2264
J. Phys. Chem. 1985, 89, 2264-2267
aggregates. The energy required for the aggregation process is provided by the collision energy of the condensing molecules as well as the latent heat of fusion. Since such processes are obviously too complex to treat rigorously, only a simplified version of the model is subject to analysis. Moskovitz and Hulse presented a quenched reaction modelZ5 for the aggregation processes of metal clusters in low-temperature rare gas matrices. The model assumes that the products are formed during a short period after condensation while guest and noble gas molecules are still mobile. The scheme is given by the following:
The numerical solutions obtained by solving the rate equations predict that a double logarithmic plot of the absorbance of M, (relative to that of M ) vs. matrix ratio M / S gives a linear line with the slope of n - 1. The experimental plots for the ammonia clusters in the region of large M / S ratios show linear relationships with slopes 1 and 2 for the peaks at 999 and 1018 cm-I, respectively, as shown in Figure 6. Therefore the quenched reaction model seems to be applicable in the present case in the region of large M/S. The obtained slopes also provide further support for our assignment of the dimer and trimer. The experimental plots, however, show serious deviations at low M/S ratios. In the above model the dimer and trimer are assumed not to be mobile, but this assumption may not be valid because the rigidity of the matrix is lowered by collisions with the impinging molecules. Therefore, we have modified the scheme by allowing
the dimers and trimers to diffuse. The net scheme is now given by M3 *21iM2
M
r
M
M6
k/+%
*2/+.2 *I
*,p3 M4
M5
M4
z
A
-
+M3
kl
M3
*2/+.2
77 M4
-
q+M3
M5
M6
The rate equations for the monomer, dimer, and trimer are d[MI/dt = -ki[MI(2[Ml + [Mzl + [M,] + [MA + [ M d ) - kz[M1 [Mz1 - k3[M1 [M31 d[Mzl/dt = -k,[MI([Mzl - [MI) kz[M,I([M] + 2[Mz1 + [M31 + [M41) - k,[Mz1[M,1 d [ M d / d t = -ki[Ml([M,] - [Mz]) - k2[Mz]([M31 - [MI) k,[Md([Ml + [M21 + 2[M,1) The rate equations are integrated by using a Runge-Kutta algorithm. The integration is taken over a reaction time T in which aggregation processes are completed. The resultant curves obtained with T = 0.1 s, kl = 2500 M-' 8,kz = 2000 M-' &, and k3 = 1500 M-' s-l are drawn in Figure 6. The calculated curves adequately reproduce the experimentally observed trend. It is considered that the model correctly represents the aggregation processes. Registry No. NH3, 7664-41-7.
Ion Chemistry and Electron Affinity of WF6 A. A. Viggiano,*+John F. Paulson, Fred Dale, Michael Henchman,t Air Force Geophysics Laboratory, Hanscom Air Force Base, Massachusetts 01 731
N. G. Adam, and D. Smith Department of Space Research, University of Birmingham, Birmingham, England (Received: December 17, 1984)
The rate coefficients and branching ratios have been measured for the reactions of WF6 with F,C1-, Br-, I-, CN-, NO,-, NOz-, SFS-,and SF,-. WF, was found to react rapidly with these ions. Three channels were observed: association, charge transfer, and fluoride ion transfer. The reactivity of WF6-and WF; with several neutrals was also investigated. No reaction was observed for either of these ions reacting with NzO, 02,Cot, NO2,SO2,HCI, or NO. The measurements indicate that the electron affinity of WF6 is 3 . 3 6 2 t 4 eV.
Introduction The electron affinities of the third-row transition-metal hexafluorides are known to be large (3-10 eV).' However, accurate determinations of the values of the electron affinities have been difficult to make. In the case of WF, the range of reported values is from 2.75 to 5.5 eV. The techniques used to make these measurements include surface ionizatioq2 collisional ionization of WF, by fast alkali beam^,^-^ electron attachment at high temperatures: calculations based on alkaline hypochlorite hydrolysis,' and bracketing by ion chemistry.s In this paper, we report studies of the ion chemistry of WF6, WF6-, and WF,-. This chemistry is then used to derive a value of the electron affinity of WF,. 'Air Force Geophysics Scholar. AFSC-URRP Visiting Professor, 1984-1985. Permanent address: Department of Chemistry, Brandeis University, Waltham, MA 02254.
*
0022-3654/85/2089-2264$01.50/0
Experimental Section The experiments were performed on the selected ion flow tube (SIFT) at the Air Force Geophysics Laboratory and the selected ion flow drift tube (SIFDT) at the University of Birmingham. Both instruments are temperature variable. The kinetic energy dependences of the reactions of F and Br- with WF6 were studied (1) L. N. Sidorov, Russ. Chem. Rev. (Engl. Trans/.),51, 356 (1982). (2) F. M. Page and G . C. Goode, "Negative Ions and the Magnetron", Wiley, New York, 1969. (3) R. N. Compton, P. W. Reinhardt, and C. D. Cooper, J. Chem. Phys., 68, 2023 (1978). (4) B. P. Mathur, E. W. Rothe, and G.P. Reck, J . Chem. Phys., 67,377 (1977). ( 5 ) H.Dispert and K. Lacmann, Chem. Phys. Lett., 45, 311 (1977). (6) V. H. Shui, P. I. Singh, B. Kivel, and E. R. Bressel, AIAA J., 17, 1178 (1979). (7) J. Burgess and R. D. Peacock, J. Fluorine Chem., 10, 479 (1977). (8) P. M. George and J. L. Beauchamp, Chem. Phys., 36,345 (1979).
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 11, 1985 2265
Ion Chemistry and Electron Affinity of WF6 TABLE I: Reactions of WF6 reaction
10I0k. cm3 s-l 12 7.2 6.0 4.5 4.5 5.5 9.5 5.5 5.9
klk, I
TABLE II: Rate Coefficient (cm3 s-') Limits for Reactions of WFsand WF7- with Neutral Molecules
.
1" 0.80 0.89 0.83 0.72 0.76 1.2 1.o 1.1
0 2
WF7
