J. Phys. Chem. 1981, 85,3557-3560
Molecular Oxygen as Energy Acceptor Oxygen is unique as an acceptor in that its highest occupied and lowest vacant orbitals (T*) are degenerate. A treatment13of the anthracene-O2 (oxciplex) orbitals in Czu symmetry indicates that the states 31'(S1,32), 31'(T1,1A*), and 3r(T1,32) belong to the same (A,) symmetry species whereas 31'(Tl,1A),lI'(S0,lA*), and lI'(S0,lA)transform as B1,B,, and Al, respectively. Here S1and T1denote lowest excited singlet and triplet states of anthracene (donor) and lA* and 'A are components of the singlet delta state of oxygen which are distinguishable only in complex formation.14 The singlet quenching processes D(S1)
+
02(3.Z)
-
should therefore be adiabatic or with gf = 1,k(AE>>RT) = k, (eq 8). This is consistent with the observed15independence of oxygen quenching rate constant k for a series of aromatic hydrocarbons on the singlet-triplet energy separation AEST = E(Sl) - E(Tl), and provides experimental access to the quantity k , = 4?rDppo 2.8 X 1O'O M-l s-l, for these systems in benzene solution.12 The further observation16J7that the quantum yield of 02(lA) production from quenching of singlet states with PEST> E(lA) is -0.5 has been interpreted16p22 in terms of a reencounter quenching of O&A) by D(Tl) following process 1, with probability p -0.5, represented by the sequence
-
I
D(T1)
+
/
*
3A2(T1, A )
3
AZ(T1,
3
C)
-+
D(T1)
Od3X)
Oz('A)
-
If the states 3Azand 3B1are formed with equal probability then a = g = 0.5 and an estimate of p o 213 is afforded by eq 1. On the other hand, the triplet energy transfer processes (13)B. Stevens, J. Photochem., 3,393 (1974/5). (14)D.R. Kearns, J. Am. Chem. SOC.,91,6554(1969). (15)C. S.Parmenter and J. D. Rau, J. Chem. Phys., 51,2242(1969). (16)B.Stevens and R. D. Small, Jr., Chem. Phys. Lett., 61,233(1979). (17)K. C. Wu and A. M. Trozzolo, J. Phys. Chem., 83, 2823,3180 (1979).
+
D(T~)
o,(~z)
3557
-
'A2iTi3X)
-
'Af(So, 'A) 1
*
Al (S0,'X)
-
DISOI
+
O,( 'A]
D(S,)
+
02i1Z)
are expected to be nonadiabatic for aromatic hydrocarbon donors such as anthracene, as evidenced by the observed12 reduction in rate constants k with increase in AE = E(TJ - E(l0,) (eq 9). The upper experimental limit k < k,/10 for this process is described by eq 9 with AE = E,, and gf = 119, if p o = 2 / 3 and u p uJ2 loll s-l. Larger rate constants reported for the oxygen quenching of high-energy 3n,?r*states of acetone18 ( k = 8 X log M-' d),xanthonelg (k = 5.6 X lo9 M-' s-l), and a~etophenone'~ ( k = 4.0 X lo9 M-l s-l) in benzene have been attributed to the additional quenching process D(TJ + 02(32) + 0~32) (B) which reduces the yield yA of O2(lAg)from these sensitizers.20 However, these rate constants do not exhibitlg the dependence on E(Tl) observed for aromatic hydrocarbons, and energy transfer to 02(lAg) from 3n,7r* states is expected to be adiabatic if the oxciplex orbitals originate in the linear combinations &,*(?r*) f +02f(?rX*!and $D(n) f 402(?r,*). The appropriate use of eq 8 in this case with gf = 119, Po = 213, and It, = 4?rDppo= 2.8 X 1O'O M-' s-l for dynamically equivalent aromatic hydrocarbon-02 systems in this solvent, provides an upper limit of k I8 X log M-' s-l for the quenching rate constant which accomodates the reported values without the intervention of the alternative quenching process B. Moreover it has recently been shown that the sensitized yield of 02(lAg) by xanthone and acetophenone increases with dissolved oxygen concentration to a limiting value of unity.21 Acknowledgment. The continued support of this research by the National Science Foundation under Grant CH-78-01578 is gratefully acknowledged.
- -
- w,)
(18)T. Wilson and A. M. Halpern, J. Am. Chem. SOC., 102, 7279 (1980). (19)A. Garner and F. Wilkinson, Chem. Phys. Lett., 45,432 (1977). (20)A. Garner and F. Wilkinson, "Singlet Oxygen", B. Ranby and J. F. Rabek, Ed., Wiley, New York, 1978,p 48;A.A. Gorman, G. Lovering, and M. A. L. Rodgers, J. Am. Chem. SOC.,100,4527(1978);A.A. Gorman, I. R. Gould, and I. Hamblett, Tetrahedron Lett., 21, 1087 (1980). (21)B. Stevens and K. L. Marsh, manuscript in preparation. (22)This is, however, kinetically indistinguishable from the simultaneous operation of quenching processes A, 1 and 2, with kl k2 in the absence of reencounter effects.
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Pulsed Laser Photolysis of Chromium Hexacarbonyl in the Gas Phase W. H. Breckenridge" and N. Sinait Department of Chemistv, University of Utah, SaR Lake City, Utah 84112 (Received: August 6, 1981; In Final Form: October 12, 1981)
Transient absorption spectra observed after pulsed laser photolysis of Cr(COI6in the gas phase are assigned to Cr(CO),-Q complexes formed by reaction of Cr(CO)5with Q = Ar, CHI, Cr(CO)6,N2, and NHa The lifetimes of the Cr(CO),-Q complexes increase substantially in the order listed.
Introduction Species resulting from the dissociation of one 01 more carbon monoxide molecules from metal carbonyls are
considered to be important as catalytic precursors,l and as reactive intermediates in ligand substitution processes in s ~ l u t i o n and ~ - ~ in solid mat rice^.^^^ Because of this,
Present address: Department of Chemistry, Brandeis University, Waltham, MA 92054.
(1)I. Wender and P. Pino, Ed., "Organic Synthesis Via Metal Carbonyls", Vol. I, Interscience, New York, 1968.
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The Journal of Physical Chemistry, Vol. 85, No. 24, 1981
there has been a great deal of effort in the last decade directed toward the identification and characterization of such coordinatively unsaturated metal carbonyl compounds. Solid-phase photochemical studies,,s6 in which a parent compound is photolyzed in an inert matrix, have been particularly useful in characterizing these fragments. Flash photolysis and laser photolysis studies of the production and reaction of such intermediates have also been re~orted.~-’O There is a large body of matrix isolation5i6and solution4Jp8J0experimental work as well as theoretical calculations”J2 concerning the M(CO), species produced from the group VI hexacarbonyls. The pioneering matrix isolation studies of Turner, Poliakoff, and co-workerse have revealed a wealth of interesting photochemistry. Of particular interest was the observation that, just as is thought to occur in solution, the primary photochemical acts are net dissociation of one CO molecule to produce the resultant M(C0)5fragments. These fragments have been thoroughly characterized in the matrices by infrared and visible spectroscopic techniques. The most startling observations were the large shifts in the visible absorption spectrum of ‘‘Cr(CO)5))when different matrix materials were used. Very careful mixed-matrix experiments showed that the shifts were not due to ordinary matrix environment effects, but were specific molecular interactions in which an atom or molecule formed a very weak complex at the “vacant” coordination site in the Cr(C0)6fragment. Even matrix materials as inert as argon showed a measurable spectral shift from that in neon (considered to be the case closest to the “free” Cr(C0)5fragment); i.e., a shift in absorption maximum from 6240 to 4890 A in the matrix series Ne, SF6, CF4, Ar, Kr, Xe, and CH4. Subsequent theoretical calculations on the ground and excited states of Cr(CO)5have provided a plausible explanation for the drastic shifts in absorption maxirna.l1J2 The electronic transition observed produces a diffuse excited-state orbital which interacts strongly with any “solvent” molecule occupying the vacant sixth coordination site. Upper-state destabilization thus results in spectral shifts which are unusually large, and provides an ideal spectroscopic “marker” for the presence of a particular “solvated” Cr(CO)Sspecies. Experiments involving laser photolysis of Cr(C0)e in solution have provided further evidence for the importance of stereospecific “solvation” of Cr(CO)& Kelly, Hermann, and Koerner Von Gustorf‘ showed, by pulsed laser photolysis of Cr(CO)6 in highly purified cyclohexane, that transients observed earlier by others were not in fact Cr(CO), as claimed but Cr(CO),-X complexes, where X was a trace unsaturated impurity. They observed a transient formed in less than 50 ns which had a maximum absorption at 5030 f 50 A and a lifetime of several hundred (2)M. Wrighton, Chem. Rev., 74, 401 (1974). (3)A. Vogler, “Photochemistry of Carbonyl Complexes” in “Concepts of Inorganic Photochemistry”, A. Adamson and P. Fleishauer, Ed., Wiley-Interscience, New York, 1975. (4)D. R.Tyler and D. Petrylak, J. Organomet. Chem., 212,389(1981). (5) J. J. Turner, J. K. Burdett, R. N. Perutz, and M. Poliakoff, Pure Appl. Chem., 49,271 (1977),and references therein. (6) R.N. Perutz and J. J. Turner, J.Am. Chem. SOC., 97,4791(1975). (7)J. M.Kelly, H. Hermann, and E. Koerner Von Fustorf, J. Chem. Soc., Chem. Commun., 105 (1973). (8)J. M. Kelly, 1).Bent, H. Hermann, D. Schulte-Frohlinde, and E. Koerner Von Gustorf, J. Organomet. Chem., 69,259 (1974). (9)J. L. Hughey, C. R. Bock, and T. J. Meyer, J. Am. Chem. SOC.,97, 4440 (1975). (10)R.Bonneau and J. M. Kelly, J.Am. Chem. Soc., 102,1220(1980). (11)P. J. Hay, J. Am. Chem. Soc., 100,2411 (1978). (12)J. Dernuynck, E.Kochanski, and A. Veillard, J. Am. Chem. SOC., 101,3467 (1979).
Letters
microseconds. They assigned this transient absorption to free Cr(CO)S. It should be noted, however, that the maximum is very near that obtained for Cr(CO)s-CH4 in the matrix, suggesting that the transient was a Cr(CO),-cyclohexane complex rather than “free” Cr(C0)6. Recent work by Bonneau and Kelly’O is consistent with this interpretation. These workers, reasoning that fluorocarbons would be more inert toward Cr(CO)Sthan cyclohexane, photolyzed Cr(CO)6 in C7F14 and observed a short-lived transient spectrum with maximum absorbance at 620 nm (close to that of Cr(CO), in a Ne matrix) and assigned this as due to nearly free Cr(CO)& They were also able to show that this transient spectrum is rapidly converted to the same spectrum as observed in the cyclohexane studies when small amounts of cyclohexane were added. In pure C7F14, the Cr(CO), decay rate was found to depend on the concentration of Cr(CO)6, and the following process was postulated to occur: Cr(CO), + Cr(CO)6 Cr2(CO)11
-
A subsequent long-lived absorption with a maximum near 4850 A was assigned to Cr2(CO)11.It was also shown that Cr(CO), in C7F14 reacts much more rapidly with CO than does the Cr(C0)6-cyclohexane complex in cyclohexane. The studies discussed above definitely show the existence of weak solvent complexes of metal carbonyl fragments and point to their importance as reactive intermediates. We have recently initiated a study of Cr(CO)6laser photolysis in the gas phase in our laboratories, with the hope of characterizing and studying the reaction kinetics of truly free Cr(CO), and the solvent complexes of Cr(C0)6 in the complete absence of matrix or liquid environments. We report here the results of the f i s t series of experiments, which are quite consistent with the results of Bonneau and Kelly in C7F14 solvent. Experimental Section A frequency-tripled Nd:YAG laser pulse at 3550 A is reflected, using a mirror which transmits visible light, into a gas cell containing Cr(CO)6at its vapor pressure at 70-80 OC, which corresponds to a concentration of -3 X lo4 M. A 1000-W continuum Xe arc is used to detect transient intermediates in absorption in the visible region by means of a monochromator, phototube, and oscilloscope. The laser excitation beam and Xe lamp detection beam are aligned coaxially, with two cutoff filters (A 1 3800 A) placed in front of the monochromator to block the laser light. The system is currently limited by lamp intensity to a time resolution of a few microseconds. Absorption spectra of transient intermediates were obtained by determining, for several monochromator wavelength settings, the optical density at a given time delay. The spectral bandwidth at the slit settings used was -40 A. Several experiments at a given wavelength were performed which showed that optical densities were fairly reproducible for a given mixture and not seriously affected by the very slight photochemical decomposition of Cr(CO)6observed after several laser shots. Results and Discussion Photochemical Results and Mechanistic Explanation. When CI-(CO)~was photolyzed either alone or in the presence of up to 20 torr of He buffer gas, the only transient spectrum observed was an absorption with a maximum at 4850 f 150 A (see Figure 1, a and b) which had a lifetime of hundreds of microseconds. With Ar or CHI present at -150 torr, the 4850-A spectrum was also observed but in addition short-lived spectra with lifetimes 150 pus and maxima in the 5000-5300-A region were de-
The Journal of Physlcal Chemistry, Vol. 85, No. 24, 1981 3559
Letters
results can be interpreted qualitatively with the following simple mechanism: Cr(CO)6+ hv Cr(C0)6+ CO Cr(C0I6 + Q cr(co)6-Q Cr(CO& + Cr(CO16 Cr2(CO)11 cr(co),-Q + Cr(CO)6 Cr2(CO)11 + Q For Q = NH3, N2, or CO a long-lived, stable Cr(CO),-Q complex is formed exclusively. For Q = Ar or CHI, a weakly bound, short-lived intermediate complex Cr(CO),Q is formed, with concomitant and/or subsequent formation of Cr2(CO)11. When Q = He, or with no Q present, Cr2(CO)11is the only product observed. It should be noted that the relatively short-lived Cr2(CO)11is probably Cr(CO)6-[Cr(CO)6],with the Cr(C0)6coordinated weakly through the oxygen atom on one of the carbonyls, with no metal-metal bonding. Comparison with Recent Experiments Involving Laser Excitation of Gaseous Metal Carbonyls. It is interesting to compare our preliminary results with those reported recently by others in laser photolysis studies of metal carbonyls in the gas p h a ~ e . ' ~ -In~ most ~ cases, the phenomenon observed was the production of electronically excited metal atoms or metal ions, presumably by means of sequential or direct multiphoton processes. At first glance, such results would seem to indicate that multiphoton effects could cause serious complications in the experiments reported here. However, a careful examination of the work in this area published to date shows either that the observed phenomena are easily detected (Le., by atomic fluorescence) and thus may be the result of minor pathways in competition with simple one-photon photochemistry, or that the particular experimental conditions (i.e., focused laser radiation and/or high absorption cross sections for the primary or secondary photochemical processes) favor multiphoton rather than single-photon photochemistry. In our experiments, a system in which such effects are minimized has deliberately been chosen, and it is important to emphasize this point. For example, there are three studies in which Cr atomic states or Cr+ were observed when laser radiation was absorbed by Cr(CO)6.16J7J3The observation of large concentrations of Cr+ when Cr(CO)6was photolyzed at 2800 A with intense, focused dye-laser radiatiod6 can be rationalized by the fact that both Cr(CO)6 and Cr(C0)5e absorb strongly in this wavelength region, maximizing the probability of multiphoton, high-energy pathways. In the two other studies, Cr(CO)6 was irradiated with higher wavelength laser radiation, and electronically excited Cr atoms were observed. Hopkirk et al.18 photolyzed Cr(C0)6 with unfocused N2 laser radiation at 3371 A and observed fluorescence from only a few excited Cr atom states. Garrity et al.19 detected production of several excited Cr atom levels when visible dye-laser radiation was passed
--
Cr(CO), only
a,)
+
+
c.)
d.)
Cr(CO),+Ar
Cr (CO),+CH,
5 n U
Flgure 1. Translent absorption spectra observed after pulsed laser photolysis of Cr(CO)Eunder various conditions: (a) Cr(CO), only, delay time 15 ps; (b) Cr(CO)e 200 torr of He, delay time 10 ps; (c) Cr(CO), 4- 200 torr of Ar, closed circles, solM line: delay time 200 m; open circles, dotted line: spectrum at delay time 10 ps from which has been subtracted the spectrum at delay time 200 1s; (d) Cr(COIE 160 torr of CH,, closed circles, solid line: delay time 100 ps; open circles, dotted line: spectrum at delay time 20 m, from which has been subtracted the spectrum at delay time 100 ps.
+
+
Cr(CO),+NH,+He O
"
O
Wavelength
1
(A)
Flgurs 2. Long-lived spectrum observed after the pulsed laser photolysis of Cr(CO)e in the presence of NH3. The mixture was Cr(CO)E 4 torr of NH3 380 torr of He. The delay time was 5 ms.
+
+
tected, with a low signal-to-noiseratio (see Figure 1,c and d). The short-lived spectra were determined by subtraction of the long-lived component, resulting in considerable uncertainty. The short-lived spectra are undoubtedly the gas-phase equivalents of the Cr(CO)& and Cr(C0)5XH4 spectra observed in the matrix studies.6 Because of the observations of Bonneau and KellylO in the fluorocarbon solvent, the common long-lived spectrum with a maximum at 4850 A is assigned to Cr2(CO)11. With NH3 present, the 4850-Aabsorption was not detected, but a very long-lived product spectrum with a maximum at -4400 A was recorded (see Figure 2). This spectrum is assigned to the stable Cr(CO)6-NH3 compound, which has a spectra maximum at 4200-4300 A in matrices? and 4350 A in cyclohexane solution.6 With excess N2, again the 4850-Atransient was not observed, but a long-lived spectral absorption with a maximum at 14000 A was recorded, tentatively assigned to Cr(CO)sN2which has a maximum at -3700 A in a matrix.13 With excess CO present, no transient spectra were observed. These (13)J. K. Burdett, A. J. Downs, G. P. Gaskill, M. A. Graham, J. J. Turner, and R. F. Turner, Znorg. Chem., 17,523(1978).
(14)Z. Karny, R. Naaman, and R. N. Zare, Chem. Phys. Lett., 69,33 (1978). (15)M. A. Duncan, T. G. Dietz, and R. E. Smalley, Chem. Phys., 44, 415 (1979). (16)P.C. Engelking, Chem. Phys. Lett., 74,207 (1980). (17)D. P.Gerritz, L. J. Rothberg, and V. Vaida, Chem. Phys. Lett., 74,l(1980). (18)A.Hopkirk, W.H. Breckenridge, C. Fotakis, and R. J. Donovan, submitted for publication. (19)J. Krasinski, S.H. Bauer, and K. L. Kompa, Opt. Commun.,35, 363 (1980). (20)D. W.Trainor and S. A. Mani, J. Chem. Phys., 68,5481(1978); Appl. Phys. Lett., 33, 31 (1978). (21)G. Nathanson, B. Gitlin, A. M. Rosan, and J. T. Yardley, J. Chem. Phvs.. 74. 361 11981). 122)J.'T. Yhdley; B. Gitlin, G. Nathanson, and A. M. Rosan,J. Chem. Phys., 74,370 (1981).
J. Phys. Chern. 1981, 85,3560-3563
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through Cr(CO)6,using multiphoton ionization detection. In both experiments, the detection techniques are very sensitive and capable of detecting very minor photoprocesses. Also, it is known that Cr(COI5has no absorption bands from 3000 A into the visible spectral region over which the dye laser was scanned.6 The results reported here with laser irradiation at 3550 A also indicate that simple one-photon photodissociation of Cr(C0)6 is the dominant process. Because of the moderate extinction coefficients for the Cr(CO),-Q and Cr2(CO)11complexes (-3000), most of the photolytic energy must result in the one-step production of Cr(CO), or the optical densities of the intermediates would be below our detection level. As in the Hopkirk et al. experiments,ls it was observed that a single sample of Cr(CO)6,with or without inert gases, could survive a great many laser shots at 3550 A, again consistent with formation and recombination of Cr(C0)5+ CO as the primary photolytic process. In one experiment with Cr(C0)6 only, it was also observed that, when the incident laser power was reduced by a factor of 2.0, the optical density of Cr2(CO)11decreased from 0.08 to 0.04, consistent with a single-photon precursor event. Multiphoton processes are undoubtedly occurring in competition with the single-photon molecular processes we observe, but as minor pathways. Finally, it is important to discuss the possibility of multiligand dissociation by single-photon excitation in these experiments, i.e. Cr(CO)6 + hv Cr(C0)4 + co Cr(C0)3 + 2CO From the average bond energy of Cr(CO)6, production of Cr(C0)4and perhaps CI-(CO)~ is energeticallypossible with 3550-A radiation. There is no evidence in our preliminary work for the occurrence of such processes, and all the observations can be rationalized by single-ligand photodissociation, similar to what occurs in fluorocarbon solvent.1° Also, Cr(C0)4 is known to have an absorption maximum at about 4000 A in inert-gas matrices? and we see no evidence for an absorption in this region of the spectrum when Cr(CO)6or Cr(C0)6 + inert gas mixtures are photolyzed at 3550 A.
--
Our observations indicate that Cr(CO)6 ultraviolet photochemistry may be substantially different from that of Fe(CO)% Yardley and co-workers have recently published two apers on Fe(COI5photolysis in the gas phase with 2480- KrF laser radiation.21i22Using a PF3chemical trapping technique, they present evidence for one-photon, multiligand dissociation of Fe(C0)5under their conditions to produce large fractions of Fe(C0)3and Fe(C0)2in addition to Fe(CO)& There is no reason to believe, however, that excitation of Cr(CO)6 would result in the same decomposition pathways as Fe(CO), excitation. Decomposition of an initially excited M(CO), species must depend intimately on both the initial photodissociation process (i.e., how much energy goes into internal excitation of M(CO),-l and how much into relative translation or CO vibration) and the bonding, structure, and complexity of the M(CO),+ M(C0),.-2, etc. fragments. Also, Cr(C0)6is more complex than Fe(C0)4and will fragment unimolecularly at a slower rate. Future Experiments. Experimental improvements underway, including the construction of a feedback stabilized high-intensity pulsed Xe lamp detection source, will allow the detection and kinetic study of weakly absorbing intermediates in this system with time resolution in the 10-ns range. This should facilitate the determination of the absorption spectrum of truly “free” Cr(C0)5as well as a range of weakly bound “solvent” complexes Cr(CO),-Q in the gas phase, and allow firm conclusions about primary photochemical events which are not possible with the present time resolution. Kinetic studies are planned in which the reactivity of free Cr(C0)5with various substrates can be compared to that of Cr(CO),-Q complexes, thus allowing direct determination of the effect of stereospecific “solvation” of metal carbonyl fragments on their reactivity.
R
Acknowledgment. Acknowledgments are made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. The authors thank Professor W. A. Guillory and his research group for the use of their Nd:YAG laser.
Ionization Potential of the Benzene-Argon Complex in a Jet K. H. Fung, W. E. Henke, T. R. Hays, H. L. Selrle,” and E. W. Schlag Physlkalische Chemle, Technische Universltat Munchen, 8046 arching, West Germany (Received: August 10, 198 1; In Final Form: September 18, 198 1)
The ionization potentials of ultracold benzene (Trot= 3 K), and its van der Waals complex, Le., benzene-Ar, have been measured in a seeded supersonicjet by a two-color resonance-enhancedtwo-photon ionization technique. The ionization potential of benzene is found to be 9240 meV with an onset of 3.2 meV, and that of the benzene-argon complex is red shifted by 21.2 meV. This directly identifies the nature of the new species formed in the hypersonic jet. Introduction Ionization of molecules with multiphoton absorption by way of resonant intermediate states has been shown to be a very facile process.’ In principle, one can scan a high-
intensity laser over the ionization threshold, and thus obtain highly accurate ionization potentials, structure near these, etc. This technique should be about two orders of magnitude more accurate than ionization with present synchrotron radiation sources.2 The difficulty with the
(1) P. M. Johnson, M. R. Berman, and D. Zakheim, J. Chern. Phys., 62,2500 (1975); G. Petty, C. Tai, and F. W. Dalby, Phys. Rev. Lett., 34, 1207 (1975); L. Zandee and R. B. Bernstein, J. Chern. Phys., 71, 1359 (1979); U. Boesl, H. J. Neusser, and E. W. Schlag, 2. Naturjorsch. A, 33,
(2) R. Frey, B. Gotchev, D. F. Kalman, W. B. Peatman, H. Pollak, and E. W. Schlag, Chern.Phys., 21,89 (1977); V. Saile, P. Giirtler, E. E. Koch, A. Kozevnikov, M. Skibowski, and W. Steinmann, Appl. Opt., 15,2659
1546 (1978).
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(1976).
Q 1981 American Chemical Society
