J . Phys. Chem. 1985,89, 569-570 to be quantitatively meaningful. In Figure 3 these results are compared to the x2I3increase expected for hard-sphere cross sections. Thus while larger clusters are generally more reactive in these elementary reactions, clusters as small as x = 4-7 appear to be more reactive when normalized to hard-sphere sizes. It is of great interest to leam whether any of these points will generalize to other reactants.
Conclusion The results presented here on reactions of free Fe clusters with small molecules demonstrate the potential for chemical probes of the nature of transition-metal clusters, and promise quantitative efforts to come. It will be especially significant to establish the similarities and differences between elementary reactive processes of metal atoms or surfaces with those of clusters, as suggested
569
here by the nonreactivity of the Fe atom. Another startling preliminary conclusion is that the Fe cluster reactivity levels off very early, and above n = 6 grows with size below the rate of increase in hard-sphere cross sections. This result may be evidence for nonbulk geometries (e.g., linear or planar) of small clusters, or is perhaps related to the important resultlo that Fe cluster ionization thresholds drop very rapidly toward the Fe work function with increasing cluster size and are within 1 eV of that value for x > 6.
Acknowledgment. The authors thank Professor R. E. Smalley for important discussions on experimental design and preliminary experiments. The contributions of K. C. Reichmann and R. 0. Brickman in developing the secondary valve/reactor are also gratefully acknowledged.
Dynamics of Hydrogen Atom Abstraction in the Reaction O(3P)
+ Ethanol
N. J. Dutton, I. W. Fletcher, and J. C. Whitehead* Department of Chemistry, University of Manchester, Manchester. MI 3 9PL, U.K. (Received: November 12, 1984) The reactions O(3P) + C2H,0H and C2H50Dhave been studied by measuring the OH and OD product internal state distributions by laser-induced fluorescence under crossed-molecular beam conditions. Hydrogen atom abstraction takes place from both C-H and 0-H bonds. The amount of OH rotational excitation is low (-4%) indicating that the reaction dynamics are direct and collinear approach of the oxygen atom to the bond under attack is favored. OH in u = 1 can only be formed by abstraction from an ethyl proton and the amount of vibrational excitation observed (u(u=l)/u(u=O) = 0.22) is typical of abstraction from a secondary C-H bond.
Introduction Considerable insight has recently been obtained into the dynamics of the hydrogen atom abstraction reactions of ground-state oxygen atoms, O(3P), with organic molecules by combining the crossed-molecular beam technique which is necessary to provide sufficient translational energy to overcome the reaction activation energies with laser-induced fluorescence detection as a means of determining the product OH internal state distributions. Studies have been made of the reactions of O(3P) with saturated,' unsaturated,2 and cyclic3 hydrocarbons, amine^,^ and aldehyde^.^ In this Letter, we report results for the reaction of ground-state oxygen atoms with ethanol. There are two possible abstraction channels:
-
0 + C2H50H
C 2 H 4 0 H+ OH AHr = -52kJ mol-'
0 + C2H50H
+ C 2 H 5 0+ OH
AHr
-
0 kJmol-'
(1) (2)
For reaction 1, there are two different types of C-H bond and the quoted reaction exoergicity relates to abstraction of the weaker a-bonded hydrogen6 Kinetic studies'J in the temperature range 300-430 K indicate that abstraction of the a-bonded hydrogen is the dominant process and that reaction proceeds with an activation energy of -6 kJ mol-' and a preexponential factor of -7 X cm3 molecule-' s-I. A further studyg at a higher temperature (923 K) found that the reactivities of the various sites (1) P. Andresen and A. C. Luntz, J. Chem. Phys., 72, 5842 (1980). (2) K. Kleinermanns and A. C. Luntz, J . Chem. Phys., 77, 3533 (1982). (3) N. J. Dutton, I. W. Fletcher, and J. C. Whitehead, Mol. Phys., 52,475 (1984). (4) K. Kleinermanns and A. C. Luntz, J. Chem. Phys., 77, 3537 (1982). (5) K. Kleinermanns and A. C. Luntz, J. Chem. Phys., 77, 3774 (1982). (6) J. A. Kerr, Chem. Reu., 66, 465 (1966). (7) N. Washida, J . Chem. Phys., 75, 2715 (1981). (8) C. M. Owens and J. M. Roscoe, Can. J . Chem., 54, 984 (1976). (9) Z. G. Dzotsenidze, K. T. Oganesyan, G. A. Sachyan, and A. Nalbandyan, Arm. Khim. Z h . , 20, 983 (1967).
in ethanol were in the ratio 3.3:2.8:1.0 for CH3:CH2:OHand that abstraction of the hydroxyl proton was associated with an activation energy of -23 kJ mol-' and a preexponential factor of -3 x 1O-" cm3 molecule-' s-I. We have studied the reaction O(3P) + C 2 H 5 0 Hin a crossedmolecular beam experiment at a collision energy of 30 kJ mol-' with laser-induced fluorescence detection of the OH product. Experiments have also been performed with C2H50Dto determine the relative rates of abstraction from the C-H and 0-H bonds.
Experimental Section The crossed-molecular beam laser-fluorescence apparatus used in these experiments has been described in detail el~ewhere.~ A supersonic beam of oxygen atoms produced by microwave discharge of a high-pressure (-80 mbar) mixture of 17% O2in He was crossed by a supersonic beam of ethanol seeded in hydrogen. This beam was prepared by flowing hydrogen over ethanol at 273 K to give a total source stagnation pressure of -400 mbar with the nozzle heated to -350 K to eliminate the formation of ethanol complexes or clusters. The peak velocities of the beams, vpk, and the widths of their velocity distributions (fwhm A v / v ) were 1850 m s-I, 20% wide for the 0 atom beam and 1270 m ,I'[ 22% wide for the ethanol beam. This gives a center-of-mass collision energy of 30 kJ mol-'. Ethanol was obtained from James Burrough (Analar grade > 99.7%) and C2H50Dfrom Aldrich (stated purity > 99.5%). The isotopic purity of the C 2 H 5 0 D sample was confirmed by NMR spectroscopy, and the purity of the beams was checked by direct observation with a quadrupole mass spectrometer. The OH and O D products were detected by recording the laser-induced fluorescence spectra of their A2.Z X211systems in the region 306-314 nm by using the frequency-doubled output of a nitrogen-pumped dye laser system operating at -73 s-I. The reactive O H fluorescence signal was -0.06 counts per laser shot at the peak and the noise which was mainly laser scattered light was of a similar magnitude. The O H laser-induced fluorescence
0022-3654/85/2089-0569$01.50/00 1985 American Chemical Society
-
-
570 The Journal of Physical Chemistry, Vol. 89, No. 4, 1985
Letters from a C-H bond (reaction 1) and not from the 0-H bond. The value obtained for the vibrational partitioning in the OH product is typical of that for abstraction from a secondary C-H as might be expected if abstraction takes place from the cy C-H bond in C2H50H. In order to determine the relative importance of the two possible abstraction sites in C2H50H,experiments were performed on the system O(3P) C2H50D. Signal was observed from both O H and OD, indicating that abstraction takes place from both the ethyl protons and the hydroxyl proton. By measuring the relative OH and OD laser-induced fluorescence signals and knowing the relative O H and O D transition probabilities,]' it is possible to determine the relative reactivities of the ethyl and hydroxyl 2.2. This is to be compared with the protons, u(OD)/a(OH) room temperature value of -0.027 and the 923 K value of 0.16.9 This increasing trend can be rationalized by noting that abstraction of the ethyl protons has a low activation energy and a relatively low preexponential factor7 whereas abstraction of the hydroxyl proton has a higher activation energy but a much higher preexponential f a ~ t o r .In ~ our beams experiment, the collision energy is greater than the activation energies for both processes and abstraction of the hydroxyl proton (reaction 2) will then be expected to dominate because of its higher preexponential factor. Indeed, in the high-energy limit, the ratio of the reactivities at the two sites will be given by the ratio of the two preexponential factors (u(OD)/a(OH) 40). Abstraction of both the ethyl and hydroxyl protons is also found in the reactions of F C2HsOD (a(OD)/u(OH) 0.32)12 and of O*('D) C 2 H 5 0 D (u(OD)/a(OH) 1.9).13 In the latter case, abstraction takes place by insertion into the C-H or 0-H bond rather than by direct collinear attack as seen for the reaction of O(3P).
+
-
i
1
I
I
I
1 2 3 4
1
lx\J-a-=
I
5 6 7 8 910
R o t a t i o n a l Quantum Number, N Figure 1. The relative OH rotational cross sections, a(N,o=O) for the reaction OOP) + C2H50H. The dashed line represents the mean OH rotational distribution from all previous studies (ref 1-5).
signals were converted into relative cross sections for each O H state in the usual ways3 Under similar conditions, no OH was detectable from the reaction O(3P) methanol, but comparable signals were detected for the reactions O(3P) propan-2-01 and 2-methylpropan-2-01.
+
+
Results and Discussion The OH rotational state distribution for the reaction 0 + C 2 H 5 0 His shown in Figure 1. It shows a form similar to that obtained for OH from the reactions of O(3P) with other organic mole cue^'-^ having a monotonic decrease from N = 1, although it is somewhat broader than the previous distributions (dashed line in Figure 1). Correspondingly, the mean OH rotational energy at 3.7 kJ mol-' is a little larger than the average for the other reactions (3.4 kJ mol-'). Nevertheless, the amount of rotational excitation is still very small (-4% of the total energy available for reaction 1). This is characteristic of a direct collinear ab(A straction mechanism as has been noted in previous recent theoretical calculationlo has confirmed a collinear transition state for the abstraction channel in O(3P) + H2CO). Clearly in this case OH can arise by abstraction from both a C-H and an 0-H bond (reactions 1 and 2) and it is possible that the increased breadth of the rotational distribution is due to a contribution from the second reaction pathway. The possible sites for abstraction will be discused below in connection with the results for 0 + C2H5OD. OH is produced in both its ground and first vibrationally excited states with a relative cross section of u ( u = l ) / u ( u = O ) = 0.22. Energetically, O H in u = 1 can only be produced by abstraction (10) M. Dupuis and W. A. Lester, J . Chem. Phys., 81, 847 (1984).
--
-
+
+
Conclusions In the reactions OOP) + C 2 H 5 0 Hand C2H50D rotational distributions were measured for O H in u = 0 and 1 and for OD in u = 0. Hydrogen atom abstraction takes place from both C-H and 0-H bonds. The amount of rotational excitation is very small (-4% of the total available) and indicates that abstraction is a direct process and takes place only for collinear configurations. Energetically, OH(u= 1) can only be formed by abstraction of an ethyl proton and the observed population (a(o=l)/a(u=O) = 0.22) is typical for abstraction from a secondary C-H bond. Abstraction of the hydroxyl proton is about twice as favorable as abstraction of the ethyl protons at the energies of the beam experiment (a translational temperature of 3500 K). Thus in high-temperature combustion systems subsequent reactions of ethoxy radicals will be as important as those of ethanol radicals (C2H40H).
-
Acknowledgment. This work was supported by the Science and Engineering Research Council. ~
~~
~~~
(11) W. L. Dimpfl and J. L. Kinsey, J . Quam. Spectrosc. Radiat. Transfer, 21, 233 (1979). (12) B. Dill, B. Hildebrandt, H. Vanni, and H. Heydtmann, Chem. Phys., 58, 163 (1981). (13) N. Goldstein and J. R. Wiesenfeld, J . Chem. Phys., 78, 6725 (1983).
