J. Phys. Chem. 1988, 92, 3869-3874 Conclusion
The UV laser photolysis of W(CO)6 in the gas phase was investigated by using a laser-based TRIS technique. Stoichiometric assignment of infrared absorption bands based on the kinetics of CO addition reactions showed that W(CO), was exclusively produced in XeF and XeCl laser photolysis and that W(CO)4 was mainly produced in KrF laser photolysis, both via a single-photon mechanism. The photolysis wavelength depen(45) Tyler, D. R.; Petrylak, D. P. J . Organomer. Chem. 1981, 212, 389. (46) Herman, H.; Grevels, F.-W.; Henne, A.; Schaffner, K. J. Phys. Chem. 1982, 86, 5151.
3869
dence of the primary fragment distribution and the internal energy content of nascent "hot" W(CO)5 are consistent with a sequential C O elimination mechanism proceeding unimolecularly through singlet ground states of W(CO)6 and W(CO),. The structural assignments deduced from IR spectrum measurements in lowtemperature matrices, C , for W(CO), and C, for W(CO)4, can be applied to these compounds in gas phase. Both W(CO), and W(CO)4 are highly reactive. They react with CO about 1 in every 10 collisions in the high-pressure limit (99.87%) was supplied by Matheson of Canada, HPLC grade tetrahydrofuran by Fisher Scientific, and reagent grade diethyl ether by Anachemia Chemicals. All ethers were extensively degassed before use.
Results Time-concentration profiles for removal of ground-state A1 atoms by reaction with DME, DEE, and T H F under pseudofirst-order conditions were recorded for different pressures of ether between 0.1 and 10 Torr. The concentration of AI atoms was monitored by saturated laser-induced fluorescence (LIF) excited by a probe pulse at various time delays following production of A1 by the photolysis pulse. All time-concentration profiles obtained could be fit through nonlinear, least-squares analysis to an expression of the form [All = A exp(-Bt) + C (1) where C represents a constant residual background of A1 atoms which remain due to the equilibrium established between AI atoms and the ether. The exponential contribution represents the rate of attainment of equilibrium following initial formation of free AI atoms in the presence of a complexing ether. The ratio CIA is required for evaluation of the equilibrium constant, as discussed below, and represents the magnitude of the background signal due to free A1 atoms with respect to the amount of A1 which is consumed in the establishment of the equilibrium condition. For all ethers studied, the parameter B displayed a linear dependence, and the ratio CIA an inverse linear dependence, upon the pressure of the reactant. The pressure-dependent behavior is shown for DEE in Figures 1 and 2. The parameter B also showed a nonlinear dependence upon the Ar buffer gas pressure which approached saturation a t higher Ar pressure, with each ether exhibiting a characteristic pressure dependence curve. These experimental observations are all consistent with a bimolecular association reaction in which an equilibrium between dissociated and complexed A1 atoms and ether exists and in which the production of a stabilized collision complex involves a third-body collision. The behavior observed here is analogous to that observed previously with certain alkenes and arenes such as tetramethylethylene and benzene,' except that room-temperature equilibria for all ethers favored the free atom to a much greater extent (Le., a weaker complex is formed with ethers than with unsaturated hydrocarbons). Furthermore, saturation in the rate of the initial exponential decay with respect to Ar buffer gas pressure was not
0
400
800
1200
1600
2000
I/P (at"') Figure 2. Plots illustrating the inverse linear dependence of the ratio CIA on the pressure of diethyl ether at temperatures between 5 and 30.5 O C . The Ar buffer gas pressure is 200 Torr for all cases.
TABLE I: Effective Bimolecular Rate Constants for A1 + Q Association Reactions at 296 K Q kQ(2)(PA,/Torr)" k600(2)b DME 0.26 f 0.07 (300) 0.426 DEE 0.90 f 0.07 (400) 1.53 THF 0.80 f 0.04 (400) 0.98
In units of cm3molecule-' s-' at the indicated pressures of Ar. Uncertainties represent two standard deviations on the least-squares slope for plots of the pseudo-first-order rate constant B vs PQ. *Bimolecular rate constants in units of cm3 molecule-' s-l at 600-Torr Ar buffer gas pressure.
achieved for any ethers studied at 600-Torr of Ar buffer gas pressure, such that limiting high-pressure rate constant values could not be obtained. Such a saturation effect was observed for several of the A1 atom reactions with unsaturated hydrocarbons.' Effective bimolecular rate constants for the reaction of A1 atoms with added ether, Q, were obtained from the slope of a plot of B, the pseudo-first-order rate constant for removal of A1 atoms, vs pressure of Q. Bimolecular rate constants obtained in this way for DME, DEE, and T H F are given in Table I under the heading kQ(2),for reaction at the pressure of Ar buffer gas noted there. Also listed are bimolecular rate constants for reaction at 600-Torr Ar pressure (k600(2)). Measurement of binding energies for Al-ether complexes using the van't Hoff relationship (discussed below) requires a knowledge of the temperature dependence of the equilibrium constant as observed from the variation in the CIA ratio with reactant pressure. Therefore, the effects of temperature variation on the ratio CIA vs the reciprocal reactant pressure (l/PQ) was studied over a 25-30 O C range. Results for diethyl ether are given in Figure 2. In all cases studied, linear plots of CIA vs 1/PQwere observed at all temperatures examined.
Kinetic Analysis The detailed kinetic analysis of reactions of A1 atoms with complexing molecules has been reported recently.' Therefore, we give here a less detailed summary of the essential aspects. Consider an equilibration process in which some initial concentration of A1 atoms, [All,, is formed and subsequently, in the presence of an excess of a free complexing ligand Q, attains equilibrium with a monoligand complex A1Q. The time dependence of A1 atom concentration is then given by
This is of the form [All = A exp(-Bt) ka[Q] and CIA = k4/kQ[Q]. Thus
+ C, where B = k, +
C/A = K J [ Q I = & / P Q
(3)
where Kc is the concentration equilibrium constant for dissociation, K, is the same in pressure units, and PQis the pressure of Q.
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3871
A1 Atom Association Complexes with Ethers TABLE II: Equilibrium Constants, Partition Functions, and Binding Energies for AIQ Complexes' A / 1030 K
