15708
J. Phys. Chem. 1996, 100, 15708-15715
Photodissociation of Cu2NH3 and Ag2NH3† S. A. Mitchell,* L. Lian, D. M. Rayner, and P. A. Hackett Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario, K1A 0R6, Canada ReceiVed: April 24, 1996; In Final Form: July 24, 1996X
We report on the reactions of copper and silver dimers with ammonia in the gas phase near room temperature and on photodissociation of the 1:1 complexes. Photodissociation action spectra in the range 295-360 nm and emission spectra of metal dimers produced by photodissociation are described. The binding energies of NH3 on Cu2 and Ag2 have been determined, and spectroscopic data and theoretical results have been used to infer a quasi-linear or end-on coordination geometry for the complexes. A simple picture of the bonding mechanism is presented, and comparisons are made with ammonia adsorbed on copper and silver atoms and bulk surfaces.
Introduction
Experimental Section
Complexes (1:1) of bare metal clusters with simple molecules are important model systems for fundamental studies of metalligand bonding in cluster and surface chemistry. Of particular interest is the variation in kinetic and thermochemical parameters of cluster reactions with cluster size, and the relationship between cluster chemistry and bulk surface chemistry. One approach that appears promising is to focus on very small metal clusters, including atoms, dimers, and trimers, to build up our understanding of the effects of increasing cluster size in a systematic manner. The application of spectroscopic methods in this area is difficult due to problems of preparation and detection of specific cluster complexes. In this work, we have obtained photodissociation action spectra of Cu2NH3 and Ag2NH3. Clusterspecific spectra were obtained by using sample preparation conditions that led to the exclusive formation of monoligand complexes and by detection of known emission spectra of excited metal dimer photofragments. In related work from this laboratory, the pulsed-field ionization-zero electron kinetic energy (PFI-ZEKE) spectrum of Nb3O was reported, which led to a definitive structural assignment of the complex.1 The spectra reported for Cu2NH3 and Ag2NH3 are broad and featureless, but they carry information on the energetics of ammonia ligand binding on the dimers and on photodissociation dynamics. They also provide indirect information on the structures of the complexes, which are quasi-linear with endon coordination of ammonia on the metal dimer. In previous reports, we described kinetic studies of the reactions of Cu2,2 Ag2,3 and Mo24 with ammonia. We have also reported on infrared multiphoton dissociation of Ag2NH3.5 Recently, the electronic structure and properties of Cu2NH36,7 and Ag2NH35,7,8 have been the subject of theoretical work. In this paper, we describe new measurements of the binding energies of ammonia on Cu2 and Ag2 and comparisons with theoretical estimates. The nature of the bonding of ammonia on coinage metal atoms and dimers is discussed from a thermochemical perspective and in terms of qualitative molecular orbital concepts.
The flow-tube reactor with a laser vaporization cluster source and a laser-induced fluorescence (LIF) detector has been described in detail previously.4 Copper or silver dimers were produced in a narrow channel (0.2 × 4.0 cm) in the presence of ∼50 Torr of helium at room temperature, following pulsed 308 nm vaporization at the surface of a rotating copper or silver rod. Metal dimers were entrained in a steady flow of helium (15 000 sccm) and thermalized to the temperature of the 7.5 cm diameter flow tube by ∼104 collisions with helium prior to entering the reaction zone 61 cm downstream from the source. The temperature ((297-353) ( 2 K) and pressure ((0.45-8.0) ( 0.02 Torr) of the reaction zone of length 72 cm were monitored continuously. Ammonia gas was introduced through a perforated inlet ring at the entrance of the reaction zone and monitored by using a mass flow controller (0-146 sccm). Reaction of metal dimers with ammonia was observed by monitoring the depletion of the dimer LIF signal as a function of ammonia pressure in the reaction zone. Cu2 was monitored by excitation at 454.65 nm (B-X, 1-0),2 and Ag2 was monitored by excitation at 429.36 nm (A-X, 2-0).3 Copper and silver atoms were monitored by excitation at 324.754 (Cu 4P-4S) and 328.068 nm (Ag 5P-5S). The excitation laser pulse was timed to coincide with the peak of the arrival time distribution of atoms or dimers at the exit of the flow tube. The transit time between the vaporization and detection points was in the range 5.585.5 ms for total pressure in the flow tube in the range 0.458.0 Torr.4 Fluorescence emission spectra were recorded by using a scanning monochromator (ISA Model H-20, focal length ) 20 cm) and photomultiplier tube (PMT). The spectral band pass of the monochromator was 4 nm FWHM. Photodissociation action spectra of Cu2NH3 and Ag2NH3 were recorded by monitoring the spontaneous emission signal of Cu2 or Ag2 photofragments and scanning the excitation wavelength in the range 295-365 nm, using a frequency-doubled, excimer laser pumped dye laser (Lumonics HD-300). All such action spectra were normalized to the excitation laser pulse energy to correct for variations in laser energy with wavelength across several dye tuning ranges. The signals due to fluorescence emission and laser pulse energy were averaged by using boxcar integrators and then digitized for storage and analysis with a laboratory computer. Time-resolved fluorescence decays were observed
† X
Issued as NRCC No. 39097. Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01181-1 CCC: $12.00
Published 1996 by the American Chemical Society
Photodissociation of Cu2NH3 and Ag2NH3
J. Phys. Chem., Vol. 100, No. 39, 1996 15709
Figure 1. Kinetic data for the association reaction of Cu2 with NH3 at the indicated temperatures in He buffer gas at 6.0 Torr of total pressure. S and S0 are LIF signals for Cu2 in the presence and absence of NH3, respectively. τ is the residence time of Cu2 in the reaction zone (τ ) 38 ( 2 ms). The slopes of the plots give second-order rate coefficients.
Figure 3. Kinetic data for the association reaction of Ag2 with NH3 in the temperature range 297-317 K in He buffer gas at 6.0 Torr of total pressure. Kp is the equilibrium constant for the association reaction in units of bar-1. The slope of the plot gives the enthalpy change for the association reaction.
TABLE 1: Kinetic Data for Association Reactions of Cu2 and Ag2 with NH3 in He Buffer at 6 Torra Cu2 + NH3 T (K) 296 348
ka
(10-12
Ag2 + NH3 cm3 s-1)
1.75 ( 0.26 0.81 ( 0.12
T (K)
Keq (10-15 cm3)
297 303 308 312 317
4.19 ( 0.42 2.35 ( 0.23 1.82 ( 0.27 1.02 ( 0.20 0.78 ( 0.15
a
ka is the second-order rate coefficient for the association reaction and Keq is the equilibrium constant for the association reaction (see the text). Uncertainties are estimated relative errors.
zone: Figure 2. Kinetic data for the association reaction of Ag2 with NH3 at the indicated temperatures in He buffer gas at 6.0 Torr of total pressure. S and S0 are LIF signals for Ag2 in the presence and absence of NH3, respectively. The slopes of the plots give equilibrium constants for the association reaction.
by coupling the output of the PMT through 50 Ω to a Tektronix Model 7D20 programmable digitizer. Results and Discussion Reaction of Cu2 and Ag2 with NH3. Kinetic data for association reactions of Cu2 and Ag2 with NH3 in helium at room temperature have been reported previously.2,3 Figures 1-3 show data that extend the previous work to include variable-temperature measurements in the range 296-348 K. The results are summarized in Table 1. For an association reaction of type 1, where M is a metal atom or cluster, flow-tube kinetic measurements may be made under limiting conditions of kinetic or equilibrium control: ka
M + NH3 + He {\ } MNH3 + He k
(1)
d
Under kinetic control, the association reaction with second-order rate coefficient ka is much faster than the dissociation reaction with first-order rate coefficient kd. In this case, and under pseudo-first-order reaction conditions, the depletion ratio of M depends on the concentration of NH3 in the reaction zone, as shown in eq 2, where τ is the residence time of M in the reaction
[M]/[M]0 ) exp(-ka[NH3]τ)
(2)
Under equilibrium control, the rates of the dissociation and association reactions are comparable. If, in addition, the reaction rate is fast compared to the rate of transport of M through the reaction zone, then the depletion of M is governed by the equilibrium constant (eq 3). The depletion ratio depends on the concentration of NH3, as shown in eq 4, and is independent of τ:
Keq ) [MNH3]eq/[M]eq[NH3]eq
(3)
[M]0/[M] ) Keq[NH3] + 1
(4)
Equation 4 applies in the case where the only reaction of M is formation of the monoligand complex MNH3 and further reaction of MNH3 with NH3 is unimportant. In Figure 1, kinetic data for the reaction of Cu2 with NH3 at 296 and 348 K are plotted in the form of eq 2. In previous work2 it was shown that the second-order rate coefficient at 296 K was directly proportional to the pressure of helium buffer gas, as expected for an association reaction in the limit of low buffer gas pressure. This is a clear indication of kinetic control. If the assumption is made that Cu2 binds only one NH3 ligand at 296 K, then the observation of kinetic control implies that the binding energy E0 is at least 16 kcal mol-1 (E0 ) -∆E0° for the association reaction eq 1). This follows from an estimate of the standard entropy change for the association reaction, ∆S°296 ) -30.1 cal K-1 mol-1 at 296 K, which is obtained in
15710 J. Phys. Chem., Vol. 100, No. 39, 1996
Mitchell et al.
TABLE 2: Binding Energies Measured in This Work for Monoligand Complexes of Atomic and Diatomic Copper and Silver with Ammonia (kcal mol-1)a atom Cu Ag a
dimer e11 360 nm. It can be noted that the occurrence of two transitions with roughly equal weights in the excitation spectrum suggests that the upper states of Cu2NH3 have Cu2 parentage A and B. This follows because the radiative lifetimes of these states of Cu2 (115 and 40 ns, respectively)10 indicate comparably large oscillator strengths, while that of the C state (800 ns)10 indicates a substantially smaller oscillator strength. The schematic poten-
tials in Figure 10 associate the higher energy transition of Cu2NH3 near 340 nm with the A state of Cu2. Bonding in Cu2NH3 and Ag2NH3. A summary of binding energies of ammonia on metal atoms, dimers, clusters, and surfaces is presented in Table 4. For the complexes CuNH3, MoNH3, AgNH3, and Ag2NH3, experimental measurements of binding energies are in good agreement with recent theoretical estimates. The theoretical estimates are given in parentheses in Table 4. For Cu2NH3, there is relatively poor agreement between theory and experiment. A significant feature of the results in Table 4 is the ordering of ammonia binding energies in the sequence dimer > atom for copper, silver, and molybdenum. A similar ordering of binding energies has been observed for complexes of Ag and Ag2 with rare gas atoms.13,14 The ammonia binding energies in Table 4 are further ordered as copper > silver ∼ molybdenum for both atoms and dimers. With regard to the structures of the dimer complexes, quantum chemical calculations have predicted pseudolinear or end-on coordination geometries for Cu2NH36,7 and Ag2NH3,5,7,8 with the nitrogen atom of ammonia bonded to a single metal atom. A further general observation can be made concerning the large blue shift of the A-X absorption bands of Cu2 (∼8000 cm-1) and Ag2 (∼10 000 cm-1) upon complexation with ammonia and the dissociative character of the excited state. All of these observations are consistent with a simple model of bonding in the ammonia complexes based on extended Hu¨ckel (EH) molecular orbital calculations. The main trends in the ammonia binding energies, dimer > atom for Cu, Ag, and Mo and Cu > Ag ∼ Mo for atoms and dimers, were reproduced by the EH calculations.15 The calculations also indicated a clear preference for end-on over side-on coordination geometry for all dimer complexes. The metalligand bonding can be described in terms of two effects: (1) a dative interaction involving the lone pair of electrons on ammonia and the lowest unoccupied orbital of the metal, (2) mixing of orbitals on the metal to reduce repulsion between the lone pair and occupied orbitals of the metal. The trends in the bonding energetics can be understood in simple terms by considering the relative importance of these effects. The stronger binding of ammonia on the metal dimers in comparison with the atoms arises because (1) the lowest unoccupied orbital of the dimer (σ*) is at lower energy than the corresponding orbital of the atom (p orbital), which leads to more favorable dative bonding for the dimer, and (2) mixing between the valence σ and σ* orbitals of the dimer is more efficient than sp hybridization in the atom for alleviating electronic repulsion by polarization of charge on the metal away from ammonia. A representative molecular orbital energy level diagram for Ag2NH3 in pseudolinear geometry (point group C3V) is shown in Figure 11. It was found that the bonding energetics were in all cases insensitive to the presence of d orbitals on the metal, so the d orbitals of Ag are omitted from the diagram. Polarization of the sσ orbital of Ag2 by mixing with sσ* in the
TABLE 4: Binding Energies for Monoligand Complexes of Ammonia with Metal Atoms, Dimers, and Clusters in Comparison with Theoretical Predictions (in Parentheses) and Adsorption Energy of Ammonia on Copper and Silver Surfaces copper Cua a
Cu2
Cu(100)j a
i
E0 (kcal mol-1) e11 (12)b 20 ( 1 (27.2)f (25)g 13.6
silver Aga a
Ag2
Ag10i Ag16 i Ag(110)k
E0 (kcal mol-1)
molybdenum
E0 (kcal mol-1)
atom for copper and silver. Also, copper > silver holds for surfaces as well as for atoms and dimers. The results for the silver clusters16 Ag10 and Ag16 show that the variation in binding energy between dimer and bulk surface is nonmonotonic. Simplified models of cluster-ligand interactions of the type used in the present study may be useful for correlating and interpreting such data.
Figure 11. Molecular orbital energy level diagram for Ag2NH3 in the end-on coordination geometry from extended Hu¨ckel calculations. The plot of the wave function for the highest occupied molecular orbital shows how the orbital is polarized away from the ammonia ligand. In order of increasing energy, the illustrated molecular orbitals of Ag2NH3 have predominant character of ammonia lone pair, Ag2 sσ, and Ag2 sσ*.
end-on configuration is illustrated in Figure 11. This produces a substantial dipole moment on the dimer, which stabilizes the complex by interaction with the dipole on ammonia. The results of high-level quantum chemistry calculations6,7 are consistent with this interpretation. Due to the polarization of the dimer in the complex, bonding of a second ammonia ligand on the opposite side of the dimer is highly unfavorable.4 This explains the observation that Ag2 and Mo24 bind only a single NH3 molecule under roomtemperature conditions. In the case of side-on coordination of ammonia on the dimer, symmetry considerations dictate that the acceptor orbital on the dimer is pπ. This orbital is at higher energy than sσ*, so that the dative interaction is favored for end-on relative to side-on coordination. The relative energies, spatial extent, and symmetry of the orbitals are such that mixing between sσ and sσ* is more efficient than sσ/pπ mixing for polarizing charge on the metal away from ammonia, so that the end-on structure is favored in this respect as well. The trend in binding energies, Cu > Ag ∼ Mo for atoms and dimers, can similarly be understood from considerations of orbital overlap and energetics as described at the EH level. A general feature of the molecular orbital energy level diagram for the dimer complex is a large destabilization of the sσ* orbital compared with sσ, as illustrated in Figure 11. This is due to mixing of these orbitals in the end-on coordination geometry and results in a substantial blue shift of the A-X (sσ2sσ1sσ*1) absorption band, as observed. Note that a red shift of this absorption band is expected in the case of side-on coordination of ammonia. In this case sσ mixes with pπ rather than sσ*, so that sσ is destabilized and sσ* is relatively unperturbed. The experimental observation of large blue shifts for Cu2NH3 and Ag2NH3 thus is consistent with end-on coordination geometries for the complexes, as predicted by highlevel calculations.6,7 The dissociative character of the excited states can also be understood from the molecular orbital analysis. In the simplest description the lone pair orbital of ammonia interacts with sσ and sσ* on the dimer, and these dimer orbitals become nonbonding and antibonding, respectively, with respect to ammonia. The excited state of the complex with configuration sσ1sσ*1 therefore is unstable and dissociates to produce ammonia and the excited (sσ1sσ*1) metal dimer.
Conclusion Binding energies have been estimated for monoligand complexes of copper and silver atoms and dimers with ammonia. The binding energy of a second ammonia ligand on Ag2 is much lower than that of the first. The dimer complexes are more strongly bound than the atom complexes, and copper binds ammonia more strongly than silver for both atom and dimer complexes. The relative stabilities of the copper and silver complexes follow the desorption energies of ammonia on copper17 and silver18 surfaces. This is consistent with the concept of localized bonding19 of ammonia at on-top sites on the surfaces. The dimer complexes have end-on coordination geometries. Photoexcitation in the region of the perturbed (blueshifted) A-X transitions of the dimers results in prompt dissociation of the complexes to ammonia and excited metal dimers. Thermochemical data and photodissociation action spectra have been used to construct potential energy curves for the ground and excited states. The end-on coordination geometry of the dimer complexes, their enhanced stability relative to the atom complexes, and the main features of their spectroscopic and photochemical properties can be understood in terms of a simple model of bonding based on extended Hu¨ckel calculations. References and Notes (1) Yang, D. S.; Zgierski, M. Z.; Rayner, D. M.; Hackett, P. A.; Martinez, A.; Salahub, D. R.; Roy, P. N.; Carrington, T., Jr. J. Chem. Phys. 1995, 103 (13), 5335-5342. (2) Lian, L.; Akhtar, F.; Hackett, P. A.; Rayner, D. M. Int. J. Chem. Kinet. 1994, 26 (1), 85-96. (3) Lian, L.; Hackett, P. A.; Rayner, D. M. J. Chem. Phys. 1993, 99 (4), 2583-2590. (4) Lian, L.; Mitchell, S. A.; Rayner, D. M. J. Phys. Chem. 1994, 98 (45), 11637-11647. (5) Rayner, D. M.; Lian, L.; Fournier, R.; Mitchell, S. A.; Hackett, P. A. Phys. ReV. Lett. 1995, 74 (11), 2070-2073. (6) Fournier, R. J. Chem. Phys. 1995, 102 (13), 5396-5407. (7) Boussard, P. J. E.; Siegbahn, P. E. M.; Svensson, M. Chem. Phys. Lett. 1994, 231 (4-6), 337-344. (8) Personal communication from Rene Fournier, 1996. (9) Franck-Condon factors and related quantities were calculated by using a revised version of the program by Zare (Zare, R. N., University of California Radiation Laboratory Report, UCRL-10925, 1963) kindly provided by P. Bunker, Steacie Institute of Molecular Sciences, NRC, Ottawa. (10) Morse, M. D. In AdVances in Metal and Semiconductor Clusters; Duncan, M. A., Ed.; JAI Press: Greenwich, CT, 1993; Vol. 1, pp 83-121. (11) Herzberg, G. Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules, 2nd ed.; Van Nostrand Reinhold: New York, 1950; pp 388-394. (12) Bondybey, V. E.; Schwartz, G. P.; English, J. H. J. Chem. Phys. 1983, 78 (1), 11-15. (13) Brock, L. R.; Duncan, M. A. J. Chem. Phys. 1995, 103 (21), 92009211.
Photodissociation of Cu2NH3 and Ag2NH3 (14) Robbins, D. L.; Willey, K. F.; Yeh, C. S.; Duncan, M. A. J. Phys. Chem. 1992, 96 (12), 4824-4829. (15) Extended Hu¨ckel calculations were performed by using the program CACAO, described in Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67 (5), 399-402. (16) Lian. L.; Mitchell, S. A.; Hackett, P. A.; Rayner, D. M. J. Chem. Phys. 1996, 104 (13), 5338-5344. (17) Wu, K. J.; Kevan, S. D. J. Chem. Phys. 1991, 94 (11), 74947498. (18) Thornburg, D. M.; Madix, R. J. Surf. Sci. 1989, 220, 268-294.
J. Phys. Chem., Vol. 100, No. 39, 1996 15715 (19) Siegbahn, P. E. M.; Wahlgren, U. Metal-Surface Reaction Energetics; Shustorovich, E., Ed.; VCH: New York, 1991; Chapter 1, pp 1-52. (20) Papai, I. J. Chem. Phys. 1995, 103 (5), 1860-1870. (21) Blomberg, M. R. A.; Siegbahn, P. E. M.; Svensson, M. Inorg. Chem. 1993, 32 (20), 4218-4225. (22) Ritze, H.-H.; Radloff, W. Chem. Phys. Lett. 1996, 250, (3-4), 415-420.
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