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J . Phys. Chem. 1984,88,4494-4497
of both F and C1 compared to Xe, we believe the same result would be obtained for CsCl as well. Planar three-dimensional trajectory calculations9 of Xe colliding with CsF, CsC1, and CsBr demonstrate the formation of CsXe’ at Erel= Eo with zero initial internal energy in the cesium halide. Collisions of Xe with CsBr, in particular, showed a high propensity for CsXe+ formation over a wide range of collision parameters, which correlates well with the particularly large cross section for CsXe+ formation obtained experimentally for this system. This suggests that the statistical assumption is most likely valid in this case as well. Trajectory calculation on CsI yielded no CsXe+ formation under the same initial conditions in agreement with the experimental results.’
Results The bond dissociation energies of the cesium halides derived from the above analysis are given in Table I, along with values obtained from several other techniques. The parameters in eq 2 required to convert Eo into Do are Zc8= 3.893 eV and the electron affinities of the four halogens which are 3.399, 3.615, 3.364, and 3.061 eV for F, C1, Br, and I atoms, respectively.1° For CsF three values of Eo were obtained from Ar, Kr, and Xe collisions; they are 5.83, 5.79, and 5.81 eV, re~pectively.~ The average was used to determine Do in Table I. For CsCl the three values of Eo were 4.87, 4.85, and 4.85 eV for Ar, Kr, and Xe, respe~tively.~For CsBr the step behavior at threshold was only observed for Xe collisions, and thus only a single value of Eoequal to 4.53 eV was ~ b t a i n e d . As ~ mentioned earlier, the bond energy of CsI was obtained from reaction la, since the step behavior of the CsXe’ cross section in this case was strongly distorted due to threshold effects involving the CsI internal energy! An analysis of reaction l a for the other three halides would give Eo values which are entirely consistent with those derived from reaction 2a, but with larger error limits. The error limits given in Table I reflect a combination of curve-fitting errors coupled with an -0.5% error limit in the measurement of the velocity of the projectile. The assigned error
limit of 0.08 eV is an absolute error limit. The relative errors between the different cesium halides are likely to be somewhat smaller. The present experiments are in excellent agreement with the photofragmentation measurements of Su and rile^.^-^ The photofragmentation method is analogous to the collisional dissociation method, with a photon providing the energy input rather than the rare gas atom collision. The bond energy is obtained in the former not by measuring the threshold for dissociation (to neutrals in this case) but by measuring the relative translational energy of the dissociated Cs and X atoms coupled with the photon energy. Also included in Table I is a calculation of the bond energies from the T-Rittner model of Brummer and Karp1~s.l~(The T-Rittner model yields Eo,from which one calculates Do via eq 2 . ) The input to the model includes the CsX equilibrium internuclear separations and vibrational frequencies given by Huber and Herzberg,14the ion polarizabilities given by Coker,15and the halogen electron affinities given above. The coefficient of the R6 term in the T-Rittner model was obtained with the London approximation (see ref 13). The average difference between the measured and calculated values for Eo is 2.2%.
Acknowledgment. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US. Department of Energy, under Contract W-31109-Eng-38. Registry No. CsF, 13400-13-0; CsC1, 7647-17-8; CsBr, 7787-69-1; CSI, 7789-17-5. (13) P. Brummer and M. Karplus, J. Chem. Phys., 58, 3903 (1973). (14) K. P. Huber and G. Herzberg, “Molecular Spectra and Molecular Structure”, Van Nostrand-Reinhold, New York, 1978. (15) H. Coker, J . Phys. Chem., 80, 2078 (1976). (16) L. Brewer and E. Brackett, Chem. Reu., 61,425 (1961). (17) J. Berkowitz, J . Chem. Phys., 50, 3503 (1969); Adu. High Temp. Chem., 3, 158 (1971).
Dynamics of the Chemlionization Reaction of Antimony Pentafluoride J. A. Russell, J. F. Hershberger, J. J. McAndrew, R. J. Cross,* and M. Saunders* Department of Chemistry, Yale University, New Haven, Connecticut 0651I (Received: July 1 I , 1983)
-
-
Using crossed molecular beams we have measured the product angular and energy distributions for SbFs + C6H5COC1 SbF5CI- + C6H5COtand for SbzFlo+ C6H5CHzC1 SbF6- + C7H7++ SbF,Cl. In both cases the product distributions are symmetric about the center of mass which indicates that the reaction proceeds by way of a long-lived collision complex. However, in the first reaction, this symmetry appears to be broken at the highest energy studied so that the reaction becomes direct at higher energies. The first reaction has a threshold at roughly 2.9 eV.
Introduction In the past 20 years much has been learned about how a chemically reactive collision occurs in detail,’ how bonds are made and broken, and how energy is transferred from one part of the system to another during the reaction. Most of these studies have necesarily focused on the simple systems of three or four atoms or atomlike groups. However, most chemical reactions are more complicated than this. The potential-energy surfaces have a higher dimensionality and may exhibit complicated features not seen in simpler systems. Therefore, we must examine more complicated systems to see how these basic ideas need to be extended. Fur(1) D. R. Herschbach, Faraday Discuss. Chem. Soc., 55,233 (1973). R. B. Bernstein, “Chemical Dynamics via Molecular Beam and Laser Techniques”, Oxford University Press, Oxford, 1982.
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thermore, the organic and inorganic chemists will not seriously think in terms of chemical dynamics until chemical reactions of interest to them are successfully studied by modern techniques in the field of dynamics. With these goals in mind, we have been using crossed molecular beams to study organic reactions.24 We have recently investigated the halide abstraction reactions of antimony pentafluoride and its polymers3r4 SbF5
+ RX
-
SbF5X- + R+
(1)
~~~
(2) K. T. Alben, A. Auerbach, W. M. Ollison, J. Weiner, and R. J. Cross, 1.Am. Chem. SOC.,100, 3274 (1978). (3) A. Auerbach, R. J. Cross, and M. Saunders, J . Am. Chem. Soc., 100, 4908 (1978). (4) L. Lee, J. A. Russell, R. T. M. Su, R. J. Cross, and M. Saunders, J. Am. Chem. Soc., 103, 5031 (1981).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol, 88, No. 20, 1984 4495
Chemiionization of Antimony Pentafluoride
3
N 7
!, I !
'7
Sh
Figure 1. Schematic of the central portion of the apparatus: C, grid cage; G1 and G2, extraction and energy analyzer grids; MS, to mass spectrometer;N, nozzle; S, skimmer; Sh, shield plate. The ionizer filament is above the beams with electrons traveling down through the beam
intersection region. where R is an organic or inorganic group. The reaction has long been used to produce organic cations in solution for spectroscopic and kinetic studies., Many of these cations are known to be intermediates in important organic reactions. Arnett6 has measured the heats of reaction for (1) in various solvents. We have recently shown that the reaction occurs in the gas phase and have sketched out the basic chemistry involved. Monomer SbF, reacts as in (1). SbF5 is known to form dimers and higher polymers in the gas phase.' The dimers react by a more complicated reaction
+
-
Sb2Flo R F
+
SbzFlo RX
-
Sb2Fll-
SbF6-
+ R"
+ SbF4X + Rf
(3)
Fluorides react to form the well-known and stable ion Sb2F11-. For some reason, the analogue of this ion with one F replaced by C1, Br, or I is much less stable, and (3) is the preferred pathway. However, we have recently found Sb2FloC1-as a minor product so that the ion is not actually unstable. We have seen reactions with trimer, but they are difficult to study since large amounts of SbF5 are used up in the process. The reactions of monomer and dimer do not appear to be very different chemically, since all substances which react with monomer also react with the dimer. Rather, the dimer reaction appears to have a lower threshold energy so that the dimer is more reactive. As one step in the study of the detailed dynamics, we report here the angular and velocity distributions of the products of (1) and (3). These were obtained by conventional crossed molecular beams. The reactions are particularly well suited for such a study because both reactants are stable molecules so that intense supersonic nozzle beams of each can be made. Yet the products are ions which can be detected very efficiently.
Experimental Section Most of the details of the experiment are contained in a previous paper4 (hereafter referred to as I). The center of the beam machine is shown in Figure 1. The two nozzle beams are produced in separately pumped chambers and intersect at right angles. The detector region is attached to the rotatable lid of the vacuum chamber, and the reaction takes place in the grid cage. For these (5) G. A. Olah, E. B. Baker, J. C. Evans, W. S. Tolgyes, J. s. M c W r e , and I. J. Bastien, J. Am. Chem. SOC., 86, 1360 (1964). M. Saunders, P. Vogel, E. L. Hagen, and J. Rosenfeld, Acc. Chem. Res., 6, 53 (1973). (6) E. M. Arnett and D. Petro, J . ~ m Chem. . SOC., 100, 5402 (1978). (7) M.J. Vasile, G . R. Jones, and W. E. Falconer, Chem. Commun., 1355 (1971). J. Fawcett, A. J. Hewitt, J. H. Holloway, and M. A. Stephen, J . Chem. SOC.,Dalton Trans., 23, 2422 (1976).
experiments the grid on the first plate of the lens system is held at the cage potential so that the product ions are formed in a field-free region. Ions at the selected angle pass through the retarding grid on plate 2, go through an ion lens, and are mass selected by a quadrupole mass spectrometer. The ions are detected by an electron multiplier which is used in a pulse counting mode. By rotating the lid we can take the angular distribution of the product ions, and, by varying the voltage on the retarding plate, we can measure the distribution in translational energies. All the plate voltages can easily be reversed so that we can study both positive and negative ions. The production of a beam of antimony pentafluoride is difficult. Those wishing to attempt it should consult the details in I. Briefly, the carrier gas (He or H,) is bubbled through liquid SbF,. The resulting vapor is passed through a second trap whose temperature determines the partial pressure of SbF, in the beam. The vapor is then sent through glass, stainless steel, and teflon tubing to the glass nozzle. By controlling the partial pressure of SbF, and the temperature of the nozzle, we can vary the monomer/dimer ratio over a wide range, although we have no quantitative measure of it. The crossed beam is made by injecting liquid RX through a rubber septum into the flowing carrier gas (see I). In a typical experiment we stabilize the beams, and tune the various plate voltages. First, we take an angular distribution with the voltage on the retarding plate at the cage potential so all ions pass through it. This takes only a few minutes during which time the beam intensities are stable. Then we sit at each angle and measure the energy distribution by scanning the voltage on the retarding plate. This voltage is controlled by a PDP 11/23 computer which also records the number of product ions detected. The computer sweeps the voltage several times and then smooths and differentiates the retarding curve to obtain the product intensity vs. LAB translational energy. The angular and energy distributions are a LAB differential cross section ZLAB(Etr,e,@)dE,, dnLAB. By dividing ZLAB by the LAB velocity, vUB, we obtain a Cartesian probability in volocity,8 Pc(vx,vy,v~)dv, dvYdvz, the probability of finding a product ion in the region around (vx, vy, vz). Because the Jacobian for a translation and rotation of the coordinate system is unity, Pc is also the Cartesian probability in the center-of-mass system. The conventional center-of-mass differential cross section is ZCM(U,O,6)
=
u2pC(uxiuy,uz)
= u2pC(~x,vy,~z)
(4)
where u is the product velocity referred to an origin at the center it must of mass, u = v vCM. Pc has all the symmetries of IcM; be axially symmetric about the relative velocity, v,.. If the reaction proceeds by way of a long-lived complex, Pc must have a plane of symmetry through the center of mass perpendicular to v,,. In our case, as in most others, there is a distribution of beam velocities and therefore a distribution of centers of mass. The calculation of ZCM requires that one knows the position of the center of mass precisely, and by multiplying by u2, the distribution goes to zero at the assumed center of mass. To avoid these problems, we have plotted our data as Cartesian probabilities.
+
Results Figures 2 and 3 show Cartesian distributions for the reaction of benzoyl chloride and monomer SbF,
+
C6H~COCl SbFs
+
C6H5COt
+SbFQ
(5)
Each contour is a line of constant Pc(ux,uy,uz). The horizontal axis is the relative velocity vrel, and the X is the position of the center of mass. The dot is the maximum in Pc (100% contour) with the succeeding contours at 90%, 80%, ..., 10% of the maximum. For low energies the negative ion suectrum shows a moderate amount of S6F6-which Tesults from-the reaction with Sb,Flo. Because the threshold for reaction with dimer is lower than that with monomer, the reactive cross section for the dimer is much higher than that with monomer just above the monomer (8) R. Wolfgang and R. J. Cross, Jr., J . Phys. Chem., 73, 743 (1969).
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The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
Russell et al.
Figure 4. Positive and negative ion Cartesian product contours for reaction 6 at 5.3 eV.
Figure 2. Negative ion Cartesian contours of product intensity for reaction 5 with relative energies of 3.3-6.8 eV: x, center of mass; *, peak of product intensity. The contours are at 10% intervals with the peak being 100% and the next at 90%, 80%, ..., 10%. The abscissa is the relative velocity vector with the SbFS beam traveling to the right.
t
I
5 10
crn/sec
roughly true as can be seen in Figure 3. These facts show that the products are formed in reaction 5 as written and not in a more complex mechanism or by reactions with polymers in either beam. The contours at the lower energies show a reflection plane of symmetry around the center of mass. This strongly indicative that the reaction proceeds by way of a long-lived complex. The data at high energy suggest that this symmetry is lost with the SbF,Clproduct moving toward the SbF5beam and the R+ product toward the RX beam. This is not unexpected since the lifetime of the complex decreases as its energy is raised, and, eventually the lifetime becomes comparable to a rotational period and the forward/backward symmetry is no longer observed. We can get a rough estimate on A H for the reaction. The 20% contour for the product at 3.3 eV corresponds to a final relative translational energy of 0.4 eV and thus a AH of 2.9 eV. Much of the spread of the contour may be due to the distributions in angle and velocity of the two beams so that the true final energy may be less than 0.4 eV, but the products probably have some internal energy, and this would decrease AH. Figure 4 shows a pair of contours for the dimer reaction with benzyl chloride C6H5CH2Cl
E,,I
=6.8 e V
Figure 3. Positive and negative ion Cartesian product contours for reaction 5 at 6.8 eV.
threshold. Thus the dimer reaction may contribute significantly even though the fraction of the dimer in the beam is very small. Therefore, we show only the negative ion distributions at low energies. At higher energies, the dimer contribution is negligible, and Figure 3 shows the distributions for both ions. The beam velocities are calculated by using standard methodsg and include Anderson's slip corre~tion.~ We have checked these calculations by mounting a filament above the beam center to ionize each beam and measure the angular and energy profiles of it. The measured and calculated velocities differ by 10% at most. A further confirmation of this is the axial symmetry about v,, which is shown in the contour diagrams. Conservation of momentum predicts that the product momenta in the center-of-mass system should be equal and opposite. The distribution of one product can be obtained by reflecting that for the other product through the center of mass and multiplying by the ratio of masses. This is at least
+ SbzFl0
B. Anderson, Gasdynamics (N.Y.),4, 1 (1974).
+ SbFC + SbF4CI
Discussion The data in Figure 3 show that reaction 5 goes as written rather than by way of a more complicated pathway or by way of reaction with polymers of SbF,. The product distributions indicate that the reaction proceeds by way of a long-lived collision complex at the lower energies. At higher energies products appear to be moving away from forward/backward symmetry about the center of mass so that by 6.8 eV the lifetime of the complex has decreased to a time comparable to the rotation period. It is also apparent from Figure 2 that the shape of the distribution changes as the relative energy increases. At 3.3 eV it is roughly spherically symmetric about the center of mass. As the energy is increased, it becomes more and more elongated. The basic model of a long-lived complex assumes that most of the detailed information in the reactants is randomized in the complex so that only the total energy and the total angular momentum are carried over from reactants to products. The total angular momentum d = J, J, L. The rotational angular momenta of the reactants J1 and J2 are presumed to be small due to the
+ +
(9) J.
C7H7'
(6) Again the beam velocities are calculated, but, in this case, there is a larger uncertainty in the velocity of the Sb2FIobecause there is a large slip correction for a molecule this massive. The curves are axially symmetric about vreI and are roughly symmetric about the center of mass. While one might quibble about how symmetric the data are, the indications are that the reaction complex has a lifetime at least comparable to its rotational period. -+
J. Phys. Chem. 1984, 88, 4497-4502 rotational cooling of the nozzle expansion and are isotropically distributed, since there is no preferred orientation. The orbital angular momentum is given by L = pr X vrel = p b X vrel, where p is the reduced mass and b is the impact parameter. L must be perpendicular to vrel, and thus the distribution in L has only cylindrical symmetry about vrel. Our data indicate that the cross section for the reaction drops rapidly as the energy is decreased toward the threshold. We hope to measure this explicitly in the near future. At low energies the reaction takes place at small b (small L), and the distribution in d will be roughly spherically symmetric and so will the product distribution. As the energy is raised, the reaction takes place at larger b so that the distribution in J is constrained to be more nearly perpendicular to vreI. If the complex dissociates so that a large fraction of d goes into the final orbital angular momentum L', then L' will be roughly perpendicular to vRI. The final relative velocity vie{ must be perpendicular to L'. A careful analysis1° of the probabilities shows that, in the limit where L' = L, the C M cross section will be proportional to (sin e')-', where 8' is the angle between vreI and vre,', that is, Pc will be largest near vIel, as we observe at higher energies. It is no great surprise that the reaction occurs by way of a long-lived complex because the product ions are attracted by a strong Coulombic force. Using a model of spherical ions with radii of 2 A each, with the charge localized at the centers, we find that the Coulomb well is roughly 3.5 eV deep. Because of the long range of the Coulomb potential, there can be no centrifugal barrier in the exit channel. It is not likely that there is a chemical (electronic) barrier or surface crossing either. It is not clear yet what the surface looks like on the entrance side of the well. We find that the reaction is rougly 3-eV endothermic from reactants to products which would indicate that the reaction is exothermic (10) W. B. Miller, S. A. Safron, and D. R. Herschbach, Discuss. Faraday SOC.,44, 108 (1967).
4497
from reactants to the bottom of the well. This is substantiated by Arnett's measurements in solution.6 When the product ions are solvated, the dissociation energy of the complex is much less than 3.5 eV, and the overall reaction is exothermic for tertiary and secondary carbocations and endothermic for primary ions. The measure of product angular and energy distributions gives information on the product side of the potential-energy surface. There are several beam experiments which probe the reactant side. One of these is the measurement of the cross section vs. translational and vibrational energy of the reactants. We have done similar experiments on another system and hope to start them on the present system soon. We have additional, preliminary evidence that an ionic-ovalent surface crossing occurs in the reactant side of the surface. We have found several organic bases which undergo charge exchange with SbF,
SbFS + B
-
SbF5-
+ B+
(7)
In a few cases the charge exchange and the halide abstraction reaction both occur. This strongly suggests that the reaction starts with an electron jump to form an ion pair (RX+)(SbF,-) held in a Coulomb well. This may dissociate back to reactants or to the ion pair, or it may exchange a halogen to give R+ and SbF5X-. In the case of benzyl and benzoyl chloride the R+ ion is very stable, and (1) and (3) are the only reactions seen. The reactions of SbF5 and organic halides form one of the few examples of a gas-phase chemiionization reaction that involves a chemical exchange, rather than just charge transfer. The reactions exhibit complicated, yet tractable dynamics.
Acknowledgment. Research support from the National Science Foundation under Grants CHE-7826137 and CHE-8201164 is gratefully acknowledged. Registry No. SbF5, 7783-70-2; C6HSCOC1,98-88-4; Sb2F,,,, 8904329-8; C6H5CHSC1, 100-44-7.
Photoionization of Isolated Nickel Atom Clusters E. A. Rohlfing, D. M. Cox, and A. Kaldor* Corporate Research-Science Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: March 23, 1983)
We have used a combination of photoionization and time-of-flight mass spectrometry to obtain a coarse-grained measure of the ionization potential of nickel clusters (Nix) for n = 2-23 as a function on ionizing laser intensity and frequency. The IPSof Nix do not monotonically approach the work function as x increases but exhibit an oscillation as a function of cluster size. Changes in the intensity dependences of the Nix+ signals are interpreted with the aid of two proposed mechanisms.
Introduction Considerable research, both theoretical and experimental, has been carried out to characterize reactions with, by, and on supported transition-metals clusters.' However, the intrinsic properties of isolated naked metal atoms clusters (M,) in the size range from two to several hundred atoms have been elusive to obtain. This size range is of considerable scientific importance because the electronic, structural, magnetic, and chemical properties are expected to change from predominately molecular to bulk in character as the clusters increase in size. Most experimental work on bare metal atom clusters to date has, of necessity, been focused upon low boiling point (e2000 "C) metals (alkalies,2 Pb,'Sb? ( 1 ) T. N. Rhcdin and G. Ertl, Ed., "The Nature of the Surface Chemical Bond", North Holland, Amsterdam, 1979.
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14) since they can be easily vaporized in a hot oven and formed into a molecular beam for subsequent study. More recently metal cluster beams for somewhat higher boiling point (e3000 "C) metals have been produced by using an improved design5 for a (2) ):( E. Schumacher, W. H. Gerber, H. P. Harri, M. Hoffman, and E. Scholl, Metal Bonding and Interactions in High Temperature Systems", J. L. Gole and W. C. Stwalley, Ed., American Chemical Society, Washington, DC, 1982, ACS Symp. Ser. No 179, p 83. (b) A. Hermann, E. Schumacher, and L. Woste, Helu. Chim. Acta, 61, 453 (1978). (c) M. M. Kappes, R. W. Kung, and E. Schumacher, Chem. Phys. Lett., 91,413 (1982), and references therein. (3) K. Sattler, J. Muhlbach, and E. Rechnagel, Phys. Reu. Left.,45, 821 (1980). (4) A. Hoareau, B. Cabaud, and P. Melinon, Surf.Sci., 106, 195 (1981). ( 5 ) S. J. Riley, E. K. Parks, C. R. Mao, L. G. Dobo, and S.Wexler, J . Phys. Chem., 86, 3911 (1982).
0 1984 American Chemical Society
