h m e n t of e x c e s s electrons to the carbonyls f o l l o w e d by neutralization was the main m e c h a n i s m for pentacarbonyl formation.
(1) (2)
e - + M(CO)6---) M(CO)6M(CO)6- + RH÷---) M(CO)~ + C O + RH.
In order to further elucidate the role of odd electron species in the c h e m i s t r y of these important c o m p o u n d s , additional tests of this m e c h a n i s m were made. First, the rates of electron a t t a c h m e n t to the carbonyls were m e a s u r e d with a d.c. conductivity technique. Second, the pulse radiolysis of solutions of the carbonyls in 2,2,4-trimethylpentane was studied at low doses to obtain spectral e v i d e n c e for the f o r m a t i o n of the pentacarbonyls by ion recombination. Finally, the laser p h o t o d e t a c h m e n t m e t h o d <2~ was e m p l o y e d to obtain the visible spectra of the carbonyl anions. The laser m e t h o d has the a d v a n t a g e of high sensitivity which permits the use of v e r y low ion c o n c e n t r a t i o n s ( - 1 0 p M o l a r ) w h e r e the ions are sufficiently long-lived to facilitate the determination of spectra. tResearch carried out at Brookhaven National Laboratory under contract with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences. ~Department of Chemistry, Jackson State University, Jackson, MI 39217, U.S.A.
EXPERIMENTAL The carbonyls were from Pressure Chemical Co. and were used as received. The 2,2,4-trimethylpentane was purified by passage through activated silica gel, degassing and storage over Na and NaK mirrors. Details of the conductivity technique to measure electron rate constants and of the laser photodetachment technique to determine spectra are published: 2) For pulse radiolysis, 3 mM solutions of the metal carbonyls in purified 2,2,4-trimethylpentane were prepared and degassed with Ar. Solutions were exposed to a 3 o.sec pulse (-1000rad) of electrons from a 2 MeV van de Graaff accelerator. Low doses were necessary to permit electron attachment because electron-ion recombination is very fastt3) (k = 2.8x 10~ M-~ sec-l) in 2,2,4trimethylpentane. The photodetachment spectra were obtained by measuring the conductivity induced by a pulse of light from a visible dye laser in a sample exposed to a prior X-ray pulse as a function of wavelength. The magnitude of the conductivity is proportional to the number of electrons photo-detached from anionic species. The anions are formed in the sample by an X-ray pulse from a 2 MeV van de Graaff accelerator; the X-ray pulse precedes the laser pulse by 100 ~.sec. The very high sensitivity of the laser conductivity method permits the use of a low dose of X-rays which produces a concentration of ions of only about 10picomolar. Under these conditions the lifetime of the ions is a few tenths of a second. Thus, the photodetachment spectrum observed 100/zsec after the X-ray pulse can be attributed to the hexacarbonyl anion, assuming that no reaction with solvent molecules or first order decomposition of the anion occurs. The validity of this assumption is borne out by the lack of any change in the detachment spectra when the time delay between X-ray and laser pulses is changed. RESULTS AND DISCUSSION Electron attachment rates. The high reactivity of the carbonyls necessitated m e a s u r e m e n t s of rates on v e r y dilute (~<0.1/~M) solutions. Rate constants for the second order electron a t t a c h m e n t reaction (l) w e r e derived from the pseudo-first order rate of d e c a y of the conductivity and the c o n c e n t r a t i o n of metal carbonyl 237
238
Y. S. KANG and R. A. I-IOLROYD
TABLE 1.
ELECTRON
A'I'rACHMENT
AND
PHOTODETACH-
MENT DATA
ke-+M(CO~-. M-~ sec -~ 7.4 x 10~3 7.8 X 1013 5.0 x 10~
Cr(CO)6
Mo(CO)6 W(CO)6
Eth eV 1.35 1.25 1.60
P-
E.A.
-0.95 -0.93 -0.92
+0.6 + 0.5 + 0.9
measured by optical absorption. The values of k~ are summarized in Table 1. These rate constants are among the largest ever reported for electron attachment to solutes in 2,2,4-trimethylpentane. For comparison, the value for attachment to SF6 is 5.8 x 10 ~3M -~ sec -~, (4) and that for attachment to aromatic hydrocarbons is generally between 1 and 2 x 10 ~3M -~ sec-L ~2) Mo(CO)6 In the pulse radiolysis of a 3 mM solution of Mo(CO)6 in 2,2,4-trimethylpentane a transient is observed immediately after the pulse; its absorption spectrum, shown by the solid line in Fig. 1, has a maximum at 412nm. A very rapid conversion occurs during the first 10 fisec to a second longer-lived species absorbing at 405 nm. A fast grow-in is observed at wavelengths ~<410 nm and a fast decay occurs at longer wavelengths. The average lifetime of these changes is 8/~sec, which is comparable to the expected lifetime of ion recombination: at the doses employed, ion concentrations are ~0.4/xM and the theoretical rate of ion recombination (reaction 2) in 2,2,4-trimethylpentane is 3.2 x 101~;¢5)thus, ion lifetimes would be expected to be about 7 t~sec. Hence we assign the initial species,
absorbing at 412 nm, to the anion Mo(CO)6- which on neutralization forms Mo(CO)5, absorbing at 405 nm. The results provide additional support for the previously suggested tt) mechanism of pentacarbonyl formation. The photodetachment spectrum of Mo(CO)6observed for a dilute solution of Mo(CO)6 in 2,2,4trimethylpentane is shown by the circles in Fig. 1. The shortest wavelength studied was 435 nm and cr~ falls off rapidly with increasing wavelengths above 435 nm, consistent with a peak in the spectrum at A ~<435 nm. As shown in Fig. 1, the shape of the photodetachment spectrum of Mo(CO)6- is very similar to that of the initial species observed by pulse radiolysis with h,,ax --- 412 rim, which is suggested to be the molecular anion. The quantum yield for photodetachment (~b~) is given by the ratio: tru/o,~, where tr, is the absorption cross section and from the data in Fig. 1, ~b,-> 0.1. This is a lower limit because the overlap of the absorption spectrum of the pentacarbonyl leads to an overestimation of the absorption by the anion. At longer wavelengths, the cross section for photodetachment is small and decreases monotonically with increasing wavelength. Cr(CO)6 The end of pulse spectrum observed in the pulse radiolysis of Cr(CO)6 in 2,2,4-trimethylpentane is shown by the dashed line in Fig. 2 with Area,= 500rim. This spectrum is assigned to the pentacarbonyl. ~) There is a rapid conversion, with a lifetime of 33/zsec, of this spectrum to a secondary species with Am~x= 470 nm. This change has been attributed to a rapid isomerization or reaction i
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F~G. 1. Spectra observed for Mo(CO)6 in 2,2,4-trimethylpentane: pulse radiolysis; - - , absorption at the end of a 3/zsec pulse; - - - , absorption at t = 100/.Lsec; 6) photodetachment spectrum.
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FIG. 2. Spectra observed for Cr(CO)6 in 2,2,4-trimethylpentane: pulse radiolysis; - - - - , absorption at the end of pulse; absorption at t =400p.sec; 6) photodetachment spectrum.
Electron reactions in the pulse radiolysis of metal carbonyls with a solvent molecule. (6) Because of this conversion it is difficult to find fast spectral changes to associate with ion recombination. However, 510 nm is an isosbestic point for conversion of the pentacarbonyl to the secondary species and at this wavelength a fast grow-in is observed within the first 10~sec. Furthermore, this initial grow-in, which accounts for a maximum of 15% of the signal, follows second-order behavior; that is, the observed rate constant increases linearly with absorbance (and concentration). From this dependence a second order rate constant of 5.6× 10" M -~ sec -~ was derived which is close to the expected rate for ion recombination in 2,2,4-trimethylpentane and confirms the formation of Cr(CO)~ by reaction (2). The photodetachment spectrum of Cr(CO)6- is shown in Fig. 2 and is similar to that of Mo(CO)6- in that ~rd decreases with increasing wavelength throughout the visible. However, the cross section at 435 nm for photodetachment is less for Cr(CO)6than for Mo(CO)6-. Data on photodetachment are limited to the visible because of the laser employed but ~he data suggest that the maximum in the spectrum of Cr(CO)6- is at a wavelength ~<435 nm.
acarbonyl anion and no spectral changes could be observed to associate with this conversion. The photodetachment spectrum of W(CO)C is shown by the circles in Fig. 3. Again a maximum is indicated at short wavelengths (~<435nm). The cross sections (tra) in the visible are much less than those observed for the other two anions.
Photodetachment thresholds The "long tails" observed at longer wavelengths in the photodetachment spectra are typical of the behavior of anions near threshold, c2'7) Such cross section data have been found to obey a 3]Ts power law; that is, a plot of (trlEhv) 2t3 versus Ehv is linear and the intercept of such a plot on the energy axis is the threshold for photodetachment (E,h) from the anion. Plots of the cross section data for the carbonyl anions are indeed linear for the data at A t> 500nm and the thresholds determined are given in Table 1, column 3. The thresholds are related to the electron affinities (E.A.) of the parent carbonyls by equation (3); where P - is the polarization energy of the anion (3)
W(CO)6 The dashed line in Fig. 3 is the end of pulse spectrum observed in the pulse radiolysis of W(CO)6 in trimethylpentane. The maximum is at 415 nm and there is little change in the absorption over the first 100/~sec. In this case the absorption of the pentacarbonyl overlaps that of the hex-
'F'\
iI I I
I0 'E
Eth = E . A . - P - + Vo
and Vo is the energy level of the quasi-free electron in 2,2,4-trimethylpentane. The values of P - , as estimated with the Born equation and the molecular radius, are given in Table 1. Vo is -0.24 eV for this solventfl ) The electron affinities of the three carbonyls, derived from this data using equation (3), are given in Table 1°(9)
A c k n o w l e d g e m e n t - - T h e help of H. A. Schwarz, whose computer programming facilitated the acquisition and analysis of pulse radiolysis data, and the help of M. Andrews, who provided the metal carbonyl samples, are both gratefully acknowledged.
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239
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REFERENCES
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nm
Note added in proof: A recent report(9) confirms the existence of stable negative ions of the metal carbonyls.
500
o 600
FIG. 3. Spectra observed for W(CO)6 in 2,2,4-trimethylpentane: - - end of pulse spectrum in pulse radiolysis; Q photodetachment spectrum.
1. L. FLAMIGNI,Radiat. Phys. Chem. 1979, 13, 133. 2. U. SOWADAand R. A. HOLROYD,J. Phys. Chem. 1981, 85, 541. 3. A. O. ALLEN and R. A. HOLROVD, J. Phys. Chem. 1974, 78, 796. 4. A. O. ALLEN, T. E. GANGWERand R. A. HOLROVD,J. Phys. Chem. 1975, 79, 25.
240
Y. S. KANG and R. A. HOLROYD
5. A. O. ALLEN, M. P. DE HAAS and A. HUMMEL, J. Chem. Phys. 1976, 64, 2587. (This value is obtained by assuming kll.L = 10-6 V cm and using the average mobility of ions in 2,2,4-trimethylpentane of 5.5x 10-4 cm2//zsec reported). 6. A. VOGLER: in Concepts in Inorganic Photochemistry (Edited by A. Adamson and P. D. Fleischauer), Wiley-Interscience, 1975, p. 277.
7. U. SOWADAand R. A. HOLROYD,J. Phys. Chem. 1980, 84, 1150. 8. R. A. HOLROYD, S. TAMES and A. KENNEDY, J. Phys. Chem. 1975, 79, 2857. 9. J. C. GIORDAN,J. H. MOOREand J. A. TOSSELL, J. Am. Chem. Soc. (1981), 103, 6632.
0146-5724/82/090237-04503.00/0 Pergamon Press Ltd.
ELECTRON REACTIONS IN THE PULSE RADIOLYSIS OF METAL CARBONYLS DISSOLVED IN 2,2,4-TRIMETHYLPENTANEt Y. S. K A N G * and R. A. H O L R O Y D Chemistry Department, Brookhaven National Laboratory, Upton, N Y I1973, U.S.A. (Received 8 October 1981) Abstract--The role of electron reactions in the radiolysis of solutions of chromium, molybdenum and tungsten carbonyls in 2,2,4-trimethylpentane was investigated. These compounds are very reactive with the electron and observed rate constants for attachment are: 7, 8 and 5 x 10t3 M- I sec- I for Cr, Mo and W hexacarbonyls, respectively. The spectra of the metal carbonyl anions were studied by both pulse radiolysis and laser photodetachment methods. The data indicate.that there is a maximum in the absorption spectra of Mo(CO)~- at 412 nm and that the anions of the other hexacarbonyls also absorb in this region. Photodetachment from M(CO)6- occurs throughout the visible with thresholds in the IR. The electron affinities of the metal carbonyls are obtained from 3/2's power law plots of the cross section data. INTRODUCTION A RECENT STUDYtl~ of the pulse radiolysis of c y c l o h e x a n e solutions of chromium,- m o l y b d e n u m and tungsten carbonyls indicated that a t t a c h m e n t of e x c e s s electrons to the carbonyls f o l l o w e d by neutralization was the main m e c h a n i s m for pentacarbonyl formation.
(1) (2)
e - + M(CO)6---) M(CO)6M(CO)6- + RH÷---) M(CO)~ + C O + RH.
In order to further elucidate the role of odd electron species in the c h e m i s t r y of these important c o m p o u n d s , additional tests of this m e c h a n i s m were made. First, the rates of electron a t t a c h m e n t to the carbonyls were m e a s u r e d with a d.c. conductivity technique. Second, the pulse radiolysis of solutions of the carbonyls in 2,2,4-trimethylpentane was studied at low doses to obtain spectral e v i d e n c e for the f o r m a t i o n of the pentacarbonyls by ion recombination. Finally, the laser p h o t o d e t a c h m e n t m e t h o d <2~ was e m p l o y e d to obtain the visible spectra of the carbonyl anions. The laser m e t h o d has the a d v a n t a g e of high sensitivity which permits the use of v e r y low ion c o n c e n t r a t i o n s ( - 1 0 p M o l a r ) w h e r e the ions are sufficiently long-lived to facilitate the determination of spectra. tResearch carried out at Brookhaven National Laboratory under contract with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences. ~Department of Chemistry, Jackson State University, Jackson, MI 39217, U.S.A.
EXPERIMENTAL The carbonyls were from Pressure Chemical Co. and were used as received. The 2,2,4-trimethylpentane was purified by passage through activated silica gel, degassing and storage over Na and NaK mirrors. Details of the conductivity technique to measure electron rate constants and of the laser photodetachment technique to determine spectra are published: 2) For pulse radiolysis, 3 mM solutions of the metal carbonyls in purified 2,2,4-trimethylpentane were prepared and degassed with Ar. Solutions were exposed to a 3 o.sec pulse (-1000rad) of electrons from a 2 MeV van de Graaff accelerator. Low doses were necessary to permit electron attachment because electron-ion recombination is very fastt3) (k = 2.8x 10~ M-~ sec-l) in 2,2,4trimethylpentane. The photodetachment spectra were obtained by measuring the conductivity induced by a pulse of light from a visible dye laser in a sample exposed to a prior X-ray pulse as a function of wavelength. The magnitude of the conductivity is proportional to the number of electrons photo-detached from anionic species. The anions are formed in the sample by an X-ray pulse from a 2 MeV van de Graaff accelerator; the X-ray pulse precedes the laser pulse by 100 ~.sec. The very high sensitivity of the laser conductivity method permits the use of a low dose of X-rays which produces a concentration of ions of only about 10picomolar. Under these conditions the lifetime of the ions is a few tenths of a second. Thus, the photodetachment spectrum observed 100/zsec after the X-ray pulse can be attributed to the hexacarbonyl anion, assuming that no reaction with solvent molecules or first order decomposition of the anion occurs. The validity of this assumption is borne out by the lack of any change in the detachment spectra when the time delay between X-ray and laser pulses is changed. RESULTS AND DISCUSSION Electron attachment rates. The high reactivity of the carbonyls necessitated m e a s u r e m e n t s of rates on v e r y dilute (~<0.1/~M) solutions. Rate constants for the second order electron a t t a c h m e n t reaction (l) w e r e derived from the pseudo-first order rate of d e c a y of the conductivity and the c o n c e n t r a t i o n of metal carbonyl 237
238
Y. S. KANG and R. A. I-IOLROYD
TABLE 1.
ELECTRON
A'I'rACHMENT
AND
PHOTODETACH-
MENT DATA
ke-+M(CO~-. M-~ sec -~ 7.4 x 10~3 7.8 X 1013 5.0 x 10~
Cr(CO)6
Mo(CO)6 W(CO)6
Eth eV 1.35 1.25 1.60
P-
E.A.
-0.95 -0.93 -0.92
+0.6 + 0.5 + 0.9
measured by optical absorption. The values of k~ are summarized in Table 1. These rate constants are among the largest ever reported for electron attachment to solutes in 2,2,4-trimethylpentane. For comparison, the value for attachment to SF6 is 5.8 x 10 ~3M -~ sec -~, (4) and that for attachment to aromatic hydrocarbons is generally between 1 and 2 x 10 ~3M -~ sec-L ~2) Mo(CO)6 In the pulse radiolysis of a 3 mM solution of Mo(CO)6 in 2,2,4-trimethylpentane a transient is observed immediately after the pulse; its absorption spectrum, shown by the solid line in Fig. 1, has a maximum at 412nm. A very rapid conversion occurs during the first 10 fisec to a second longer-lived species absorbing at 405 nm. A fast grow-in is observed at wavelengths ~<410 nm and a fast decay occurs at longer wavelengths. The average lifetime of these changes is 8/~sec, which is comparable to the expected lifetime of ion recombination: at the doses employed, ion concentrations are ~0.4/xM and the theoretical rate of ion recombination (reaction 2) in 2,2,4-trimethylpentane is 3.2 x 101~;¢5)thus, ion lifetimes would be expected to be about 7 t~sec. Hence we assign the initial species,
absorbing at 412 nm, to the anion Mo(CO)6- which on neutralization forms Mo(CO)5, absorbing at 405 nm. The results provide additional support for the previously suggested tt) mechanism of pentacarbonyl formation. The photodetachment spectrum of Mo(CO)6observed for a dilute solution of Mo(CO)6 in 2,2,4trimethylpentane is shown by the circles in Fig. 1. The shortest wavelength studied was 435 nm and cr~ falls off rapidly with increasing wavelengths above 435 nm, consistent with a peak in the spectrum at A ~<435 nm. As shown in Fig. 1, the shape of the photodetachment spectrum of Mo(CO)6- is very similar to that of the initial species observed by pulse radiolysis with h,,ax --- 412 rim, which is suggested to be the molecular anion. The quantum yield for photodetachment (~b~) is given by the ratio: tru/o,~, where tr, is the absorption cross section and from the data in Fig. 1, ~b,-> 0.1. This is a lower limit because the overlap of the absorption spectrum of the pentacarbonyl leads to an overestimation of the absorption by the anion. At longer wavelengths, the cross section for photodetachment is small and decreases monotonically with increasing wavelength. Cr(CO)6 The end of pulse spectrum observed in the pulse radiolysis of Cr(CO)6 in 2,2,4-trimethylpentane is shown by the dashed line in Fig. 2 with Area,= 500rim. This spectrum is assigned to the pentacarbonyl. ~) There is a rapid conversion, with a lifetime of 33/zsec, of this spectrum to a secondary species with Am~x= 470 nm. This change has been attributed to a rapid isomerization or reaction i
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F~G. 1. Spectra observed for Mo(CO)6 in 2,2,4-trimethylpentane: pulse radiolysis; - - , absorption at the end of a 3/zsec pulse; - - - , absorption at t = 100/.Lsec; 6) photodetachment spectrum.
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FIG. 2. Spectra observed for Cr(CO)6 in 2,2,4-trimethylpentane: pulse radiolysis; - - - - , absorption at the end of pulse; absorption at t =400p.sec; 6) photodetachment spectrum.
Electron reactions in the pulse radiolysis of metal carbonyls with a solvent molecule. (6) Because of this conversion it is difficult to find fast spectral changes to associate with ion recombination. However, 510 nm is an isosbestic point for conversion of the pentacarbonyl to the secondary species and at this wavelength a fast grow-in is observed within the first 10~sec. Furthermore, this initial grow-in, which accounts for a maximum of 15% of the signal, follows second-order behavior; that is, the observed rate constant increases linearly with absorbance (and concentration). From this dependence a second order rate constant of 5.6× 10" M -~ sec -~ was derived which is close to the expected rate for ion recombination in 2,2,4-trimethylpentane and confirms the formation of Cr(CO)~ by reaction (2). The photodetachment spectrum of Cr(CO)6- is shown in Fig. 2 and is similar to that of Mo(CO)6- in that ~rd decreases with increasing wavelength throughout the visible. However, the cross section at 435 nm for photodetachment is less for Cr(CO)6than for Mo(CO)6-. Data on photodetachment are limited to the visible because of the laser employed but ~he data suggest that the maximum in the spectrum of Cr(CO)6- is at a wavelength ~<435 nm.
acarbonyl anion and no spectral changes could be observed to associate with this conversion. The photodetachment spectrum of W(CO)C is shown by the circles in Fig. 3. Again a maximum is indicated at short wavelengths (~<435nm). The cross sections (tra) in the visible are much less than those observed for the other two anions.
Photodetachment thresholds The "long tails" observed at longer wavelengths in the photodetachment spectra are typical of the behavior of anions near threshold, c2'7) Such cross section data have been found to obey a 3]Ts power law; that is, a plot of (trlEhv) 2t3 versus Ehv is linear and the intercept of such a plot on the energy axis is the threshold for photodetachment (E,h) from the anion. Plots of the cross section data for the carbonyl anions are indeed linear for the data at A t> 500nm and the thresholds determined are given in Table 1, column 3. The thresholds are related to the electron affinities (E.A.) of the parent carbonyls by equation (3); where P - is the polarization energy of the anion (3)
W(CO)6 The dashed line in Fig. 3 is the end of pulse spectrum observed in the pulse radiolysis of W(CO)6 in trimethylpentane. The maximum is at 415 nm and there is little change in the absorption over the first 100/~sec. In this case the absorption of the pentacarbonyl overlaps that of the hex-
'F'\
iI I I
I0 'E
Eth = E . A . - P - + Vo
and Vo is the energy level of the quasi-free electron in 2,2,4-trimethylpentane. The values of P - , as estimated with the Born equation and the molecular radius, are given in Table 1. Vo is -0.24 eV for this solventfl ) The electron affinities of the three carbonyls, derived from this data using equation (3), are given in Table 1°(9)
A c k n o w l e d g e m e n t - - T h e help of H. A. Schwarz, whose computer programming facilitated the acquisition and analysis of pulse radiolysis data, and the help of M. Andrews, who provided the metal carbonyl samples, are both gratefully acknowledged.
I I I
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239
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400
REFERENCES
\ \
°°°pOOOOlO
nm
Note added in proof: A recent report(9) confirms the existence of stable negative ions of the metal carbonyls.
500
o 600
FIG. 3. Spectra observed for W(CO)6 in 2,2,4-trimethylpentane: - - end of pulse spectrum in pulse radiolysis; Q photodetachment spectrum.
1. L. FLAMIGNI,Radiat. Phys. Chem. 1979, 13, 133. 2. U. SOWADAand R. A. HOLROYD,J. Phys. Chem. 1981, 85, 541. 3. A. O. ALLEN and R. A. HOLROVD, J. Phys. Chem. 1974, 78, 796. 4. A. O. ALLEN, T. E. GANGWERand R. A. HOLROVD,J. Phys. Chem. 1975, 79, 25.
240
Y. S. KANG and R. A. HOLROYD
5. A. O. ALLEN, M. P. DE HAAS and A. HUMMEL, J. Chem. Phys. 1976, 64, 2587. (This value is obtained by assuming kll.L = 10-6 V cm and using the average mobility of ions in 2,2,4-trimethylpentane of 5.5x 10-4 cm2//zsec reported). 6. A. VOGLER: in Concepts in Inorganic Photochemistry (Edited by A. Adamson and P. D. Fleischauer), Wiley-Interscience, 1975, p. 277.
7. U. SOWADAand R. A. HOLROYD,J. Phys. Chem. 1980, 84, 1150. 8. R. A. HOLROYD, S. TAMES and A. KENNEDY, J. Phys. Chem. 1975, 79, 2857. 9. J. C. GIORDAN,J. H. MOOREand J. A. TOSSELL, J. Am. Chem. Soc. (1981), 103, 6632.