1620
J. Phys. Chem. 1981, 85,1620-1622
Organometallic Bond Dissociation Energies. Laser Pyrolysis of Fe(CO), Gregory P. Smith” and Richard M. Laine Chemical Kinetics and Physical Organic Chemlstry Departments, SRI International, Menlo Park, California 94025 (Recelved: February 9, 198 1; I n Final Form: April 6, 198 1)
The gas-phase decomposition of iron pentacarbonylwas studied by using the technique of pulsed-laser-powered homogeneous pyrolysis. By measuring the infrared fluorescence of methane bath gas, we were able to characterize temperature profiles under various experimental conditions. By using the IR signal as a temperature standard and measuring iron pentacarbonyl decomposition yields, we determined a first bond dissociation energy of 48 f 4 kcal/mol. Bond homolysis steps are important in many organometallic and catalytic reaction mechanisms, but experimental difficulties have to date precluded the gas-phase measurement of almost all relevant neutral single-bond dissociation energies-l We have recently begun a program to determine these values using a cold-walled, laser-initiated, flash-pyrolysis technique,2 and report here our preliminary results for iron pentacarbonyl, Do[Fe(C0)4CO] For these experiments, utilizing the pulsed-laser-powered homogeneous pyrolysis method, we have irradiated a mixture of 3 torr of SF6, 0.5 torr of Fe(C0)5,11 torr of CH,, and 86 torr of SO2 with the PzOline of a Lumonics pulsed C02 laser. The SFGabsorbs the laser energy and heats the bath gas (in this case, SOz) by various collisional energy-transferprocesses on a microsecond time scale. The sample molecule, Fe(C0)6,which does not directly absorb the laser radiation, then decomposes at a rate determined by the gas temperature thus attained. Relative rate s t ~ d i e sindicate ~ * ~ that a true and well-defined kinetic temperature is established. The heated mixture is then cooled by a shock wave. Cold walls prevent heterogeneous decomposition typical of organometallics, and the short reaction times quench any extensive chain mechanisms. After several laser pulses, decomposition yields can be determined by gas-phase mass spectrometric analysis of the mixture. We have used the Fe+ peak to monitor the Fe(C0)5concentration. The only other possible contributors to this peak are potential pyrolysis products, which were observed to condense. Even the lightest possible product, Fe2(C0)9,has a much lower vapor pressure than the 0.5 torr pressure of the Fe(C0)5sample, and thus could only make a negligible contribution to the Fe’ peak. Accretion of product on the walls did not affect decomposition yields in this “wallless” reactor. By measuring the Fe(C0I5 decomposition yield relative to that of an appropriate known chemical standard or relative to some physical temperature measurement at several different laser intensities (temperatures), the activation energy for Fe(C0)5 decomposition can be determined. The temporal evolution of the temperature is indicated in Figure 1, which presents the 3.3-pm CH4 infrared fluorescence (v3 = 1 0), detected by an InSb detector (Infrared Associates, 1-MHz bandwidth). In the upper trace, an optically thick mixture (5 cm) is irradiated in a 3.8-cm diameter cylindrical cell by a concentric, 1.2-cm diameter laser beam. The detector is situated perpendi-
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(1) Connor, J. A. Top. Current Chem. 1977, 71, 71. Skinner, H.A. Adv. Organometal. Chem. 1965,2, 49. (2) Steel, C.; Starov, V.; Leo, R.; John, P.; Harrison, R. G. Chem. Phys. Lett. 1979,62,121. Lewis, K. E. McMillen, D. F.; Golden, D. M. J. Phys. Chem. 1980,84, 227-8. (3) Lewis, K. E.; McMillen, D. F.; Golden, D. M., unpublished results. 0022-3654/81/2085-1620$01.25/0
culm to the axis of the cell and views a crosssection of the irradiated region 0.5 cm wide midway down the axis. The resulting shock waves, axial (window-to-window)and radial, are clearly evident, and propagate at approximately the speed of sound. After the initial absorption and energy transfer, the irradiated central cylindrical region of the cell is at much higher temperature and pressure than the surrounding region. A shock wave travels radially outward into this area, heating the gas somewhat in ita wake, though not to the extent of the original heated region. Concurrently, a rarefaction wave travels inward and cools the central hot zone, i.e., the volume of hot, reactive molecules contracts with time. The shock wave reflects off the outer cylinder walls and later returns to the central region, reheating the mixture at a time of -60 ps. A similar temperature-pressure gradient is established along the axis of the optically thick cell, and the centrally positioned IR detector records the heating effects of the axial shock wave passage at 0.5 and 1.5 transits of the cell length. To avoid these reheating effects, we utilize a thin (1.2 cm) cell, a rear-reflecting mirror, and off-axis irradiation, with the results illustrated in the lower trace. By moving the laser beam to within 0.3 cm of the cell walls (toward the IR detector), the reflected radial shocks no longer constructively reheat the reaction volume. The thin cell and mirror prevent significant axial thermal gradients. (This particular run used CO rather than CH,; both gave similar results.) At the speed of sound, cooling to chemically unreactive temperatures will be completed, at the center, in approximately 10 ps, as the figure indicates. The rate equation for the laser pyrolysis decomposition is dX( V)/dt = -Ae-E/kTX(V) (1) where E and A are the Arrhenius activation energy and preexponential factor for Fe(C0)5 decomposition, and X(V)is the Fe(C0)5concentration in a given volume element V. Integrating In (X(V)/Xo) = -Ae-E/kTt(V) (2) where t(V)is the time volume element V stays heated at temperature T before shock wave cooling. If the total heated volume VL decreases linearly with time over time to
t(V) = t o - V(tO/Vd
(3) (Other functional forms produce minor differences.) Substituting (3) into the exponentiated form of (2) X(V)/Xo = exp[-Ae-E/kTto(l- V/VL)I Integrating over the total irradiated volume VL VLX/Xo = -VL(Atoe-E/kr)-l[exp(-AtOe-E/kT) - 11 0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 12, 1981 1621
Letters
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Flgure 2. Log plot of Fe(CO), decomposition yield vs. CH, infrared fluorescence.
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Flgure 1. Infrared fluorescence vs. time in a long and short cell. R and A are radial and axial shock waves. Fluorescing gases are CH, (top) and CO (bottom).
where X is the average concentration over this volume. If we expand the exponential by a power series
X/Xo = eElkT/Ato -Atoe-E/kT+
A2t02 2
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1
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If the single-shot yield (1 - X/Xo) is expressed in terms of the N-shot yield XN, with a ratio VT/VL of the cell volume to the irradiated volume
At low pressures, this reduces to
The total cooling time to was roughly constant over the limited temperature range used. As the temperature standard, we used the maximum of the CH, infrared signal intensity. (Calculations of the shock phenomena, which match the observed behavior of the IR signal, Figure 1, indicate the peak intensity before the heated volume decreases significantly is a good measure of the kinetic temperature. Details will be reported elsewhere.) The In of the CH4 infrared signal is, from the Boltzmann population for V = 1, proportional to E V / k T (E, = 3020 cm-l = 8.7 kcal). When data at various temperatures (laser intensities) are used, the slope of a plot of the left-hand side of eq 8, the In of the yield, vs. the In of the IR peak, will give Note that knowledge of absolute temperatures is not needed to determine E. Current evidence indicates this activation energy is associated with the first bond scission. Photochemical experimentd indicate Fe(C0)4is an initial product in the UV (4) Engelking, P.;Lineberger, W. J.Am. Chem. SOC. 1979,101, 5569.
photolysis of Fe(C0)5,and that subsequent recombination produces Fe2(C0)9and Fe3(C0)12. We have observed a dark, magnetic product which condenses on the walls. The decomposition yield does not vary with added CO, so recombination to Fe(C0)5 does not occur. Thus no equilibrium between Fe(C0)5and Fe(C0I4is established, decomposition of smaller Fe(CO), fragments cannot be rate determining for Fe(C0)5decomposition, and the measured value corresponds to the first bond scission. Given the low second-bond energy [Do(Fe(CO),-CO)],4 subsequent decomposition should be fast in our hot system. Calculations7 indicate the reaction is in the high-pressure limit. The value of 48 kcal/mol for the Fe(C0)5bond energy is much greater than the average bond energy1p4 of 28 kcal/mol, and illustrates the importance, particularly for kinetics, of determining individual rather than average values. This contrasts with the conclusions of Day et ala8 that the Ni(CO), bond dissociation energy is 22 kcal/mol, less than the average bond energy of 35 kcal/mol. However, that study gave a low A factor (log A = 14.5). We can estimate log A for Fe(C0)5from (2), given an estimate of the absolute temperature for one point in Figure 2. From a measurement of the absorbed laser energy and the known heat capacity of the mixture, the reaction temperature for the run of highest decomposition in Figure 2 is -850 K. For t = 10 ps, log A = 16.8, or 16.5 if a second Fe(CO)5molecule is consumed via recombination. This is high compared to the maximum of -16.2 calculated from transition-state theory and the expected value of 16 from known bond scissions to a diatomic plus polyatomic p r o d ~ c t . Note, ~ however, that if the temperature is actually only 50 K higher (900 K), log A is a reasonable 15.8-16.1. The cause of the high first- and apparently low second-bond dissociation energy of Fe(C0)5 is uncertain. (Both anion and cation also have low second-bond dissociation energies.lO) These results, however, correspond
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(5) McKinney, R. J.; Pensak, D. A. Inorg. Chem. 1979,18,3413. (6) Wrighton, M. Chem. Rev. 1974, 74,401. Nathanson, G.; Gitlin, B.; Rosan, A.; Yardley, J. T. J. Chem. Phys. 1981, 74,361. (7) Troe, J. J. Chem. Phys. 1977,66, 4758. (8) Day, J. P.; Pearson, R. G.; Basolo, F. J.Am. Chem. SOC.1968,90, 6933. (9) Benson, S. W. "Thermochemical Kinetics"; Wiley: New York, 1976. (10)Compton, R. N.; Stockdale, J. A. D. Int. J.Mass Spectrom. Ion Phys. 1966,22,47. Winters, R. E.; Kiser, R. W. Inorg. Chem. 1964,3,699.
1622
J. Phys. Chem. 1981, 85, 1622-1624
to a relatively unstable Fe(C0)4 (high AHf) only if the reverse recombination reaction has no significant activation energy, Le., if there is no large steric and electronic reorganization energy. Relaxation of the Fe(C0)4fragment during the bond break may contribute little energy, since calculations5 indicate the CZugeometry of singlet Fe(C0)4 and the C3"geometry likely in the Fe(C0)6 dissociation transition state lie very close in energy. Matrix experimen@ report that the ground state of Fe(C0I4is high spin (triplet), but the bond cleavage should produce a singlet. Irradiation at 1880 cm-l, and particularly at 9000-13000 cm-l, isomerizes triplet Fe(C0)4and promotes the recombination reaction,l' suggesting a reorganization energy less
than 29 kcal and possibly less than 5 kcal. The possibility of a significant electronic reorganization energy limits any thermodynamic conclusions to AHf(Fe(CO)J I -99 kcal/mol, although the important kinetic value for the first bond cleavage of 48 kcal/mol is not affected.
Acknowledgment. This work was supported, in part, by the National Science Foundation, Grant CHE-7923569. (11)Davies, B.; McNeish, A.; Poliakoff, M.; Turner, J. J. J. Am. Chem. SOC.1977,99, 7573. Barton, J. J.; Grinton, R.; Thomson, A. J.; Davies, B.: Poliakoff. M. J. Chem. SOC..Chem. Commun. 1977. 841. Davies. B.: McNeish, A.; Poliakoff, M.; Tranquille, M.; Turner, J. J. Chem. Phys: Lett. 1978, 52, 477.
Direct Observation of the Equilibrium between Allyl Radicals, O p , and Allylperoxy Radicals Rennie P. Ruiz, Kyle D. Bayes," Department of Chemistry, University of California, Los Angeles, California 90024
Martyn T. Macpherson, and Michael J. Pllling Physical Chemistry Laboratory, Oxford University, Oxford OX 1 302,United Kingdom (Received: March 17, 198 1; In Final Form: April 29, 1981)
Allyl radicals have been formed in low concentrations in the gas phase and detected with a photoionization mass spectrometer. With 0 2 also present, the form of the allyl decay suggests that an equilibrium is established between C3H5and C3H50p The relative amplitudes of the signals of free allyl and total allyl, as obtained from an analysis of the decay data, are used to calculate the equilibrium constant. Measurements on the rate of approach to equilibrium give both the forward and reverse rate constants and the ratio k l / k l gives an equilibrium constant in good agreement with that determined from amplitude measurements. Use of the equilibrium constant together with an estimated ASo for the reaction yields a C3H5-02bond energy of 17.2 f 1.0 kcal/mol. Reactions of hydrocarbon radicals with O2 are very common in combustion and air pollution chemistry.lI2 At 300 K the reaction of radical R with O2 is thought to form the peroxy radical, R02. Since the carbon-oxygen bond in R02 is weak, the ROz molecule becomes unstable toward decomposition at high temperatures. This transition from stable to unstable R 0 2 has been involked as the cause of the changeover from cool flame combustion to high-temperature cracking comb~stion.~Benson has estimated these bond energies, but they have never been measured dire~tly.~ We have been able to follow the reaction of the allyl radical with 02, reaction 1, and to observe the equilibrium
between R and ROz in the gas phase. The allyl radicals were formed by flash photolyzing 1,5-hexadiene4at 193 nm using an ArF eximer laser. The radical concentration was followed as a function of time by using a photoionization mass spectrometer, as described previ~usly.~When using the xenon resonance line at 147 nm (8.4 eV) only the allyl (1) M. F. R. Mulcahy, "Gas Kinetics", Wiley, New York, 1973. (2) K. L. Demerjian, J. A. Kerr, and J. G. Calvert, Adu. Enuiron. Sci. Technol., 4, 1 (19j4). (3) S. W. Benson, J. Am. Chem. SOC.,87, 972 (1965). (4) C. L. Currie and D. A. Ramsay, J. Chem. Phys., 45, 488 (1966). ( 5 ) E. A. Ogryzlo, R. Paltenghi, and K. D. Bayes, Znt.J. Chem. Kinet., in press. 0022-3654/81/2085-1622$01.25/0
radical contributed to the signal at m l e 41; at room temperature and with O2 present the m / e 41 signal decayed completely to the background count rate, indicating no contribution from the allylperoxy radical or from the parent 1,Bhexadiene. The allyl radical concentrations were kept low, typically l o l l ~ m - in ~ order , to simplify the kinetics by avoiding radical-radical reactions. In the absence of 02,the allyl radical signal decayed slowly with a rate equal to the rate of pumpout of the cell, approximately 10 s-l. This pumpout rate is obtained by dividing the flow rate of the gas in cms s-l at the pressure and temperature in the reaction vessel by the cell volume (52 cm3). Since the observed decay rate is equal to that calculated from the flow rate, other radical loss processes, such as reaction on the cell walls or radical-radical reactions, are not significant. In the presence of oxygen the behavior is more complex. Figure 1 shows signal decays for three different temperatures. At room temperature and with 11.5 mtorr of O2 present (Figure la), the allyl signal decays exponentially, as expected for pseudo-first-order kinetics. The rate of this exponential decay at room temperature can be increased or decreased by increasing or decreasing the O2 concentration. At 94 "C (Figure lb), the signal shows a rapid initial decay followed by a slower decay at the pumpout rate. At a temperature of 119 "C (Figure IC),the decay reverts to first order and the decay constant now corresponds to the pumpout rate, i.e., oxygen has little effect on the decay of the allyl radical. This behavior is 0 1981 American Chemical Society
